^^^^?'pil

VOLUME II

Findings

RINCON BAYOU DEMONSTRATION PROJECT

Concluding Report

UNITED STATES DEPARTMENT OF THE INTERIOR
Bureau of Reclamation

In Cooperation >^ith

University of Texas Marine Science Institute

September 2000

Cover: Rincon Overflow Channel, September 1997

RINCON BAYOU DEMONSTRATION PROJECT

Concluding Report

VOLUME II

Findings

September 2000

Citation

Bureau of Reclamation. 2000. Concluding Report: Rincon Bayou
Demonstration Project. Volume II: Findings. United States
Department of the Interior, Bureau of Reclamation, Oklahoma-Texas
Area Office, Austin, Texas.

Abstract

The Rincon Bayou Demonstration Project significantly lowered the minimum flooding threshold of
the upper Nueces Delta, thereby increasing the opportvinity for larger, more frequent diversions of
fresh water from the Nueces River. During the 50-month demonstration period, the amount of
fresh water diverted into the upper Nueces Delta was increased by about 732%. Five fireshwater
inflow events were sufficient to activate the project's Rincon Overflow Channel and inundate, to
varying degrees, the tidal flats of the upper delta. These tidal flows would not have otherwise been
direcdy fireshened. As a result, in a relatively short period of time (only 4.2 years after the opening of
the project's Nueces Overflow Channel), the average salinity gradient in the upper delta reverted to a
more natural form, with average salinity concentrations in upper Rincon Bayou becoming the lowest
in Nueces Delta.

The effects of the demonstration project on the ecology of Rincon Bayou and the upper Nueces
Delta were positive to the environment. Single-celled plant communities in the water column
(phytoplankton) and on the surface of the sediments (microphytobenthos) evidenced increases in
primary productivity with the reduction of salinity concentrations. Benthic communities (composed
of bottom-dwelling organisms) evidenced increases in abundance, biomass and diversity. And,
vegetation communities evidenced increases in plant cover and decreases in bare area. In summary,
it was observed that freshwater inflow controlled, to a great extent, the ecological fiinction of the
upper delta ecosystem by regulating critical biological mechanisms.

A significant degree of ecological function was returned to the Nueces Delta and Nueces Estuary
ecosystems by the demonstration project. Prior to the project, persistendy high salinity
concentrations severely inhibited the function of the Nueces Delta, and the delta's natural
contribution to the greater estuary ecosystem was limited to infrequent periods when natural flow
events occurred. With the restored regular interaction between the Nueces River and Rincon Bayou,
fresh water and nutrients were more consistendy introduced into the upper delta. As a resvilt,
estuarine habitat in the delta component of the Nueces Esmary improved in both quality and
quantity, and foraging opportunities for many estuarine species were increased.

Contributors

PRINCIPAL INVESTIGATORS

Kenneth H. Dunton, Ph.D.

University of Texas

Marine Sdence Institute

750 ChanneHview Drive

Port Aransas, Texas 78373

George H. Ward, Ph.D.

Uruversity of Texas

Center for Research in Water Resources
University of Texas, PRC-119
Austin, Texas 78712

Paul A. Montagna, Ph.D.

University of Texas

Marine Science Institute

750 Channelview Drive

Port Aransas, Texas 78373

Terty E. Whitledge, Ph.D.

University of Alaska Fairbanks
School of Fisheries and Ocean Sciences
P.O. Box 757220
Fairbanks, Alaska 99775-7220

CO-AUTHORS

Headier D. Alexander-Mahala

University of Texas

Marine Science Institute

750 Channelview Drive

Port Aransas, Texas 78373

Richard D. Nelson, Ph.D.

U.S. Bureau of Redamation
Dakotas Area Office
304 East Broadway Avenue
Bismarck, North Dakota 58502

Michael J. Irlbeck

U.S. Bureau of Redaniation

Oklahoma-Texas Area Office

300 East 8th Street, Room 801

Austin, Texas 78701

Christine Ritter, Ph.D.

Texas Water Development Board
Environmental Section
P.O. Box 13231
Austin, TX 78711-3231

Richard D. Kalke

University of Texas

Marine Science Institute

750 Channelview Drive

Port Aransas, Texas 78373

Richard A- Roline

U.S. Bureau of Reclamation
Denver Center, Code D-8820
P.O. Box 25007
Denver, Colorado 80225

Jonnie G. Medina

U.S. Bureau of Redamation

Denver Center, Code D-8510

P.O. Box 25007

Denver, Colorado 80225

Dean A. StockweU, Ph.D.

University of Alaska Fairbanks
School of Fisheries and Ocean Sdences
P.O. Box 757220
Fairbanks, Alaska 99775-7220

EDITOR

Robert G. Harris

U.S. Bureau of Reclamation

Great Plains Regional Office

316 North 26th Street

Billings, Montana 59101

PROJECT DIRECTOR
Michael J. Irlbeck

Contents

Page

1-1 Chapter One - Introduction

1-2 Backgrovind

1-4 The Rincon Bayou Demonstration Project

1-4 Project Features

1-9 Participants

1-9 Authority and Funding

2-1 Chapter Two - Study Area

2-1 Background

2-1 The Nueces Estuary

2-2 The Nueces Delta

2-3 Hydrography of the Nueces Estuary and Delta

2-4 Water Level

2-6 Salinity and Freshwater Inflow

2-8 Ecology of the Nueces Estuary and Delta

2-8 Estuarine Habitats in the Nueces Delta

2-11 History of the Nueces Estuary and Delta

2-14 Changes in Hydrography

2-18 Changes in Ecology

3-1 Chapter Three - Hydrography

3-1 Introduction

3-1 Objectives

3-2 Methods and Approach

3-2 Data Sources

3-3 Quantification of Hydrographic Interactions in the

Study Area

3-4 Identification of Hydrographic Events During the

Demonstration Period

3-7 Results

3-7 Overview of Hydrographic Events that Occurred

During the Demonstration Period

3-9 Summary of Selected Individual Events

Contents V i

3-20

Discussion

3-20

Flooding Thresholds for the Upper Nueces Delta

3-20

Activation and Behavior of Demonstration Project

Features

3-22

Summary

3-23

Exchange Events

3-23

Positive-Flow Events

3-24

Tidal Flat Inundation Events

4-1

Chapter Four - Water Column Productivity

4-1

Introduction

4-1

Objectives

4-2

Methods and Approach

4-2

Study Design

4-3

Measurements

4-4

Results

4-4

Hydrography

4-5

Nutrients

4-10

Phytoplankton Pigments

4-12

Water Column Production

4-14

Nutrient Amendment Bioassays

4-18

Microphytobenthic Sediment Biomass and Production

4-22

Discussion

4-24

Summary

5-1

Chapter Five - Benthic Communities

5-1

Introduction

5-2

Objectives

5-2

Methods and Approach

5-2

Study Design

5-4

Measurements

5-6

Results

5-6

Salinity

5-7

Temperature

5-8

Dissolved Oxygen

5-8

Benthos

5-21

Discussion

5-21

Macrofauna and Meiofauna

5-22

Trophic Links

5-24

Effects of Diversions as Disturbances

5-24

Summary

ii ^ Contents

Page

6-1 Chapter Sex - Vegetation Communities

6-1 Introduction

6-2 Objectives

6-2 Materials and Methods

6-2 Monitoring Stations

6-4 Open Water and Pore Water Chemistry

6-4 Transect Sampling

6-5 Percent Cover and Leaf Area Index

6-5 Analyses

6-6 Biomass

6-6 Results

6-7 Salinity

6-12 Ammonium

6-13 Nitrite+Nitrate

6- 1 5 Large-Scale Whole Transect vVnalyses: Individual Species

Responses to Events

6-21 Large-Scale Whole Transect Analyses: Leaf Area Index

6-21 Small-Scale Analyses

6-37 Biomass

6-43 Root: Shoot Ratios

6-45 Discussion

6-45 Salinity

6-45 Inorganic Nitrogen

6-46 Vegetation

6-48 Summary

7-1 Chapter Seven - Synthesis and Conclusions

7-2 Changes in Hydrography

7-2 Effects on Salinity

7-5 Biological Responses

7-5 Water Column Productivity

7-6 Benthic Communities

7-6 Vegetation Commumties

7-7 Integration of Project Effects

7-8 A Conceptual Model

7-10 Summary

8-1 Chapter Eight - Future Opportunities

8-2 Opportunities for a Permanent Diversion Project

8-2 Opportunities for Further Ecological Study

8-2 Selection and Monitoring of Indicator Species

8-3 Modeling

8-4 Opportunities for Integration with Bay and Esmary

Release Schedules

8-4 Opportunities for Adaptive Management

Contents

9-1

Chapter Nine - Literature Cited

9-1

Chapter 1

Introduction

9-2

Chapter 2

Study Area

9-4

Chapter 3

Hydrography

9-4

Chapter 4

Water Column Producti\'ity

9-5

Chapter 5

Benthic Communities

9-6

Chapter 6

Vegetation Communities

9-8

Chapter 7

Synthesis and Conclusions

9-8

Chapter 8

Future Opportunities

Appendices

A-l A Technical Notes ON THE RiNCON Gauge AND Data

B-1 B Hydrography OF the Nueces Delta AND ESTUARY:
1992-1999

C-1 C Analysis OF THE Historic Flow Regime OF THE

Nueces River into the Upper Nueces Delta and of
THE Potential Restoration Value of the Rincon
Bayou Demonstration Project

D-l D Recent Trends IN Precipitation Occurring on THE
Nueces River Watershed of South Texas

E-l E Utilization OF EsTUARiNE Organic Matter During
Growth and Migration byJuvenile Brown Shrimp

PENAEUS AZTEOJS IN A SOUTH TEXAS ESTUARY

F-1 F EFFECTS OF Temporality, Disturbance Frequency
and Water Flow on an Upper Estaurine
Macroinfauna Community

G-1 G Field Notes AND Observations FROM Benthic
Sampling Trips: October 1994 - December 1999

iv ^ Contents

Tables

Tahk Page

1 - 1 Summary of the annual monitoring program

conducted as part of the demonstration project. 1-7

2-1 Summary of mean annual flow of the Nueces River
into the Nueces Estuary (1940 to 1996) and upper
Nueces Delta (1940 to 1999). 2-18

3-1 Summary of hydrographic data sources. 3-2

3-2 Criteria used to define hydrographic events in the

data record by response variables. 3-5

3-3 Summary of hydrographic events which occurred

during the demonstration period: October 1, 1994,
through December 31, 1999. 3-8

4-1 Hydrographic events (from Table 3-3) occurring

prior to each water column sampling period. 4-6

5-1 Hydrographic events (from Table 3-3) occurring prior

to each benthic sampling period. 5-9

5-2 Average benthos characteristics at all stations during

the demonstration period. 5-9

5-3 Parameters from nonlinear regressions to predict

macrofauna characteristics from salinity (Figure 5-8). 5-13
5-4 List of species average abundances over the course

of the study at each station. 5-15

5-5 Species overall dominance. 5-16

5-6 Temporal species abundance (» individuals found

in all 6 stations, 18 samples) per sampling period. 5-20

5-7 Composition of the meiofauna community. 5-21

6-1 Hydrographic events (from Table 3-3) occurring

prior to each vegetation sampling period. 6-7

6-2 Mean open water (OW) and pore water (PW)

salinity values at each station. 6-8

6-3 Mean open water ammonium (NH4*) and nitrite+

nitrate (NOj -I-NO3') concentrations at each

station. 6-12

6-4 Mean pore water ammonium (NH4*) and nitrite+

nitrate (NO2+NO3 ) concentrations at each

station. 6-13

6-5 Summary of species percent cover results from

GIS analyses. 6-23

6-6 Summary of LAI results from small scale

GIS analyses. 6-38

6-7 Average total biomass values for four halophjrte

species at the three stations on each sampling date. 6-42
6-8 Average (n=4) rootrshoot ratios for four halophyte

species at the three stations on each sampling date. 6-44

7-1 Summary of the effects of the demonstration project

on the upper Nueces Delta. 7-8

Contents

Figures

Figure Page

1-1 Location of the Nueces Delta along the

Coastal Bend of Texas. 1-2

1-2 Historical population trend for the areas

served by the Nueces River at Calallen. 1-2

1-3 The Nueces River Basin, including major

drainages and reservoirs. 1-3

1-4 Study area for the Rincon Bayou Demonstration

Project and location of project features. 1-5

1-5 View of the Nueces Overflow Channel. 1-5

1-6 View of the private road crossing separating the

upper and central Rincon Bayou channels. 1-6

1-7 View of the Rincon Overflow Channel. 1-6

1-8 View of the low water crossing at the head of

Rincon Bayou. 1-7

1-9 Overview of the monitoring sites and stations

established for the Rincon Bayou Demonstration

Project. 1-8

2-1 The Nueces Estuary and Delta. 2-2

2-2 The Nueces Delta. 2-2

2-3 Typical view of the upper Nueces Delta. 2-3

2-4 Typical view of the lower Nueces Delta. 2-3

2-5 Nueces River (looking downstream) just below the

IH 37 bridge under flood conditions (April 1992). 2-7

2-6 Natural flooding of the upper Nueces Delta,

April 1992. 2-7

2-7 View of salt marsh habitat in the upper Nueces Delta. 2-9

2-8 View of water column habitat in central Rincon Bayou. 2-10
2-9 View of muddy bottom habitat in upper Rincon

Bayou during a period of low water. 2-10

2-10 View of algal mat habitat near South Lake. 2-11

2-11 Example of concrete rubble found at several points
along the north bank of the Nueces River
downstream from the IH 37 bridge. 2-12

2-12 Example of Rangia middens (i.e., piles of bi-valve

shells) found in Nueces Delta. 2-13

2-13 Typical view of the Nueces River floodplain

upstream of the Nueces Delta. 2-14

2-14 The Nueces River Basin, including major drainages

and reservoirs. 2-16

2-15 Annual precipitation trends at four gauges about the

greater Nueces River watershed since about 1900. 2-17

2-16 Mean annual precipitation of available data at four

gauges about the greater Nueces River watershed. 2-18

Contents

Piffi" Page

3-1 Location of hydrographic gauging stations in the

Nueces Delta and upper Nueces Bay. 3-3

3-2 View of the Nueces River at Calallen Diversion Dam. 3-4

3-3 View of the Rincon gauge (Station 0821 1 503). 3-4

3-4 Hydrographic data for 1994. 3-10

3-5 Hydrographic data for 1995. 3-11

3-6 Hydrographic data for 1996. 3-12

3-7 Hydrographic data for 1997. 3-13

3-8 Hydrographic data for 1998. 3-14

3-9 Hydrographic data for 1999. 3-15

3-10 Selected hydrographic data for Events 12, 13 and 14

(October 2 through November 3, 1996). 3-16

3-11 Selected hydrographic data for Event 23

(September 1 through 31, 1998). 3-18

3-12 Discharge into the tidal flats area through the road

crossing at the north end of the Rincon Overflow
Channel during Event 16 (June 27, 1997). 13-19

3-13 Visual satellite image showing the location of
Hurricane Bret in relation to the Nueces
Watershed just before landfall (August 22, 1999). 3-20

3-14 The Nueces Overflow Channel looking southwest. 3-21

3-15 The Rincon Overflow Channel. 3-22

3-16 The low water crossing at the head of the upper

Rincon Bayou channel. 3-22

3-17 Typical view of the Nueces Overflow Channel

during tidal exchange. 3-23

3-18 View of the upper Rincon Bayou (background)

during a typical positive-flow event (Event 16). 3-23

3-19 View of diverted fresh water in the tidal flats area in
the upper Nueces Delta during activation of the
Rincon Overflow Channel (Event 16). 3-24

4-1 Location of water column sampling stations in the

upper Nueces Delta. 4-2

4-2 Salinity at all water column stations (except

Station 62) for each sampling date. 4-5

4-3 Average salinity at each sampling site for each

sampling date. 4-7

4-4 Nitrate concentrations at all water column stations

(except Station 62) for each sampling date. 4-8

4-5 Ammonium concentrations at all water column

stations (except Station 62) for each sampling date. 4-9

4-6 Nitrate concentrations at all water column stations

(except Station 62) for each sampling date. 4-9

Contents V vii

Figure Page

4-7 Dissolved inorganic nitrogen concentrations at all

water column stations (except Station 62) for

each sampling date. 4-10

4-8 Percent of dissolved inorganic nitrogen contributed

by ammonium at all water column stations (except

Station 62) for each sampling date. 4-11

4-9 Phosphate concentrations at all water column stations

(except Station 62) for each sampling date. 4-11

4-10 Nitrogen to phosphorous (N:P) ratios at all water

column stations (except Station 62) for each

sampUng date. 4-12

4-11 Chlorophyll concentrations at all water column

stations (except Station 62) for each sampling

date. 4-13

4-12 Primary production at all water column stations

(except Station 62) for each sampling date. 4-13

4-13 Assimilation index at all water column stations

(except Station 62) for each sampling date. 4-14

4-14 Results from nutrient amendment bioassays for

selected stations: March 7, 1997. 4-15

4-15 Results firom nutrient amendment bioassays for

selected stations: April 22, 1997. 4-16

4-16 Results from amendment bioassays for

selected stations: August 7, 1997. 4-17

4-17 Sediment chlorophyll concentrations at all water

column stations (except Stations 62 and 68)

for each sampling date. 4-18

4-18 Sediment chlorophyll to phaeopigments ratio

at aU water column stations (except

Stations 62 and 68) for each sampling date. 4-19

4-19 Total sediment and water column chlorophyll

and salinity. 4-19

4-20 Sediment chlorophyll and water column

chlorophyll. 4-19

4-21 Sediment primary productivity' concentrations at

all water column stations (except Stations 62

and 68) for each sampling date. 4-20

4-22 Sediment chlorophyU and sediment primary

productivity. 4-21

4-23 Sediment primary productivity and water

column productivity. 4-21

4-24 Total primary productivity (sediment and water

column) and salinity. 4-21

4-25 Assimilation index at all water column stations

(except Stations 62 and 68) for each

sampling date. 4-22

Contents

Pifft" Page

5-1 Locations of benthic sampling stations. 5-3

5-2 View of benthic Station C in upper Rincon Bayou

under dry (above) and wet (below) conditions. 5-6

5-3 Salinity at all benthic stations (A through F) for

each sampling date. 5-7

5-4 Marsh-wide average salinity at each sampling site

for each sampling date. 5-8

5-5 Dissolved oxygen at all benthic stations

(A through F) for each sampling date. 5-10

5-6 Average macrofauna biomass (a), abundance (b)

and diversity at each station (A through F). 5-11

5-7 Marsh-wide averages of macrofauna biomass,

abundance and diversity for all stations. 5-12

5-8 Relationship between average macrofauna

characteristics and salinity. 5-14

5-9 Stehlospio benedicti. 5-17

5-10 Principal components analysis of 16 most common

species, including species loadings (a) and station

score plots (b). 5-18

5-11 Average meiofauna abundance for each station (a)

and marsh-wide (b). 5-19

5-12 Marsh-wide average abundance of meiofauna

and macrofauna. 5-21

5-13 Comparison of marsh-wide average chlorophyll

biomass with macrofauna biomass (a) and

meiofauna abundance (b). 5-23

6-1 Location of vegetation sampling stations in the

upper Nueces Delta. 6-3

6-2 The layout and dimensions of a typical vegetation

transect. 6-5

6-3 Total monthly precipitation during the demonstration

period. 6-8

6-4 Salinity for open water (a) and pore water (b) at each

sampling site for each sampling date. 6-9

6-5 Correlation between total flow through Rincon

Bayou and open water salinity for each station. 6-10

6-6 Mean pore water ammonium values for each station. 6-14

6-7 Reference Station average total transect percent cover
for the five dominant species and bare area on
each sampling date. 6-16

6-8 Station II average total transect percent cover for
the five dominant species and bare area on each
sampling date. 6-17

Contents ^ ix

Figure Page

6-9 Station III average total transect percent cover for the

five dominant species and bare area on each

sampling date. 6-18

6-10 Total transect leaf area index (LAI) for each sampling

date at each transect. 6-22

6-11 Reference Station percent cover maps for the five

springtime sampling periods. 6-24

6-12 Station II percent cover maps for the five springtime

sampling periods. 6-25

6-13 Station III percent cover maps for the five springtime

samphng periods. 6-26

6-14 Reference Station percent cover maps on the

sampling date prior and three sampling dates

following the July 1997 composite

hydrographic event. 6-28

6-15 Station II percent cover maps on the sampling date

prior and three sampling dates following the

July 1997 composite hydrographic event. 6-29

6-16 Station III percent cover maps on the sampling

date prior and three sampling dates foOowing

the July 1997 composite hydrographic event. 6-30

6-17 Reference Station percent cover maps on the

sampling date prior and three sampling dates

following the October 1998 composite

hydrographic event. 6-31

6-18 Station II percent cover maps on the sampling date

prior and three sampling dates following the

October 1998 composite hydrographic event. 6-32

6-19 Station III percent cover maps on the sampling date

prior and three sampling dates following the

October 1998 composite hydrographic event. 6-33

6-20 Reference Station percent cover maps on the

sampling date prior and three sampling dates

following the September 1 999 composite

hydrographic event. 6-34

6-21 Station II percent cover maps on the sampling

date prior and three sampling dates following the

September 1999 composite hydrographic event. 6-35

6-22 Station III percent cover maps on the two sampling

dates prior and two sampUng dates following the

September 1999 composite hydrographic event. 6-36

6-23 Reference Station leaf area index (LAI) firom

GIS analyses. 6-39

6-24 Station II leaf area index (LAI) from GIS analyses. 6-40

6-25 Station III leaf area index (LAI) from GIS analyses. 6-41

Contents

7-1 Selected stations used for analysis of long-term

salinity changes in Rincon Bayou. 7-3

7-2 Long-term average salinity values for selected stations
in the Nueces River and Rincon Bayou before (a)
and after (b) implementation of the demonstration
project. 7-4

7-3 Total annual cumulations of selected freshwater
sources affecting the Nueces Delta during the
period 1992 through 1999. 7-5

7-4 Conceptual model of the Nueces Delta ecosystem. 7-9

8-1 View of the lower Nueces Delta with the City of

Corpus Christi in the background. 8-5

-S3U.

^t^l^* .^^

Findings

» -■

\\

CHAPTER ONE

Introduction

"These waters of the seaboard exert an
influence upon the affairs of mankind far out
of proportion to their size, for it is here that
land and sea meet, and the fresh and salt
waters of the earth intermingle."

♦ E.J. Perkins

The Rincon Bayou Demonstration Project was
conducted from 1994 through 1999 within the upper
Nueces Delta, located northwest of the city of Corpus
Christi, Texas. The delta was formed and is supported
by the Nueces River, which passes along its southern
edge as it empties into Nueces and Corpus Christi bays
(Figure 1-1). Because of the dynamic nature of this
natural system, the delta consists of a variety of
habitats, including open water, marshes and mud flats
and possesses a unique array of hydrological and
biological characteristics.

Ecologically, the Nueces Delta is part of the greater
Nueces Estuary, which is a brackish transitional zone
situated between the freshwater riverine habitats of the
Nueces River and the marine habitats of the Gulf of
Mexico. Salt water from the bays regularly inundate the
lower reaches of the delta during a variety of tidal and
atmospheric events. The delta is also periodically
inundated with fresh water when the Nueces River
occasionally spills out of its banks. Such freshwater
flooding events significandy contribute to the biological
productivity of the delta by providing a medium for
nutrient exchange and biochemical cycling, supplying
fresh water to marsh plant communities, transporting
detrital and other nutrient materials from the
established marsh vegetation and sediments to the bay,
and buffering bay salinity (Longley 1994). The Nueces
Delta has therefore been considered one of the most
important sources of nutrient material for the entire
Nueces Estuary system (Texas Department of Water
Resources 1981), supporting numerous plant and
animal communities, including commercially important
marine life.

Chapter One ♦ 1-1

BACKGROUND

NUECES BAY

DEMONSTRATION PROJECT
STUDY AREA

Aransas
Bay

Aransas
Pass

CORPUS CHRISTI BA Y

RBDFISH
BAY

Approximate Scale

Kilometers

0 S

OSOBAY

Gult of Mexico

lUarim ' %■■

Figure 1-1 : Location of the Nueces Delta along the Coastal
Bend of Texas.

Since the beginning of the 20th century, the human
population along the Coastal Bend region of Texas has
substantially increased (Figure 1-2). As a result, the
demand for fresh water to meet the growing municipal
and industrial needs of the region also increased
significandy. In response, two large reservoirs were
constructed in the Nueces Basin for the purpose of
storing flood flows. The first was Wesley Seale Dam
(Lake Corpus Christi), constructed on the Nueces River
by the City of Corpus Christi in 1958, and the second
was Choke Canyon Dam (Choke Canyon Reser\'oLr),
constructed in 1982 on the Frio River by the
U.S. Bureau of Reclamation (Reclamation) (Figure 1-3).
Choke Canyon Dam was designed to be operated in
conjunction with Lake Corpus Christi as part of one
reservoir system.

During the period when these two reservoirs were
being planned, particularly in the case of Choke
Canyon Dam, the potential adverse impacts of
reservoir operations on the bay and estuary systems
were a concern. It was generally suspected that these
impoundments would reduce the amount of fresh
water entering the Nueces Estuary and upper Nueces
Delta, adversely affecting the natural productivity of
these systems. However, there was very Utde specific
information available regarding the needs of delta and
estuary systems, or of their responses to changes in

freshwater inflows. In recognition of the bay and
estuary resources, the State of Texas required that, once
Choke Canyon Dam was completed and filled, a total

500 -

■D

c

1

o

S 300

/

1

Q.

Q. 200 -

/

/

o

1-

100 -

/

0 -

n

f

1860 1880 1900 1920 1940 1960 1980 2000

Figure 1-2: Historical population trend for the areas
served by the Nueces River at Calallen.

Sources: City of Corpus Christi 1981 and Texas Water Board
1992.

1-2 V Introduction

MEXICO

Gulf

of

Mexico

Figure 1-3: The Nueces River Basin, including major drainages and reservoirs.

Source of topographic base map: U.S. Geological Survey 1997.

of 186,274 10' m' (151,000 acre-ft) of water per year
would be provided to the estuaries by a combination
of releases and spills from the reservoir system at Lake
Corpus Christi Dam and return flows to Nueces
Estuary (Texas Water Rights Commission 1976).
Although the dam was completed in 1982, flow into
the reservoir was minimal for the first several years.
However, in June 1987, record rainfall over the Frio
River watershed filled the reservoir, and water was
released as flood control for the first time.

During the mid-1980's, several entities initiated regular
monitoring programs in the region. Salinity in the
Nueces Estuary began to be sampled continuously by
several State agencies and universities. Fish, shrimp,
crabs and oysters in Nueces Bay were sampled during
routine. State-wide coastal inventory surveys. Modest

research efforts were also undertaken by university
researchers to monitor the effect of the water releases
on hydrography and benthos in the estuary.

By the early 1990's, it had become apparent that there
had been a notable reduction in the freshwater inflow
to the delta and esmary systems. For example,
historically Nueces Bay had supported large popula-
tions of shrimp and oysters, which generally require
salinity concentrations in the range of 10 to 20 parts
per thousand (ppt) salt. During the relatively dry
period of the late 1980's and early 1990's, the salinity of
the bay had increased to hypersaUne conditions
(> 36 ppt), and consequendy the shrimp and oyster
populations were reduced. For example, from 1984
through 1989, shrimp har%'est declined in Nueces

Chapter One ♦ 1-3

Estuary, even though it was stable in the Aransas Bay
ecosystem to the north and the upper Laguna Madre to
the south (Montagna et al. 1998).

Because of contention regarding the amount of fresh
water dedicated to the bays and estuary from the
reser\'oir system, releases had not been made for that
purpose since Choke Canyon Reser\^oir filled. In 1992,
the Texas Natural Resources Conservation
Commission (TNRCC) implemented an operating plan
requiring minimum mandator)' inflows on a monthly
schedule totaUng 186,274 10^ m' (151,000 acre-ft). As
part of the same order, TNRCC also created the
Nueces Estuary Advisory Council to provide oversight
of the releases and monitor the operating plan, and to
make recommendations for improving the plan. In
April 1995, the original TNRCC order was revised to
adopt a "target" minimum monthly inflow plan instead
of a mandatory inflow amount. This change, which
resulted in an annual inflow target of 112,258 10^ m^
(91,000 acre-ft), was intended to mimic natural
hydrographic conditions in the Nueces Basin while
providing some relief to the water customers of the
resen'oir system.

THE RINCON BAYOU
DEMONSTRATION PROJECT

Beginning about 1993, a consortium of local, State and
Federal entities began investigating alternatives to
restore fresh water to the greater Nueces Estuary. In
1993, as part of this initiative, the Reclamation
undertook a temporary demonstration project to
provide detailed scientific information regarding the
freshwater needs of the delta and its response to
changes in freshwater inflows. The Rincon Bayou
Demonstration Project had two primary objectives:

1) To increase the opportunity for natural
freshwater flow events into the upper Nueces
Delta; and

2) To monitor any resulting changes in the hydro-
graphy and biological productivity of the delta.

Project Features

The area selected for the demonstration study
encompassed the northwestern portion of the upper
Nueces Delta, or that area generally north of Rincon
Bayou and west of the eastern-most railroad crossing
(Figure 1-4). This area represents both the historic
location of river inundation events and the western
limit of the Nueces Estuary {i.e., the tidally influenced
portions of Rincon Bayou and a large area of tidal
flats).

Water Diversion

Reclamation decided on a final demonstration project
design after reviewing several different alternatives.
The selected alternative provided an uncomplicated
means of increasing the opportunity for freshwater
diversion and distribution in the upper delta, while
preser%'ing the natural "event" mechamsm to which the
system had adapted. The physical aspects of the
project included two principal features: the Nueces
Ch^erflow Channel and the Rincon Overflow Channel
(Figure 1-4).

The primary feature of the demonstration project was
an overflow channel (Nueces Overflow Channel)
excavated from the Nueces River to the headwaters of
Rincon Bayou (Figures 1-4 and 1-5). The channel was
located approximately 60 m downstream of the
Interstate Highway 37 (IH 37) bridge along the north
bank. The design dimensions of the Nueces Overflow
Channel were approximately 274 m long and 12 m
wide, with a bottom elevation of 0.6 m (2.0 ft) mean
sea level (msl). This bottom elevation was selected so
as to prevent regular tidal exchange between the
Nueces River and the upper delta. However, early into
the demonstration period, the channel's effective
bottom elevation was lowered by flow events and tidal
exchange to approximately mean sea level. Minor
excavations were also made at two sites along the
headwater channel of Rincon Bayou to remove
channel-constricting sediment deposits, thereby
allowing a higher diversion volume during flood events.
Construction of the channel was completed
on October 26, 1995. The purpose of the Nueces
Overflow Channel and associated channel

1-4

Introduction

Northern
bluff line

Calallen
Diversion Dam

Figure 1-4: Study area for the Rincon Bayou Demonstration Project and location of project features. The

pre-existing private road crossing separating the upper and central Rincon Bayou channels was not part of the
demonstration project.

Figure 1-5: Viewof the Nueces Overflow Channel. The

view is looking northeast, with the Nueces River in the
foreground, and Rincon Bayou in the distant background.
The photo was taken on June 26, 1997, during the first
significant flow event.

Photo courtesy of the Bureau of Reclamation.

improvements was to lower the flooding tlireshold of
the Nueces River, thereby increasing the opportunity
for more frequent and higher magnitude flow events
into the upper Nueces Delta.

During the design phase of the project, Reclamation
determined that an existing private road crossing over
Rincon Bayou would act as a partial dam during larger
flood events, limiting the volume of water diverted into
die delta and substantially reducing the proposed
project's effectiveness (Figures 1-4 and 1-6). There-
fore, a second overflow channel (Rincon Overflow
Channel) upstream of this road crossing was designed
which would connect Rincon Bayou to the tidal
mudflat areas in the northern part of the delta
(Figure 1-7).

The design dimensions of the Rincon Overflow
Channel were approximately 610 m long and 30 m

Chapter One

1-5

Figure 1-6: View of the private road crossing
separating the upper and central Rincon Bayou
channels. This pre-existing structure (constructed
between September 1992 and March 1993) was not part of
the demonstration project. The photo was tal<en on June
26, 1997, during the first significant flow event. During
larger events, the structure backed water up in the upper
Rincon Bayou (left), as indicated by the difference in water
levels on either side of the road.

Photo courtesy of the Bureau of Reclamation.

Figure 1-7: View of the Rincon Overflow Channel. The

view is looking northeast from Rincon Bayou (foreground),
showing the channel's outlet into the tidal flats
(background). The photo was taken on June 26, 1997,
during the first significant flow event.

Photo courtesy of the Bureau of Reclamation.

wide, with a bottom elevation of 1 .22 m (4.0 ft) msl on
the upstream (south) end and 0.91 m (3.0 ft) msl on
the downstream (north) end. In addition, the
downstream end of the channel was crossed with an
elevated road over eight 24'-diameter HDPE culverts.
The acmal bottom elevations of the channel at the end
of the demonstration period were about 1.14 m
(3.75 ft) and 0.76 m (2.50 ft) msl, respectively. The
primary purpose of the Rincon Overflow Channel was
to provide a "spillway" during larger flow events that
would divert floodwater around the private road
crossing, thereby improving diversion and distribution
of fresh water within the delta during larger events.

In addition to these two principal features, numerous
access road improvements were made as part of
Reclamation's agreement with the pri%'ate landowners
in the delta. These included the installation of cattle
guards, placement of culverts in low areas, and
rehabilitation of a low water crossing over the upper
end of Rincon Bayou (Figures 1-4 and 1-8). This last
feature was improved by raising the crest elevation
from about 1.5 m (5.0 ft) msl to about 2.1 m (7.0 ft)
msl, and adding thirteen 36"-diameter HDPE culverts
in two locations to allow passage of flow events. The
purpose of these road improvements was to preserve
landowner access to the upper delta during the term of
die demonstration project and to minimize the
resistance to water moving into the delta during
discharge events.

Several other diversion alternatives were considered at
the beginning of the study. These included: 1) either a
total or partial diversion of the Nueces River into the
delta, 2) delivery of a continuous flow of fresh water
from the existing municipal infrastructure of either the
San Patricio Municipal Water District or the
O.N. Stevens Treatment Plant, 3) diversion of either
river or groundwater through wind or solar pumping,
or 4) some combination of the above. A detailed
analysis of each of these alternatives was presented in
the Plan of Study for the demonstration project
(Bureau of Reclamation 1993). Each of these alterna-
tives would have supplied some measure of fresh water
into the upper delta and esmary. However, none of the
alternatives would have adequately met the first
objective of the demonstration project, which was to

1-6

Introduction

restore the opportunity for natural freshwater flow
events from the Nueces River into the delta.

Monitoring Program

The second objective of the demonstration project was
to monitor any resulting changes in the hydrography
and productivity of the delta from freshwater flow
events. The data collection program was therefore
designed to monitor hydraulic conditions in the study
area, as well as those biological parameters that would
be most responsive to project diversions: namely,
water column productivity, benthic communities and
vegetation communities. Each of these monitoring
elements were regularly sampled at various stations in
the upper delta (Table 1-1). The biological monitoring
program was initiated in October of 1994, some
12 months before the Nueces Overflow Channel was
opened. This initial 12-month period served as a
baseline period before the effects of the demonstration
project began. The hydraulic monitoring began with
the installation of gauging instrumentation in April of
1996. In addition to the data direcdy collected as part
of the demonstration project, the measurements of
other monitoring and research efforts were also

Figure 1-8: View of the low water crossing at the head
of Rincon Bayou. This access road was one of several
road improvements made as part of the demonstration
project. The photo was taken on June 26, 1997, during the
first significant flow event.

Photo courtesy of the Bureau of Reclamation.

Table 1-1: Summary of the annual monitoring program conducted as part of the demonstration project.

Monitoring
Element

Response Variables

Schedule

Stations

Hydrography

Water Column
Productivity

Benthic
Communities

Vegetation
Communities

precipitation; and stage, velocity and calculated disctiarge through the
Nueces Overflow Channel

water quality (conductivity, temperature, depth, dissolved oxygen,
calculated density, total suspended solids and water clarity)

nutrients (orthophosphates, dissolved silicon, nitrate, nitrite,
ammonium, particulate carbon and nitrogen (PC/PN), dissolved
organic nitrogen (DON))

phytoplankton (species composition and size fractionation of major
producing groups, biomass and growth rate of suspended species and
of microphytobenthos (benthic phytoplankton))

macrofauna (species composition, density and biomass)

meiofauna (species composition, density and biomass)

pore water and open water (salinity, nitrate, nitrite, ammonium and
temperature)

macrophyte communities (species composition, percent cover and
leaf-area)

macrophyte communities (above-ground biomass, below-ground
biomass and calculated root/shoot ratios)

Continuous

1

Monthly

8

Monthly

8

Monthly

Quarterly

6

Quarterly

6

Quarterly

3

Quarterly

3

Bi-annually

3

Chapter One ♦ 1-7

utilized when available. These additional data sources
included the Texas Coastal Ocean Obsen'^ing Network
marine monitoring system of Texas A&M University-
Corpus Christi Conrad Blucher Institute, the weather
station network administered by the National Weather
Service, the national stream flow gauging program
conducted by the United States Geological Survey and
unpublished data from faculty at the University of
Texas Marine Science Institute.

en\'ironmental conditions {e.g., tide, evaporation,
precipitation, runoff, etc) as the treatment sites but
would not be affected by the project's diversions. This
site, located in the upper delta, ser^'ed as an acceptable
"reference" to which data from treatment sites could
be compared. Therefore, the demonstration study
included a total of four monitoring sites: one reference
and three treatment (upper Rincon Bayou, central
Rincon Bayou and tidal flats) (Figure 1-9).

The study "treatments" were considered to be
freshwater diversions into Rincon Bayou either
through the Nueces Gh^erflow Channel or over the
bank of the Nueces River. A comparison site was
needed that would be subject to the same general

This mofiitoring aspect of the demonstration project
involved the time obsen'ation of responses in the
biological resources of the delta to intended (though
not fully controlled) applications of fresh water. As
with many biological studies, assessment of treatment

NUECES :

OVERFLOW 4'

CHANNEL i^^

68^1 Rincon:
gaugeT

Hondo IH37
Creek

Calallen
Diversion Dam

^'ffincon Bayou

central Rincon Bayou Site
(treatment)

Figure 1-9: Overview of the monitoring sites and stations established for the Rincon Bayou Demonstration
Project. In general, the study design included one reference site and three treatment sites (upper Rincon Bayou,
central Rincon Bayou and tidal flats). The location and number of sampling stations at each site is indicated by
numerals (60 through 68) for water column productivity, letters (A through F) for benthic communities, and Roman
numerals (I through III) for vegetation communities.

Introduction

applications is often complicated by the difficulty in
establishing a suitable experimental control. Here, the
establishment of a random control area was not
practical. Consequendy, the basis of comparison of
responses in biological productivity and species
diversity to freshwater applications was fashioned
according several considerations. First, hydrographic
and biological data collected from the delta and
surrounding area in years prior to the start of the
current study offered some credible information on the
status of the delta. Analyses of such pre-study data
have been or are in the process of being published
elsewhere, thus providing some characterization of
conditions in the delta prior to the start of this study.
Second, studies by scientists on this project and others
have previously established some biological response
relationships to fresh water in Gulf Coast estuaries, and
in particular, responses to changes in salinity. These
known responses were an important tool in assessing
the effects of the demonstration project. The
experience of project scientists with conditions in the
Nueces Delta and similar estuaries offered highly
relevant expertise to project studies. Third, a site was
selected as a reference {i.e., comparison) point based on
location and lower likelihood of impacts from
freshwater inundations {i.e., treatments). Obviously,
the more affected by project diversions, the less useful
the reference site would be in evaluating treatment
effects. Finally, the duration of data collection (over
five years) and the varying freshwater inputs to the
upper delta allowed the study of relationships to the
varying biological responses {e.g., consistency in
response). Consequendy, the demonstration project
monitoring program was quasi-experimental in design.
Nevertheless, previously established biological and
ecological relationships applied here enabled the
opportunity for deductive conclusions central to the
goals of this study.

In designing the monitoring plan for the demonstra-
tion project. Reclamation again considered several
different alternatives. Most of these alternatives
included using data on wildlife or fish use of the area
{e.g., mammals, reptiles, waterfowl, shorebirds, fin-fish,
shellfish, etc.) as a measure of project success.
However, considering the extensive non-estuarine
migratory range of many of the species which utilize

the study area, it was determined that these organisms
would be subject to a variety of other forces not related
to the demonstration project. Using such data to
establish relationships between the direct effects of the
project and observed changes in wildlife or fish
populations or habitat use would have been difficult.
For this reason, vertebrate wildlife use was considered
to be an ineffective tool in evaluating the direct effects
of the project on delta productivity.

Participants

There were several key participants in the Rincon
Bayou Demonstration Project. The primary
participants of the study, without whom this project
would not have been possible, were the private
landowners in the delta who granted Reclamation
temporary permission to make the modifications and
conduct the monitoring program. The design,
coordination and funding of the project was provided
by Reclamation. All of the biological data collection
and analysis activities were conducted by researchers
from the University of Texas Marine Science Institute,
with support from the Texas Water Development
Board. The hydraulic monitoring was conducted by
the U. S. Geological Survey, who installed and main-
tained the data collection equipment, and hydrographic
data analysis was provided by the University of Texas
Center for Research in Water Resources.

Authority and Funding

The Rincon Bayou Demonstration Project was
conducted under the authority of the Federal
Reclamation Act of June 17, 1902, as amended.
Funding for the smdy was appropriated on a yearly
basis by the United States Congress under
Reclamation's General Investigations and Wetland
Development programs.

Chapter One ♦ 1-9

CHAPTER TWO

Study Area

BACKGROUND

The Nueces Estuary

"If there are seventy-five square miles on this
earth that disgrace it, those seventy-five square
miles may be found here, Nueces Bay being
one big slimy slough, only fit for the habitation
of alligators and mud-snakes"

♦ Dr.A.C. Peirce(1894)

There is no universally accepted definition of an
estuary, though these systems are generally considered
to share the following properties: a 1) coastal water
body, that is 2) semi-enclosed, with 3) free connection
to the open sea, with both 4) an influx of seawater, and
5) an influx of freshwater, and which is 6) of small to
intermediate scale {e.g., Pritchard 1967; Ward and
Montague 1996). The property of scale differentiates
an estuary from larger systems, such as the
Mediterranean Sea, the Baltic Sea, and the Gulf of
Mexico, which satisfy the other properties but clearly
are not estuaries. These properties do not necessarily
occur all of the time, as in many estuaries, the relative
influence of freshwater and seawater influxes varies
with season. In essence, an estuary is a transitional
system between a purely freshwater and a purely
marine system. It is, therefore, influenced by processes
that are terrestrial and marine, but there are also
hydrographic features unique to the estuarine
environment and a consequence of its transitional
character.

The boundaries of the Nueces Estuary include four bay
systems (Texas Department of Water Resources 1982):
one primary bay (Corpus Christi), one secondary bay,
(Nueces), and two tertiary bays (Oso and Redfish)
(Figure 2-1). In terms of geomorphic classification, the
estuary is considered a coastal plain estuary (Pritchard
1967), being composed of a drowned river valley lying
perpendicular to the coastline. However, the Nueces
Estuary also shares characteristics of lagoons with
large, bar-built bays parallel to the coastline, like

Chapter Two ♦ 2-1

N,^--..,-, Nueces Delta

^y?^— ^C^

Nueces Bay

Redfish
Bay

i

Corpus Chnsli Bay

Approximate Scale

Kilometers

■ ■ ■
0 5

Oso Bay

Gulf

of

Mexico

Figure 2-1: The Nueces Estuary and Delta.

Redfish Bay. The total estuary has an average depth of
about 2 meters (m) and covers approximately
500 square kilometers (km^ (Orlando etal 1993). The
bays are protected from the Gulf of Mexico by a
system of barrier islands, and the only sigmficant
tributary of the estuary is the Nueces River. As a
transition between continental and oceanic environ-
ments, the Nueces Estuary is subject to the effects of

both marine and fluvial (riverine) elements. Included
in the boundary of the Nueces Esmary are the tidally
influenced portions of the Nueces Delta, which lies
immediately west of upper Nueces Bay.

The Nueces Delta

The Nueces Delta (or Nueces marsh) is a complex area
of vegetated marshes, mudflats and open water that
covers approximately 75 square kilometers (km"^
(Figure 2-2). Along its northern boundary, the delta is
separated from a large expanse of agricultural land by a
steep bluff that reaches heights of about 20 m. To the
south, the delta is separated from the municipal and
industrial areas of the City of Corpus Christi by a
similar bluff. The eastern limit of the delta is
delineated by the upper segment of Nueces Bay, and
the western limit by Interstate Highway 37 (IH 37)
where it crosses the Nueces River. Upstream of this

DEMONSTRATION PROJECT
STUDY AREA

Approximate Scale
Kilometers

Figure 2-2: The Nueces Delta. Generally depicted are open water (shaded) and tidally influenced areas.

Source of base map: Salas 1993.
2-2 ♦ Study Area

point, the broad Nueces floodplain extends for several
miles in a northwest direction, being confined to a
width of approximately 2 to 5 km.

Along the southern edge of the delta, the Nueces River
flows west to east for approximately 15 km and
empties into Nueces Bay. The banks of the river along
this reach, which are generally about 1.5 to 2.5 m high,
are steep and wooded. At higher flows, the river spills
into the delta through numerous depressions along its
northern bank. Within the delta itself, the two
dominant hydraulic features are Rincon Bayou and
South Lake (Figure 2-2). Rincon Bayou, which was
likely once a course of the Nueces River, stretches
along the entire northern portion of the delta to the
bay. The depth and width of this channel varies gready
along this reach, at times being confined to just a few
meters wide and at other times opening up into several
large, shallow lakes or pools {e.g., upper Rincon Bayou,
central Rincon Bayou, and North Lake). South Lake is
a large pool located in the south-central portion of the
delta and is connected to the upper Nueces Bay by
numerous tidal channels.

Figure 2-3: Typical view of the upper Nueces Delta.

The view is looking northeast, with North Lake in the
background.

Photo courtesy of the Bureau of Reclamation.

brackish and saltwater marshes (Figure 2-4). Along
both the nordiem and southern bluffs, remnants of
diverse coastal forests exist where they have not been
displaced by agriculture or development (Salas 1993).

The delta is crossed latitudinally by two Missouri-
Pacific (MoPac) railroads. The western-most railroad
is located just downstream of the IH 37 bridge,
crossing only the extreme western portion of the delta.
The eastern-most railroad divides the delta roughly in
half, with a majority of the lower (eastern) half being
regularly inundated by water from upper Nueces Bay.
The upper (western) half of the delta is generally higher
in elevation, and only the lower channels and pools are
regularly inundated by the bay. Both railroad crossings
are elevated 3 to 5 m above tlie ground by fill material
for most of their span, with the exception of a few
bridged crossings over the more sigmficant channels.

Generally, habitat feamres are quite diverse within the
Nueces Delta, ranging from wooded upland areas to
open bay waters. The western (upper) half of the delta
is primarily dominated by rangeland and improved
pastures, with brush thickets dominating the higher
elevations, and a mixture of tntertidal mud flats,
marshes, shallow pools and channels dominating the
lower elevations (Figure 2-3). The eastern (lower) half
of the delta is almost exclusively dominated by

HYDROGRAPHY OF THE NUECES
ESTUARY AND DELTA

The Nueces Estuary and associated river delta are
gready influenced by water originating from riverine.

Figure 2-4: Typical view of the lower Nueces Delta.

Nueces Bay is a short distance to the right, and the bluff
along the northern boundary of the delta is in the distant
background.

Photo courtesy of the Bureau of Reclamation.

Chapter Tivo ♦ 2-3

estuarine and marine aquatic environments. There-
fore, key hydrographic indicators like water level
and salinity represent the combined effect of
hydro-meteorological forces, including tides, wind,
precipitation and local runoff, and river inflow.

Water Level

Water level variations in the Nueces Estuary are
generally the result of interactions between
(astronomical) tides, long-term secular excursions of
the Gulf of Mexico and meteorological forcing (winds)
(Ward 1997).

Tides

Of the many Fourier tidal components, four in
particular account very well for the obser\'ed variation
in the Nueces Estuary (\)C^ard 1997). The three
astronomical tides, which are generally short-term
influences, include a semidiurnal (12.4-hour) and
diurnal (24.8-hour) tide, each of which is then
modulated by a 27.2-day lunar tide (i.e., the variance of
water level resulting from variations in the declination
of the moon). The physical causes of fourth tidal
component, a long-term secular variation in the Gulf
of Mexico, are not well understood. In a "normal"
year, there are clear maxima in the spring and autumn,
and clear minima in winter and summer. The
semi-annual variation is generally considered to be
dominated by the winter minimum and autumn
maximum (Chew 1964). Ward (1997) has summarized
the featvures of this secular variation to include the
following characteristics: 1) there are considerable
year-to-year differences in the seasonal water-level
variation, 2) the autumn maximum is usually the
highest mean water elevation of the year and the winter
minimum is usually the lowest, 3) the summer
minimum in July and the autumn maximum in October
are the most consistent in terms of seasonal regularity,
4) both the winter (December to March) and spring
(April through June) extremes have a considerable
seasonal range in which they occur and can exhibit
multiple extremes during these periods, and 5) despite
reference to a semiannual period, the signal is not
harmonic, as both the autumn maximum and the

summer minimum, especially the former, tend to be
more sharply focused in time and extend over two-six
weeks in duration.

This semi-annual variation is a prominent component
of water level variation in the Nueces Estuary, and
becomes increasingly important, compared to the other
tidal components, with distance into the upper reaches
of the bays, especially in Nueces Bay and Delta. The
filtering properties of the inlets, shallow bays and
channels gready attenuate the shorter period
frequencies {e.g., semidiurnal and diurnal signals), but
pass the longer periods with virtually no attenuation.
As a result, water level variations caused by the cyclical
lunar tide and the semi-annual secular rise and fall of
water level in the Gulf represent the predominant long-
term mechanism of water exchange with the Nueces
Estuary. These two factors also account for most of
the (gready attenuated) tidal variation of water level
within the upper Nueces Bay and Delta.

Wind

On a shorter time scale, meteorological forcing (wind)
is a prominent mechanism for water-level variation.
The principal process is the response of a free surface
to an imposed stress, referred to more colloquially as
"setdown" or "setup" of water levels. Where there are
adjoining bay systems (such as Nueces and Corpus
Christi bays, or Corpus Christi Bay and the Gulf of
Mexico), winds cause a direct downwind set-up of
water levels across component bays. This difference in
water levels causes a direct and an indirect water
exchange between the water bodies. The direct effect
is that of wind stress on the bay itself The indirect
response of the same bay is that resulting from wind
stress on the adjacent water body, which forces water
into or out of the former. The area over which the
winds operate and their duration are both important in
the magnitude of the estuary's response.

There are three types of meteorological forcing
elements common to the Nueces Estuar^': frontal
passages, sea breezes and storm surges. Frontal
passages produce water level variations and
accompanying transports of water. The primary
mechanism is the change in direct wind stress on the

2-4 ♦ Study Area

water surface. As the front approaches the coastline,
onshore wind flow is increased, setting up water levels
along the coastline. With the frontal passage, winds
turn abrupdy to the northern quadrant, reversing the
direction of stress. Ward (1997) differentiates two
classes of fronts: the relatively short-lived low-energy
"equinoctial" frontal passages that do not force a
response in the large water body of the Gulf, and the
large-scale, longer duration "outbreak" fronts that
result in exchange between the Gulf and the interior
bays.

The response of the Gulf of Mexico during frontal
passages is the single most important factor
determining the total response of the interior bay
systems. For Nueces Bay there is a two-step response,
the response of the bay to setdown of Corpus Christi
Bay, and the response of Corpus Christi Bay to
setdown in the Gulf of Mexico. The cross-bay
transports are about the same magnitude for both
equinoctial and polar-outbreak fronts. However, the
volume of water exchanged is much greater for the
polar-outbreak fronts since a response to the Gulf is
involved (Ward 1997). The cross-bay transports, on
the other hand, occur much more quickly but entail
smaller water level changes and smaller volumes
(generally on the order of 1% of the volume of the
bay).

Another short-period force affecting water levels in the
Nueces Estuary and Delta is the sea breeze cycle. A
sea breeze is a solenoidal circulation produced by the
diurnal variation in density of the lower atmosphere
resulting from the surface temperature differential of
the land and sea (Haltiner and Martin 1957). It is
ultimately caused by the difference in thermodynamics
of sea water and land surface and is most pronounced
along their boundary. As the sea breeze circulation
begins to develop, it imposes an organized circulation
in the lower atmosphere that spreads inland and
increases the wind speed. The reverse circulation
develops in the evening as a land breeze, spreading out
to sea from the coastline. In the coastal zone itself, the
sea breeze is manifested as a diurnal variation in wind
velocity superposed on the normal onshore flow from
the Gulf of Mexico. The familiar freshening of winds
in the afternoon and the increase of short-crested

wind-waves (chop) are well-known features of summer
hydrography in these bays attending the sea breeze.

The sea breeze is a relatively weak circulation, and its
importance depends on other factors affecting wind.
The effect of the sea breeze on water levels in the
estuary may be minimized by more dynamic
atmospheric processes, such as airmass replacement or
interception of radiation by clouds and can be masked
even by the prevailing onshore flow. The sea breeze is
therefore best developed during conditions when these
other influences are uncommon, which is typically
during summer.

Finally, the most extreme water level responses of the
Nueces Estuary to meteorological forcing are
associated with the storm surges of tropical
depressions. These intense organized tropical systems,
the most extreme representative being the hurricane,
are characterized by a cyclonic circulation with intense
swirhng winds around a center of extremely low
pressure. The low pressure center and the circulating
winds combine to create an elevated mound of water
(storm surge) that moves with the depression, but the
wind stress on the water's surface is the more
important determinant of the magnitude of the surge.

As the cyclone and associated storm surge make
landfall, the volume of water in the surge behaves as
any long-period shallow-water wave, slowing due to
shoaling water depths and steepening. As the surge
propagates into bays and estuaries, it is subjected to
various local physiographic modifications. In some
regions, this can lead to further amplification of the
surge height, and some of the highest recorded surges
on the Texas coast have occurred on the inland side of
the bays, the largest being Hurricane Carla surge
measured at Port Lavaca in excess of 6.7 m. While a
hurricane can inflict damage through high winds,
tornadoes and intense rainfall, it is the surge, perhaps
in concert with wave attack, that is responsible for
most of hurricane-related impacts on the Texas coast.

In extreme instances, tropical storms may also have a
secondary effect upon water levels in the estuary due to
heavy precipitation. Such was the case with Hurricane
Beulah, which made landfall in the autumn of 1967.

Chapter Tm ♦ 2-5

Inflow resulting from the record-setting flood event in
the Nueces River caused a substantial rise in water
levels in Nueces and Corpus Christi bays of up to
0.6 m above that of the Gulf of Mexico (Grozier et al.
1968; Corps of Engineers 1968). Grozier et al. (1968)
went on to report that water levels in the bay were slow
to fall (it took several days) because of the enormous
amount of runoff from the rains inland, even though
Corpus Christi Pass and two other new channels were
opened between the bays and the Gulf by the storm.

Salinity and Freshwater Inflow

In general, salinity concentrations in estuary systems
are stratified horizontally and vertically, both of which
are primarily determined by the size and location of the
freshwater source(s). Over the area of the estuary
(horizontal), the gradient of fresh to brackish to marine
salinity concentrations (which is 0 to 1 5 to 30 parts per
thousand (ppt) salt, respectively) progresses from the
areas closest to the freshwater source {i.e., river's delta
and mouth), to the secondary bay(s), then to the
primary' bay. The degree of salinity stratification in the
water column (vertical) depends upon the intensity of
mixing.

In Nueces Estuary

As in most estuaries, there are substantial spatial
gradients in salinity across the Nueces Estuary, not
only because of the great range in hydro-chmatology,
but also because of the location of the river drainages
and the variable influence of the sea (Ward and
Armstrong 1997). Based on evaluation of long-term
data, seasonal variations in the salimty of the Nueces
Estuary, other than a proclivity for sUghdy higher
salinity concentrations in the summer, were not readily
evident (Vt'ard 1997). Salinity variations within the
water column {i.e., stratification) in the Nueces Estuary
was found to be minimal (less than 0.5 ppt), but the
largest values of this (small) gradient typically occurred
in Nueces Bay, nearer to the freshwater source (Ward
1997). One exception to this generalization is
evaporation stratification, which t)'pically occurs in
stagnant portions of Corpus Christi Bay during the
summer. In this case, the salinity gradient in the water

column can be as much as 6 ppt and often associated
with hypoxia (Ritter and Montagna 1999).

Short-term vacillations of salimt)^ values within the
greater Nueces Estuary are primarily in response to
water-mass changes. One of the most common and
important contributors to this vacillation is the
response of salinit)' to an influx of freshwater
(extrusion) and the subsequent recover)^ of salinit)- as
the inflow event diminishes (intrusion). These two
mechanisms involve different physical processes, and
therefore are not symmetnc events. Extrusion is
effected by a replacement of water volume due to the
rapid influx of the inflow (river) hydrography,
commonly called a "freshet" event. Intrusion is
accomplished by the internal circulations gradually
returning higher salimty water to the upper reaches of
the bay and delta. Extrusion typically occurs quickly,
within a few days to several weeks (depending upon
the region and the size of the freshet event), while
intrusion typically requires weeks to months. Because
the synoptic events producing the runoff are also
frequendy accompanied by a frontal passage and
regional rainfall, displacement of salt water by fresh is
assisted by both wind forcing (frontal cross-bay
transports and efflux to the Gulf) and surface
precipitation surfeit. Intrusion, occurring over a longer
time frame, is assisted by the t}'pical evaporation deficit
at the surface.

One factor of the regional hydro-chmatolog}' that
facilitates identification of these kinds of events is that
the largest freshets (which therefore have the potential
for the greatest salinity response) are very widely
spaced in time. Occasionally several such freshets
occur closely enough in time that intrusion from one is
superposed on extrusion from the other, making
interpretation of the salinity response quite
complicated. From a physical %'iewpoint, extrusion is
an advective process, invoh'ing the wholesale
displacement of water from the upper bay to the lower,
or (for extreme events) into Corpus Christi Bay or
even the Gulf of Mexico. Intrusion, on the other
hand, is the combined effect of smaller water mass
transfers, such as tidal exchanges and internal
transports, whose cumiolative behavior is more of a
diffusive process, operating to mix out and reduce the

2-6 ♦ Study Ana

overall salinity gradient. At any point in time, the
salinity is a result of the combined effects of the two
types of processes, but their relative importance
depends upon the characteristics of the freshet
response.

In Nueces Delta

In the Nueces Delta, variations in salinity
concentrations of the pools, channels and marshes are
driven by the same mechanisms as in the bays (i.e.,
water-mass exchanges invohring both intrusion and
extrusion processes), with two sigtiificant differences.
First, because much of the water in the delta is
segregated into shallow pools and channels by higher
land formations, many of these become frequently
isolated from the bay due to water level fluctuations.
This seclusion magnifies the effect of evaporation,
resulting in the concentration of salts from bay water
into the soils and water of the delta. The opportunity
for continuous dilution by very large volumes of bay
water enjoyed elsewhere in the estuary is not readily
available in the delta.

Second, the magnitude, frequency, duration and timing
of freshwater flow events (or freshets) into the Nueces
Delta are burdened by one additional condition not
applicable to Nueces Bay. This condition is that the
stage (water level) attained by the flood event in the
river must exceed the minimum flooding threshold of
the river in the delta segment of the stream
(determined by the elevation and dimensions of the
lowest portions of the river bank) (Figure 2-5).
Therefore, unlike Nueces Bay, the opportunity for
freshets in the delta are limited to discrete periods of
time when the river hydrograph sufficiendy meets this
condition. If the flooding threshold is not met, the
flow event in the river will bypass the delta, providing
Nueces Bay with an intrusion event without the same
courtesy for the delta.

These periodic deltaic inundation events usually coin-
cide with tropical storm activity in early autumn or
witii the passage of frontal systems in late spring
(Texas Department of Water Resources 1982). Such
flooding events flush the numerous channels and
ponds of the delta, and inundate large areas to an

Figure 2-5: Nueces River (looking downstream) just
below the IH 37 bridge under flood conditions (April
1992). During such high-flow events, the river spills fresh
water into the upper Nueces Delta from several low points
along the north (left) bank.

Photo courtesy of the Bureau of Reclamation.

extent and depth governed by the volume and duration
of die flood event (Ward 1985) (Figure 2-6). These
discrete events also serve as a mechanism whereby
salts, as well as organic materials and other nutrients
produced in the delta marsh, are exported from the
delta into the greater Nueces Estuar)'. Because of

Figure 2-6: Natural flooding of the upper Nueces
Delta, April 1992. Such freshwater inundation events
significantly contribute to the biological productivity of the
delta by providing a medium for nutrient exchange and
biochemical cycling, supplying freshwater to marsh plant
communities, transporting detrital and other nutrient
materials from the established marsh vegetation and
sediments to the bay, and buffering bay salinity.

Photo courtesy of the Bureau of Reclamation.

Chapter Tm ♦ 2-7

these events, the Nueces Delta is one of the most
important sources of nutrient material for the entire
estuary system (Texas Department of Water Resources
1981).

ECOLOGY OF THE NUECES
ESTUARY AND DELTA

Montagna et al. (1996) has described eight different
t)^es of habitat subsystems occurring in the Nueces
Estuary: salt marshes, beaches, the water column,
muddy bottoms, sandy bottoms, oyster reefs, seagrass
beds and algal mats. Although each subsystem is
described separately, it is noted that there are many
interconnections among them, as water cxirrents, waves
and tides transport organic matter, energy and even
animals pass between habitats (Montagna et al. 1996).

According to a conceptual ecosystem model developed
for the Nueces Estuary (Montagna et al. 1996), these
eight habitat subsystems may be organized by their
relative relation to the tidal water level. Intertidal
habitats (those within the range of high and low tides)
include salt marshes and beaches. Salt marshes are
important sources of organic matter for the estuary,
serve to buffer shorelines and provide habitat for
important fish and wildlife species. Beaches support a
low diversity of species because they experience high
energy from waves, wind and currents which mix and
transport detrital matter (plant and animal tissue) from
the estuarine and marine environments.

Subtidal habitats (those below the average low tide
level) include the water column, muddy bottoms, sandy
bottoms, oyster reefs and seagrass beds. The water
column {i.e., the vertical portion of open water areas)
supports a complex and productive food-web
comprised of small, often single-celled organisms
(plankton) that move with currents, and their
predators, which are larger organisms (nekton), like
fish. Beneath the water column, the substrate of the
estuarine floor may be muddy or sandy. Muddy
bottoms are more common in the estuary and support
shrimp and other commercially important species,
while sandy bottoms, which occur near the shoreline,
are less common and support a number of large

animals. Oyster reefs are associated with high diversity
because they provide substrate, shelter and foraging
opportumries for many different species. Seagrass
beds are very diverse and productive, and serve as an
important nursery ground for larval crustaceans, fish
and invertebrates.

Supratidal habitats (those above the average high tide
level) include algal mats. Algal mats occur in isolated
pools only periodically inundated by tide or rainfall and
are comprised of nitrogen-fixing mats of filamentous
blue-green algae living in colonies on the sediment
surface. These habitats provide nutrients to shoreline
environments and foraging habitats for wading birds.

Estuarine Habitats in the Nueces
Delta

The most predominant estuarine habitat subsystems in
the Nueces Delta are salt marshes, the water column,
muddy bottoms and algal mats. The following more
detailed description of these habitat types were
primarily derived from Montagna et al (1996), except
where noted.

Salt Marsh

Salt marshes are located in the shallow or intertidal
regions of the estuary and delta, often near a source of
freshwater input (e.g., river mouths and secondary
bays), and are dominated by marsh grasses and other
plants (Figure 2-7). The salt marsh of the Nueces
Delta is the most extensive within the Nueces Estuary.
Salt marshes trap soft sediment and organic material
from the water column between individual plants.
Beneath the plants are strong reducing conditions, and
often low oxygen levels due to decomposition of
organic matter. Areas with a higher frequency of
freshwater inflow have higher diversity, higher rates of
primary production and higher net community
production. The biomass of producer and consumer
organisms can also be high, but species diversity can be
low because of fluctuating salinity.

2-8 ♦ Study Area

Figure 2-7: View of salt marsh habitat in the upper
Nueces Delta. The vegetation transect for Station I is in
this vicinity.

Photo courtesy of the Bureau of Reclamation.

Like seagrass beds, salt marshes are also an important
nursery and feeding grounds for a variety of
invertebrates and fish. Because of the amount of dead
and decaying plant matter in salt marshes, their
contribution of detritus to the food-web of adjacent
habitats is important. The plant litter is uti]i2ed by
micro-organisms and other small estuarine animals
(Marples 1966) and serves as a critical link between
primary and secondary trophic levels (Burkholder and
Burkholder 1956; Odum and Wilson 1962; Teal 1962).
Marsh plants also provide shelter for a variety of small
organisms, including crustaceans, avians and mammals,
and serve to stabiLi2e marsh sediments. Because of
these functions, the marsh subsystem in the Nueces
Delta supports numerous estuarine organisms,
including shrimp, crabs and fin-fish, by providing large
amounts of food and structure.

One of the primary variables affecting the distribution
and abundance of vegetation species in the delta marsh
is salt. Salt is generally imported into the delta from
Nueces Bay by a variety of tidal and wind forces, and
generally exported from the delta by infrequent
freshwater inundation events resulting from over-
banking floods in the Nueces River. Water and soil
salinity can be highly variable, depending upon
precipitation, tide level and temperature (Henley and

Rauschbauer 1981), especially because evaporation
often exceeds precipitation for several months a year
(Longley 1994). Elevation and proximity to channels
and creeks are also factors that affect salinity, which
often results in distinct vegetation zones (Chapman
1960; Chapman 1974; Nixon 1982).

Although most marsh plant species in the delta are salt-
tolerant (halophytic), excessive salt concentrations can
cause hypersaline conditions which are adverse to plant
survival. For example, although halophytic species can
survive in intermittent hypersaline environments,
prolonged periods of salinity stress can stunt active
growth and reproduction, leading to decreases in
abundance and productivity (Deegan et al. 1986;
Bertness ^/ «/. 1992). Ultimately, decreased vegetative
coverage can create bare areas, which are then direcdy
exposed to evaporation, leading to further increases in
salinity (Zedler 1983; Bertness 1991). Successful re-
colonization of bare areas requires at least a short-term
lowering of salt concentrations in the soils, which
allows re-invasion by vegetative (rhizomes) or
reproductive (seed germination) growth.

Water Column

The water column habitats of the Nueces Estuary and
Delta are shallow and are often only as productive as
their substrate (Figure 2-8). This is in contrast with
marine environments, such as the Gulf of Mexico,
where the water column habitat is very deep and more
productive than the bottom. Because fresh water
mixes with salt water in the bays, the resulting salinity
concentrations are often brackish (10 to 25 ppt).
However, when evaporation exceeds freshwater inflow
and flushing by the ocean, salinity concentrations can
become saltier than the ocean (> 35 ppt). During dry
periods in the delta, where supratidal pools and
channels may become isolated for extended periods of
time, salinity concentrations may exceed that of the
ocean by several times. The estuarine water column is
usually well oxygenated but can become quite turbid as
sediment is re-suspended by wind or human actixdties.
Mixing, due to the consistent high winds and shallow
depths, prevents significant stratification of the bay
water column.

Chapter Tm ♦ 2-9

Figure 2-8: View of water column habitat in central
Rincon Bayou.

Photo courtesy of the Bureau of Reclamation.

The water column food-web consists of phytoplankton
(single-celled plants) being eaten by zooplankton
(single-celled animals), which in turn are eaten by fish.
Primary production (the conversion of solar energy to
chemical energy by photosynthesis) by phytoplankton
in estuanne water can be relatively high but is typically
much less than in salt marshes. There are two cycles of
energy in the water column food- web. The first may
be referred to as the "microbial loop," which includes
only small flagellated phytoplankton and zooplankton
and bacteria. Flagellates are a very diverse group of
plankton that can travel short distances by beating their
whip-Hke flagella. The small phytoplankton are preyed
upon by small zooplankton, and, when the small
phytoplankton and small zooplankton die, they may be
decomposed by bacteria in the water. This food-web
c\'cle is small, transfers energy rapidly, and its
components are tighdy coupled. However, energy
cycled in this microbial food-web are not usually
transferred to laigher trophic levels (e.^., fish).

The second food-web cycle of the estuarine water
column consists of the larger phytoplankton, such as
diatoms, which are eaten by zooplankton and some
fish. In addition, some zooplankton are eaten by larger
zooplankton and the lar\'al and adult forms of some
fish. Even larger predatory fish (e.g., red drum,
Sdaenops ocellatus) then eat these plantkivourous fish.

This food-web cycle is larger, and it transfers energy
slower than the microbial loop cycle, but both cycles
are important in nutrient processing within the estuary.

Open Water, Muddy Bottom

The most common benthic habitat in the Nueces
Estuar)- is the unvegetated muddy bottom (Figure 2-9).
Movement of water over the surface of the mud keeps
the sediment oxygenated to about one centimeter in
depth. Below this region is a strongly reduced
en\aronment due to the presence of ox)'gen-consuming
microbial animals. Mud is easily re-suspended, and
muddy bottoms may therefore experience frequent
erosion or deposition of sediment. Turbidit)' tends to
be high at the surface, which restricts the presence of
light-dependant primarj' producers and filter feeders.
Deposit feeders, however, can be present in high
abundance, diversity and biomass.

Detritus is the most important source of carbon for
muddy bottom habitats. This material may originate
from terrestrial sources transported by freshwater
inflow, marine sources derived from marshes,
seagrasses or sedimented phytoplankton. In the
benthic muds, there are three types of animals which
utilize detritus, including non-selective deposit feeders,

Figure 2-9: View of muddy bottom habitat in upper
Rincon Bayou during a period of low water.

Photo courtesy of the Bureau of Reclamation.

2-10 ♦ Study Area

selective deposit feeders and omnivores. Non-selective
deposit feeders (e.^., polychaetes) process bulk
sediment by extracting organic matter from the mud.
Selective deposit feeders (e.^., moUusks) usually have
tentacles to pick and choose specific particles of
material for ingestion. Omnivores (e.^., edible shrimp,
Petiaeus sp.) eat detritus, microphytes or any small
animals that they can catch. These benthic animals, in
turn, provide prey items for many other estuarine
animals, particularly fish.

Algal Mat

vMgal mats are unusual features of the supratidal zone
that occur in some locations within the Nueces Estuary
and Delta (Figure 2-10). They occur when rain, wave
surges or higher tides collects in low spots near the
shore, often in areas with higher elevation than salt
marshes. The trapped water is very shallow, and often
becomes quite warm and saHne with solar radiance and
evaporation. However, these conditions allow a bloom
of photosynthetic bacteria (cyanobacteria, or blue-
green algae) that live on the sediment surface. These
producers are very important to die bay ecosystem
because they have the abiUty to fix atmospheric
nitrogen (Nt) into forms more usable by other
producers and bacteria like ammonia (NHj), nitrate

Figure 2-10: View of algal mat habitat near South
Lake. This mat is comprised of colonies of filamentous
blue-green, unicellular green, flagellated and
diatomaceous algae, bacteria and a minor assemblage of
worms, crustaceans and ciliates. The vegetation is
approximately 6 to 10 inches tall.

Photo courtesy of U.S. Fish and Wildlife Service.

(NOj) or nitrite (NO2). When this material is
transported back into the bays, it represents a nutrient
spike that can enhance primary productivity in the
estuary. However, aside from the cyanobacteria, there
are not many species endemic to the relatively harsh
conditions of the algal mats.

HISTORY OF THE NUECES ESTUARY
AND DELTA

Just over a century ago. Dr. A.C. Peirce, a Bostonian
naturalist intent on collecting bird species from the
coastal regions of Texas, traveled to the Corpus Chrisd
area. At that time (about 1890), Corpus Christi was
still a primitive setdement on the western bayfcont,
accessible to the mainland by ferry across the Nueces
near San Patricio, by fording El Rincon at Indian Point
or by rail (Ward 1998). Peirce's book (1894) describes
the travels of he and his guide (a local Corpus Christi
resident and hunter), and includes several accounts of
excursions into parts of the Nueces Estuary,
particularly die Nueces Delta and upper bay. Although
it reflects the values and judgements of the late 19th-
century, the work pro\'ides a picture of the study area
prior to many of the human activities that have
changed it since:

"About a week after my arrival at Corpus Christi,...
we started for the Nueces Flats. Our road was mostly
through a country covered with a low growth ofmesquite
andweesatche [sic] brush, where pasture fences were
much more numerous than houses, of which we saw
few. . .. The land all about this part of the country is
divided up into pastures containing many square miles
each, which are occupied by thousands of sheep, goats,
horses and neat cattle. . .. Twelve or fifteen miles from
our starting place, we left the beaten road, and traveling
four or five miles over a rough and hilly stretch of land,
crossed the Nueces River and camped a few miles
beyond

"Above the function of the river with the bay is a large
area of low marshy surface; this is the Nueces Flats,
which include several thousand acres of land and water
In hundreds of places on the north side of the river, the
earth is depressed below the level of the stream; and
these depressions, filled with water, are, in places, only

Chapter Two ♦ 2-11

separated from each other and the large stream by slight
elevations. . .. As a rule, the bottoms of these small
bodies of water are firm, but a few of those nearest the
river an decidedly bogg)i. On each side of the river, and
between the water and the grass-covered land, is a space
perhaps twenty yards in width, which is made up of
bottomless mud To venture on to this mud is simply to
venture into it, and as it is seemingly without limit in
depth, one might better try to walk on the ocean, so far
as danger is concerned

"We found wild geese and ducks in abundance; nearly
every one of these small ponds was well stocked with
them.... Gulls and terns were also plentiful. . .. These
birds frequent the place in search of food, which they
find about the strip of mud next the river. " (Peirce
1894).

From Peirce's description, the north bank of the
Nueces River within the delta was very low. At
present, the river bank in the upper delta is about
1.5 to 2.1 m (5 to 7 ft) above that of the elevation of
the river under low flow conditions. Several factors
could have contributed to this change, including river
channelization and the intentional deposit of fill
material. Evidence of the later, in the form of
scattered concrete rubble and re-bar, was found by
Reclamation during construction of the Nueces
Overflow Channel (Figure 2-11).

Given the lower flooding threshold for delta
inundations and the expanse and condition of the
described mudflats adjacent to the river, the occurrence
of freshets into the delta from the Nueces River were
likely much more common at the turn of the century
than at present. However, as can be observed today,
the Nueces River in the delta was not continually fresh,
as Peirce testifies that, 'ihe water of the river was not strong
of salt, but was just brackish enough to fail completely to quench
thirst" (18,94). That the Nueces River frequendy
experienced dry or low-flow periods prior to 1900 was
also observed by others (Hollon 1956; Collins 1878).

Other evidence that the delta was much fresher than
at present is the presence of Rangia middens
(Figure 2-12). These piles of bivalve shells are the

Figure 2-1 1 : Example of concrete rubble found at
several points along the north bank of the Nueces
River downstream from the IH 37 bridge. It is

speculated that this material was intentionally placed in
low portions of the bank to reduce flooding of adjacent
pastures and was probably acquired from the highway
bridge renovation during the late 1950's.

Photo courtesy of U.S. Bureau of Reclamation.

remains of foraging activities by Native Americans and
may be found in the Nueces Delta at several locations
along Rincon Bayou. Rangia cuneata is the dominant
animal in Gulf coast estuaries where salinity
concentrations continuously range from 0 to 1 5 ppt
(Hopkins et al. 1973). Although adults can sur\ave
higher salinity values, lan^ae require concentrations in
the range of 2 to 10 ppt for survival. The presence of
large adult Bjingia in the Nueces Delta indicates that the
habitat there had been primarily oligohaline. At
present, adult Rangia are only found only in the Nueces
River just below die Calallen Dam (Kalke 2000).

Several weeks later, Peirce and his guide made a second
excursion, only this time they ventured into the delta
by boat from Nueces Bay:

2-12 ♦ Study Area

Figure 2-12: Example of Rangia middens {i.e., piles of bi-valve shells) found in Nueces Delta. This location
is along the southern edge of the central Rincon Bayou channel.

Photo courtesy of University of Texas Marine Science Institute.

'From a man who lived by the abaters of Nueces Bay
we rented a sail boat, at four bits a day. . .. Into this
hay empties the Nueces River, and at the junction of the
two is a mud flat, miles in extent The river is deep
and narrow, but at its mouth spreads out, as it were, to
cover this great surface with an inch or two of water.
The amount of water over the flat depends in a great
measure upon the wind; a hree;^efrom the east sending
the water from the bay over the shoals, while a strong
current of air from the west will have an opposite effect
and leave the crest of the flat above the water's edge.

"From various sources I learned that years ago the bay
had extended several miles further back than now, and
that the boggji soil on the sides of the river was at that
time just such aflat as the one I have described If this
is true there is no reason why the whole bay may not in
time be replaced by land. Such a radical change as this
is to be hoped for, for if there are seventy five square
miles on this earth that disgrace it, those seventy five
square miles may be found here, Nueces Bay being one
big slimy slough, only fit for the habitation of alligators
and mud-snakes " (Peirce 1984).

Peirce's description of the dynamic rate of siltation in
the delta indicates that there was, during that period, a
very large amount of sediment was moving down the
Nueces watershed and into Nueces Bay. In fact,
Morton and Paine (1984) have reported that the
shoreline of Nueces Bay had been accreting

(advancing) into the bay for much of the period
between 1867 xmtil the time of their research in 1982.
White and Calnan (1990), who compared historical
photographs of the lower Nueces Delta (below the
eastern-most MoPac railroad) with those taken in 1959
and 1979, also reported an increase in total delta area.
Between 1930 and 1959, the total area of the lower
delta (which included both vegetated area and barren
flats) increased by 164 hectares (ha) (405 acres), and
between 1959 and 1979, increased by 52 ha (133 acres)
(White and Calnan 1990). This process of down-basin
transport of sediment was also subjectively verified in
the 1930's and 1940's, when La Fruta Dam,
constructed in 1935 on the lower Nueces River, began
to lose a significant amount of its storage capacity due
to siltation (Corpus Chris ti 1990).

However, although in recent years the total area of
the lower delta has increased, the total vegetated area
has decreased and the total water and barren areas
have increased. During the 49-year period between
1930 and 1979, the total area increased by 216 ha
(534 acres), but the water/barren areas increased by
345 ha (852 acres), for a net loss of 129 ha (318 acres)
of vegetation (White and Calnan 1990). Subsidence
(Brown etal 1976) and the loss of fluvial sediments
from the Nueces River were identified as possible
mechanisms causing the increase in water and
barren areas and the loss of marsh vegetation
(White and Calnan 1990).

Chapter Two ♦ 2-13

In yet another venture, Peirce and his guide traveled
west from Corpus Christi by land, crossed the Nueces
River from the south and entered into the upland
Nueces floodplain several miles upstream from the
delta:

"Above the junction of the Nueces River with the bay
the river is bordered on each side by a strip of timber
several miles in width; the Nueces Bottoms. The
bottoms are dark and gloomy, every particle of ground
not occupied by the large and magnificent trees being
covered with shrubs and tall palmetto leaves; while the
direct sunlight is almost completely shut out by the long
and flowing Spanish moss which covers every tree,
weaving their twigs and leaves together in a tangled and
matted web. But for the many roots which form a
network beneath the surface, all this land would be too
bogQi to uphold any living creature, and at the channel's
sloping sides, where but few roots are present, it is
dangerous to venture away from snags and logs.

"Our course through the timber was anything but
straight, and it was full two hours before the second river
was before us. As was expected, here we met with
trouble; for although the stream was narrow, it was not
supplied with snags upon which we could cross. My
partner predicted, that further down its course we should
find the water-way much wider and more snagQi.
Beating our way through the deep-tangled wildwoodfor
two or three miles toward the bay, we reached that part
of the swamp where the two rivers united at an acute
angle. Here there was an entire change of scene.
Instead of a narrow channel bordered by steep banks,
there was a spread of mire acres in extent, wherein
thousands and thousands of snags and water soaked
logs were piled in confusion. More than this, hundreds
of snakes were to be seen about and upon the driftwood,
where they had come to bask in the sunshine and to feed
in the shallow pools. Nearly everything supported at
least one reptile, and I thought that before attempting to
cross we would have to clear our intended path with
powder and shot These serpents were not all of one
species, but moccasins were the predominating kind "
(Peirce 1894).

From Peirce's description, the Nueces Delta at the turn
of the century began where the heavily forested

bottomland of the Nueces River ended. The present
floodplain, however, is significandy less wooded than
what Peirce observed (Figure 2-13), likely as a result of
decades of agriculture and ranching activities. The
"second river" encountered tn this floodplain was Hkely
Hondo Creek, which at present is only a remnant braid
of the lower Nueces that joins the river almost
perpendicularly just downstream of Calallen Diversion
Dam. The description of "thousands and thousands of
snags and water soaked logs" at the upper end of the
delta gives testimony to the size and effect of the
immense floods that must have undoubtedly ravaged
the heavily-wooded lower floodplain of the (reservoir-
free) Nueces River watershed.

Changes in Hydrography

Water Level

There have been numerous physical changes in Nueces
Estuary during the 20th-century that have altered, to
some degree or another, the historical response of
water level to tides and wind. These modifications

Figure 2-13: Typical view of the Nueces River
floodplain upstream of the Nueces Delta. A flooded
Hondo Creel< is visible in the lower right. The photograph
was taken on June 26, 1997, during which a flood in the
lower watershed had activated secondary channels in the
floodplain. The telephone pole in the center of the
photograph provides an approximate scale.

Photo courtesy of U.S. Bureau of Reclamation.

2-14 ♦ Study Ana

include such activities as the stabilization and jettying
of inlets, dredging of deep shipping channels in the bay
and through the barrier island system, and installation
of additional barrier features. Other factors include
commercial shell dredging activity, which has been
attributed for removing about 50% of the volume of
Nueces Bay during the period of 1950 to 1968 (Ward
1997), and the conversion of tidal marshes to upland,
particularly along the south shore of Nueces Bay,
which has transformed approximately 15% of the area
of Nueces Bay since 1925 (Ward 1997).

Salinity and Freshwater Inflow

After compiling and analyzing salinity data throughout
the Nueces Estuary from the 1950's through the
1990's, Ward and Armstrong (1997) found diat the
much of the estuary exhibited a well-defined increasing
trend in salinity, including Nueces Bay and most of the
open areas of Corpus Christi Bay. The average rates of
increase were not considered insigmficant. For
example, over ten years, the average rates of salinity
increases would result in an increase of average saHmty
of Corpus Christi Bay by 0.5 ppt and Nueces Bay by
2.5 ppt (Ward and Armstrong 1997). After inspection
of the data, these authors determined that the greatest
contributor to the declining trend was the reduced
frequency of occurrence of high-flow events in the
Nueces River. Several factors have been identified as
having possibly contributed to reduced stream flow in
the Nueces River, including an increase in water
storage and evaporation due to the construction of
reservoirs, an increase in the consumption of water
withdrawn from streams or shallow aquifers and a
possible decrease in rainfall within the greater
watershed (Asquith et al. 1997).

Since 1935, three main-stem reservoirs have been
constructed on the Nueces River and its tributaries.
These dams include La Fruta Dam, Wesley Seale Dam
(Lake Corpus Christi), which replaced La Fruta Dam,
and Choke Canyon Dam (Figure 2-14). A direct result
of their construction was that the combined basin
storage capacity dramatically increased over a relatively
short period from 67,848 10' m' (55,000 acre- ft) in
1935 to over 1,221,264 10' m' (990,000 acre-ft) in 1982
(Figure 2-14). Wesley Seale Dam lies on the lower

reach of the Nueces River, and Choke Canyon Dam
lies on the lower Frio River, the largest tributary to the
Nueces.

Developed for the purposes of providing a reliable and
municipal water supply and flood protection, these
dams have contributed to reduced streamflow in the
lower Nueces River by their diminutive influence on
larger river hydrographs, and through direct water loss
to consumptive uses and evaporation. The present
permitted firm yield of the reservoir system is
171,470 10' m' (139,000 acre-ft) for mumcipal and
industrial use, and a portion of delivered water returns
to the estuary through treated return flows. Because of
the relative shallow depth of the two reservoirs and the
relatively hot climate, evaporation from these two
water bodies can remove a significant amount of water
from the river system. For example, during 1999
alone, over 217,730 10' m' (176,500 acre-ft) were lost
to evaporation from the combined reser\^oir system
(Hilzinger 2000).

Another factor possibly contributing to decreased
stream flow in the lower Nueces River include
increased non-reservoir surface water withdrawals in
the greater watershed. For example, long-term (1940
to 1990) analysis of reported surface water withdrawals
in the basin upstream of the reservoirs indicates an
increase of about 60 % from 1965 to 1990 (Green and
Slade 1995), which could have reduced the amount of
inflow to the reser\^oLrs.

Finally, a decreasing precipitation trend in the Nueces
watershed would be expected to reduce streamflow.
Howerver, after analyzing rainfall data from four south
Texas gauges (Cotulla, Beeville, Sabinal and Corpus
Christi) reflecting conditions for the Nueces watershed,
Medina (2000) found that annual precipitation (using a
base period that consisted of data since 1900)
produced no particular trend (Figures 2- 1 5a through
2-1 5c). Using a baseline that began during the late
1940's {e.g.. Figure 2-15d), annual precipitation
portrayed an increasing trend (Medina 2000). The
most prominent and common feature of the
precipitation data at all stations was the drought of the
late 1940's and early 1950's. Similarly, Asquith etal.

Chapter Two ♦ 2-15

Figure 2-14: The Nueces River Basin, including major drainages and reservoirs.

Source of topographic base map: U.S. Geological Survey 1997.

(1997) found little evidence for statistical trends in
precipitation along the Coastal Bend from 1968
through 1993.

Recendy, two independent investigations were
conducted to quantify' changes in streamflow of the
Nueces River since 1940, one regarding inflow into
Nueces Estuary and the other regarding inflow into
Nueces Delta. The first study was conducted by
Asquith et al. (1997), who used streamflow in the
Nueces River near Mathis (located immediately below
Lake Corpus Chrisd) to generally represent estuary
inflow. After performing a statistical trend test on
daily flow values, it was concluded that the data

showed strong exddence for a downward trend for the
period of 1940 to 1996. When analyzed in the
historical context of reservoir construction, it was
reported that the change in mean annual streamflow
during the period after the construction of Lake
Corpus Christi (1958 to 1982) was negligible (a
decrease of about 1%), but that the change during the
period after construction of Choke Canyon Dam (1982
to 1 996) was large (a decrease of about 55%) (Asquith
et al. 1997). Although their analysis used the
construction dates of large reser\'oirs to delineations of
the record, it was explained that the effects of
reservoirs were only partially responsible for the
observed differences between the time periods.

2-16 ♦ Study Ana

(a) Cotulla

50| 1 1 1 1 1 1 1 1 r

40

c
o

ra 30

20

10

P^^<i^^^<i^^^<ii^^<i^^^<i^^^#VVVV^

(b) Beeville

50ri — I — I — I — I — I — I — I — r

20

10

J I I I I I L

^<i^^^<i^\<i'V^^<iiV''^<i^^^<J^^^<i\^^^*VV''

(c) Sabinal

50| — I — r-

40

10

1 — I — I — I — r

• • •

J I I I

,<iO°^<,^^^<,^\<J^,<,^V'',<i^^^<j^^^#^<i<^V^

(d) Corpus Christi

50| — I—

40

ra 30

Q.

'o

20

10

1 1 1 1 1 1 r

I I L

,<4^\<i^W\<^^\<i^\'*^\'i'^V\'*"^V

i

Approximate Scale

Kilometers

0 50

• Beeville

Figure 2-15: Annual precipitation trends at four gauges about the greater Nueces River watershed since
about 1900 Source: Medina 2000.

Note: 1 inch = 2.54 cm

Chapter Tm ♦ 2-17

The second study was conducted by Irlbeck and Ward
(2000), who used river over-banking events into the
upper Nueces Delta (i.e., Rincon Bayou) to generally
represent delta inflow. After analyzing the flow regime
characteristics of deltaic inundation events, it was
concluded that the magnitude of such events has also
decreased during the period of 1940 to 1999. Again, in
the context of reservoir construction, it was reported
that change in the annual mean flow into the delta
from the river during the period after the construction
of Lake Corpus Christi (1958 to 1982) was large (a
decrease of about 39%), and that the change during the
period after construction of Choke Canyon Dam (1982
to 1999) was very dramatic (a decrease of over 99%)
(Irlbeck and Ward 2000).

The comparative results of these two investigations
(Table 2-1) show a marked difference in the response
of the Nueces Delta to reductiotis in stream flow over
the past sixty years when compared to that of the other
two periods (1958 through 1981, and 1982 through
1999) (Figure 2-16). The distribution of large
precipitation events were also found to be less frequent
during the drought years of the late 1940's and early
1950's than in the latter two periods, which were not
significandy different from each other (Medina 2000).

This information is relevant because this first period,
which was used as the baseline for calculating percent
changes in estuary and delta inflow by Asquith et al.
(1997) and Irlbeck and Ward (2000), respectively, likely
under-represents to some degree the actual baseline
conditions for freshwater inflow to the Nueces Estuary
and Delta. In addition, the second period, which

1940-1957 1958-1981 1982-1999

H Sablnal H Beeville 5 NE

n Corpus Christi CD Cotulla

Figure 2-16: Mean annual precipitation of available
data at four gauges about the greater Nueces River
watershed. Source: Medina 2000.

Note: 1 inch = 2.54 cm

showed the smallest percent change in both analyses,
also experienced the largest mean watershed
precipitation (Figure 2-16).

Changes in Ecology

The history of the Nueces Estuary is not dissimilar
from that of many other estuaries, in that it includes
man-made alterations such as diverting or removing
freshwater sources, creating channels, extracting
sediments, disposal site of waste materials and
harvesting plants and animals. These changes have

Table 2-1 : Summary of mean annual flow of the Nueces River into the Nueces Estuary (1 940 to 1 996)' and upper
Nueces Delta (1940 to 1999)^ Time periods in both studies were based upon the construction dates of large reservoirs in
the watershed.

Time Period

Mean annual river flow

into Nueces Estuary

(acre-ft)

Percent change
from Period 1

Me<
into

m annual river

upper Nueces

(acre-ft)

flow
Delta

Percent change
from Period 1

1940-1957
1958-1982
1983-1996(9)

619,000
614,000
279,000

-0.8%
-54.9%

127,997

77,989

537

-39.1%

-99.6%

' Source: Asquith ef a/ 1997.
^ Source: Irlbeck and Ward 2000.

Note: 1

acre-ft =1.2336 10' m'

2-18 ♦ Study Area

often greatly affected the general ability of the estuary
to offer the diverse range of habitats that allow a wide
range of organisms to thrive. Major alterations
affecting water-mass exchanges between the estuary
and the Gulf (and therefore the response of water
levels in the Nueces Estuary and Delta) include
alterations of the Nueces River channel, oyster shell
mining and the excavation of shipping channels, barrier
structures and artificial passes. The primary alteration
affecting salinity concentrations in the estuary and delta
is reduced freshwater inflows from to a combination of
dam construction and water extraction for agriculture,
industry and municipal uses.

Data records do not exist to accurately document all of
the resulting alterations to the ecology of the Nueces
Estuary and Delta over the past century. However, it
is generally agreed that the combined effect of these
activities has resulted in an estuary and delta with

higher salt concentrations in the soils and water,
channelized flow that reduced flooding by freshwater
overflow and tides, species shifts to more salt tolerant
forms and probably reductions in the total number and
biomass of plants and animals. The most notable and
evident changes have been reductions in oyster and
shrimp harvests (Montagna et al. 1998), each of which
requires a low salinity regime at some point in their life
cycle (Moffet 1970). With few exceptions (e.g., the
monitoring work conducted by the Texas Parks and
Wildlife Department in Nueces Bay), available data has
been primarily derived from subjective comparisons of
historical sources with present conditions, inferences
drawn from similar ecosystems, which have endured
comparable change processes, and direct observations
from recent monitoring activities.

Chapter Tmo ♦ 2-19

GEORGE H. WARD

Center foi Research in Water Resources,
University of Texas, Austin

MICHAEL J. IRLBECK

U.S. Bureau of Reclamation, Austin

CHAPTER THREE

Hydrography

INTRODUCTION

"Water is the blood of the earth,

and it flows through its muscles and veins.

♦♦♦ Kuantzu (late 4th Century)

The Nueces Estuary and Delta are subject to numerous
hydro-meteorological forces, including a combination
of river flow, precipitation, peripheral runoff and
forcing from wind and tide. Each of these influences,
in isolation or in combination, may gready affect the
quality and quantity of estuarine habitat available. Such
hydrographic events can alter water chemistry [e.^.,
salinity and nutrients); transport detrital material; cause
large volume exchanges between the bay, delta and
river; and make accessible or restrict habitats available
for estuarine aquatic organisms. Therefore,
interpretation and analysis of the effects of the
demonstration project and resulting biological
responses required a comprehensive understanding of
the hydrography of the area during the study period.

OBJECTIVES

1) To quantify (by direct measurement and data
analysis) the hydrographic interactions of the
upper Nueces Delta with the Nueces River and
Nueces Bay during the study period (October 1,
1994, through December 31, 1999);

2) To identify and describe significant hydrographic
events during this period; and

3) To describe the observed changes in the hydraulic
characteristics of Rincon Bayou resulting from the
demonstration project features.

Chapter Three ♦ 3-1

METHODS AND APPROACH
Data Sources

Hydrographic data for this analysis were obtained from
a variety of sources, including the Texas Coastal Ocean
Observing Network (TCOON) marine monitoring
system of Texas A&M University-Corpus Christi
Conrad Blucher Institute (CBI), the weather station
network administered by the National Weather Service
(NWS) and the national stream flow gauging program
conducted by the U.S. Geological Survey (USGS)
(Table 3-1).

Table 3-1: SummatY of hydrographic data sources.

surface in Nueces Bay is in response to meteorology,
tides and river hydrographs, this is negligible in
comparison to the temporal excursions in water level
in the river and marsh. Therefore, stage data from the
White Point gauge was regarded as an acceptable
indication of the general coincident elevation of
Nueces Bay.

The USGS gauges from which data were obtained
included the Nueces River at Calallen (Station
0821 1500) (Calallen gauge) and Rincon Bayou near
Calallen (Station 08211503) (Rincon gauge). The
Calallen gauge was located on the Nueces River, about
0.64 km upstream from Calallen Diversion Dam

Data Source

Parameter

Measurement Location

Data Type

Nueces River at Calallen,
USGS

TCOON system, CBI

Corpus Christi Bay NWS

Rincon Bayou near Calallen,
USGS

Estimated daily flow

Water level

Wind direction and velocity

Salinity

Daily precipitation

Water level
Current velocity
Calculated flow

Daily precipitation

Nueces River

Nueces and Corpus Christi
bays

Corpus Christi International
Airport

upper Rincon Bayou

Data recorded at
15-minute intervals

Data recorded as
6-mlnute averaged
values

Data archived as daily
values

Data recorded at
15-minute intervals

Data archived as daily
values

Data obtained from the TCOON system included
salimty and water level. For each of these parameters,
hourly measurements were obtained from the CBI data
archive, and, for the analysis reported here, subjected
to 24-hour averaging to obtain daily mean values. The
salinity data used were from the CBI SALT03 gauge,
which is situated due south of VCTiite Point in the
center of the bay about equidistant from the mouth of
Rincon Bayou and the mouth of the Nueces River
(Figure 3-1). This gauge responds to flow from both
conveyances. Salinity concentrations are measured by
robot conductivity sensor and converted to salinity,
reported in parts per thousand (ppt).

Water level in Nueces Bay was that measured at CBI's
White Point gauge, nearer the mouth of Rincon Bayou.
While there is no doubt some slope to the water

(Figure 3-2) and about 3.05 km upstream from the
entrance of the Nueces Overflow Channel. Flow data
were obtained from both gauges, and stage and
precipitation data were also obtained from the Rincon
gauge. There are significant limitations to the accurac)'
of the flow data from the Calallen gauge at higher
values for two primary reasons. First of all, the gauge
ceases to represent the total flow of the Nueces River
above about 56.63 cubic meters per second (m'/s)
(2,000 cubic feet per second (cfs)) due to activation of
additional flow channels in the floodplain. Second, no
reliable field observations of discharge values are
available above 77.87 m'/s (2,750 cfs), so all daily flow
values in excess of 77.87 m'/s (of which there were
3 in the record under review) were estimated by
extrapolation.

3-2

«*

Hydrography

DEMONSTRATION PROJECT
STUDY AREA

Approximate Scale
Kllomelers

Figure 3-1: Location of hydrographic gauging stations in the Nueces Delta and upper Nueces Bay. The location of
the Corpus Christ! Airport station is not shown, but is located approximately 14 l<m to the southeast of the study area.

The Rincon gauge was located in the head-water
channel of Rincon Bayou, approximately 274 m
downstream (Figure 3-3). There was a gap in the data
record from the Rincon gauge from October 26, 1995,
when the Nueces Overflow Channel was opened,
through May 15, 1996, when the Rincon gauge was
activated. The absence of data limited the analysis of
the demonstration project during this seven-month
period when no measurements of flow, water level or
precipitation at the station were available. However,
because this period proved to be relatively dry with few
hydrographic events, the missing data were not deemed
critical. Additional technical information on the
Rincon gauge instrumentation, data and observed rela-
tions with the Calallen gauge was developed separately
by Irlbeck and Ockerman (2000) and has been included
as Appendix A of this Concluding Report.
Precipitation data were also obtained from the
National Climatic Data Center records for Corpus
Christi Airport, which is located approximately

14 kilometers southeast of the study area. This gauge
supplied precipitation data for the region before the
Rincon gauge was installed.

Quantification of Hydrographic
Interactions in the Study Area

Once the hydrographic parameters of interest were
thus identified, raw data for the period of 1992 through
1999 were obtained from these sources and compiled
by Ward (2000). Prior to late- 1993, virtually the only
extant data for the study area was salinity in Nueces
Bay and inflow measured at Calallen. In 1993, reliable
records from CBI tide gauges and anemometers
became available. By the date of the breaching of the
Nueces Overflow Channel on October 26, 1995, fairly
continuous data from the region was available, with
only the USGS gauge in Rincon Bayou itself lacking. It
did not become operational until May 16, 1996.

ChapterThm ♦ 3-3

Figure 3-2: View of the Nueces River at Calallen
Diversion Dam. The photo was taken on June 26, 1997,
for which the mean daily flow rate in the river was
63.7 mVs (2,250 cfs).

Photo courtesy of the Bureau of Reclamation.

Identification of Hydrographic
Events during the Demonstration
Period

In an effort to develop a common summary' of
hydrographic events useable in the analyses of changes
in water column productivity (Chapter 4), benthic
communities (Chapter 5) and vegetation communities
(Chapter 6), the compiled hydrographic data record
was divided into separate "events" (or event-duration
analysis) (Ward 2000). A hydrographic "event" was
considered to include one or more of the following
response mechanisms: 1) a substantial volume of
freshwater flow, 2) an increase in water level (stage) or
3) a decrease in salinity. Where and how each of these
three broad response mechanisms were characterized
depended largely upon the availability' of data. In
general, the responses of freshwater inflow and water
level were each divided into two components; one for
the upper Nueces Bay (and lower delta), and one for
Rincon Bayou (and upper delta). The response of
salinity was considered for the hydrographic analysis
only in upper Nueces Bay, but was examined in each of
the subsequent chapters at the various monitoring
stations. The initial conceptual model for such an
event was a flood hydrograph translating down the

Three dme scales were considered of pertinence to the
evaluation of the compiled hydrographic data for the
Nueces Estuary: intratidal, intertidal and event-
duration. The intratidal (or intradiumal) time scale
represented short-term behavior at an hourly
resolution; the intertidal (or interdiumal) time scale
represented day-to-day variations of hydrographic
parameters averaged over 24 hours; and the event-
duration scale extended over the time period
encompassing all responses to a specific hydrographic
event, and could range from several days to many
weeks. Therefore, to facilitate detailed analysis at
varying degrees of detail, all available data was
compiled in hourly, daily and event-specific formats
(Ward 2000). Further discussion of the approach and
results of the intratidal, intertidal and event-duration
analyses conducted by Ward (2000) has been included
as Appendix B of this Concluding Report.

Figure 3-3: View of the Rincon gauge
(Station 08211503).

Photo courtesy of the Bureau of Reclamation.

3-4

Hydrography

Nueces River into the vicinity of the upper delta.
However, as became evident, other hydro-
meteorological processes would also effect similar
responses in these variables.

Criteria for Event Delineation

After inspection of the entire period of record on both
intertidal and intratidal scales. Ward (2000) formulated
criteria to identify an event based upon the separate
hydrographic behaviors of each of the key response
parameters. These response variables included stage
(in Nueces Bay, in Rincon Bayou and the super-
elevation of Rincon over Nueces Bay), flow (in the
Nueces River and in Rincon Bayou) and salinity (in
Nueces Bay) (Table 3-2). It should be emphasized that
these criteria were ultimately arbitrary but were utilized
to ensure an objective selection of candidate events for
analysis. Precipitation was not treated as a separate
hydrographic variable, though it was certainly an
important hydrographic element in understanding the
response of the delta ecology. The reason for its
exclusion as a defining parameter was that it provided
no in forma tion/)«'rj'(? on the response of the Nueces
Estuary or Delta. A similar argument was made for the
exclusion of wind as a defining criterion.

Once these criteria were established, the daily data for
the period of study was manually inspected (Ward
2000). Individual occurrences within the record which
met at least one of the six criteria were identified as
hydrographic events. Then, for each event, the
24-hour mean data for aU hydrographic variables
during the event were separated and transferred for
individual analysis. The duration period for each event
was at least that for which the defining criterion was
satisfied, though often a longer event period was
chosen to be sure that the complete response of the
bay or delta was included in the analysis. When several
hydrographic events overlapped (i.e., when several
variables each satisfied criteria separately and
simultaneously), the event duration was at least the
period from the first occurrence of the criterion
threshold for the earliest parameter to at least the last
such threshold for the latest parameter.

The greatest difficxilty reported in separating such
events was met when a time series of events occurred
in which the response of one parameter overlapped
that of the next. For example, a series of river
hydrographs might occur, each of which raised the
Rincon stage or CalaUen flow above the threshold
defining an event, and a new surge of inflow occurred

Table 3-2: Criteria used to define hydrographic events in the data record by response variables.

Source: Ward 2000.

Response
Parameter

Location

Defining Criteria of a Hydrographic Event

Flow

Stage

Salinity

Nueces River A 24-hour mean (daily) flow in the Nueces River at Calallen exceeding

14.2 mVs (500 cfs).

Rincon Bayou A 24-hour mean (daily) flow in Rincon Bayou, either positive or negative,

exceeding 0.28 mVs (10 cfs).

Nueces Bay A 24-hour mean (daily) stage in the water elevation of Nueces Bay exceeding

0.30 m (1 .0 ft), referenced to the consistent CBI datum from Ward (1997),
established by "empirical leveling".

Rincon Bayou A 24-hour mean (daily) stage in the water elevation of Rincon Bayou exceeding

0.61 m (2.0 ft), relative to Rincon gauge datum, which is 422 cm above the
consistent datum for CBI gauges.

Super-elevation The difference of Rincon Bayou minus Nueces Bay daily stage values
exceeding 0.15 m (0.5 ft), referenced to common datum.

Nueces Bay Change in salinity concentrations of Nueces Bay exceeding 5 ppt over a five

day period.

Chapter Three ♦ 3-5

before the recession of the preceding event has
subsided. Separating these into individual events was
therefore rather arbitrary, and, from the estuarine
response point of view, this division may or may not
have been differentiated in the actual environment,
which may have responded as if adjacent events in the
record were a single "merged" event.

Characterization of Individual Events

Once individual events were thus delineated in the
record, integrated values for each response parameter
in each event were computed. The term "integrated"
means either averaged or accumulated, whichever was
more meaningful for the parameter under considera-
tion. The parameters compiled for each event include
the following: event number, date, duration, rainfall,
flow, stage and salinity.

Event Number — For purposes of tracking and
reference, each identified event occurring from
October 1, 1994, through December 31, 1999 (the
duration of the monitoring program), was numbered
sequentially in time.

Date and Duration — The span of each event was
specified by its starting date and its duration in days.
In some cases, the ending date of one event was
immediately before the starting day of the next, which
indicates that a subjective separation had been assumed
in the record for purposes of analysis.

Rainfall — Local precipitation for each event was
determined by summation of daily totals during the
event. For the period of October 1, 1994, through
May 15, 1996 (i.e., prior to the activation of the Rincon
gauge), precipitation was that reported at Corpus
Christi Airport, which e\'idenced a correlation with the
Rincon gauge of 0.81 for the 3.7 years of coincident
data. After May 15, precipitation was that reported by
the Rincon gauge.

Flow — Daily values for both flow variables (Nueces
River and Rincon Bayou) were converted to daily
volume and then summed to determine the cumulative
event volume. For flow in the Nueces River, daily
average flow was also computed for each event by
dividing the total flow volume by the event duration in

days. Because flow in Rincon Bayou was frequendy bi-
directional, these data were analy2ed somewhat
differendy. Total positive volume (pos) was
determined by considering only that flow directed from
the river into the delta. Total negative volume (neg)
was similarly determined but considering only that flow
directed from the delta into the river. Total exchange
(gross) was defined to be the sum of the absolute
values {i.e., ignoring signs) of the positive and negative
volumes. Finally, total net exchange (net) was defined
to be the algebraic sum {i.e., observing signs) of the
positive and negative volumes.

It is important to note that, during the demonstration
period, the Nueces River did not exceed the natural
flooding threshold for the delta, which was 1.64 m
(5.40 ft) (Bureau of Reclamation 2000), except on four
occasions. This means that, except for these events
(Events 16, 18, 25 and 36), the only water exchanged
between the Nueces River and Rincon Bayou during
the study period passed through the Nueces Overflow
Channel. During the excepted events, an additional
amount of water also entered Rincon Bayou naturally
\'ia the low depressions along the bank of the river.
Therefore, for all other events, the Rincon Bayou flow
volumes reported were those measured through the
Nueces Overflow Channel at the Rincon gauge. For
Events 16, 18, 25 and 36, an additional volume was
added to that gauged through the overflow channel.
This additional amount depended upon the daily stage
level attained by the river during the event and was
estimated using the hydraulic model developed by
Reclamation (2000). Accordingly, for Event 16, which
attained a peak daily stage of 1.70 m (5.57 ft) msl, an
additional 4 10^ m' (3 acre-ft) were added; for Event
18, which attained a peak daily stage of 1.72 m (5.65 ft)
msl, an additional 5 10^ m' (4 acre-ft) were added; for
Event 25, which attained a peak daily stage of 2.22 m
(7.28 ft) msl, an additional 1,189 10' m' (964 acre-ft)
were added; and for Event 36, which attained a peak
daily stage of 1.74 m (5.72 ft) msl, an additional 6 10'
m' (5 acre-ft) were added. The remark "natural flow
event" identifies these events in Table 3-3.

Stage — For the water level in Nueces Bay and Rincon
Bayou, both the daily average and peak daily stages
were utilized, the latter since the average alone might

3-6 ^* Hydrography

not have fairly depicted the range of water level
excursion during an event. Raw daily stage data from
the Rincon gauge, which is reported in local time, was
corrected for GMT {i.e., the 15-minute data were re-
averaged) to allow temporal consistency with the other
data variables. The super-elevation of water levels,
determined by subtracting the Nueces Bay stage from
the Rincon Bayou value, was used to determine the
predominant influence on stage in upper Rincon
Bayou. On an instantaneous basis, the super-elevation
is the direct force for discharge through the Nueces
Overflow Channel.

Salinity — The salinity response of upper Nueces Bay
to an inflow event would be expected to be an initial
drop in concentration, followed by a slow increase with
time, or "recovery". However, this approach of
integrating the salinity parameter could possibly
understate the response of salinity to lower magnitude
hydrographic events, because the entire response could
be occur within the event's duration and therefore
elude detection. Foxir separate indicators were thus
computed from the salinity data for each hydrographic
event: 1) the salinity value at the beginning of the event
(start), 2) its net incremental change at the close of the
event (chg), 3) the percent of the beginning value that
the increment represented (% chg), and 4) the
minimum value attained during the event (min). As so
defined, the sign of the incremental (percent) change
indicates whether salinity increased or decreased over
the duration of the event (negative for a decrease and
positive for an increase).

RESULTS

In this analysis, hydrographic events were delineated
according to the mechanism or feature exhibiting a
response: namely, the response of flow in the Nueces
River or in Rincon Bayou; water level in Nueces Bay or
in Rincon Bayou; and salinity in upper Nueces Bay. A
oirsory inspection of the hydrographic events
presented in Table 3-3 reveals the fact that each of
these response mechanisms can occur in isolation,
without the involvement of the others. This indicates
that a certain degree of care must be used in
determining how a hydrographic event has influenced

the chemistry and ecology of the project area. For
example, if inundation and de-watering were of
concern, then the extent to which events accomplished
a response of water level would be of central interest,
and the most important of these was the response of
water level in Nueces Bay for the lower delta and in
Rincon Bayou for the upper delta. If freshwater inflow
to Nueces Bay from the river were the major
determinant, perhaps through a sediment or chemical
load, then the flow events in the Nueces River would
be regarded as most important. If it were freshwater
flow into the upper delta, then flow in Rincon Bayou
would be critical. Finally, if the depression of salinity
were a key interest, then the salinity events in Nueces
Bay would be of primary concern.

Overview of Hydrographic Events
That Occurred During the
Demonstration Period

For the period investigated {i.e., October 1994 through
December 1999), a total of 37 hydrographic events
were identified (Table 3-3). Five of these events
occurred prior to the opening of the Nueces Overflow
Channel, and thirty-two afterward. These events were
highly variable in the magnitude of their responses,
durations and in the subset of hydrographic variables
in which a response occurred. Some were associated
with seasonal high waters in Nueces Bay, some were
salinity responses elicited only by internal circulations
of the bay, some were responses to intense rainfall and,
of course, some were in fact inflow hydrographs in the
Nueces River. Most (28) of these events occurred after
operation of the USGS Rincon gauge in the Nueces
Overflow Channel, allowing the direct measurement of
flow diverted by the demonstration project, the
associated water level rise and in situ precipitation.

Of the total 37 events observed, 1 5 met the flow
criteria for a flow event in the Nueces River, 16 met
the criteria for a stage event in Nueces Bay and 21
met the criteria for a salinity event. Of the 28 events
occurring after the installation of the Rincon gauge,
20 met the criteria for a flow event in Rincon Bayou,
27 met the criteria for a stage event in Rjncon Bayou
and 14 met the criteria for a super-elevation event.

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3-8 ♦♦♦ Hydroff-aphy

Water level in Rincon Bayou (at least since the Nueces
Overflow Channel was opened) was the most
responsive parameter to hydro-meteorological forces in
the upper delta during the demonstration period
(Table 3-3).

Summary of Selected Individual

Events

As can be observed from Figures 3-4 through 3-9,
most of the observed hydrographic events involving
large flows in the Nueces River occurred during the
latter portion of the demonstration period, particularly
during 1997, 1998 and 1999. This absence of such
events in the early part of the demonstration period
was attributable to the fact that, during the first few
years, south Texas experienced a moderate to severe
drought. However, although large freshwater diversion
events were absent during this period, modest flow
events into Rincon Bayou from the Nueces River did
occur {e.^.. Events 12, 13 and 14, and probably a few of
Events 6 through 10). The driving mechanism for
these events was not flow in the river but other hydro-
meteorological forces affecting water level in the
Nueces Estuary (Table 3-3). Several other similar
exchange events occurred throughout the
demonstration period.

Fall 1996: Events 12 through 14

One example of these kinds of small exchange events
were Events 12, 13 and 14, which resulted from a fall
maxima high water event in the Gulf of Mexico
(Table 3-3). From October 3 through 8, as the water
levels in Nueces Bay and Rincon Bayou increased, a
sustained positive flow occurred through die overflow
channel, which peaked on October 6 at 1.04 m'/s
(36.9 cfs) (Figure 3-10). During same period, only a
minimal amount of river flow (no more than
0.10 m^/s, or 3.5 cfs) passed over Calallen Diversion
Dam. However, during October 9 through 14, flow in
the river rose to about 4.67 m'/s (165 cfs) on
October 12, while water levels in the upper delta and
bay began to decrease. The surprising result was that
water diversion through the channel reversed direction
and flowed from the upper delta into the Nueces River

until October 15, when the water level in the bay again
began to increase. This behavior, which continued
through Events 13 and 14, demonstrated that, at low
flow volumes in the Nueces River, diversions through
the overflow channel were driven primarily by water
level variations in Nueces Bay and the upper delta.

The USGS made several salinity measurements at the
Rincon gauge during Event 12. Salinity was measured
on October 4, 5 and 6, which was the period of
sustained positive flow through the overflow channel.
The salinity values for each day were 2.0, 3.9 and
7.2 practical saHnity units (psu) (which are approxi-
mately equivalent to parts per thousand), respectively.
During this period, no flow occurred in the Nueces
River on October 4 and 6, and only 0.10 m'/s (3.5 cfs)
occurred on October 5. The increasing salinity values
over this 3-day period indicates that flow was moving
up the Nueces River channel and into Rincon Bayou as
a result of the rising water level in Nueces Bay.
Therefore, the total net diversion into Rincon Bayou
during these three events (289 10' m', or 234 acre-ft)
was relatively fresh water.

Summer 1997: Events 16 and 17

The first significant occurrence of freshwater flow
during the demonstration period occurred from June
21 through July 27, 1997 (Events 16 and 17). These
two events occurred one immediately after the other,
and were derived from the same basin-wide
precipitation event. This storm was one of the many
tropical/middle latitude heavy rain events common to
south Texas. On June 21, a near-stationary low
pressure system over south-central Texas began to
move east and north, causing scattered showers and
thundershowers over a large part of north Texas, the
Texas Hill Country and central Texas. This movement
allowed tropical moisture to move in from the south
and feed into the area of instability, lift and daytime
heating in the afternoon, which resulted in a second
round of locally heavy rain in the greater Nueces
watershed.

Rainfall amounts with this second rain event varied
from 23 to 58 centimeters (cm) (9 to 23 inches) over
the Texas Hill Country, and between 13 and 25 cm

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Chapter Tine ♦ 3-15

2.25

1 r

10 12 14 16 18 20 22 24 26 28

30 1 3
Nov

Flow in Rincon Bayou (Rincon gauge)

Flow in Nueces River (Calallen gauge)

— o— Water level in Nueces Bay (White Point gauge)

Figure 3-10: Selected hydrographic data for Events 12, 13 and 14 (October 2 through
November 3, 1996). During this period of low flow in the Nueces River, water level in Nueces Bay
(not flow in the Nueces River) was the predominant factor in determining the rate and direction of
flow through the Nueces Overflow Channel and Rincon Bayou.

(5 to 10 inches) over central and south-central Texas,
including the headwaters of the Nueces and the Frio
drainages. A similar pattern, on a larger scale, affected
central and south central Texas in December of 1991,
causing 30 to 51 cm (12 to 20 inches) of precipitation
over much of south Texas. Direct precipitation in the
upper delta from the 1997 storm totaled only 4.9 cm
(1.94 inches) through June 21-23, -with the only other
measurable precipitation being 0.1 cm (0.04 inches)
fromjuly20to21.

These two hydrographic events (16 and 17)
exemplified a common resulting flow pattern in the
lower Nueces River from precipitation events in the
greater watershed: namely, a duel-peak hydrograph in
the river at the point of diversion into the delta

(Figure 3-7). The first principal peak (Event 16)
arrived at the upper delta on June 26, and represented
runoff primarily in the lower (eastern) watershed.
Approximately 20 days later, during which time the
Nueces River at Calallen experienced a sustained flow
of about 39.6 m'/s (1,400 cfs), the second principal
peak (Event 17) arrived on July 15. This peak
represented runoff from the same storm but from the
upper (western) watershed. As a result of the total
river inflow into the bay system, the salinity of Nueces
Bay was reduced from 26.8 ppt at the beginning of
Event 16, to 3.4 ppt at the end of Event 17.

Because the average ambient water level in Nueces Bay
was so low and the average stage in the river at the
point of diversion was so high, the hydraulic head

3-16 ♦♦♦ Hydrography

imposed by the river (as represented by mean super-
elevation) was considerable (greater than 1.21 m, or
4.0 ft) (Table 3-3). This indicates, unlike during Events
12 through 14, the diversion rate into Rincon Bayou
during Events 16 and 17 was predominandy a fiinction
of the elevated stage in the Nueces River. As each
principal peak receded, discharge through the overflow
channel dropped sharply (Figure 3-7). At the event's
conclusion, the loss of stage in the river immediately
resulted in a sharp decrease in the rate of discharge,
and the upper delta discharged a significant amount of
water back into the river. This pattern, the reversal of
diverted flow at the end of large river flow events,
would become common in subsequent events. During
both events, the Rincon Overflow Chaimel was
activated, diverting water into the extensive tidal flat
area of the upper delta.

FaU1997: Event 18

The fall event of 1997 (Event 18) resulted from very
heavy precipitation in the lower Nueces Watershed.
This event, although comparable to Event 16 in
regards to positive flow and maximum stage in Rincon
Bayou, was different from the previous two events in
one significant way. Unlike Events 16 and 17, which
were events responding to precipitation higher in the
basin. Event 1 8 was a hydrographic response to
intense local precipitation direcfly on the study area
and lower watershed. Approximately 23 cm
(9.17 inches) fell within the area during the first 6 days
of the event, contributing to heavy local runoff and
artificially elevated water levels in the upper delta
and Nueces Bay (as reflected in mean and peak stage).
As a result, discharge into the delta from the river
actually began before the river (gauged at Calallen)
began to respond to the event (Figure 3-7).

As with the two previous events, the rate of diversion
into the delta fell off sharply as soon as stage in the
river began to drop, and, like Event 17, remained
negative for the concluding days of the event.
Although the total positive flow diverted into Rincon
Bayou during this event was about the same as that for
Event 16, over 60% of this volume flowed back into
the river at the end of the event. This difference was
attributable to the fact that water levels in the Nueces

River at the point of diversion (and therefore, the
hydravilic head) were not maintained after the river
crested, as was the case with Event 16.

FaU 1998: Events 21 through 27

For purposes of event analysis, the fall of 1998 was a
very complicated period. The careful observer of
Table 3-3 and Figure 3-8 will recognize that, as with
Events 12 through 14, Events 21 through 27 were
essentially contiguous, or that the ending day of each
event immediately preceded the beginning day of the
next. The challenge in interpretation was separating
the effects of a bewildering number of influencing
factors. For example, during this 96-day period, two
tropical storms made landfall in the region, a fall-
maxima high water event occurred in the Gulf of
Mexico, over 56 cm (22 inches) of local rainfall was
recorded in the study area, over 219,820 10' m'
(178,194 acre- ft) flowed from Nueces River into the
bay and over 5,092 10' m' (4,128 acre- ft) was diverted
into the upper delta. For purposes of interpreting
biological responses in the delta, one may justifiably
consider each numbered event as a mere temporal
component of the greater autumn occurrence of 1998.

Beginning on August 17, heavy rain episodes caused by
a cool air mass sagging into central Texas began to
occur over much of south Texas. Because the rainfall
followed record drought conditions between April and
July, very httie runoff resulted. In fact, over 15 cm
(6 inches) of precipitation feU over a 2,000-km" area in
the Frio River watershed and produced absolutely no
flooding (Patton 1998). The rainfall continued
throughthe day on August 1 8, with a wide-spread area
in the greater Nueces watershed receiving 20 to 33 cm
(8 to 13 inches).

Tropical Storm Charley made landfall near Port
Aransas the night of August 21 (Events 21 and 22).
As the storm center moved slowly inland toward the
Hill Country west of San Antonio, it produced 5 to
8 cm (2 to 3 inches) of rain in a three-hour period,
with the heaviest rainfall resulting from "feeder" bands
that wrapped around the center. These bands were
moving very slowly and dropped several inches of rain

Chapter Thne ♦ 3-17

before they departed an area. Primarily because the
region had received large rainfall totals for several days
prior, significant and fatal flooding on the upper
Frio and Nueces rivers resxilted. Direct precipitation
in the delta from the storm's landfall totaled only
about 0.6 cm (0.24 inches) (August 21 to 23).
Nevertheless, a modest peak in stage and discharge in
Rincon Bayou occurred on August 23 and 24, primarily
due to the storm surge (Figure 3-8). Because a
relatively small amount of precipitation fell in the lower
Nueces watershed associated with the landfall of
Charley, the Nueces River at Calallen recorded only
a modest peak on August 8.

However, as previously mentioned, Charley did result
in a broad flooding event in the western basin. The
Nueces River at Calallen began to respond to this
flooding on September 4, and crested on September 14
(Event 23). Flow through the Nueces Overflow
Channel generally followed that of the river, but
fluctuated on a hourly and daily basis. This oscillation

was primarily due to the complicating effect of a
second storm surge associated with Tropical Storm
Frances, which made landfall near San Antonio Bay on
September 10. The arrival of Frances may be observed
in Figure 3-11, when, during September 7 through 10,
stage and flow in Rincon Bayou increased dramatically,
while stage in the Nueces River remained essentially
constant. Similarly, when the surge subsided after
September 10, discharge through the overflow channel
dropped sharply, even though flow in the Nueces River
at Calallen increased significantiy. Direct precipitation
in the delta from Tropical Storm Frances totaled
5.7 cm (2.26 inches) from September 9 to 12.
Flow in the Nueces River continued to remain above
19.8 mVs (700 cfs) (Events 23 and 24) for several
weeks after the landfall of Frances as a result of the
storm's heavy rain in the upper watershed. Flow in
Rincon Bayou was also generally positive during this
period but not substantial (less than 0.28 m'/s, or
10 cfs). As with the previous events, flow in Rincon

300

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Flow in Rincon Bayou (Rincon gauge)

Flow in Nueces River in 10's of cfs (Calallen gauge)

Water level in Nueces Bay (White Point gauge)

Figure 3-11: Selected hydrographic data for Event 23 (September 1 through 31, 1998).

3-18 ♦ Hydrography

Bayou became negative immediately after stage in the
river began to receded beginning October 13
(Figure 3-8).

Due to significant runoff from the large amounts of
tropical moisture and associated precipitation in the
upper Nueces Basin during this period, the Nueces
River again began to rise, cresting on October 21
(Event 25). This crest resulted in an estimated daily
flow rate of 73.6 m'/s (2,600 cfs) in the Nueces River
at Calallen, the highest flow rate attained during the
demonstration period. The corresponding water
elevation in Rincon Bayou was also the highest daily
stage recorded during the study, 2.22 m (7.28 ft msl),
which was 0.48 m (1.56 ft) higher than the next highest
event daily peak (Event 36). Because of this. Event 25
was the only event to witness the Nueces River
meaningfully exceed the natural {i.e., without-project)
diversion threshold of the upper Nueces Delta, which
was 1.64 m (5.40 ft) msl. As a result, the estimated
amount of delta inflow over the natural river bank
(1,204 10' m\ or 976 acre-ft) was about 50% as much
as that which was gauged through the overflow
channel (2,560 10' m', or 2,075 acre-ft). It was during
this event that the road crossing at the lower end of the
Rincon Overflow Channel washed out (Figure 3-12).

Events 26 and 27 concluded the fall occurrence of
1998, resulting in two more smaller peaks in river and
diversion flows as the greater watershed finally drained
the accumulated runoff of the preceding months.

FaU1999: Events 35 and 36

Hurricane Bret was the first major hurricane of the
Adantic 1999 season (Figure 3-13). The storm formed
in the southern Gulf of Mexico and moved slowly
northward along the Mexican Coast. As it approached
south Texas, it rapidly intensified into a Category 4
hurricane. Landfall was made as a Category 3 storm on
August 23, in a remote area between Brownsville and
Corpus Christi (Event 35). The storm surge from Bret
was substantial, and combined with the 14.43 cm
(5.68 inches) of local precipitation that fell on August
22 and 23, the Rincon gauge recorded a maximum
elevation of 1.76 m (5.79 ft) msl (August 23). This
value was the highest value recorded during the study

Figure 3-12: Discharge into the tidal flats area through
the road crossing at the north end of the Rincon
Overflow Channel during Event 16 (June 27, 1997).

Each of the 10 corrugated HOPE culverts shown were 24"-
diameter. This structure was subsequently washed out
during Event 25 (October 1998).

Photo courtesy of the Bureau of Reclamation.

period without a corresponding flow ev^ent in the
Nueces River. Once the low pressure system had
moved on-shore, the surge tide and local runoff in the
upper delta began to drain out, resulting in a net
negative flow through the overflow channel for Event
35 (Figure 3-9).

As a result of heavy rainfall in the watershed associated
with Hurricane Bret, particularly in the southwestern
portion, the Nueces River again responded with a large
hydrograph beginning on September 5 (Event 36).
The amount of diverted flow through the overflow
channel was the second largest recorded for any event,
with a net flow of 1,052 10' m' (853 acre-ft) over
17 days. The salinity of Nueces Bay, which had already
been lowered by the landfall of Hurricane Bret some
three weeks before, continued to decrease as a result of
the event.

Chapter Thm ♦ 3-19

Figure 3-13: Visual satellite image showing the
location of Hurricane Bret in relation to the Nueces
Watershed just before landfall (August 22, 1999).

Having threatened to lay waste to much of south Texas,
the storm made landfall as a Category 3 Hurricane.
Meteorologists were surprised, however, by Bret's
lackluster performance inland. "We ain't surprised," said
one Texas native. "You think something named 'Bret' is
gonna do much here? I have breakfast cereal with more
hair on its chest."

Source of Image: National Environmental Satellite, Data and
Information Service of the National Oceanic and Atmosptieric
Administration.

DISCUSSION

Flooding Thresholds for the Upper
Nueces Delta

For over 1 3 years prior to the opening of the Nueces
Overflow Channel {i.e., October 26, 1995), the
minimum flooding threshold required for the Nueces
River to spiU freshwater into the upper Nueces Delta
was rarely attained (Irlbeck and Ward 2000). Based
upon recent estimates, a flow in the Nueces River at
Calallen greater than about 59.5 m'/s (2,100 cfs) was
required to breach the lowest portion of the northern
river bank (Irlbeck and Ockerman 2000). However,
excavation of the overflow channel fundamentally
changed this condition. The minimum flooding
threshold was thus lowered from 1.64 m (5.4 ft msl) to
about 0.0 m msl. As can be observed from the
suinmary of hydrographic events in Table 3-3, this
change not only allowed more frequent diversions of

fresh water into Rincon Bayou and the upper delta, but
also provided the opportunity for other non-riverine
elements like wind and tide to force water exchanges
between the upper delta and the Nueces River. As a
result, near continual {i.e., daily) exchange between the
Nueces River and the upper delta was observed during
the demonstration period, whereas before the
interaction was limited to only extremely rare river
inflow events.

During the 50-month period when the overflow
channels were open, over 8,810 10' m' (7,142 acre-ft)
was diverted from the Nueces River into Rincon
Bayou and the upper delta. Using the hydraulic model
developed by the Bureau of Reclamation (2000), it was
estimated that, of this total amount, only about 1,204
lO' m' (976 acre-ft) would have been diverted without
the demonstration project features. Therefore, during
the demonstration period, the total volume of
freshwater inflow into the upper Nueces Delta was
increased by about 732% from what would have
occurred without the project. In a longer-term
examination, Irlbeck and Ward (2000) analyzed the
inflow patterns of the upper delta assuming that
demonstration project features had been in place since
the completion of Choke Canyon Dam in 1982
through 1999. These authors concluded that the
average annual inflow amount to the upper delta
during this 17.6-year period would have been increased
from about 666 10' m' (540 acre-ft) to approximately
4,219 10' m' (3,420 acre-ft), or by over 633% from that
which would have occurred without the project
(Irlbeck and Ward 2000). This long-term analysis has
been included as Appendix C of this Concluding
Report.

Activation and Behavior of
Demonstration Project Features

Nueces Overflow Channel

Because the controlling (bottom) elevation of the
Nueces Overflow Charmel was at or near mean sea
level, it was activated (or it allowed the exchange of
water) when there was a change in water level in either
bay or the river. Throughout most of the

3-20 V Hydrog'aphy

demonstration period, bi-directional flow occurred
almost daily. Changes in the physical condition of the
Nueces Overflow Channel included the encroachment
of vegetation along the water's edge, and a slight
narrowing of the bottom of the channel due to erosion
from its banks (Figure 3-14).

Since the primary purpose of the demonstration
project was to divert a portion of the flow in the
Nueces River through the diversion channel, a logical
inquiry was the proportion of such a flood so diverted.
Upon examination of the relation between the total
event flow volume in the Nueces River and in the
Nueces Overflow Channel, it was determined that the
volume diverted into Rincon Bayou increased generally

Figure 3-14: The Nueces Overflow Channel ioo,„.i9
southwest. Note the changes in vegetation and channel
characteristics from October 1995, immediately after
construction (above), to June 1999, towards the end of the
demonstration period (below).

Photos courtesy of the Bureau of Reclamation.

with the flow in the river, and the actual proportion of
the flow amount diverted was on the order of 2% of
that in the river (Ward 2000). The actual rate of
discharge, however, varied considerably between
events depending upon the water level in Nueces Bay
and Rincon Bayou.

That the relation between Nueces River event volume
and the volume transported through Rincon Bayou
should depend upon water level was not unexpected,
based upon hydraulic considerations. Unlike a river
channel system in which the head gradient and the
water level (stage) are closely related, there is no direct
relation between water level and flow in the Nueces
River below Calallen Diversion Dam because of the
corrupting effect of tidal and meteorological
water-level variations. For the events observed, the
Nueces River hydraulic head was superposed on
whatever water level was present in Nueces Bay, which
affected how the river head could drive flow through
the overflow channel. Deeper water made available a
greater cross-section area of the channel and lowered
the frictional resistance. Therefore, a given hydraulic
head in the Nueces River drove a greater flow through
the diversion channel when the Nueces Bay water level
was higher.

Rincon Overflow Channel

In addition to increased inflow, the demonstration
project features also increased the distribution of
diverted fresh water within the tidal flats of the upper
delta. The controlling elevation of the Rincon
Overflow Channel was about 1.14 m (3.75 ft) msl.
When water levels in Rincon Bayou exceeded this
threshold, flow would pass through the chaimel and
across the tidal flats to the northeast. Without this
overflow channel, total diversions through the
demonstration project would have been lower, and
most of the freshwater diverted would have remained
channelized in the upper delta.

Although no direct gauging data were available to
determine exactiy when and to what degree the Rincon
Overflow Channel was activated during the
demonstration period, it is certain that, on at least two
occasions, the channel passed a significant amount of

Ch^ter Three ♦ 3-21

freshwater into the tidal flats. The first ocairrence was
during Event 16, when such discharge was observed in
the field, and the second was during Event 25, when
the road structure at the north end of the channel was
washed out into the tidal flats (Figure 3-15). Based
upon comparable event stages, super-elevations and
flow volumes, it was strongly suspected that the
Rincon Overflow Channel also passed some amount of
diverted fresh water during Events 17, 18 and 36,
although this was not visually verified.

Figure 3-15: The Rincon Overflow Channel. The road
crossing structure, as shown from the air during Event 16
in June 1997 (above), w/as washed out during Event 25 in
October 1998, as shown in June 1999 (below). The
HOPE culverts (foreground of the lower photo) were
24"-diameter, and the livestock (background of the
lower photo) were Red Brangus.

Ptiotos courtesy of the Bureau of Reclamation.

Figure 3-16: The low water crossing at the head of the
upper Rincon Bayou channel. The integrity of this
structure, although significantly over-topped during Event
25 (October 1998), remained unchanged throughout the
demonstration period. Each HOPE culvert was
36"-diameter. The photo was taken on June 1999.

Photos courtesy of thie Bureau of Reclamation.

Low Water Crossing

The low water crossing at the head of Rincon Bayou
performed exceptionally well. All flows diverted from
the river (either naturally or through the Nueces
Overflow Channel) passed through this structure
(Figure 3-16). On at least one occasion (Event 25), a
significant amount of water passed over the top of the
crossing, with no damage visible. At the end of the
demonstration period, the structure was essentially the
same as at the beginning.

SUMMARY

The demonstration project features significandy
lowered the minimum flooding threshold of the upper
Nueces Delta, and thereby increased the opportunity
for larger, more frequent hydrographic events. Based
on observations and data analysis during the
demonstration project, such events were categorized
into three general types: small "exchange" events,
"positive-flow" events and "tidal flat inundation"
events.

3-22 ^* Hydmgraphy

Exchange Events

Exchange events were considered to be frequent, low-
volume interactions between the channels and pools of
Rincon Bayou and either adjacent water body (Nueces
Bay or Nueces River) (Figure 3-17). Exchange events
were primarily caused by daily differences in water level
elevations, although these differences were in turn the
result of a variety of other factors like tide, wind, river
inflow, etc. The net flow volume for these types of
events was generally low (less than 123 10' m', or
100 acre- ft), and could be either positive or negative
through the Nueces Overflow Channel. Although the
effects of exchange events were confined to the
channels of Rincon Bayou, they provided considerable
dilution and mixing of ambient waters, especially in the
upper delta. Events 11, 15, 22 and 31 were typical
examples of exchange events (Table 3-3). Prior to the
demonstration project, the Rincon Bayou and the
upper delta were completely isolated from such daQy
interactions with the river.

Positive-Flow Events

Figure 3-17: Typicai v:i.. of the Nueces Overflow
Channel during tidal exchange. The view is looking
downstream (east) from the Nueces River.

Photos courtesy of the Bureau of Reclamation.

the demonstration period, the amount of fresh water
diverted into the upper Nueces Delta was increased by
about 732%, and most of this increase was attributable
to positive-flow events.

The second event type, positive-flow events, were
considered to be infrequent, large-volume events which
resulted in a positive flow of water from the Nueces
River into Rincon Bayou (Figure 3-18). Because these
events were primarily driven by flow events in the
Nueces River, they typically occurred during the spring
or faU. Unlike exchange events, the volumes associated
with positive-flow events (usually greater than
123 10' m', or 100 acre-ft) did not simply dilute water
in Rincon Bayou but also displaced it to a considerable
extent. Because of their magnitude, positive-flow
events were not confined to Rincon Bayou but
frequendy affected the lower adjacent flats, channels
and pools. Depending upon their magnitude, such
events might or might not have also inundated the
higher marshes and tidal flats of the delta. As
previously discussed, the actual diverted volume and
effectual extent of any one such event was greatiy
dependant upon the ambient water level in Nueces
Bay. Events 26, 27, 29 and 34 were typical
examples of positive-flow events that did not inxmdate
higher adjacent marshes and flats (Table 3-3). During

Figure 3-18: View of the upper Rincon Bayou
(background) during a typical positive-flow event
(Event 16). The head-water channel of Rincon Bayou
(about 300 m downstream of the Nueces Overflow
Channel) is in the foreground, and the western-most
MoPac Railroad bridge is center. Upper Rincon Bayou is
in the background. The photograph was taken on
June 26, 1997.

Photo courtesy of the Bureau of Reclamation.

ChapterThree ♦ 3-23

Tidal Flat Inundation Events

Tidal flat inundation events were considered to be
large, positive- flow events during which the Rincon
Overflow Channel was activated, diverting fresh water
into the tidal flats of the upper delta and immersing, to
some degree, those higher marshes (Figure 3-19).
These events (Events 16 and 25 confirmed, and
Events 17, 18 and 36 strongly suspected) were
relatively rare during the demonstration period.
Although these tidal flats were also periodically
inundated by other hydro-meteorological forces {e.g.,
the storm surge of Hurricane Bret during Event 35),
such non-riverine events were not considered to be
tidal flat inundation events because fresh water was not
significandy involved in the mechanism. Without the
demonstration project, these tidal flats would not have
been direcdy freshened, as the largest of the natural
diversions that would have occurred (Event 25) would
not have exceeded the confines of the Rincon Bayou
channel in the upper delta.

V 'l^^^^^^^^^^^^^^^^l

^^■■■■PP

^ "^•jy^^-Vgj^^B^^B^B

JUJPH^g

MV

'^^^^smHi

B

n

Figure 3-19: View of diverted fresh water in the tidal
flats area in the upper Nueces Delta during activation
of the Rincon Overflow Channel (Event 16). The view is
looking east from the outfall of the overflow channel. The
photograph was taken on June 27, 1997.

Photo courtesy of the Bureau of Reclamation.

3-24 ♦♦♦ Hydrography

TERRY E. WHITLEDGE

Institxite of Marine Science,
University of Alaska Fairbanks

DEAN A. STOCKWELL

Institute of Marine Science,
University of Alaska Fairbanks

CHAPTER FOUR

Water Column
Productivity

"In water all hath had its primal source;
and water still keeps all things in their
course."

♦ Johann Wolfgang von Goethe (1749-1832)

INTRODUCTION

Observations of nutrient and primary productivity
changes in aquatic environments have been used to
assess many aquatic ecosystems for changes of primary
inputs or suspected ecosystem alterations (Boynton et
al. 1982; Pennock et al. 1999). Estuarine areas, such as
the Nueces Delta, depend upon the mixing of fresh
water with sea water to maintain biological
productivity. Specifically, fresh water imports nutrients
and dilutes salinity of the receiving sea water.
Therefore, increased freshwater inflow from the
demonstration project should have a large impact on
the water column and its biological processes. The re-
introduction of fresh water from the Nueces River into
the upper Nueces Delta offered an opportunity to
monitor nutrient and primary productivity responses in
a historic river delta that had been altered by lack of
freshwater inflows from small and medium runoff
events. In the recent past, fresh water flow has been
limited to only large events that have flooded the delta
every several years.

OBJECTIVES

1) To assess the effect of the demonstration project
on salinity and nutrient availability to the water
column of the study area;

2) To assess the response of water column
phytoplankton populations to changes in salimty
and nutrient availability; and

Chapter Four ♦ 4-1

3) To assess the response of the sedimented

phytoplankton (microphj'tobenthos) populations
to changes in salinity and nutrient availability.

METHODS AND APPROACH
Study Design

Water column processes were examined at eight
sampUng sites located throughout the upper Nueces
Delta (Figure 4-1). At each sampling station,
hydrographic data, water samples and mud samples
were collected to measure temperature, salinity,

inorganic nutrient concentrations, phytoplankton
pigments and plankton growth (productivity) rates.
Samples were collected monthly at most sites during
the demonstration period unless extreme flooding
prevented access to the stations or they were dry. A
total of 493 samples were taken for most of the
measurements. TTie eight sampling stations were
chosen to represent the various segments within the
project area, including one reference site and three
treatment sites (upper Rincon Bayou, central Rincon
Bayou and Tidal Flats). One other station (68) in the
Nueces River near the Nueces Overflow Charmel was
used to provide a characterization of the riverine
source.

Northern
bluff line

Like

Rincon Bayou

central Rincon Bayou Site
(treatment)

Figure 4-1 : Location of water column sampling stations in the upper Nueces Delta. The four sites included one
reference site (Stations 62 and 63) and three treatnnent sites: upper Rincon Bayou (Stations 65 and 66), central Rincon
Bayou (Stations 60 and 61) and Tidal Flats (Station 62). In addition to these, one sampling station (Station 68) was
occupied in the Nueces River near the Nueces Overflow Channel.

4-2

Water Column Pmdudivity

Because phytoplankton are relatively short-lived and
are mobile within the water column, they integrate the
effects of changes in their environment over very small
temporal scales. Therefore, nutrient and primary
production responses in the water column (e.^., by
phytoplankton) and on the surface sediments (e.^., by
microphytobenthos) often occur within the time
period of a few hours to several days. This relatively
short response time creates a challenge to collect
measurements in appropriate time scales. A water
column monitoring approach sensitive enough to
record bi-weekly or even weekly changes in water
column productivity could result in a major imposition
on project budgets and collection logistics. After
consideration of available resources and funding for
monitoring activities, sampling of water column
productivity during the demonstration project was
scheduled on a monthly basis. This schedule, although
possibly limiting the ability to assess the immediate
effects of project diversions on phytoplankton and
microphytobenthos, provided an opportunity to assess
changes in broader column productivity characteristics.

Measurements

Hydrography

Physical hydrographic measvirements were made at the
surface at each sampling site. Parameters recorded
were sampling location, date, time, latitude, longitude,
sample depth(s), temperature, salinity, dissolved
oxygen, per cent oxygen saturation, pH, Secchi depth,
water depth and weather conditions. A multi-
parameter YSI model 610 profiler instrument was used
for /« situ measurements of salinity and depth
parameters. The units of measure (and their nominal
accuracy) were: salinity, after conversion from in situ
conductivity and temperature (0.2 practical salinity
units (psu)) and depth (1 cm). Salinity was also
measured by field refractometer in many samples and
reported as parts per thousand (ppt) for comparison
purposes. Differences as large as 4 ppt are often
observed due to the high variability of salinity in
estuarine waters, the difficulty of maintaining
calibration in the electronic instrument and keeping the
field refractometer clean and dry. Dissolved oxygen.

pH and Secchi depth were not analyzed, but the data
were logged for the sake of completeness.

Nutrients

Ambient Nutrient Concentrations - The

concentrations of nitrate, nitrite, ammonium,
orthophosphate and silicate were determined in all
water samples according to published methods of
Environmental Protection Agency (1983) and
Whidedge et al. (1981) using automated continuous
flow analyzers. All nutrient samples were analyzed
with a Technician AutoAnalyzer II. The water samples
were collected in pre-numbered polyethylene bottles
and immediately placed in the dark and on ice.
Chemical analysis of the samples occurred within
24 hours of collection and was often completed within
5 to 6 hours. Calibration of the automated nutrient
channels were performed with each set of samples. A
series of five concentrations for each analyte was
analyzed prior to analysis of field samples in order to
ascertain proper operation. A detailed protocol of
standards and their preparation are described by
Whidedge et al. (1981) and have been used for
estuarine/marine samples from 1975 through the
present. All standards were prepared in the laboratory'
using either ultra-pure grade deionized distilled water
or, as a standard addition, low nutrient sea water.

Nutrient Amendment Bioassay Studies — Bioassay
techniques were employed in the field to evaluate the
relative influence of nitrogen, phosphorus, or trace
metal additions to changes in phytoplankton biomass
[i.e.. Chlorophyll A). These botde assays were
enrichment modifications of the productivity estimates
and are useful to determine possible nutrient
limitations. The bioassay amendment studies were
accomplished in screw cap test tubes that contained
50 milliliters (ml) of sample. Initial samples were
analyzed for extracted chlorophyll content. Four
replicates of each sample were amended with 10 micro-
moles per liter (jimole/l) of ammonium, 10 (omole/l of
phosphate, 10 jimole/l of ammonium plus 10 ^mole/1
of phosphate or 100 micro-liters (jj) of "f/2" trace
metal stock solution. Four replicates of a control
sample with no additions were also utilized. After the
additions, the caps were tightened and in vivo

Chapter Four ♦ 4-3

fluorescence readings were taken on samples. The
amended samples were placed in diffuse lighted
incubators at 25 °C and additional fluorescence
measurements were taken daily for 3 to 4 days. The
mean of the four in vivo fluorescence samples was used
to represent the effect of the amendment additions.
No readings were discarded. VClien the incubations
were terminated, the samples were analyzed for
extracted {in vitro) chlorophyll content.

Phytoplankton Pigments

Changes in phytoplankton biomass were determined
using Chlorophyll A as the index of biomass. The
chlorophyll and pigment samples were analyzed
with a model 10-005RU Turner Designs fluorometer
which was specifically designed for pigment analyses
using the methodology of Hohn-Hansen et al. (1965).
Calibration of the in vitro chlorophyll analysis was
accomplished with pure chlorophyll obtained
commercially and standardized with a
spectrophotometer. Phaeopigment concentrations
were also determined in the same samples after
addition of a small amount of hydrochloric acid.

Primary Production

Rates of phytoplankton primary production were
monitored using replicate '^C incubations and natural
sunlight using the method of O'Reilly and Thomas
(1983). This method has been used for all
measurements in South Texas bays over the previous
8 years. The procedure consists of collection of
duplicate water samples that were inoculated with
'■"C isotope and incubated in a water bath for 2-3 hours
in fuU sunlight. Dark botde uptake was measured for
corrections and '''C inoculation volumes were checked
with replicate initial blanks. Primary production
'■"C measurements were analyzed with a Beckman
model LS5801 liquid scintillation counter that
employed self-calibration with known sources and
calculates counting efficiency. Initial carbonate
alkalimty was analyzed by standardized methodology of
25 ml of 0.01 M hydrochloric acid additions to 100 ml
of sample. The extremely high alkalinity of Rincon
Bayou often required additional aUquots of acid
addition until a proper pH of <3.9 was obtamed.

Sedimented Plankton (Microphytobenthos)

Chlorophyll — The chlorophyll content of sediment
was determined at each site by sub-sampling a 5-cm
core collected by hand. A 1-cc syringe was also used to
collect the sample from the upper 0.5 cm of the
sediment surface. Extraction and analysis of the
chlorophyll/phaeopigment content were conducted
accordifig to the same procedures as the water samples.

Primary' Productivity — The primary production of
microphytobenthos was determined on 1-cc mud
samples from the top of 5-cm cores collected by hand.
The sediment was suspended in 25 ml of filtered water
collected at the site. Replicate '''C incubations were
incubated in natural sunlight for 3 to 4 hours.

RESULTS
Hydrography

Temperature

Temperature is generally not a strong controUing factor
on water column primary production, but seasonal
temperature changes may have a secondary effect on
production processes. Temperature data were most
useful in characterizing rapid changes in environmental
conditions, such as sudden changes of weather during
winter cooling events [e.g., fironts). Long extended
periods of high temperature were used to identif)'
periods of drought and other times of stress on the
plant and animal populations in the upper delta.

Salinity

Salinity is a conservative variable because its
concentration is altered only by physical processes. As
precipitation or evaporation occurs, salinity can be
used to produce an accurate estimate of the quantity of
water added or subtracted from an estuary. In
addition, salinity values also give a good indication as
to the spatial extent of freshwater inflow events.

In general, the upper Nueces Delta experienced a wide
variance of salinity concentrations during the

4-4 V Water Column Productivity

demonstxation period, ranging from over 120 ppt
(Station 65) to less than 1 ppt {e.g.. Station 66)
(Figure 4-2). The incomplete salinity graphs with
missing data mosdy resulted from dry periods when
there was no standing water at the stations. The
Nueces River site (Station 68) had the lowest salinity at
all times except on September 16, 1999. This sampling
date was at the conclusion of Event 36 (Chapter 3),
when a large volume of water previously diverted into
the Rincon Bayou during the event had flowed back
into the Nueces River (Figure 3-9) likely transporting
acquired salt from the upper delta.

Measurements of freshwater flow into and out of
Rincon Bayou (Chapter 3), and direct precipitation
were compared with water column saUruty data
(Table 4-1 and Figure 4-3). Because salinity at the
water column stations was measured monthly, daily
rain and inflow data were summed by water column
sampling dates. The variations in average salinity at
each site over time clearly showed the liighly variable
amounts of precipitation and evaporation over the five
year sampling period (Figure 4-3). During the months
following high freshwater inflow periods in summer
1997 (Events 16 and 17), fall 1998 (Events 23 dirough

27) and fall 1999 (Events 36 and 37), the upper Rincon
Bayou site (Stations 65 and 66) had lower salinity
values than those in the central Rincon Bayou site
(Stations 61 and 62), and often times lower than the
Reference sites (63 and 64). This condition {i.e., salinit)'
concentrations lowest in the upper delta) represents a
"normal" estuary salinity gradient typically found in
unperturbed systems. During dry periods and drought
{e.g., the last part of 1995 through the first part of 1996,
as well as summer 1998), a "reverse estuary" condition
was observed {i.e., the highest salinity concentrations
were found in the upper delta), where the upper
Rincon Bayou site was predominandy saltier than
Reference or central Rincon Bayou sites (Figure 4-3).
These observations conformed closely to the project
design in that medium to high river flow events
circulated tiirough the historical Rincon Bayou
channel.

Nutrients

Inorganic nutrients are utilized by plants to produce
organic matter through the process of photosynthesis.
The concentrations of nutrients available and amount

I I I
1996

I I I I I I I I I I I I I I I I I I I I I I I I I I I
1998 1999

Figure 4-2: Salinity at all water column stations (except Station 62) for each sampling date. Data from Station
62 were not plotted because the Tidal Flat site was frequently dry resulting in significant data gaps.

Chapter Four ♦ 4-5

Table 4-1: Hydrographic events (from Table 3-3) occurring prior to each water column sampling period. Daily
values for net flow and total precipitation were summed between sampling intervals. In some cases, sampling occurred
during an event. The Nueces Overflow Channel was completed on October 26, 1995, so flow prior to that date was zero.
Precipitation data for sampling dates prior to May 16, 1996 were that recorded at the Corpus Christi International Airport.
Hyphens (-) indicate missing or incomplete data.

Water

Hydrographic

Total Net Flow

Total

Water

Hydrographic

Total Net Flow

Total

Column

Events

Through Rincon

Precipitation

Column

Events

Through Rincon

Precipitation

Sampling

(Table 3-3)

Bayou

(inches)

Sampling

(Table 3-3)

Bayou

(inches)

Date

(acre-ft)

Date

(acre-ft)

28-Oct-94

0

7.37

18-Jun-97

-9

1.32

17-NOV-94

0

0.20

14-JUI-97

16, 17

1386

1.95

07-Dec-94

0

2.17

07-Aug-97

163

0.08

11-Jan-95

1

0

6.35

17-Sep-97

13

0.19

13-Feb-95

0

0.44

28-Oct-97

18

265

11.36

15-Mar-95

0

6.33

04-Dec-97

-6

2.66

17-Apr-95

0

0.99

17-Dec-97

0

0.08

16-May-95

0

0.48

27-Jan-98

3

0.56

17-Jun-95

2

0

4.10

24-Feb-98

35

3.01

20-Jul-95

3

0

1.14

24-Mar-98

-3

2.01

17-Aug-95

4

0

4.46

08-Apr-98

19

19

0.00

27-Sep-95

5

0

4.81

02-May-98

21

0.15

25-Oct-95

0

0.36

05-Jun-98

4

0.00

28-NOV-95

6

-

12.83

08-Jul-98

20

-9

0.82

14-Dec-95

7

-

0.04

19-Aug-95

21

-2

4.67

17-Jan-96

8,9,10

-

0.51

29-Sep-98

22, 23, 24

649

4.39

17-Feb-96

-

0.04

24-Oct-98

25

4246

9.15

20-Mar-96

-

0.00

18-NOV-98

26,27

130

3.95

10-Apr-96

-

1.11

18-Dec-98

-14

0.60

15-May-96

-

0.47

12-Jan-99

-3

0.50

17-Jun-96

21

0.66

24-Feb-99

-6

0.51

21-Jul-96

0

1.58

18-Mar-99

28

13

0.06

27-Aug-96

11

18

4.94

15-Apr-99

29

130

2.29

30-Sep-96

21

4.54

24-May-99

30,31

-135

3.81

30-Oct-96

12, 13

247

0.80

09-Jun-99

32

2

1.08

18-NOV-96

14

47

0.18

21-Jul-99

33,34

15

6.89

10-Dec-96

14

1.16

19-Aug-99

35

-16

0.26

30-Jan-97

2

2.32

16-Sep-99

36

821

7.25

06-Mar-97

20

1.41

28-Oct-99

37

-54

3.60

24-Mar-97

19

5.05

17-NOV-99

-5

0.00

21-Apr-97

15

-15

4.95

08-Dec-99

18

0.28

29-May-97

33

5.99

4-6 V Water Column Productivity

50
45
40
35

E 30
o

15
10

5 -]
0

e

o

5000

4000

3000 -

2000

1000 -

150

120

Q.

>.

60 ™

1995

1996

1997

1998

1999

H Total rainfall

■ Total flow into Rincon Bayou

■ Average salinity: Reference Site (Stations 63 and 64)

- Average salinity: central Rincon Bayou Site (Stations 60 and 61]

- Average salinity: upper Rincon Bayou Site (Stations 65 and 66)

Figure 4-3: Average salinity at each sampling site for each sampling date. Cumulative daily rainfall and inflovt/ into
Rincon Bayou also plotted for each monthly period between sampling dates.

of sunlight often determine the amount of biological
productivity in an estuary. Organic matter from die
plants may go through several padiways depending on
whether it was eaten, decomposed or buried in the
sediments. Organic matter that is consumed may be
excreted back into the environment or be incorporated
into the animal tissue. Microbial populations can also
absorb or breakdown organic matter and return it to
the environment. Both organic matter pathways,
through higher animals or microbes, produce
regenerated nutrients which enhance nutrient
concentrations in the water and again become available
for uptake by plants. The relative amounts of the
original nutrients and the regenerated nutrients can
often provide rate process estimates for turnover of
organic matter in estuaries.

The relative importance of different nutrients varies in
freshwater and marine environments. As a result, the

importance of phosphorus nutrients are often clearly
obser\^ed in freshwater segments, while nitrogen
nutrients are most important in saline segments. These
differences are caused by many physical and biological
processes that vary over short time and spatial scales.

Nitrate (NO,)

Nitrate is the most common form of nitrogen nutrient
in oxygenated environments. The concentrations of
nitrate in many rivers has increased over the past five
decades as a result of increased usage of agricultural
fertilizer and wastewater effluents.

The nitrate content of the Nueces River and the
Nueces Delta stations (Figure 4-4) were relatively low
(< 2 ^mole/l). Less dian 1% of the data were >
2 |jmole/l and usually occurred during flooding events.
The largest concentrations of nitrate (> 20 ^xmole/I)

Chapter Four *♦* 4-7

were only found at the Reference site (Stations 63 and
64) and only during 1999. The relatively low nitrate
concentrations obsen^ed throughout the data record
would provide a low level maintenance for
phjlioplankton growth but would certainly not provide
nutrients needed to fiiel bloom conditions.

Ammoniutn (NH4)

Ammonium is the form of nitrogen most commonly
released by rec)'cling of organic matter or excretion by
higher organisms. Accordingly, ammonium is often
called "regenerated" nitrogen. Shallow environments
such as the Nueces Delta and Nueces Bay tend to
increase the relative amounts of regenerated nitrogen
compared to nitrate (which is often called "new"
nitrogen because it has been newly added to the
ecosystem by advection or inflow).

The concentrations of ammonium observed during the
demonstration period were usually larger than nitrate
concentrations (Figure 4-5). The Nueces River station
(Station 68) often had the smallest ammonium levels,
while the upper Rincon Bayou stations (65 and 66)
were sometimes largest. The central Rincon Bayou
stations (60 and 61) occasionally contained the largest

ammonium concentrations. There was no obvious
direct relationship between salinity and ammonium at
any of the station sites.

Nitrite (NOj)

Nitrite is the form of nitrogen that is intermediate
between nitrate and ammonium for its valence state
and is not typically observed in large concentrations in
most estuarine or marine environments.

During the demonstration period, the small concen-
trations of nitrite resulted from either nitrification or
denitrification processes (Figure 4-6). Estuarjr
ecosystems tend to have both nitrification and
denitrificaion processes occurring at the same time by
specific microbes for each. Large concentrations of
nitrite {e.g., >1 jimole/1) indicate that special conditions
existed for a short time.

Dissolved Inorganic Nitrogen (DIN)

DIN is the sum of the nitrate, nitrite and ammonium
forms of nitrogen. This amount represents the total
inorganic nitrogen nutrients available for uptake by
plants. Because all three forms of nitrogen are readily

1995

1996

1997

1998

1999

Figure 4-4: Nitrate concentrations at all water column stations (except Station 62) for each sampling date.

4-8 ♦♦♦ Water Column Product! viiy

I I I I I I

1999

Figure 4-5: Ammonium concentrations at all water column stations (except Station 62) for each sampling date.

T-r

1998

Figure 4-6: Nitrite concentrations at all water column stations (except Station 62) for each sampling date.

Chapter Four ♦ 4-9

utilized by photosynthetic processes, DIN is often
used to estdmate nitrogen nutrient availability.

The concentration of DIN at most stations was
< 10 ^imole/l, and, on only six sampling dates, it was
larger than 20 |imole/l, of which five were at the
Reference site (Figure 4-7). In general, the Reference
site (Stations 63 and 64) displayed most of the largest
concentrations of DIN, which likely was a result of
unusually large inputs from local runoff The Nueces
River (Station 68) generally had the lowest DIN
concentrations.

The largest contributor to the DIN pool was
ammonium, as indicated by its percentage of DIN
(Figure 4-8). In only a relatively few samples (about
6%) was ammonium less than 40% of the DIN.
During 1999, a relatively large number of samples
contained 100% ammonium.

5 |imole/l or lower, although several samples had
values near 30 |jmole/l (Figure 4-9).

Also, these concentrations represented high
concentrations of phosphorus relative to nitrogen.
For example, most values for the nitrogen to
phosphorous (N:P) ratio calculated from phosphate
and DIN concentrations were below 3.0 (Figure 4-10).
A typical value of the N:P ratio for organic matter is
about 15. During the demonstration period, only
5 samples had values larger than 15.0, indicating a
possible shortage of phosphorus in the system. In
contrast, there were a large number of samples with
values < 5.0 for N:P, indicating that phosphorus was
ver^' abundant and nitrogen may be limiting
autotrophic processes.

Phytoplankton Pigments

Phosphate (PO4)

Phosphate (orthophosphate) is the primary
phosphorus nutrient, and it's dynamics vary widely in
fresh and salt waters. During the demonstration
period, phosphate concentrations were primarily about

Phytoplankton pigments are useful indicators of plant
biomass. Chlorophyll A has been the primary pigment
traditionally used, but better analytical methods since
1985 have allowed a large number of other pigments to
be identified and measured accurately. Chlorophyll A

-r-rvr I I I I I I I I I T I I I

1995 1996

1997

1998

1999

Figure 4-7: Dissolved inorganic nitrogen concentrations at all water column stations (except Station 62) for each
sampling date.

4-10 ♦ Water Column Productivity

100

0 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
1995 1996 1997 1998

1999

Figure 4-8: Percent of dissolved inorganic nitrogen contributed by ammonium at all water column stations
(except Station 62) for each sampling date.

I I I I I I I I I I I I I I I I I I I I I
1 998 1 999

Figure 4-9: Phosphate concentrations at all water column stations (except Station 62) for each sampling date.

Chapter Four ♦ 4-11

c
o

ro

3

(/)
O

Q.
O

C
Q)
O)
O

1995

Figure 4-10: Nitrogen to phosphorous (N:P) ratios at all water column stations (except Station 62) for each
sampling date.

remains the typical pigment measured by fluorometric
techniques but is often accompanied by a
phaeopigment estimate after the addition of acid.

Chlorophyll

Chlorophyll concentrations (which are an estimate of
plant biomass in the water) were quite large during the
demonstration period, with a large number of values
> 100 |j,g/l (Figure 4-11). The smallest concentrations
were mosdy measured in the Nueces River (Station 68),
but aU other sites had some values > 1 50 (ig/1, which
could be described as bloom conditions. These
biomass accumulations of plankton partially drive the
large rates of primary production in the Nueces Delta.

Water Column Production

Water column primary production is a combination of
phjrtoplankton biomass and the growth rate of
individual cells. Therefore, combinations of high
biomass or high growth rates can produce large rates
of primary production. The rate of primary
production is a function of nutrient availability and
incident radiation to the phytoplankton cells.

The '''C experiment conducted as part of this
demonstration project indicated that the average rate
of primary production is about 3 gC/m'/day
(Figure 4-12), but higher values were typically as large
as 10 gC/m"/day. One extremely high rate
(38 gC/m"/day) was obtained at Station 63 before the
opening of the Nueces Overflow Channel as a result of
a bloom of filamentous blue-green algae. Frequendy
the Nueces River station (68) had the lowest rates, but
all Rincon Bayou stations showed large responses
during inflow events, particularly during the summers
of 1997 (Events 16 and 17) and 1999 (Events 29 and
33). The observed rates of primary production in the
upper Nueces Delta were larger than values for Nueces
Bay (StockweU 1989), and the highest rates were equal
to the largest observed in up-welling areas in the ocean.

Assimilation Index

The assimilation index is the rate of primary
production normali2ed to the chlorophyll biomass.
This index is useful to determine if the specific growth
rates of phytoplankton cells are large enough to create
bloom conditions. Typical assimilation index values of
50 to 100 mgC/m^/day/fjgChl are observed for many
phytoplankton spring blooms.

4-12

Water Column Productivity

1000

1995

1996

1997

1998

1999

Figure 4-1 1 : Chlorophyll concentrations at all water column stations (except Station 62) for each sampling date.

Figure 4-12: Primary production at all water column stations (except Station 62) for each sampling date.

Chapter Four ♦ 4-13

The assimilation index for most stations had values of
20 to 100 mgC/mVday/^gChl (Figure 4-13). Many of
the highest values obsen^ed before appreciable inflow
passed through the Nueces Overflow Channel (i.e.,
prior to October 1996) were measured at the Nueces
River station (68). Later during the demonstration
period, the upper Rincon Bayou stations (65 and 66)
were much higher compared to earlier years, and most
station samples gave higher results during the final
year. This apparent increase at most of the sampling
stations during the final year might be due to increased
freshwater diversions.

Nutrient Amendment Bioassays

The nutrient amendment studies were undertaken to
determine whether phytoplankton growth widiin the
Nueces Delta could be stimulated by the addition of
nitrogen, phosphorus or trace metal nutrients. The
biomass of the phytoplankton as measured by
chlorophyll was used to indicate biological response.
The amount of change may be influenced by the
incubation conditions, but the relative responses can
provide valuable clues to the degree of limitation of an
addition or set of additions. Two nutrient amendment

experiments were performed in March and April 1997
during what was a relatively dry period in the delta. A
final amendment experiment was undertaken in August
1997, after a large amount of summer-time freshwater
inflow (Events 16 and 17).

Amendment Series 1: March 7, 1997

Station 68 — The two phosphate amendment additions
responded equivalent to the control for the four day
period (Figure 4- 14a). All other additions showed
increased chlorophyll concentrations compared to the
controls. Trace metals, nitrate, nitrate plus
phosphorus and silicon, and nitrate plus phosphorus
had approximately twice the response compared to
ammonium and ammonium plus phosphorus.

Station 63 — AH amended samples showed a
chlorophyll response for days 1 and 2 that was
equivalent to the control with no additions
(Figure 4- 14b). On day 3, both concentrations of
phosphate had no effect compared to the control, but
all other additions enhanced chlorophyll production.

Station 66 — The two phosphate amendment additions
showed responses equivalent to the control for the

O

CD
T3

o

E,

X

0)
■D

C

c
o

<fl

CO

<

250

200

150

100

r T I 1 I I f r I I I

1995

1 1 I I T

1996

I I I I I I I I I I I I I I I I I I I I I I I I
1997 1998 1999

Figure 4-13: Assimilation index at all water column stations (except Station 62) for each sampling date.
4-14 •♦* Water Column Productivit)!

(a) Station 68

5

% 4
>^

X

% 3

c

0)

o

^ 2

O
(/)
0)

o

3

1 -

DayO
Day 1
Day 2
Day 3
Day 4

Control

NH4

N03

NH4+P

N03+P NH4+P+SI N03+P+SI

Tm

O
>^

X

0)
•D

c

<u
o

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0)

o

CO

o

3

(b) Station 63

9
^ 8
7
6
5
4
3
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1
0

^^ DayO

1 1 Day 1
■■■ Day 2
1 1 Day 3

1 B 1

Fl

III

1

ll

ll

1,

ll

1

ll

1

i "

ll

ll

ll

■■ i

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Control NH4

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P2 NH4+P N03+P NH4+P+SI N03+P+Si Tm

(c) Station 66

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o

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00

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r 1 Day 3

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1.

14.

^

1

pill

n'

pii

11 — lijj

—

12
10 -

8 -

6

4 -

2 -

n -

1

Control

NH4

N03

P2

NH4+P

N03+P NH4+P+SI N03+P+SI

Tm

Figure 4-14: Results from nutrient amendment bioassays for selected stations: March 7, 1997. Chlorophyll
response indicated by fluorescence. Each daily value represents the mean of four replicate samples.

Chapter Four *■

4-15

three day period (Figure 4- 14c). All other additions
increased chlorophyll concentrations compared to the
controls.

Amendment Series 2: April 22, 1997

Station 66 — All additions showed an increase in
chlorophyll compared to the control except phosphate
additions (Figure 4-15c). VChile all samples showed a
decreasing trend, the chlorophyO enhancements were
approximately a 30% increase over the control.

Station 68 — All additions showed an increase in
chlorophyll compared to the control except phosphate
additions (Figure 4-15a). The chlorophyll
enhancements were approximately doubled compared
to the control.

Station 60 — AH additions showed an increase in
chlorophyll compared to the control except phosphate
additions (Figure 4-15d). The chlorophyll
enhancements were approximately double when
compared to the control.

Station 63 — All additions showed an increase in
chlorophyll compared to the control except phosphate
additions (Figure 4- 15b). The chlorophyO
enhancements were increased by approximately 50%
over the control.

Amendment Series 3: August 7, 1997

Station 68 — Additions of ammonium, nitrate, silicate
and ammomum plus P showed mcreased chlorophyll
responses compared to the control (Figure 4-16a).

(a) Station 68

6

« 5

X 4 -F

C

s

^ 1

DayO
Day 1
Day 2
Day 3

(b) Station 63

14 T

•ST 12

i 10

o

lI 2

DayO
Dayl
Day 2
Day 3

(c) Station 66

11

(d) Station 60

10 -

7

3
2 -

1
0

DayO
Day 1
Day 2
Day 3

P NH4+P

•« 6
o

i 5

£ 2+-

o

c 1 4

DayO
Day 1
Day 2
Day 3

Figure 4-15: Results from nutrient amendment bioassays for selected stations: April 22, 1997. Chlorophyll
response indicated by fluorescence. Each daily value represents the mean of four replicate samples.

4-16

W^ater Column Productivity

(b]3tMton»

40

30

I 10

OayO
Day4

OMm NH4 N09

l'*H4»P

CiaM NhH

NC'2

f*4*tP Bt4

Tm

[cf Station M

»

K

20..

1»
10
& -I

DayO
^w^ i

ll I M

(d) Stalian 60
>>

JO - ■■

» 14 •

1 U

!K ■

e -
o
3 *

DayD
Day 2
D8y4

l

COIM NhH N03

WH4+P BM

Tm

CoiAOl hH4

NO!

NM'tP SN

Tm

Figure 4-16: Results from nutrient amendment bioassays for selected stations: August 7, 1997. Chlorophyll
response indicated by fluorescence. Each daily value represents the mean of four replicate samples.

Phosphate and trace metals stimulated small
chlorophyll responses and were probably not
significant.

Station 63 — No chlorophyll increases were observed
with any of the amendment additions (Figure 4- 16b)

Station 60 — All additions showed an increase in
chlorophyll compared to the control except phosphate
additions (Figure 4-1 5d). The chlorophyll
enhancements were approximately double when
compared to the control.

Amendment Series 3: August 7, 1997

Station 68 - Additions of ammonium, nitrate, silicate
and ammonium plus P showed increased chlorophyll
responses compared to the control (Figure 4- 16a).

Phosphate and trace metals stimulated small
chlorophyll responses and were probably not
significant.

Station 63 — No chlorophyll increases were observed
with any of the amendment additions (Figure 4-1 6b)

Station 66 - No chlorophyll increases compared to the
control were observed with any of the amendment
additions (Figure 4- 16c).

Station 60 - Only trace metal additions enhanced
chlorophyll compared to the control (Figure 4-16d).
Other additions produced no significant effects in
chlorophyll concentration.

Chapter hour ♦♦♦ 4-17

MiCROPHYTOBENTHIC SEDIMENT BlOMASS

AND Production

The shallow water environment of the Nueces Delta
allows a large fraction of water column phytoplankton
to settle to the sediment. These benthic cells mav
become attached to the sediment surface or remain
unattached to potendally re-suspended with wind
induced mixing.

The absence of data during the period with the most
freshwater inflow (e.^s.. summer 1997 through fall 1999)
precluded a full assessment of the demonstradon
project's effects on microphytobenthic communides,
but some results were observed from the data available.

Sediment Chlorophyll

The biomass of microphytobenthic cells is often
estimated by chloroph\'Ll concentradons, but there is
no method to determine if the cells are attached and
growing in place or merely deposited temporarily until
re-suspension. Therefore, estimates of
microphytobenthos producdon include both types of
plankton.

The sediment chlorophyll declined at all stations during
the course of the demonstration period (Figure 4-17),
but no causal mechanisms were apparent. Because two
stations at the Reference site (63 and 64) also exhibited
the decline, it is likely that the trend was not related to
the effects of the demonstration project.

The ratio of chlorophyll to phaeopigments in the
sediment varied within a tjipical range during the
demonstration period (Figure 4-18). The several high
ratios of chlorophyll compared to phaeopigments at
Stations 65 and 66 (upper Rincon Bayou site) during
1996 indicate that chlorophyll production occurred
more rapidly than did pigment decomposition during
that period.

The inventory of chlorophyll in the water column and
sediment was not closely related to saEnit}', but the
combined chlorophyll biomass was largest at salinit}-
concentrations below 60 psu (Figure 4-19). This
obsen-ation can partially be attributed to reciprocal
concentrations of high sediment and low w-ater column
chlorophyll (Figure 4-20). Much of the time, sediment
chlorophyll was high when water column chlorophyO
was low, but at other times, low sediment chlorophyll
occurred when the water column chlorophyll
concentration was large.

iqg<i

lOQA

inn?

Figure 4-17: Sediment chlorophyll concentrations at all water column stations (except Stations 62 and 68) for
each sampling date.

4-18

Water Column Productivity

1995

1996

1997

1998

1999

Figure 4-18: Sediment chlorophyll to phaeopigments ratio at all water column stations (except Stations 62 and 68)
for each sampling date.

400 -

•

E

"3) inn

•

0

ophyll (m

•
••

•

•

5
O

100 -

4

0 -

* "^ 1 ] 1 ! \

20 40 60 80 100

Salinity (psu)

120 140

180

160

^ 140 -

E, 120

■& 100
o

Z 80

<D 60

I 40 4

20
0

.

•

• t

•• •

•
•

••f

'V-

*fw .

ftr>:- . • .

100

200

300

400

500

Water column chlorophyll (mg/m )

Figure 4-19: Total sediment and water column
chlorophyll and salinity.

Figure 4-20: Sediment chlorophyll and water column
chlorophyll.

Chapter Four ♦ 4-19

Microphytobenthic Producrion

The primar)' producrion of microphytobenthos was
generally similar to water column rates, although the
range of water column primary production values was
more than two times larger (compare Figures 4-21 and
4-12). There was some indication that sediment
production was more likely to be high during winter
months, while water column production tended to be
highest in the summer, especially at the two Reference
stations.

The sediment chlorophyll was not strongly related to
sediment primary production values (Figure 4-22).
This indicates that chlorophyll accumulation in the
sediments did not necessarily dominate the flux rates
of sediment primary production and was probably
attributed to a rather short residence time for
chlorophyll in the sediments before degradation
occurred. A direct comparison of water column and
sediment primary production rates indicates that there
was no correlative relationship (Figure 4-23) and that
most of the water column rates were 2 to 4 times larger
than the sediment rates.

The total combined water column and sediment
primary production rates did not have a strong
relationship to salinity (Figure 4-24), but there was
certainly a general trend that showed the largest
production rates were at salinity concentrations below
60 psu. The upper limits of total primary production
over the entire range of salinity clearly showed an
inverse relationship with saltmty.

Benthic Assimilation Index

The primary production assimilation index for the
microphytobenthos (Figure 4-25) was not nearly as
dynamic as the water column index. During only one
monthly sampling period in the summer of 1995 did
the sediment assimilation index exceed a value of
30 gC/m"/day/Chl. This low range probably indicates
that severe conditions of limited Light, low nutrients
and possibly high temperatures found in the sediments
were not conducive to high rates of primary
production per unit of chlorophyll.

1995

1996

1997

1998

1999

Figure 4-21 : Sediment primary productivity concentrations at all water column stations (except Stations 62
and 68) for each sampling date.

4-20 ♦ Water Column Productivity

E 4
O

= 3
o

"D
O

CO

E

2 -

c

(D

E

_; M ! M

20 40 60 80 100 120 140 160 180

Sediment chlorophyll (mg/m^)

Water column primary productivity (gC/m /day)

Figure 4-22: Sediment chlorophyll and sediment
primatv productivity.

Figure 4-23: Sediment primary productivity and water
column productivity.

14

>■ 12

CD '^
"O

O 10

>.

> 8
t3

■D

2 6

E 4 4

TO

o 2

1-

•

•
•

1

1

•

•

•

• *

1

•

••
• •

•w

■

•1

^

• •••

1

'••.

i 1

20

40 60 80 100 120 140

Salinity (psu)

Figure 4-24: Total primary productivity (sediment and
water column) and salinity.

Chapter Four ♦ 4-21

I I I I I I I I I I I I I I
1998 1999

1 1 I I r

Figure 4-25: Assimilation index at all water column stations (except Stations 62 and 68) for each sampling date.

DISCUSSION

The extreme environment of Rincon Bayou is readily
apparent when the wide ranges of temperature and
salinit}' are considered. The nutrient content of the
water column required for primary' production to occur
was quite variable during the demonstration period.
The combination of water column inventories of
nutrients and chlorophyll pigments with instantaneous
measurements of primary production provided some
indications of resources available and their rates of
utilization. However, the relatively small area and
volume of water contained in the Rincon Bayou
ecosystem reduced residence times and accelerated
fluxes through the marsh. The monthly sampling
schedule was therefore not generally sensitive enough
to fully analyze the effects of freshwater inflow events
on phytoplankton production and growth. However,
even witli the constraints of monthly sampling
intervals, it was quite apparent that positive water
column nutrients and primary production effects
resulted from the demonstration project.

First of all, nutrient amendment experiments were
performed in March and April 1997 to ascertain what
element(s) Hmited phytoplankton production in the

waters of Rincon Bayou. A subsequent amendment
experiment was undertaken in August 1997, after a
large amount of freshwater inflow (Events 16 and 17),
to determine if nitrogen was still the limiting nutrient.
Before freshwater inflow, ammonium additions
increased chlorophyll production compared to control
samples from several sites. Those experiments also
indicated that vital trace metals used in "f ' media also
stimulated chlorophyll production. Additions of
nitrate also stimulated chlorophyll production to about
the same extent as ammonium, but very small ambient
concentrations of nitrate were observed at the
sampling sites. In August, after a significant inflow of
riverine fresh water, no nutrient additions increased
chlorophyll production at any station in the delta,
indicating that nutrients were no longer limiting
phytoplankton production.

These nutrient amendment bioassays were the primary
indicator of the relatively rapid nutrient utilization and
phytoplankton response. Initial responses to the single
and multiple nutrient and/or trace metal additions
were on the order of a day and continued for 2 to
3 days. It has been clearly demonstrated from other
analyses of freshwater inflows that nitrate utilization
increased primary productivity' in the Nueces River and

4-22

Water Column Productivity

Nueces Bay (AJCTiitledge and Stockwell 1995).
Therefore, the rapid uptake of nitrate due to freshwater
inflow very likely occurred in the upper Nueces Delta
shordy after numerous hydrographic events during the
demonstration period, even if these responses were not
readily observed within the (infrequent) sampling
intervals.

Second, the total of water column and sediment
primary production in the delta had an inverse
relationship with salinity. Although the water column
contributed the largest fraction of the total ica. 75%),
the sediment production rates also provided a
significandy large amount. As salinity values in Rincon
Bayou declined below 60 psu, the range of primary
production increased. Therefore, the diversion of an
estimated 8,810 10' m' (7,142 acre-ft) of water into
Rincon Bayou during the demonstration period
lowered salinity concentrations, the osmotic stress on
individual organisms lowered and almost certainly
increased primary production in those waters.

Third, although the increase of N:P was not strongly
correlated widi inflow events, at least four sampling
dates had values near or exceeding 1 5 which were likely
the result of new nitrogen being added to the study
area by river inflow. In general, the nutrient
concentrations in the water column, especially
dissolved inorganic nitrogen, were within the range to
allow large amounts of primary production. Most
(50 to 90%) of die DIN was in the form of
ammonium, which is readily utilized by both water
column and benthic phytoplankton. The shallow water
environment enhanced nutrient remineriaUzation, so
both the ability to utilize and produce nutrients were
relatively large. The N:P ratio indicated that nitrogen
was typically the nutrient in lowest concentration.

Fourth, the species composition of phytoplankton
apparently remained dominated by primarily small
diatoms during most of the monitoring period.
However, several observations of blooms of other
phytoplankton were noted immediately after
freshwater inflow events. These blooms were typically
comprised of single celled blue-green algae (not the
filamentous cyanobacteria of algal mats) normally
present in fresh water or very low salinity

environments. Although t)'picaUy short-lived,
(persisting no more than a few days), the presence of
these blooms did not frequentiy occur in the study area
prior to the demonstration project except under natural
freshening events that occurred every several years.
The more frequent presence of these blooms in the
upper and central Rincon Bayou was an indication that
the water column ecosystem was showing a more
typical response to freshwater inflow.

Finally, the lack of regularly observed chlorophyll
biomass accumulation was Ukely a result of the rapid
flux of water through the monitoring area during
inflow events. However, the occasional obsen^ation of
high chlorophyll biomass during periods of moderate
salinity concentrations of 30-60 psu showed that
phytoplankton could remain in die monitoring area for
a sufficient time period to accumulate biomass. This
analysis is supported by the assimilation index, which is
the rate of carbon growth per unit of chlorophyll. The
assimilation index for sampling stations in Rincon
Bayou was often in the range of 50-100 mg C/m2/day
per unit of chlorophyll, which is a typical value for
estuarine systems. There were also numerous
assimilation values > 100 mg C/m2/day per umt of
chlorophyll at several sampling stations. These values
indicate that rapid primary production per unit of
phytoplankton biomass was occurring, especially
during 1999, which was a consistendy wet year in terms
of project diversions (Chapter 3).

In general, the temporal responses of the
phytoplankton were too rapid to intensely observe
nutrient accumulation, chlorophyll increases or primary
production stimulation to specific inflow events.
Therefore, the nutrient amendment bioassays were the
primary data to demonstrate immediate phytoplankton
responses to freshwater inflow events in the Rincon
Bayou. Future monitoring of nutrient and water
column primary production should employ automated
sampling instrumentation that responds to inflow
events with increased sampling frequency. The
increased number of samples plus the temporal
correlation with other recorded variables, such as
salinity, would gready improve the understanding the
dynamics and importance of deltaic habitat for
phytoplankton.

Chapter Vour ♦ 4-23

SUMMARY

Fresh water diverted by the demonstration project
stimulated primary productivity in the water column
and on the sediment surface by importing nutrients
required for plant growth and lowering salinity
concentrations. During the demonstration period,
phytoplankton and microphytobenthos rapidly
responded to the inputs of riverine nutrients with

increased growth rates and accumulation of biomass.
The increased primary production rates were especially
prominent during periods when sahnit}^ was less than
60 psu. The assimilation index {i.e., the relative amount
of growth per cell) was also generally higher dunng
periods of low salinity, indicating that inherent growth
rates were also increased by project diversions.

4-24

Water Column Pnductivity

PAUL A. MONTAGNA

Manne Science Institate
University of Texas, Austin

RICHARD D. KALKE

Marine Science Institute
University of Texas, Austin

CHRISTINE RITTER

Texas Water Development Board, Austin

"The bottom of an estuary regulates or modifies
most physical, chemical, geological, and
biological processes throughout the entire
estuarine ecosystem via what could be called a
benthic effect."

*>Y:>3Yetal. (1989)

CHAPTER FIVE

Benthic
Communities

INTRODUCTION

The three major habitats in estuarine marshes are the
vegetated tidal marshes, the water column and the
sediments. The sediments are both tidal (ranging
between the tides) and subtidal (below the tidal
elevation range). Benthos (bottom dwelling organisms)
live in association with sediments. Benthic
invertebrates live either in (infauna) or on (epifauna)
the sediments. Estuarine benthic infauna are
particularly susceptible to major changes in salinity
regimes in the environment because of limited mobility
(Kalke and Montagna 1991; Montagna and Kalke 1992;
1995; Mannino and Montagna 1997). Freshwater
species, which tolerate salinity concentrations, ranging
from 0 to 0.5 parts-per-thousand (ppt), are typically
found where rivers meet marshes. OUgohahne species
live in the upper reaches of estuaries where salinity
ranges from 0.5 to 5 ppt. Brackish or estuarine species
can accommodate large variations in salinity ranging
from 5 to 25 ppt. Marine species generally can not
accommodate salinity values lower than 25 to 30 ppt
and are limited to the more saline portions of the
estuary. Salinity is temporally dynamic at any given
location, changing with floods and droughts. Thus,
locations of salinity preference zones for various
organizations change within an estuary throughout the
year. The interaction between dynamic hydrography
and salinity preference means the spatial and temporal
dynamics of benthic infaunal populations are sensitive
indicators of freshwater inflow effects (Montagna and
Kalke 1992; 1995).

Abundance and biomass of infauna may increase if
nutrient loading from river input is transformed into
food for benthic animals (Montagna and Yoon 1991).

Chapter Five ♦ 5-1

This occurs when nutrients introduced from a river
stimulate primary production (Deegan et al. 1986;
Nixon et al. 1986). The primary' production can be
deposited, but it may also be advected and deposited
further downstream, potentially increasing benthic
productivity away from the river inflow source. This
assumes that fresh water and low salinity do not have a
negative effect. Salinity stress on physiology (Finney
1979) and hypoxia (Ritter and Montagna 1999) could
reduce benthic populations. The net effect of
freshwater inflow on biological processes {i.e.,
enhanced productivity, recruitment gains and losses ■via
low-saHnity intolerance) is therefore a function of the
interaction between physical processes {i.e.,
sedimentation, re-suspension, advection and seawater
dilution) and chemical processes {i.e., nutrient
enrichment and cycling).

If freshwater inflow enhances benthic producti^'ity,
then increased abundance and biomass should be
found if inflow were re-introduced into Rincon Bayou
and the upper Nueces marsh. Benthic infauna are
useful indicator species in studies of long-term effects,
because they are relatively immobile and long-Uved
compared to plankton of similar size. The larger
macrofauna (organisms greater than 0.5 millimeters
(mm) in length) and smaller meiofauna (between 0.5
and 0.063 mm in length) have different ecological roles
in marine ecosystems (CouU and Bell 1979; Coull and
Palmer 1984). Therefore, macrofauna and meiofauna
could respond to freshwater inflow at different spatial
and temporal scales. Macrofauna, with planktonic
larval dispersal, indicate effects over larger spatial scales
and longer temporal scales. Meiofauna, with direct
benthic development and generation times as short as
one month, indicate effects over smaller spatial scales
and shorter temporal scales. Even where meiofauna
share ecological properties with macrofauna, the
meiofaunal processes operate on much smaller spatial
and temporal scales (Bell 1980).

OBJECTIVES

1) To assess the effect of the demonstration project
on benthic infauna biomass, abundance and
diversit}';

2) To assess the response of different trophic levels
by examining meiofauna and macrofauna; and

3) To assess the utilization of marsh habitats by
infaunal species.

METHODS AND APPROACH
Study Design

Increased opportunity for freshwater inflow into the
study area was accomplished by lowering the Nueces
River bank leading to Rincon Bayou (Nueces Overflow
Channel) just east of where U.S. Highway 37 crosses
the Nueces River (Chapter 1). A Before vs. After/
Control vs. Impact (BACI) experimental design (Green
1 979) was used to determine effects of the
demonstration project on benthos. Samples were
taken both before and after the Nueces Overflow
Charmel was opened. During each sampling period,
"control" and experimental impact sites were sampled
(Figure 5-1). An experimental control did not really
exist, because the system could not be sampled "with"
and "without" an overflow charmel at the same time.
In addition, there is large natural variability in
hydrographic and organismal responses in this
ecosystem. Therefore, a reference site was chosen that
reflected changes caused by natural variability but not
the overflow channel. The site, which was largely
unaffected by the demonstration project, was
considered a reference site to the sites affected by the
project. The BACI design allowed establishment of
two kinds of reference points to distinguish variability
caused by the project from natural variability.

A second component of the experimental design was
to replicate at the treatment level to avoid "pseudo-
replication." Pseudo- replication occurs when
treatments are confounded with replicates (Hurlbert
1984). For example, if each site were represented by

5-2

Benthic Communities

Northern
bluff line

Approximate Scale
Kilometers

Lite

^incon Bayou

^ central Rincon Bayou Site

(treatment)

Figure 5-1: Locations of bentliic sampling stations. The three sites included one reference site (Stations A and B) and
two treatment sites, upper Rincon Bayou (Stations C and D) and central Rincon Bayou (Stations E and F).

only one station, then it could not logically be
concluded that differences between stations were due

to site differences, as the same magnitude of change
could have occurred in two different stations within
the same site. Therefore, two replicate stations were
assigned to each site (or treatment level) within the
project.

The reference site (Stations A and B) was located
upstream from the overflow channels, reflecting
natural variability but not effects from project
diversion (Figure 5-1). Two experimental (treatment)
sites were also established: 1) the upper Rincon Bayou
site, which was nearest the overflow channel and was

most influenced by the Nueces River, and 2) the
central Rincon Bayou site, which was located farther
downstream and was less influenced by the river, but
more by the tide. Two stations were located in the
upper Rincon Bayou site (Stations C and D), and two
stations were located in the central Rincon Bayou site
(Stations E and F).

The study was a two-way factorial design where the
main sources of variation were temporal and spatial
treatments. The response variables measured were
benthic macrofaunal biomass, abundance and diversity
and meiofaunal abundance and major taxa diversity.
Three replicate samples were taken at each station
during each sampling date.

Chapter Five ♦ 5-3

Benthic samples were taken quarterly in January, April,
July and October of each year. This sampling regimen
was chosen to compliment the timing of previous and
current benthic monitoring programs (Kalke and
Montagna 1991; Montagna and Kalke 1992; 1995;
Montagna et al. 1993; Martin and Montagna 1995;
Mannino and Montagna 1997; Ritter and Montagna
1999). During the initial programs, samples were taken
monthly (Kalke and Montagna 1991) or bimonthly
(Montagna and KaUce 1992). It was discovered that
there were roughly four seasonal events each year,
including winter and summer lows, and fall and
summer highs. Based on the cyclical nature of benthic
recruitment, growth and population losses, it was
determined that seasonal sampling was sufficient to
identify annual trends in long-term sampling programs.
Sampling began (October 28, 1994) one year before the
Nueces Overflow Channel was excavated (October 29,
1995), and continued for five additional years (through
October 28, 1999). Because the first sample of the
second year was taken early (October 3, 1995), there
were five pre-treatment samples and sixteen treatment
samples.

Measurements

Hydtography

The physical hydrographic conditions of the water
column overlying sediments was measured at each
station during each sampling period. Measurements
were collected at the surface and near the bottom and
recorded on the field log sheet. Conditions recorded
during sampling included location, date, time, water
depth and weather conditions. Water quality was
measured with a multi-parameter instrument (Hydrolab
Surveyor II). A sonde unit was also lowered to just
beneath the surface and to the bottom. The
instruments allowed collection of a variety of water
quality parameters rapidly. The following parameters
were read firom the instrument's digital display unit
(accuracy and units): temperature (± 0.15 degrees
centigrade (°C)), pH (± 0.1 units), dissolved oxygen
(+ 0.2 milligrams per liter (mg/1)), specific conductivity
(± 0.015 to 1.5 (millimhos per centimeter (mmho/cm),
depending on range), redox potential (± 0.05 millivolts

(mV)), depth (± 1 meter (m)) and salinity (reported in
ppt, automatically corrected to 25 °C).

Benthos

Sediment was sampled by hand with core tubes to
measure both meiofauna and macrofauna abundances.
Macrofauna were sampled with a 6.7-cm diameter tube
and sectioned at depth intervals of 0 to 3 cm and 3
to 10 cm; meiofauna were sampled with a 1.8-cm
diameter tube and sectioned at depth intervals of 0 to
3 cm only. Samples were presented with 5% buffered
formalin. In the laboratory, meiofauna were sorted
on 0.063 mm sieves, macrofauna on 0.5 mm sieves.
Macrofauna were identified to the lowest taxonomic
level possible (usually the species level), counted and
weighed to the nearest 0.01 mg for biomass. Meio-
fauna were identified to higher taxonomic levels
(usually phylum, class or order) and counted.

Biomass of macrofauna was measured by combining
individuals into higher taxa categories {i.e., Crustacea,
Mollusca, Polychaeta and others). Samples were dried
for 24 hours at 55 °C, and weighed. Mollusks were
placed in 1 N HCl for 1 minute to 8 hours to dissolve
carbonate shells and washed before drying.

AH meiofauna and macrofauna data were digitized and
proof-read. For macrofauna, species diversity was
calculated by replicate and by pooling aU replicate cores
for each site. Diversity was calculated using Hill's
diversity number one (Nl) (Hill 1973). It indicates the
number of abundant species in a sample and is a
measure of the effective number of species (Ludwig
and Reynolds 1988). The effective number of species
is a measure of the degree to which proportional
abundances are distributed among species (Hill 1973).
It is calculated as the exponentiated form of the
Sharmon diversity index:

Nl-

H'

As diversity decreases, Nl will tend toward 1. The
Sharmon index is the average uncertainty per species in
an infinite community made up of species with known
proportional abundances (Shannon and Weaver 1 949;
Hutcheson 1970). It is calculated by:

5-4

Benthic Communities

s

/ \

/ \

■

'=E

Hi

In

^

1=1

. n,

. «.

where «, is the number of individuals belonging to the
rth of J" species in the sample, and « is the total number
of individuals in the sample. Hill's Nl was used
because it is units of numbers of species and is
therefore easier to interpret than most other diversity
indices.

All statistical analyses were performed using SAS soft-
ware (SAS Institute Inc. 1991). tMI data (except when
calculating diversity) were log-transformed prior to
analysis. A two-way ANOVA was used to test for
differences in meiofauna and macrofauna abundance,
biomass and diversity among treatments and sampling
dates. Where treatment effects were significant, Tukey
multiple comparison procedures were used to find
pairwise, a posteriori differences among sample means
within a treatment. The Tukey test finds significant
differences among sample means, while maintaining
the experimentwise error rate {i.e., the probability that
one or more erroneous statements wiU be made in an
experiment) at 0.05 (Kirk 1982). This mediod,
therefore, ensured that study data were not
(incorrecdy) analyzed independentiy.

Community structure of macrofauna species was
analy^ied by multivariate methods. TTie species data
were prepared for analysis by making a matrix where
each row represented an observation of the average
number of individuals in each station, or station-date
combination, and each column represented a unique
species. The data set was multivariate because there
were more than one species, which were the response
variables for the analysis. A common problem with
such matrices is that many of the variables {i.e.,
columns) covary. The covariance can be either positive
(two or more species responding similarly to a stimuli)
or negative (two or more species responding in
opposite fashion to a stimuli). An example of a
positive covariance is when all species increase in
response to increased food. An example of a negative
covariance is where one species competes with or preys
upon another. Complex interactions among multiple

response variables requires multivariate analysis to
illuminate the common patterns in the data set.

Principal components analysis (PCA) is a multivariate
method that is also a variable reduction technique.
PCA is a useful tool because it transforms the species
data matrix into new variables that can be: 1) mutually
orthogonal {i.e., the new variables are uncorrelated to
one another) and 2) extracted in order of decreasing
variance {i.e., much of the iaformation of the original
set, like variance, of variables is concentrated in the
first few principal components (PCS)). The PCS can
also be used as predictors in regression analysis
because they are orthogonal and collinearity {i.e., a
linear relationship between variables) does not exist.
All multivariate analyses were performed with the
SAS FACTOR procedure (SAS Institute Inc. 1991),
using the PC method on the covariance matrix. When
performing PCA on the covariance matrix, the analysis
does not treat all the variables as if they have the same
variance. All count or measurement data was log
transformed prior to multivariate analysis.

Results of the PCA are visualized in bivariate plots.
Generally, only the first two PC factors (PCI and PC2)
are used in the plots. The results are visualized in two
ways: as factor patterns and as loading scores. Each
data set is simply a matrix {i.e., rows of observations
versus columns of variables). The factor patterns are
the PC coefficients for each variable or column. These
vector patterns were used to interpret what PCI and
PC2 represent by plotting the column heading as the
symbol for each point. Next, the loading scores for
each observation were plotted using the site name as
the symbol for each point. The plot of the loading
scores allowed visualization of the relationships or
correlation among the sampling units, stations in the
present study.

Chapter Five ♦ 5-5

RESULTS

Salinity

The water column overlying sediments in the study
area changed on var)'ing temporal scales because of
natural conditions (e.g., wet and dry periods) as well as
hydrographic events through the Nueces Overflow
Channel (Figure 5-2). Salinity ranges during the
demonstration period were extreme in Rincon Bayou,
varying from freshwater conditions (< 0.5 ppt) to
hyper-saline conditions (> 36 ppt). The highest
recorded salinity was 160 ppt (Station C) during July
1 996. Salinity trends appeared similar at different
stations but were not always the same (Figure 5-3).
For example, during summer 1995, Stations A and B
had the lowest salinity values while Station E had the
highest. During summer 1996, Stations C and D had
the highest concentrations but, during summer 1998,
Stations A and B had the highest. Floods during the
summers 1997 (Events 16 and 17) and 1999 (Events 33
and 34) maintained salinity values much lower than
during the other summers.

In the analysis of the hydrographic effects of the
demonstration project (Chapter 3), a total of 37 events
were identified during the study period (Table 3-3).
Measurements of freshwater flow into and out of
Rincon Bayou, as well as direct precipitation, were
compared with benthic salinity data. Because salinity at
benthic stations was measured quarterly, daily rain and
inflow data were summed by sampling date (Table 5-1).
Although data from the Rincon gauge was not available
prior to May 1, 1996, flow into Rincon Bayou prior to
the opening of the Nueces Overflow Channel was zero
(Chapter 3). During the period after the overflow
channel was exca%'ated but before the Rincon gauge
was in place, there were some small exchange events
(e.g.. Event 6, which resulted from locally hea\7
rainfall), but these were not considered to result m
substantial inflow into Rincon Bayou (Chapter 3).
Over the entire period, rainfall events occurred
frequendy, but there were only four sampling dates
prior to which significandy large inflows of fresh water
were recorded (Table 5-1), includingjuly 1997 (Events
16), October of 1997 (Events 17 and 18), October
1998 (Events 21 dirough 25), and October 1999

Figure 5-2: View of benthic Station C in upper Rincon
Bayou under dry (above) and wet (below) conditions.

The dry conditions were during the summer of 1996. and
the flooding conditions during the summer of 1997
(Event 16).

Photo courtesy of the University of Texas Marine Science
Institute.

(Events 35). Only four of the 37 events identified
during the demonstration period (Events 16, 18, 25
and 36) were sufficiendy large to result in delta inflow
without the demonstration project and, of these, only
one did so appreciably (Ev^ent 25) (Chapter 3).

Rainfall and freshwater inflow data were used to
determine the cumulative effects of dilution on the
average sahmty at each site (Figure 5-4). Hj^ersaUne
conditions likely resulted from near zero flows and
high evaporation rates, which typically occurred in
summer. The differences between the two treatment
sites indicate that "reverse estuar)'" conditions had
existed in Rincon Bayou before freshwater diversions

5-6

Benthic Communities

180

160

140

120

3

100

-1— »

C/5

80
60

40

20

0

1995

1996

1997

2000

Figure 5-3: Salinity at all benthic stations (A through F) for each sampling date.

began (/>., higher salinity concentrations in the delta
than in the bay). Early in the study period, the upper
Rincon Bayou site often exhibited higher salinity values
than did the central Rincon Bayou site, but later in the
demonstration period, this relationship reversed
(Figure 5-4). Also, a comparison between the
reference site and the upper Rincon Bayou site indicate
the influence of freshwater diversions on the later.
Before diversions began {i.e., October 1996), the
reference site (which is subject to runoff from rainfall
northwest of Highway 77) was predominandy fresher
than either of the two treatment sites in Rincon Bayou.
Beginning with the inflow occurrence of fall 1996
(Event 12 through 14), the upper Rincon Bayou
reference site was predominandy fresher than the
reference site (Figure 5-4).

Temperature

Temperature rose in summer to 33 to 40 °C and
dropped in winter to about 12 °C following an
expected seasonal pattern. Generally, Stations E and F
had the highest temperatures, and Stations A and B
had the lowest. Although absolute differences in water
temperature among stations were quite small, the
differences are likely due to differences in water depth
because greater volumes of water change temperature
more slowly. There was considerable variation in
station differences between years. Overall,
temperature differences among stations were not
sufficient to cause differences in benthic responses.

Chapter Five ♦ 5-7

50 -,

45

40

35 -]

E 30 -

^ 25 4

C

OJ 20

15

10

5

0 J

5000

- 140

1995

1996

1997

1998

1999 2000

^m Total flow into Rincon Bayou

I Total rainfall
▼• • Average salinity: Reference Site (Stations A and B)
-o— Average salinity; upper Rincon Bayou Site (Stations C and D)
-a — Average salinity: central Rincon Bayou Site (stations E and F)

Figure 5-4: Marsh-wide average salinity at each sampling site for each sampling date. Cumulative daily rainfall and
inflow/ into Rincon Bayou also plotted for each quarterly period betw/een sampling dates.

Dissolved Oxygen

Dissolved oxygen (DO) concentrations varied widely
during the study period, from hypoxic (< 2 rng/l) to
super-saturated (Figure 5-5). Stations A and B were
most likely to be hypoxic. These stations have sea
grasses, so decay of organic matter or night time
respiration likely caused the hypoxia, which was mostly
likely to occur in summer. The solubility of oxygen in
sea water decreases with increasing temperature and
salinity, both of which increased in summer.

Benthos

To indicate the importance of freshwater inflow, the
average salinity at all stations (instead of at each site)
was evaluated against many organismal response
variables.

Macroinfauna

There was a strong significant interaction (P = 0.0001)
between stations and dates for both log-transformed
biomass and log-transformed abundance. Station C,
the most directiy affected by diversion from the
Nueces Overflow Channel, had the highest biomass
and abundance of macrofauna (Table 5-2). However,
because of the significant interactions, differences
among stations means could not be determined with a
simple post hoc comparison, and examination of the
interaction itself was necessary.

At various times. Stations E and F had the lowest
biomass, or Stations A and B had the lowest
(Figure 5-6a). Station C had the highest abundance of
all stations during periods of peak biomass blooms.
Station C was the station most directly influenced by
inflow from the project. There was a strong seasonal

5-8

Benthk Communities

Table 5-1: Hydrographic events (from Table 3-3) occurring prior to each benthic sampling period. Average
salinity concentrations reported for all benthic stations (A through F). Daily values for net flow and total precipitation
were summed between sampling intervals. In some cases, sampling occurred during an event. The Nueces Overflow
Channel was completed on October 26, 1995, so flow prior to that date was zero. Precipitation data for sampling
dates prior to May 16, 1996 were that recorded at the Corpus Christi International Airport. Hyphens (-) indicate
missing or incomplete data.

Benthic
Sampling Date

Hydrographic Events
(Table 3-3)

Total Net Flow Through

Rincon Bayou

(acre-ft)

Total Precipitation
(inches)

Average Marsh-wide

Salinity

(ppt)

28-Oct-94

0

15.61

10.6

11-Jan-95

1

0

8.68

15.5

12-Apr-95
12-Jul-95

2,3

0
0

7.72
5.72

18.0
61.7

3-Oct-95

4,5

0

9.50

34.6

8-Jan-96

6, 7, 8, 9, 10

-

13.58

27.8

9-Apr-96
12-JUI-96

:

1.15
2.78

73.2
112.6

22-Oct-96

11, 12, 13

260

10.05

38.1

6-Jan-97

14

86

3.06

48.0

23-Apr-97
2-JUI-97

15

16

26

782

12.24
9.26

14.0
2.6

28-Oct-97

17, 18

1,065

11.54

3.1

16-Jan-98

-6

3.37

21.3

8-Apr-98
9-Jul-98

19
20

54

15

5.04
1.12

20.7
75.0

28-Oct-98

21,22,23,24,25

3,900

18.53

0.8

12-Jan-99

26,27

161

4.58

19.6

14-Apr-99
7-Jul-99

28,29
30,31,32,33,34

136
-90

2.86
10.64

15.7
5.2

28-Oct-99

35

709

12.25

11.6

Note: 1 acre-ft = 1.2336 10' m'; 1 inch = 2.54 cm

Table 5-2: Average benthos characteristics at a

II stations during the

demonstration period.

Macrofauna

Meiofauna

Station

Biomass

Abundance

Diversity

Abundance

(g/m^)

(n/m')

(N1 0.1 8/m')

(n/IOcm')

A

1.82

24,900

2.1

433

B

2.31

21,200

2.2

369

C

3.23

43,700

1.5

1,240

D

2.10

35,700

1.5

1,060

E

2.42

24,500

1.9

1,540

F

1.83

20,300

1.7

1,730

Chapter Five ♦ 5-9

2000

Figure 5-5: Dissolved oxygen at all benthic stations (A through F) for each sampling date.

trend, as the highest biomass occurred in spring
(April), and lowest biomass in summer (July) or fall
(October).

There was also a strong significant interaction (P =
0.0001) between stations and dates for log-transformed
macro fauna abundance (Figure 5-6b). The nature of
this interaction was much more complex than for
biomass. No one station appeared to have the highest
peaks, but Stations A and B (reference site) had almost
always the lowest values. Again, low values had a
tendency to occur in summer and higher abundances in
fall or winter. This result indicated that Stations A and
B, most removed from demonstration project effects,
suffered from the consequences of evaporation and
concomitant high salinity concentrations.

Macrofauna diversity was very low, ranging from zero
to 11 species found at a station (Figure 5-6c). Because
diversity was so low, all three replicates were pooled
for species analyses lea\'ing no replication. An
interaction between stations and dates was e'vident.
However, in 13 of 21 sampling periods. Stations A or
B had the highest diversit}'. Diversity was highest
when abundance was highest.

The seasonal and inter-annual trends were most
evident when all samples at all stations were averaged
together to form a marsh-wide average biomass (Figure
5-7a), or marsh-wide average abundance (Figure 5-7b).
The marsh-wide averages were most useful because
station differences were obscured by interaction effects
with sampUng dates. VCTien the marsh- wide averages
were compared to average sahnit}', the effect of inflow
became apparent. VCTien salinit)^ values were high,
biomass decreased, often to near zero. During periods
following salinit\' declines due to inflow, biomass
increased as ex'idenced in the summer through fall of
1997 and 1998. In contrast, salinit}' declines due to
rainfall alone did not have the same effect. During
April 1997, there was a large local rainfall event but
Htde inflow (Table 5-1, Figure 5-4). Biomass continued
to decline during spring and summer 1997. In
contrast, when salinity declined due to an inflow event
in fall 1997 (Event 18), biomass increased. There was
a strong seasonal signal, with highest biomasses in
January' or April of each year, and lowest biomasses in
July of each year. Another important trend was less
variability in biomass fluctuations through time as the
demonstration project progressed.

The marsh-wide average abundance trend (Figure 5-7b)
was very similar to the biomass trend described

5-10

Benthic Communities

I Average macrofauna biomass (each station)

1995

1996

1997

1998

1999

2000

(b) Average macrofauna abundance (each station)

225000

200000

""e 175000

g. 150000

3^ 125000

■^ 100000

■g 75000
>

T3 50000

25000
0

1995

1996

1997

1998

1999

2000

(c) Average macrofauna diversity (each station)

1995

1996

1997

1998

1999

2000

Figure 5-6: Average macrofauna biomass (a), abundance (b) and diversity at each station (A through F).

Diversity values represent the mean of three samples at each station.

Chapter Five ♦ 5-11

(a) Average macrofauna biomass (all stations)
8
7

160

1995

1996

1997

1998

1999

2000

a.
a.

>.

'c
"to

C/3

(b) Average macrofauna abundance (all stations)

90000
80000

— • — Abundance
Salinity

180
160
140

Q.
Q.

C

"(5

C/5

1995

1996

1997

1998

1999

2000

(c) Average macrofauna diversity (all stations)

E
CO 3

— •— N1 Diversity
Salinity

140

1995

1996

1997

1998

1999

2000

Q.

>>

'c
15

05

Figure 5-7: Marsh-wide averages of macrofauna biomass, abundance and diversity for all stations.

Salinity values are averaged from all stations for each sampling period.

5-12

Benthic Communities

previously. Peak abundance occurred after inflow,
lowest abundance during peak salinity concentrations.
Blooms could result in very high abundances, greater
than 40,000 individuals per m". This ecosystem was
characterized by dominance of only a few species. The
diversity index N I was used to calculate the average
number of dominant species among all stations at each
sampling period, and then was compared to salinity
(Figure 5-7c). Generally, no more than two dominant
species were present at each sampling period.
Diversity, measured as the number of dominant
species, increased following periods of low salinity and
decreased when salinity was high.

Macrofauna characteristics have a strong non-linear
relationship with saUmty, which appeared to be a bell-
shaped curve skewed to the left with a long tail to the
right (Figure 5-8).

A three parameter, log normal model:

Y^a * exp( -0.5 * (ln(X/ c)/by)

was used to characterize the nonlinear relationship
between biological characteristic (1) and salinity (X).
The three parameters characterize different attributes
of the curves, where a is the maximum value, b is the
skewness (or rate of change) of the response as a
function of salinity, and c the location of the peak
response value on the salinity axis. The models fit the
data reasonably well, indicated by the coefficient of
variation for each parameter ranging from 7% to 31%
(Table 5-3). Using these parameters, abundance
appeared to peak at a high salinity around 32.7 ppt.

biomass at 18.7 ppt and diversity at 9.1 ppt. Lower
skewness (b) parameters indicates more narrow ranges
of responses values with respect to salinity. For the
three characteristics, the salinity range of response (b)
increased as the salinity peak value for the response
increased (c) (Table 5-3).

The direction of salinity change during a sampling
penod was important for diversity (Figure 5-8c), but
not biomass (Figure 5-8a) or abundance (Figure 5-8b).
The lowest values of all biological responses occurred
at the highest salinity concentrations, and the highest
concentrations always occurred during periods of rising
salinity (note the circle symbols in Figure 5-8). Low
diversity occurred when salinity values were decreasing
(obser\^ations on graph with diamond symbols) and
high diversity occurred when salinity values were rising
within normal salinity ranges (i.e., < 35 ppt).

In spite of the low average diversity on any given
sampling date, a total of 37 species were found over
the five year period of the study (Table 5-4). The
polychaete, Streblospio benedicti (Figure 5-9) was an
overwhelmingly dominant species at all stations and in
the marsh overall (Table 5-5). In fact, S. benedicti
represented 84% of all individuals found over the
entire course of the study. Only four other species
contributed as much of 2% of the community: the
polychaete Laeonereis culveri, the snail Assiminea sucdnea,
and unidentified species of ostracod, and unidentified
chironomid larvae.

Table 5-3: Parameters from nonlinear regressions to predict macrofauna characteristics from salinity
(Figure 5-8). Coefficient of variation for parameters in parentheses.

Characteristic

Parameter

Abundance
Biomass
N1 Diversity

45,774 (25%)
3.426 (15%)
2.361 (7%)

0.6663(31%)

0.9048 (27%)

1.699(13%)

32.7(17%)
18.7(24%)
9.08(19%)

Chapter Five ♦ 5-13

(a) Average macrofauna biomass (all stations)

8
7

.— 6 -

E

w
if)

OJ

E
o

bo

-

o

o
o

o

: y

1

/^

o
o

I

o

o

o
1

1

0°

1

o

1

20

40

60

80

100

120

(b) Average macrofauna abundance (all stations)

E
o

3^

80000 -

O

o

60000 -

o

(/)

m

40000 -

oo^

- —

— — ■

■.-...,^^^^

■D

<Py^

o

""■^ — >

®---^

>

20000 -
0 -

o

1

1

o

1

o

1

o

1

20

40

60

80

100

120

(c) Average macrofauna diversity

o

o
o

Q.

n

E

3

120

Salinity (ppt)

Figure 5-8: Relationship between average macrofauna characteristics and salinity. Circles (o) represent rising
salinity, and diamonds (< ) represent falling salinity. Line is fit with a three parameter log normal regression model.

5-14

Benthic Communities

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Chapter Five ♦ 5-15

Table 5-5: Species overall dominance. Percent of individuals in each sample.

Species Code

Taxa Name

81

Streblospio benedicti

491

Laeonereis culveri

381

Assiminea succinea

181

Ostracoda (unidentified)

487

Chironomid larvae

562

Mediomastus ambiseta

478

Paranais grandis

364

Berosus sp.

111

Capitella capitata

162

Mulinia lateralis

307

Ceratopogonid larvae

8

Oligochaetes (unidentified)

460

Hemicyclops sp.

71

Polydora ligni

734

Damselfly nymphs

201

Corophium louisianum

387

Corophium sp.

371

Tricho corixa sp.

7

Nemertinea (unidentified)

557

Rictaxis punctostriatus

595

Hydrophilidae (unidentified)

854

DIptera (unidentified)

494

Chironomid pupae

903

Pentneura sp. larvae

585

Mesocyclops sp.

488

Macoma mitchelli

802

Latonopsis occidentalis

428

Mysidopsis sp.

493

Mysidopsis almyra

345

Tipulid larvae

689

Diosaccidae sp. 132-MG

232

Callinectes sapidus

202

Gammarvs mucronatus

312

Psychodid larvae

880

Chaoboms sp. larvae

906

Helodidae larvae

905

Dragonfly nymphs

Percent

Commutative %

84.4371

84.4371

4.9874

89.4245

2.9154

92.3399

2.4330

94.7729

2.3698

97.1427

0.7671

97.9098

0.4692

98.3790

0.2636

98.6426

0.2557

98.8983

0.2030

99.1013

0.1766

99.2779

0.1028

99.3807

0.0923

99.4730

0.0791

99.5521

0.0712

99.6233

0.0501

99.6734

0.0474

99.7208

0.0448

99,7656

0.0343

99.7999

0.0343

99.8342

0.0316

99.8658

0.0290

99.8948

0.0132

99.9080

0.0132

99.9212

0.0105

99.9317

0.0053

99.9370

0.0053

99.9423

0.0053

99.9476

0.0053

99.9529

0.0053

99.9582

0.0026

99.9608

0.0026

99.9634

0.0026

99.9660

0.0026

99.9686

0.0026

99.9712

0.0026

99.9738

0.0026

99.9764

The communities within the upper and central Rincon
Bayou treatment sites were dominated by a few
common species, evidenced by the stations clustering
together on the first axis of the principal component
analysis (TCA) (Figure 5-10). The PCA 1 axis, or first
principal component (PCI), explained 70% of the
variance in the data set, and PCA 2 axis (PC2)
explained an additional 19% of the variance. The

dominant organisms Streblospio benedicti (Sb), Laeonereis
culveri (Lc), unidentified Ostracoda (Os) and
Chironomid larvae (Ch) drove the trend for PC 1
toward high positive values because they were part of
the average dominant commumty (Figure 5-lOa).
Thus, all stations had high PCI values (Figure 5-lOb).

5-16

Benthic Communities

Figure 5-9: Steblospio benedicti. This benthic organism
is approximately 1 cm long.

Photo courtesy of the University of Texas Marine Science
Institute.

Other than the four dominant species, all other species
were rare. The rare species were responsible for
regional and station clustering along the PC2 axis.
Presence oi Assiminea sucdnea (As) at Stations A and B
and oi Mediomastus amhiseta (Ma) at Stations E and F
were primarily responsible for separating stations.
Stations A and B also had a higher incidence of insect
larvae like Chironomid larvae (Ch) , Berosus sp. (Be),
Ceratopogonid larvae (Ce) and Damselfly nymphs
(Da). The three clusters (A and B, C and D, E and F)
indicated the communities within the treatments were
slighdy distinct from one another. The stations within
a treatment site {i.e., upper and central Rincon Bayou)
were more similar than the treatment sites themselves
(Figure 5- 10b). The community structure data
presented in the PCA plots were the only macrofauna
data that showed a strong treatment-site trend.

The six most dominant species (Table 5-5) were found
continuously throughout the study, except when
salinity concentrations were high (> 35 ppt)
(Table 5-6). Rare species generally occurred during low
salinity periods only. The only species to occur
consistentiy during hyper-saline conditions was the
insect, Triio corixa (SP 371), but it was also found when
salinity values were brackish, so it was not considered
an indicator species of freshwater inflow. Each

drought period {e.g., the summers of 1995, 1996 and
1998) appeared to be characterized by different species
(Table 5-6).

Meiofauna

The meiofauna community was composed of
Nematoda, Copepoda (primarily Harpacticoida) and
16 other taxa. Nematodes comprised 71% of all
organisms on average, and copepods comprised 9%
(Table 5-7). Insect larvae comprised only 0.02% of the
organisms found. The other metazoan taxa comprised
only 4% of all other organisms found and included
permanent meiofauna (Turbellaria, Gastrotricha,
Tardigrada, Cnidaria, Rotifera and Kinorhyncha) and
temporary meiofauna (Polychaeta, OUgochaeta,
Gastropoda, Bivalvia, Ostracoda and Amphipoda). In
addition, two groups of protozoans were found among
the meiofauna, Ciliata and Foraminifera, which
comprised about 1 5% of all meiofauna found.

The average total number of meiofauna was 908,000
individuals per m^ (Table 5-2). There was a significant
interaction between stations and dates (P = 0.0001). In
contrast to macrofauna, meiofauna abundance
exhibited differences among treatments (Table 5-2,
Figure 5-1 la). Stations A and B always had the lowest
abundances. In fact, the average abundance at Stations
A and B was almost three times lower than the average
at Stations C and D, and four times lower than the
average at Stations E and F.

The average abundance of meiofauna among all
stations at each sampling period changed with
changing salinity conditions (Figure 5-1 lb).
Abundances were lowest when salinity concentrations
were highest, and recovered after periods of low
salinity. In general, the pattern was similar to the
pattern for macrofauna abundances. The lowest
abundances were recorded during the dry periods of
1996. After significant freshwater inflow in 1997
(Events 16, 17 and 18), which lowered salinity,
abundances recovered and reached the highest level in
January 1998.

Chapter Five

5-17

(a) Species loadings

3

(b) Station score

< 0

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F

A
B

Abbreviation Key for species:

Sb = Streblospio benedicti

Lc = Laeonereis culveri

As = Assiminea succinea

Os = Ostracoda (unidentified)

Ch = Chironomid larvae

Ma = Mediomastus ambiseta

Pg = Paranais grandis

Be = Berosus sp.

Cc = Capitella capitate

W = Mulinia lateralis

Ce = Ceratopogonid larvae

Ol = Oligochaetes (unidentified)

He = Hemicyclops sp.

PI = Polydora ligni

Da = Damselfly nymphs

Co = Corophium

-1 0 1

PCA2

Figure 5-10: Principal components analysis of 16 most common species, including species loadings (a) and
station score plots (b).

5-18

Benthic Communities

(a) Average meiofauna abundance (each station)
10000

1995

1996

1997

1998

1999

2000

(b) Average meiofauna abundance (all stations)

5000

120

- 100

Figure 5-11: Average meiofauna abundance foreacli station (a) and marsh-wide (b). Salinity
values are averaged from all stations for each sampling period.

ChapterFive ♦ 5-19

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5-20

Benthic Communities

Table 5-7: Composition of the meiofauna community.

Taxa

Percent

Nematoda
Copepoda
Other metazoa
Insects
Protozoa

71,10
9.39
4.17
0.02

15.32

DISCUSSION

Macrofauna and Meiofauna

The upper Nueces Delta is an unusual marsh, with
extreme environmental variability. The variability is
evidenced by wide ranges in salinity (from 0 to
160 ppt), high temperatures (12 to 40 °C) and a
tendency toward reverse salinity gradients, with higher
salinity concentrations in the upper marsh area. These
environmental extremes drive the response patterns of
benthic resources. Macrofauna diversity in Rincon
Bayou was generally lower than found in either Nueces
Bay or Corpus Chris ti Bay. The average number of
dominant species (diversity index Nl) per station was
only 1.8 in Rincon Bayou, whUe, in contrast, the
average Nl value was 3.5 in Nueces Bay stations

(Mannino and Montagna 1997) and 7.0 in Corpus
Christi Bay (Ritter and Montagna 1999). For
comparison, hypoxic stations in Corpus Christi Bay
averaged an Nl value of 1.5 (Ritter and Montagna
1999). The low diversity in Rincon Bayou reflects a
relatively greater degree of stress on benthos caused by
the higher environmental variability, particularly in
salimty extremes.

Macrofaunal abundance and biomass, which are
indicators of productivity, were in a range typically
found in Nueces and Corpus Christi bays. This
observation indicates that environmental variability
affected benthic community structure but was not
likely to affect secondary productivit}'. However,
marsh habitats typically have greater productivity than
open bay habitats (Day et al. 1989), so it was not
known if production in the Nueces marsh was optimal
or suboptimal.

Overall, there is strong evidence that the demon-
stration project increased producti^^ty and ameliorated
stresses on biodiversity. These positive effects were
caused by increased opportunities for freshwater
inflow into the marsh and responses of the benthos to
this inflow. Seasonal increases of biomass occurred in
spring, when salinity values were lowest and water

100000

ri"" 80000

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X

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o

c

03

■o

X)
03

03

c

03

1000 -§

- 3000

- 2000

1995

1996

1997

1998

1999

2000

Figure 5-12: Marsh-wide average abundance of meiofauna and macrofauna.

Chapter Five ♦ 5-21

levels were highest. In contrast, during summer when
salinity values were highest and water levels were
lowest, biomass was always lowest (Figure 5-7). Inflow
events triggered bursts of productivity as indicated by
increased abundance and biomass following periods of
lower salinity concentrations. Biodiversity increases
just after inflow events and as salinity was rising again
indicated more species were utilizing the marsh habitat
as a result of increased inflow events, although
different species appeared after each inflow event
(Table 5-6). The responses to inflow were found by
following changes after inflow events that filled Rincon
Bayou with fresh water.

Interestingly, prior to overflow channel construction,
brackish conditions in April 1995 resulted in decreased
diversity. In contrast, after the Nueces Overflow
Channel was excavated, brackish conditions in April
(1997 to 1999) resulted in increased diversity
(Table 5-6). The responses of increased abundance to
inflow by macrofauna and meiofauna were similar
(Figure 5-12), indicating that both trophic levels of
benthos were responding to inflow events through the
overflow channel.

The mechanisms of infaunal response to inflow were
likely trophic as well as physiological. The physio-
logical responses were controlled by increased
survivability and tolerance to specific salinity ranges
caused by inflow events. Trophic responses were
indirect and related to responses by potential food
items. When primary producers responded to inflow
by increased biomass, then increased food levels led to
increased secondary production. Therefore, the
trophic link between primary producers and secondary
consumers was demonstrated by correlated, or lagged,
abundance patterns.

Standing stocks of macrofauna and chlorophyll were
somewhat concordant. Peaks of macrofauna biomass
followed periods of increasing chlorophyll
(Figure 5- 13a). The only exception was in summer
1997, when meiofauna abundance was more
concordant with chlorophyll biomass (Figure 5- 13b).
Peaks of meiofauna abundance followed periods with
high concentrations of chlorophyll in the overlying
water. Meiofauna are known to be grazers and to

favorably respond to the presence of chlorophyll
(Montagna 1995; Montagna et al. 1995). In
San Antonio Bay, meiofauna respond to freshwater
inflow and increased chlorophyll with increased grazing
rates (Montagna and Yoon 1991).

Trophic Links

There was evidence (Riera et al. 2000) that the
demonstration project also restored the function of the
Nueces marsh as a nursery habitat for development of
juvenile brown shrimp, Penaeus a^ecus. Brown shrimp
spawn offshore in the Gulf of Mexico. Post-lar%'ae are
carried by on-shore water movement and enter bays,
ultimately finding productive shallow estuarine waters
protected from storms and predators (Day et al. 1989).
Most of the larval brown shrimp enter marine bays
from late winter through early spring, spend about
three to four months in estuarine nursery grounds and
return to the offshore Gulf of Mexico in early summer
(Moffett 1970).

As a sub-component to this benthic analysis, the
trophic dynamic links and migratory behavior of
juvenile brown shrimp were investigated from Aransas
Pass to Corpus Christi Bay to Nueces Bay and to the
Nueces Delta (Riera et al 2000). Stable isotopes ratios
of carbon and nitrogen (6"C and 6'^N) of shrimps
and their potential food sources were measured
between December 1995 and July 1996. Stable
isotopes of carbon and nitrogen change as a function
of the food an organism eats (DeNiro and Epstein
1978; Fry and Parker 1979). Because food sources
change in different habitats, stable isotopes can also be
used to assess migration of shrimp (Fry 1981).

During the study, shrimp lengths increased from 10 to
1 1 mm when the animals entered Corpus Chnsti Bay
as larvae, to 80 to 90 mm when they returned to the
Gulf of Mexico as subadults. Brown shrimp exhibited
spatial and temporal 6"C variation (from -25.2 to
12.5%o), indicating a high diversity of food sources
throughout their migration. From examination of the
6''C values, it appears the main food sources used by
juvenile brown shrimp in Rincon Bayou were Spartina
altemiflora, Spartina spartinae, detritus and benthic

5-22 ^* Benthic Communities

(a) Average macrofauna biomass (all stations)

1000

100

3. 10

O

0.1

\

Chlorophyll
Macrofauna biomass

\ ^ /

E

4 </>
CD

E
o

- 2

1995

1996

1997

1998

1999

2000

(b) Average meiofauna abundance (all stations)
1000

100 -

3

10 -

Chlorophyll
Meiofauna abundance

1995

1996

1997

1998

1999

6000

- 5000

4000 E

O

c

- 3000 m

c
cc

T3

2000 i
<

- 1000

2000

Figure 5-13: Comparison of marsh-wide average chlorophyll biomass with macrofauna biomass (a) and
meiofauna abundance (b).

Chapter Five ♦ 5-23

diatoms (Riera et al. 2000). From 6''C and 6''N
values, it appears organic matter inputs earned by river
inflow could also contribute significandy to the feeding
of migratory brown shrimp. In the marsh habitats of
Rincon Bayou and the Nueces Delta, shrimp isotopic
ratios changed rapidly, indicating high tissue turnover
rates and rapid growth. Therefore, re-introduction of
fresh water to the marsh results in conditions within
nursery areas {e.g., higher benthic biomass) favorable
for feeding and growth of juvenile brown shrimp.
Further discussion of the approach and results of the
stable isotope analyses conducted by Riera et al. (2000)
has been included as Appendix E of this Concluding
Report.

Effects of Diversions as Disturbances

Rapid changes in salinity due to flooding events can be
classified as a disturbance. The frequency and timing
of these freshwater inflow events into Rincon Bayou
were very important. The effects of disturbance
frequency and altered flow on macro-benthic
community structure and colonization in Rincon
Bayou were also independentiy studied during the
demonstration period (Ritter and Montagna 2000).
Abundance and biomass decreased with increasing
disturbance frequency, indicating post-disturbance
community persistence is important in regulating
community structure. There was higher abundance
and biomass in defaunated sediments relative to
background sediments, indicating disturbance plays an
important role in community production of early
succession communities. The collection date was the
most important factor determining community
structure, thus natural variabilit)' overwhelmed effects
of both the flow and disturbance frequency
manipulations. The temporal changes were driven by a
Strehlospio benedicti recruitment event (resulting in
abundances as high as 1.3 10' m"^ captured June 20,
1997 (one day before the beginning of Event 16) and
during the subsequent freshwater event. After the
event, S. benedicti abundance declined rapidly, and
freshwater species invaded, leading to the progression
of three distinct community states {i.e., community
structure and species dominance changed three times)
over the 14-week period of the study. The

overwhelming significance of "temporality" {i.e.,
short-term temporal change in community structure)
was the unexplained temporal component of
commumty variation in experimental manipulations.
Temporality is simply a smaller temporal scale than
seasonality'. The importance of short term changes
relative to flood events indicated that the
demonstration project was responsible for high
productivity during the summers of 1997 through
1999. Further discussion of the approach and results
of the stable isotope analyses conducted by Ritter and
Montagna (2000) has been included as Appendix F of
this Concluding Report.

SUMMARY

Benthos in Rincon Bayou are normally under great
stress due to high salinity concentrations, especially
during summer. The benthos responded positively to
inflow events by increased biomass, abundances and
diversity. Direct and indirect mechanisms were
responsible for the benthic response. The direct
mechanism included increased physiological tolerance
to oligohaline and estuarine salinity concentrations
relative to hyper-saHne conditions. Inflow events at
various times during the year, but especially in the
spring and fall, were likely cues for benthic
reproduction and settiement of planktonic lan'ae.
Trophic interactions and habitat utilization represented
indirect mechanisms to which benthos responded.
Increased microalgal food and marsh habitat
availability stimulated secondary production. As the
frequency and magnitude of freshwater inflow events
increased due to the demonstration project, the
opportunity for positive responses also increased.
Without the demonstration project features, Rincon
Bayou would experience considerably less inflow, and
revert to a reverse estuary with consequentially less
biodiversity and productivity.

5-24

Benthic Communities

CHAPTER SIX

KENNETH H. DUNTON

Marine Science Institute
University of Texas, Austin

HEATHER D. ALEXANDER-MAHALA

Marine Science Institute
University of Texas, Austin

Vegetation
Communities

"Ye marshes, how candid and simple and
nothing withholding and free

Ye publish yourselves to the sky and offer
yourselves to the sea!

Tolerant plains, that suffer the sea and the
rains and the sun. . ."

♦ Sidney Lanier (1878)

INTRODUCTION

A primary factor affecting the growth and distribution
of coastal halophytes (salt-tolerant plants) is soil
salinity (Chapman 1974; Ungar 1974; Riehl and Ungar
1982; Clewell 1997). Halophytes are able to tolerate
relatively high concentration of sodium (Na*) and
chlorine (CI ) because of physiological mechanisms
allowing them to exclude, compartmentalize or extrude
salts (Badger and Ungar 1990). Several halophytes,
including Salkomia sp. and Suaeda sp., even exhibit
stimulated growth at some salinity levels (Ungar 1991).
However, there is for each species a salimty
concentration at which the effectiveness of these
mechanisms is compromised, and growth and
reproduction are limited (Adam 1990).

High soil salinity negatively impacts the reproductive
ability of halophytes by decreasing seed viability.
Hypersalinity can result in a reduction in the number
of seeds germinating, a delay in the initiation of
germination and an increase in the number of seeds
remaining dormant (Ungar 1962; Chapman 1974;
PhilipupiUai and Ungar 1984; Ungar 1995). Each of
these consequences ultimately leads to decreases in
plant cover and increases in bare soils, which can
persist indefinitely until fireshwater inimdation (via
either precipitation or flooding) occurs diluting the
soils, alleviating the salinity stress and breaking the
osmotically induced seed dormancy (Ungar 1962;
Ungar 1978; Ungar 1995). Once germination takes
place, successful seedling growth is also salimty
mediated. The period of seedlmg development is
probably the most sensitive time during the life cycle
of a halophyte because the seedlings develop close to

Chapter Six ♦ 6-1

the soil surface, exposing them to salinity levels 2 to
100 times that of the subsoil (Ungar 1978).

While adult plants have been reported to tolerate
salinity levels 10 to 100 times greater than seedlings
(Mayer and Poljakoff-Mayber 1963), high soil salinity
levels can negatively affect adult plant growth as well.
Plant sunaval does not necessarily mean plant growth,
as a plant may continue to sun-ive at a particular
salinity level without increasing in si2e or actively
reproducing. Several primary mechanisms have been
suggested to explain the negative effects of
hypersalinity on halophytes. These include ion toxicity
of internal cells, interference with the uptake of
essential nutrient ions, lowered external water potential
and energy constraints (e.g., a large amount of energy is
required to actively salt ions) (Greenway and Munns
1983; Yeo 1983).

Most halophytes can survive over a range of salinity
concentrations, but no species has been reported to
have maximal growth rates at salinity levels at or above
seawater concentration (35 parts-per-thousand (ppt))
(Ungar 1991). For example, Spartina foliosa, a California
salt marsh plant, was found to have 50% less dry mass
production in sea water than in fresh water, with only
39% of the plants sur%dving in the saltwater treatment
(Phleger 1971). Barbour (1970) reported growth
reductions in Salicomia virginica and Distichlis spicata at
salinity values ranging from 5 to 22 ppt. Adams (1963)
noted that in North Carolina salt marsh plants could
not tolerate soil salinity levels over 70 ppt. Allison
(1992) found a reduction in species number in a
California salt marsh after periods of low freshwater
availability suggesting that only a few stress -tolerant
species could survive the high saUnity.

In many instances, short-term freshwater flooding of
hjrpersaUne marshes leads to an increase in primary
productivity. Zedler (1983) found biomass oi Spartina
foliosa to increase 40% in the Tijuana Estuary,
CaUfomia, after two months of flooding rains. Covin
and Zedler (1988) noted a 60% increase in the stem
density oi S. foliosa after summer reservoir discharges
and sewage spills along the Mexico border. They also
found near extinctions of Salicomia higelovii and Suaeda

esteroa during a drought period in 1984 that led to
hypersaline conditions.

In the Nueces Delta, hypersalinity occurs as a result of
both natural and human-induced conditions. The
region is semiarid, having low annual rainfall (70
centimeters (cm) or 28 inches per year) and hot, dry
summers. A net annual water deficit is common, as
evaporation (1 52 cm or 60 inches per year) often
exceeds precipitation (Longley 1994). These
conditions produce hj^persaline soils that are diluted
only through direct precipitation or by flooding of the
Nueces River. The natural salinity stress is accentuated
in the Nueces Delta because considerable harnessing of
river water for municipal, agricultural and industrial
purposes has reduced the opportunity for freshwater
flooding events into the marshlands (Irlbeck and Ward
2000). In years prior to the demonstration project, the
river breached its banks and flooded the marsh only
during infrequent flooding events.

OBJECTIVES

1) To determine the effects of the demonstration
project on the open water and pore water salinity
and nitrogen levels; and

2) To determine the project effects on the
distribution and abundance of emergent marsh
vegetation at three different stations over four
growing seasons in the upper Nueces Delta.

MATERIALS AND METHODS

Monitoring Stations

The emergent vegetation and related physio-chemical
parameters (i.e., salinity and nitrogen levels) were
quantified at three sampling stations in the upper
Nueces Delta, including one reference station and two
treatment stations (Figure 6-1). Station I (Reference
Station) was located west of the tidal flats area of the
upper delta, about 0.9 km from the outfall of the
Rincon Overflow Channel. This location was selected
to limit the amount of influence by fresh water
diverted by the demonstration project, but also to

6-2

Vegetation Communities

Northern
bluff line

Approximate Scale
Kilometers

'^incon Bayou

central Rincon Bayou Site
(treatment)

Figure 6-1: Location of vegetation sampling stations in the upper Nueces Delta. The study design included
one reference site and two treatment sites (Tidal Flats and central Rincon Bayou), with one sampling station per site.

allow to the same meteorological influences as the
other two treatment stations. Station II was located in
the tidal flats area about 1.2 km downstream from the
Reference Station and about 0.5 km downstream from
the Rincon Overflow Channel. Freshwater flow
through the Rincon Overflow Channel direcdy
impacted Station II. Station III was located adjacent to
the central Rincon Bayou chamiel about 2.7 km
downstream from Station II, 3.1 km from the Rincon
Overflow Channel and 5.7 from the Nueces Overflow
Channel. Station III was closest to Nueces Bay (about
7.4 km via channels). Vegetation at Station III was
potentially influenced by fresh water entering the
marsh via the Nueces Overflow Channel and was
subjected to tidal inundation of saline waters from the
bay.

The vegetation at the three stations was diverse,
including both annual and perennial species. Annual
plants differed from perermial plants in that they
complete their life cycle within one year and reproduce
by seeds (sexual reproduction). Perennial plants can
live for more than a year and reproduce both sexually
by seeds and by asexual vegetative growth {i.e.,
expansion of above-ground tissues and below-ground
rhizomes). The dominant vegetation species at the
three stations were the perennial plants Borrichia
frutescens^ Bads maritima, Monanthocloe littoralis and
Distichlis spicata. The perennial succulent Salkornia
virginica was dominant at Station III but was rarely
found at the odier two stations. The amiual succulent,
Salkornia bigelovii, was periodically present at all three

Chapter Six ♦ 6-3

stations. The annual species Suaeda maritima, Lycium
carolinianum^ and Umonium nashii were occasionally
found at the stations.

Open Water and Pore Water Chemistry

Four replicate open water samples were collected from
the water adjacent to the sampling transects. Two
replicate sediment pore water samples were collected
using lysimeters placed within the transect sediments at
the 0, 49 and 99 m marks. Lysimeters were made of
PVC pipes (6 cm diameter, 60 cm length) with
horizontal slits cut in the lower 30 cm to allow for the
passive movement of water into the pipe. Prior to
sampling, lysimeters were pumped dry and aDowed to
refill. If water were unavailable, sediment samples
were taken and later centrifuged to extract pore water.
After being centrifuged, salinity of pore water samples
was determined using a refractometer. Oftentimes, the
sediment was too dry to extract any water, and
therefore data were not acquired on several sampling
dates. In the field, open water sahnity was recorded
witli an Orion conductivity meter and reported in ppt.
AU samples were collected in botdes and placed on ice
for later determination of ammonium (NH4*) and
nitrite plus nitrate (NO, + NO3) levels.
Concentrations of NH4*, and NO, + NO3 were
determined using standard colorimetric techniques
(Parsons ^/-a/. 1984).

Freshwater flow through Rincon Bayou and direct
precipitation were also measured during the
demonstration period (Chapter 3). Daily data for these
two freshwater variables were summed according to
the periods tmmediately preceding each vegetation
sampling date to allow comparison with salinity values.

Transect Sampling

Seasonal vegetation distribution and abundance were
quantified using transect sampling (Bertness and
Ellison 1987). Generally, sampling occurred in the late
spring (~June), late summer (~September), late fall
(~December) and mid-winter (~January). The
sampling schedule was based on a previous study in the

Nueces Delta, which suggested that the emergent
vegetation exhibits peaks and declines in growth at
different times of the year. Annual species were noted
to increase in cover during the late spring but were
non-existent throughout the summer, fall and winter.
Additionally, many species exhibited a reduction in
cover during the late fall because of the shedding of
their leaves. Late summer sampling was chosen
because the plants typically experience several months
of increased temperatures and decreased rain. Mid-
winter sampling was selected because it is the time of
year when plant cover is typically low, but annual
species seedling growth has begun.

In June 1995, three permanent transects were
established, one at each station. The transects were
99 m long and 8 m wide (792 m^ at the Reference
Station and Station II (Figure 6-2), and 103 m long and
8 m wide (824 m^ at Station III. At all three stations,
the transect lines were spaced at 3 m inter\^als for the
first 9 m of the transect. At the Reference Station and
Station II, the lines were spaced at 10 m intervals
between 9 and 99 m. Each transect extended
perpendicularly from the vegetation line at the water's
edge. The transect lines were spaced closest together
near the water's edge because this was expected to be
the part of the transect showing the greatest variation
in degree of tidal inundation and soil moisture.
Station III differed in transect size and sampling
intervals because a small channel intersected the
transect between the 47 m and 57 m marks, resulting in
the occurrence of three water's edges at this station.
The transect design for Station III was different from
the Reference Station and Station II because there was
no area large enough to encompass at least a 99 m
transect direcdy along Rincon Bayou. However, the
difference in transect design was inconsequential
because the three stations were uniquely different, and
direct statistical comparisons between the stations was
not necessary. At Station III, the lines were spaced at
10 m intervals between 9 and 39 m, 8 m between
39 and 47 m, 10 m between 47 and 57 m, 3 m inten'^als
between 57 and 63 m and 10 m inter\'als between
63 and 103 m. The transects were sampled at 2 m
intervals along the horizontal transect lines, for a total

6-4

Vegetation Communities

Water's
edge

8m

3m

10m

99 m

Figure 6-2: The layout and dimensions of a typical vegetation transect. The figure illustrates the transect design
used at the Reference Station and Station II. Each dot is a sampling point for vegetation parameters (i.e., percent cover
and leaf area index). Asterisks (*) denote the sampling locations of pore water samples.

of 65 sampling points at the Reference Station and
Station II and 75 sampling points at Station III.

Percent Cover and Leaf Area Index

Percent cover and leaf area index (LAI) were sampled
seasonally from June 1995 to December 1999, for a
total of 20 seasonal sampling dates. The three stations
were sampled one additional time in June 2000, but
only data for percent cover were acquired. However,
the Station III transect was partially destroyed by
livestock, and therefore percent cover data for that
date were not collected.

A 0.25 m' quadrat subdivided into 100 cells was used
to estimate the percent cover of each species at each
sampUng point. CeUs with no vegetation and those
covered with water or wrack (dead plant material) were
considered bare area. LAI, a measure of plant foliage
density and distribution, was quantified at each
sampling point using a LAI-2000 Leaf Canopy
Analyzer (LI-COR, Lincoln, Nebraska). LAI provided
a non-destructive means of estimating foliage density
by measuring the amount of light attenuated within the
canopy. Each LAI measurement was an average of
three individual readings taken at a sampling point.
LAI readings for 1995 were not included in the
analyses due to complications with the measuring
technique at die beginning of die sampling period.

Analyses

Vegetation parameters were analyzed at two different
spatial scales (792 or 824 m" transects and 0.25 m"
quadrats interpolated to 0.25 m'grid cells). Large-scale
analyses averaged data over an entire transect and
utilized traditional methods, including graphs and
tables. Small-scale analyses were accomplished
through the use of a Geographical Information System
(GIS). The GIS allowed each sampUng point and
corresponding data to be geographically represented,
taking into account the spatial relationships between
vegetation parameters. Analysis at two different scales
was necessary because much of the heterogeneity
foiind at small scales was lost when the data were
averaged over an entire transect. Large-scale
obser\'ations were useful in that they indicated general
patterns and trends in vegetation parameters, such as
seasonal peaks and declines, but small-scale analyses
provided detailed information regarding changes in
vegetation species distribution and abundance.

Large-Scale Analyses

Total transect average percent cover for each species
was determined by adding together the percent cover
of each species at each sampling point and then
dividing by the total number of sampling points. Total
LAI was calculated in the same manner. These total
ntimbers provided an estimate of species percent cover
and LAI on a large-scale basis.

Chapter Six •♦♦ 6-5

Small-Scale Analyses

Incli\'idual species percent cover and LAI readings
taken at each sampling point were analyzed on a small-
scale basis by the means of a GIS. Each sampling
point was given geographic coordinates (Universal
Transverse Mercator; Zone 14) based on differential
Global Positioning System (GPS) points taken in the
field at the four comers of the transects. Percent cover
and LAI data acquired in the field were assigned to the
corresponding geographic location at each sampUng
point then incorporated from tabular format in
Microsoft Excel to ArcView GIS software. The
Inverse Distance Weighting function in Arc View's
Spatial Analyst extension was used to interpolate grids
for the area within the transect using the data points
taken in the field as reference numbers. The method
assumes that each point has a local influence that
diminishes with distance so that points closer to the
cell containing the field measurement have greater
values than those farther away. A grid cell size of
0.25 m (the same size as the quadrat) was used,
providing a means of small-scale analyses.

Maps of percent cover and LAI for each transect were
analyzed to determine changes in species cover and
bare area before and after hydrographic events at each
station. For percent cover, vegetation maps for each
sampling date were created by querying the data to find
areas in the transect where cover for each individual
species were greater than 50%. This allowed all species
to be mapped together on the transect. Maps from the
sampling date prior to an event were then compared to
maps for the three sampling dates following the event.
The conversion of the point data into interpolated
surfaces allowed for mathematical manipulations and
analyses in a spatially coherent, three-dimensional
manner, which naturally represented the data.

BlOMASS

Four replicate samples of monospecific stands oi Batis
maritima, Borrichia Jrutescens, Monanthocloe litioralis and
Distkhlis spkata growing near the transect were sampled
bi-annuaUy (winter and spring) using a PVC corer
(10 cm diameter, 30 cm length). The corer was placed

around the vegetation and above-ground plant material
clipped. The corer was then dri\'en into the ground for
collection of below-ground material. Plant material
was sequentially sieved using a 1 cm sieve foUowed by
a 1 miHtmeter (mm) sieve, sorted into above- and
below-ground portions and dried at 60° C to a
constant weight. Samples were weighed to the nearest
0.1 gram (g), and biomass was converted to an area
basis. Root to shoot (RjS ratios) ratios were calculated
from biomass measurements.

RESULTS

Vegetation and physio-chemical parameters [i.e.,
salinity and nitrogen levels) were examined based on
changes observed within and between the stations
before and after periods with significant freshwater
inflow or precipitation (Table 6-1). In general,
vegetation from three different sampling periods
exhibited measurable responses to these hydrographic
influences: July 1997, October 1998 and September
1999. The major hydrographic events (Chapter .3)
preceding each of these sampling dates include Event
16 (June 21 through July 3, 1997), Event 17 Quly 4
through 26, 1997), Event 25 (October 16 through 29,
1998), Event 35 (August 19 through September 3,
1999) and Event 36 (September 4 through 20, 1999)
(Table 6-1). Where any sampling period was preceded
by more than one identified hydrographic event {e.g.,
July 1997 and September 1999), it was assumed that
any measurable change observed in the vegetation
could be due to a combination of those events.
Therefore, for the purposes of this vegetation analysis,
the events preceding each sampling period during
which a response was observed were collectively
referred to as the composite hydrographic events of
July 1997, October 1998 and September 1999. During
each of these periods, the Rincon Overflow Channel
was activated and Station II was subjected to the
influence of project diversions.

Because salt marsh vegetation also often responds to
direct precipitation, total monthly precipitation was
reported separately to allow precipitation-mediated

6-6

Vegetation Communities

Table 6-1: Hydrographic events (from Table 3-3) occurring prior to each vegetation sampling period. The three
composite hydrographic events which stimulated a vegetative response (July 1997, October 1998 and September 1999)
are indicated in bold. Cumulative flow, rainfall and average salinity concentrations also reported for all vegetation stations
(1 through 3). Daily values for net flow and total precipitation were summed between sampling intervals. In some cases,
sampling occurred during an event. The Nueces Overflow Channel was completed on October 26, 1995, so flow prior to
that date was zero. Precipitation data for sampling dates prior to May 16, 1996 were that recorded at the Corpus Christi
International Airport. Hyphens (-) indicate missing data.

Vegetation

Hydrographic Events

Total Net Flow Through

Total Precipitation

Sampling Date

(Table 3-3)

Rincon Bayou (acre

-ft)

(inches)

6/29/95

1,2,3

0

6.06

8/25/95

4

0

4.84

11/7/95

5,6

-

14.57

2/13/96

7,8,9, 10

-

3.62

5/22/96

-

1.64

9/5/96

11

41

9.37

11/6/96

12, 13, 14

250

3.26

2/17/97

79

4.29

6/2/97

15

57

16.66

8/29/97

16,17

1,542

3.35

11/26/97

18

264

14.19

1/5/98

-1

0.38

6/1/98

19

78

5.45

10/2/98

20,21,22,23,24

670

9.89

12/2/98

25, 26, 27

3,381

13.11

1/13/99

-8

1.08

6/2/99

28, 29, 30, 31

-1

7.40

9/2/99

32, 33, 34, 35

-41

13.72

12/8/99

36,37

820

4.63

changes to be identified (Figure 6-3). It should be
noted that, although direct precipitation was not an
effect of the demonstration project, it was a freshwater
source to which vegetation responded, which could
also be used to indicate the importance of freshwater
diversions.

Salinity

open Water Salinity

Open water salinity values for all three stations
increased and decreased simultaneously throughout the
study period, although the magnitude of change varied
between the stations (Table 6-2 and Figure 6-4a).
Decreases in salinity were often seen following
precipitation or flooding, and high salinity values were
common during dry, hot periods. An important
assumption was that changes in open water and pore

water salinity values at the Reference Station were not
due to the channels, being determined by
environmental conditions such as precipitation,
evaporation and run-off. This was important when
comparing the salinity values between the stations and
assessing the relative impacts of the channels.

When evaluating salinity levels, precipitation and flow
effects were analyzed independendy because changes in
salinity could be due to one or both parameters
(Table 6-1 and Figure 6-4a). The data acquired in this
study indicated that heavy precipitation could occur
independent of flow and result in low salinit)' values.

Therefore, when analyzing the salinity data, sampling
dates that experienced prior hea\7 precipitation
without heavy flow were treated separately from all
other events (Figure 6-5). When interpreted in this
manner, there was a positive correlation between
increasing flow and decreasing salinity values at both

Chapter Six ♦ 6-7

10
9
8

0)

oj 5

I—
o

4
3
2 -

1 T

0 -L

T ~

1996

J

la

25

20

15 -Si

1

c

S

10 TO
O

- 5

1997

1998

1999

2000

Figure 6-3: Total monthly precipitation during the demonstration period. Data from April 1995
through April 1996 are that reported from the Corpus Christi International Airport, and data from
May 19996 through December 1999 are that recorded at the Rincon gauge.

Table 6-2: Mean open water (OW) and pore water (PW) salinity values at each station. Pore water samples taken
at three locations (0, 49 and 99 m) along each transect. Hyphens (-) indicate missing samples because of dry open
water channels or pore water holes. Each value reported reflects the average of two individual samples.

Reference Station

Station II

Station III

Yr

Mo

OW

PW

OW

PW

OW

PW
Dm 49 m

Dm 49 m

99 m

Dm 49 m

99 m

99 m

Apr

14.8

20

-

23

36

-

30

32

-

95

Jun

-

-

-

56

50 65

69

58

-

-

Aug

10

18 53

56

26

43 68

66

28

51

70

Nov

4

29 51

36

9

30 70

47

23

38 33

36

Feb

27

-

-

-

70

68

47

-

-

96

May

-

-

-

-

-

-

57

-

-

Sep

5

44 68

74

5

28 79

-

28

-

62

Nov

49

46 64

66

58

69 70

71

54

63

71

Feb

53

49

-

65

59 76

61

47

-

-

97

Jun

13

27

-

12

13 70

-

21

-

-

Aug

43

-

-

-

-

-

22

-

-

Nov

12

26 61

52

8

8 36

54

15

-

40

Jan

14

-

-

13

-

-

18

-

-

98

Jun

45

-

-

-

-

-

49

-

-

Oct

30

46 81

-

33

40 48

62

22

52 41

73

Dec

12

37 55

-

11

34 53

69

16

-

-

Jan

18

-

-

15

-

-

18

-

-

99

Jun

23

30 72

-

47

54 80

83

44

54 51

81

Sep

6

23 83

42

7

18 21

41

12

-

11

Dec

17

29 82

87

29

60 85

82

29

46 51

92

6-8

Vegetation Communities

50

40

I 30

•S 20

10 -

(a) Open water salinity
5000

1996

1997

1998

1999

Total rainfall

Total flow into Rincon Bayou

■ ▼• ■ Average salinity;
-o— Average salinity:
-D— Average salinity:

Reference Site (Station I)
Station II
Station III

2000

50

40

§ 30

•i 20

a

10

(b) Pore water salinity (at 49 m along the transect)

5000

4000

CD 3000
o

I 2000

1000

H

^y^-^i

I

140
1- 120

100 c:-

Q.

80
60
40
20
0

Q.
>.

To
CO

1996

1997

1998

1999

2000

I I Total rainfall

Total flow into Rincon Bayou

• ▼• • Average salinity:
-o— Average salinity:
-a — Average salinity:

Reference Site (Station I)
Station II
Station III

Figure 6-4: Salinity for open water (a) and pore water (b) at each sampling site for each sampling date. Cumulative
daily rainfall and inflow into Rincon Bayou also plotted for each period between sampling dates.

Chapter Six ♦ 6-9

(a) Reference Station

l\J

60

5-. 50

Q.

|40

•
•

•

•

CO
CO

20
10

•
o

o

•
•

~~~~" ~~~-

._____j^ = 0.33

0

oo

1 1 1 1 1

(b) Station II

1

60

•

•

_ 50

Q.

B 40 ■
1 30

•

•

^""■^----.^..^^^^r^ = 0 68

'«20.

^^^^^^

10
0

o

^^-^---..•^

(c) Station III

l\J ■

60

_50

Q.

3 40

t

•

alinity

GO

o

o

•

- r2 = 0 56

"20
10

o

o

•

•

^^^^-—

_____^ •

0 J

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Flow Into RIncon Bayou (acre-ft)

Figure 6-5: Correlation between total flow through RIncon Bayou and open water salinity for each
station. Open circles (O) indicate salinity values on sampling dates following heavy precipitation (> 25 cm)
with little to no flow (< 617 10^ m^ or 500 acre-ft). Closed circles (•) indicate salinity values on all other
sampling dates. The separation of salinity values was used to distinguish the effects of freshwater inflow.
The linear regressions indicate the correlation between flow and salinity values only for events with large
inflow. As flow increased, salinity decreased at the treatment stations (Stations II and III). As expected, a
clear correlation was not found at the Reference Station.

6-10

Vegetation Communities

Station II (r = 0.68) and Station III (r = 0.56) and a
nonsignificant correlation at the Reference Station
(r^ = 0.33). The correlations were based the total
precipitation and flow experienced at the stations
between sampling periods. However, on two
occasions, sampling dates were only two months apart
(November 1997/January 1998 and December
1998/January 1999). For these two periods, flow and
precipitation for the three months prior to the
sampUng date were considered.

At the Reference Station, salinity values greater than
35 ppt (seawater concentration) were recorded 22% of
the time, while values less than 18 ppt were measured
56% of the time. The highest salinity (53 ppt)
occurred in February 1997 during a five-month period
with less than 25.2 cm (10 inches) of rain. The lowest
values recorded coincide with periods having heavy
rainfall. For example, a salinity value of 13 ppt was
recorded in June 1997, following three months with
over 37.8 cm (15 inches) of rain. However, salinity
increased rapidly afterwards to 43 ppt in August 1997.
A salinity value of 12 ppt was recorded in December
1998, following four months with over 55.4 cm
(22 inches) of rain. In this instance, salinity remained
relatively low throughout 1999, most likely because
spring rains were abundant, with the highest salinity for
the year (23 ppt) being recorded in June 1999.

At Station II, open water salinity values above 35 ppt
were recorded on three sampling dates (25% of the
samples), with the highest recording being 65 ppt in
February 1997. Low values recorded at this station
occurred on the same dates as those at the Reference
Station. Values after major hydrographic events
flooding the Rincon Overflow Channel do not differ
by more than 1 ppt from those at the Reference
Station but were 5 ppt lower than Station III after the
October 1998 and September 1999 events.

Station III had the greatest number of readings higher
than 35 ppt (35%). The relatively higher values may be
reflective of its closer proximity to Nueces Bay. The
highest value (57 ppt) was recorded in May 1996,
following six months of less than 5 cm (2 inches) total
rainfall.

Pore Water Salinity

Pore water salinity values were typically higher than
those in open water (Table 6-2 and Figure 6-4b). At
the Reference Station, pore water salinity
concentrations ranged between 20 and 87 ppt and were
as much as 14 times higher than the open water salinity
value measured on the same date. At Station II, values
ranged between 8 and 85 ppt and were as much as 16
times higher than the open water value. The range of
values at Station III was similar to the other stations
(1 1 to 92 ppt) but was measured to be only as much as
3 times higher than the open water value.

Pore water values did not always decline after a
hydrographic event, whether the event was
precipitation- or flow-mediated (Figure 6-4b).
However, as many of the values are missing,
determination of actual decrease proved difficult.
Additionally, sampling dates did not always
immediately follow times of heavy precipitation or flow
events. Consequentiy, salinity values might have
decreased, but unless the sampling date occurred soon
after the freshwater input, any pore water response
might not have been measured.

On the occasions that values could be recorded, the
response was variable. In December 1998, following
three months with over 25 cm (10 inches) rainfall, pore
water salinity values at the Reference Station decreased
by 9 and 26 ppt at two locations (0 and 49 m).
However, values at Station II were only reduced at 0 m
(the location closest to open water) and only by 6 ppt,
despite the flooding of the station during the October

1998 event. In contrast, pore water salinity values
decreased at Station II in September 1999 by 36, 58
and 42 ppt at 0, 49 and 99 m locations, respectively.
The decrease immediately followed the September

1999 event, which had not yet activated the Rincon
Overflow Channel but did see over 15 cm (6 inches)
rain in one day. During this event, a tidal surge from
the bay flooded the transects, flushing the soils and
then retreating after the surge. In this instance, the
sampHng date was only days following the event. The
spring and summer months also had relatively high

Chapter Six ♦ 6-11

rainfall compared to the other sampling years. The
range of values at the Reference Station following the
same event decreased only at 0 m by 6 ppt, but
increased at 49 m by 2 ppt and 45 ppt at 99 m.

Ammonium

Open Water Ammonium

Concentrations of NH4* at the Reference Station
exhibited the greatest variation (0.9 to 14.7 jimoles ),
with peak values occurring in September 1996
(12.1 (imoles ) and June 1999 (14.7 ^moles )
(Table 6-3). The increase in September 1996
coincided with a peak at Station II. The June 1999
peak occurred during a period of 4 months with over
30.2 cm (12 inches) of rain.

At Station II, concentrations resembled those at
Station III, except for a peak value of 9.4 |imoles in
September 1996, 51.6 (omoles in January 1997 and two

peaks of 6.4 ^imoles in December 1998 and January
1999. ITie first peak occurred during a period with
more than 22.7 cm (9 inches) of rain in two months,
and the last two peaks followed the October 1998
event, which flooded the station. Values prior to the
event were in the average range for that time of year
(2.7 (imoles), but were more than t«.ice as high
following the event. The concentration decreased by
60% in the following five months to near 4.0 ^unoles,
which represented the fall peak range under non-event
conditions.

At Station III, concentrations cycled over the study
period, with low values (~2.0 to 3.0 |jmoles) being
obser\xd in the late fall and winter. Values increased
from winter to spring and peaked in the late summer
or fall (4.0 to 6.0 (jmoles). Station III exhibited the
smallest range of values, 1.6 to 9.5 [ijnoles.

Table 6-3: Mean open water ammonium (NH/) and nitrite+nitrate (NO2 -•- NO3') concentrations at each station.

Values are reported ± SE. (n = 4). Samples acquired prior to September 1996 represent only one water sample and
therefore do not contain SE. Dashes indicate times when samples could not be taken.

Yr

Mo

Reference Station
NH/ NO2 + NO3

Station II
NH/ NOj + NO3

Station III
NH/ NO2 + NO3

Apr

3.7

0.8

5.5

5.3

2.5

2,7

Jun

4.1

1.2

3.0

20.2

5.6

2.3

95

Aug

1.7

1.3

5.4

1.3

2.9

5.6

Nov

3.1

1.7

1.7

7.1

3,0

2.7

Feb

5,0

1.8

-

-

1,6

1.1

96

May

-

-

-

-

2,7

0.7

Sep

12.1 ±1.00

0.9 ± 0.05

9.4 ±1.57

1.2 ±0.08

3,8 ±0,12

1,8 ±0,5

Nov

1.9±0.12

1.2 ±0.09

1.6±0.11

0.9 ±0.05

2,3 ±0,12

1,0 ±0,07

Feb

1.7±0.15

0.8 ±0.01

1.6±0.14

0.9 ± 0.05

1.7±0,15

0,9 ±0,04

97

Jun

2.3 ±0.05

0.5 ±0.03

3.2 ± 0.20

0.7 ±0.04

2,6 ± 0.08

0,8 ±0,05

Aug

3.5 ±0.17

0.8 ±0.08

-

-

3.9 ±0,38

0,8 ±0.03

Nov

2.6 ±0.15

0.5 ±0.10

3.7 ±0.17

0.3 ±0.01

4,1 ±0,14

0,4 ± 0,02

Jan

3.9 ±0.07

0.5±0.11

51 .6 ±2.22

1.9±0,18

2,9±0,15

0,7 ±0,17

98

Jun

0.9 ±0.18

0.8 ± 0.03

-

-

1.7±0,11

0,8 ±0,02

Oct

5.7 ±0.17

0.9 ± 0.02

2.7 ±0.09

0,5 ±0.03

2,4 ±0,10

0,5 ±0.01

Dec

2.6 ±0.37

0.7 ±0.02

6.4 ±0.44

1.9 ±0,09

2,1 ±0,03

0.9 ±0,03

Jan

2.5±0.19

0.9 ±0.01

6.4 ±1.50

1.1 ±0,03

2,3 ±0,05

1.0 ±0,01

99

Jun

14.7 ±0.67

0.9 ±0.04

3.8 ±0.27

1,1 ±0,06

5.0 ±0,23

1.2 ±0,02

Sep

2.6 ±0.06

0.4 ±0.02

2.8 ±0.08

0.5 ±0.01

2,8 ±0,39

0,5 ±0,03

Dec

8.0 ± 0.32

0.9 ± 0.03

4,6 ±0.10

1.5 ±0.09

9,5 ± 0,24

1,1 ±0,05

6-12

Vegetation Communities

Pore Water Ammonium

NiTRITE+NlTRATE

Pore water NH4* concentrations were generally much
higher than open water levels, with the highest values
recorded being in the summer and lowest values in the
late fall and winter (Table 6-4). At the Reference
Station, values ranged from 1.9 to 193 (xmoles. The
highest values were recorded in June 1999 (53 to
193 (xmoles). At Station II, values ranged between
4.5 and 197 |xmoles. High values (64 to 197 |jjnoles)
were measured over a six-month period from |une to
December 1999 and coincided with a fourteen-month
period with over 125.9 cm (50 inches) of precipitation.
The highest values recorded at Station III were also in
June through December 1999 (21 to 213 i^moles). At
all three stations, values appeared to gradually increase
over the study period (Figure 6-6), with similarly high
values seen during the second half of 1999.

Open Water Nitrite+Nitrate

At the Reference Station, NO, + NO, concentrations
exhibited the smallest range of values (0.4 to
1.8 pmoles), with the highest value being recorded in
February 1996 (Table 6-3). The highest overall values
were seen at Station III during the first sampling year
(1995), with values exceeding 2.3 jimoles at each
sampling date during that year and approaching
6 ^imoles in August 1995. The range of values for the
remainder of the study period was between 0.4 and
1.2 ^imoles. The values at Station II were between
0.3 ^imoles and 5.3 ^moles, with the highest values
recorded in April 1995. An additional small peak
occurred in December 1998 (1.9 jimoles).

Table 6-4: Mean pore water ammonium (NH/) and nitrite+nitrate (NOj" + NO3) concentrations at each station.

Pore water samples were taken at three lysimeters located at 0, 49 and 99 m from the water's edge. Two water
samples were taken at each lysimeter, and the two values averaged to determine a mean value for that distance.
The range of mean values at the three distances is reported. If only one number is listed, then samples were only
taken from one well, and dashes represent samples that could not be taken (as a result of sediments being too dry to
extract water).

Yr

Mo

Reference Station
NH4* NOj + NO3

Station II
NH4* NO2 + NO3

Station III
NH/ NOj + NO3-

Apr

11.4

3.5

28.8

1.4

17.7

0.5

Jun

2.4 to 21.8

8.4 to 38.2

37 to 48.5

2.7 to 14.4

-

-

95

Aug

4.9 to 28.4

2.0 to 3.3

16.2 to 53.7

1.5 to 6.0

7.4 to 12.9

2.8 to 11.9

Nov

1 .9 to 1 1 .4

2.7 to 12.1

4.9 to 22.9

0.1 to 4.8

2.0 to 13

0.2 to 1.0

Feb

8.6

-

6.1 to 13.2

-

-

-

96

May

-

-

-

-

-

-

Sep

4.5 to 23.5

2.4 to 31.3

6.4 to 125.4

4.1 to 49.8

3.4

0.5 to 0.7

Nov

10.9 to 66.6

1,6 to 8.9

24.7 to 36.8

0.8 to 1.2

7.8 to 9.3

0.4 to 0.8

Feb

10.7

1.3

4,5

25.1

-

-

97

Jun

48.3

0.5

5.6 to 15.3

0.8 to 1.0

-

-

Aug

-

-

-

-

-

-

Nov

6.7 to 8

0.9 to 3.6

12.2 to 17.6

1.7 to 2.6

7.9

2.9

Jan

-

-

-

-

-

-

98

Jun

-

-

-

-

-

-

Oct

19.9 to 38.2

0.9 to 4

32.8 to 131.7

0.6 to 4.7

10 to 65.9

0.5 to 5.0

Dec

10.6 to 45.6

1.2 to 10.3

48.7 to 64.7

1 to 2.8

-

-

Jan

27.2

2.3

30.7-60.5

1.0 to 2.0

-

-

99

Jun

52.9 to 107.4

-

63.7 to 182.8

1.3

79.9 to 111.4

-

Sep

26.8 to 79.7

0,5 to 1

67.1 to 106.3

0.1 to 0.8

21.1 to 55.5

0.7

Dec

58.4 to 84.5

-

85.6 to 113.1

-

86.1 to 94,8

-

ChapterSix ♦ 6-13

(a) Reference Station

120 .

100 -

^ 80
o

I. 60

+

S 40 \

20
0

,T, ■ ,

—I — I — 1 — I — I — I — r-

—\ — I — I — I — I — I — I — I — 1 — I — I — I — I — I — I — 1 — I — I — 1 — I — I — I — I — r-

(b) Station II

120

T — I — 1 — I — 1 — 1 — I — I — I — I — r— 1 — I — I — I — I — I — I — 1 — I — I—

— 1 — I — I — 1 — I

(c) Station III

120

AJAODFAJAODFAJAODFAJAODFAJAOD
95 96 97 98 99

Figure 6-6: Mean pore water ammonium values for each station. Pore water samples were taken at thiree
lysimeters located at 0, 49 and 99 m from the water's edge. Curves represent the best fit curve for the values taken at
the 49 m location. The values reported for each distance is the mean of two samples. Occasionally, one or more
locations were dry, so some sampling dates do not have measurements or have only one or two measurements.

6-14 *♦* Vegetation Communities

Pore Water Nitrite+ Nitrate

Pore water NO," + NO3 concentrations were typically

higher than open water values (Table 6-4). At the

Reference Station, values ranged from 0.5 to

38.2 (jmoles, and at Stations II and III, concentrations

ranged from 0.1 to 49.8 |jinoles and 0.4 to 11.9 ^xmoles,

respectively.

large-scale whole transect analyses:
Individual Species Responses to Events

There was considerable variability in the percent cover
of each species within and between stations before and
after composite hydrographic events (Figures 6-7
through 6-9). At the Reference Station and Station III,
vegetation changes appear to be predominantly
precipitation-mediated rather than flow generated,
while changes at Station II occurred as a result of both
flow through the channels and direct precipitation.

Reference Starion

Batis maritima - At the Reference Station, total
transect percent cover of Batis maritima cycled
seasonally with peaks generally occurring in the
summer and declines in the late fall/early winter
(Figure 6-7a). The greatest increase in cover (3.5% to
23%) occurred between February and August 1997.
Between February and June, the study area received
52.1 cm (20.7 inches) of rain, or 57% of the total yearly
rainfall. At the Reference Station, B. maritima exhibited
a decrease in cover in June 1998. From April to June
1998, there was litde precipitation (< 0.91 cm, or
0.36 inches), unlike other springs during the study
period when several inches of rain fell during the same
period.

Borrichia frutescens - Percent cover of Borrichia
frutescens at the Reference Station exhibited an inverse
correlation with Batis maritima cover (r^=0.61)
(Figure 6-7b). Generally, peaks in B. frutescens occurred
when B. maritima cover was relatively low.

Distichlis spicata - At the Reference Station, cover of
Disticblis spicata exhibited an overall decrease from

10% in February 1996 to < 1% in June 1997

(Figure 6-7c). Cover remained < 1% for the remaining

study period.

Monanthodoe littoralis - At the Reference Station,
Monanthocloe littoralis cover remained relatively constant
throughout the study period (29 to 42%), exclusive of a
23% decline in cover (from 37 to 14%) between
January and September 1999 (Figure 6-7d). The initial
decline in cover occurred followed six months with
about 45 cm (18 inches) rain. The lowest cover
measured coincided with the passing of Hurricane Bret
(August 1999). The storm released over 15 cm
(6 inches) of rain in one day. The transect was also
flooded by a tidal surge. M. littoralis cover increased
20% (14 to 34%) in the two months following the
hurricane, a period with litde to no rainfall. Similar,
but smaller-scale declines in cover were seen following
two or more consecutive months with several inches of
rainfall (August to September 1996 and February to
June 1997). The decreases in cover were always quickly
followed by an increase in growth.

Salicornia bigelovii - At the Reference Station,
Salicomia bigelovii cover exhibited spring peaks greater
than 10% cover occurring in 1996, 1997 and 1998
(Figure 6-7e). The largest peak (30%) in June 1999 was
about 20% higher than the other two peaks and
occurred following a winter and spring with consistent
rainfall (11 out of 14 months received over 5 cm
(2 inches) of rain).

Bare Area — On occasion, decreases in bare area
corresponded with increases in Salicomia bigelovii cover.
For example, S. bigelovii cover was about 29% in the
summer 1999, and bare area cover decreased by about
20% (Figures 6-7e and 6-7f). However, there was no
strong correlation between the two parameters
(r = 0.27 for S. bigelovii cover > 2%). The greatest
bare area cover at the Reference Station (69%)
occurred in September 1999 following the composite
hydrographic event in September 1999, which included
the landfall of Hurricane Bret.

Chapter Six V 6-15

(a) Batis maritima

(d) Monanthocloe littoralis

100
80
60
40
20 .

i t i

100
80
60
40
20

0 1 1 1 mill Nil iiiiiiiuiiiMiiiiiiii 1 1 1 1 1 II I ii

1 1 I n 1 1 1 1 I

^ ^ i

I I II I I I I I I M I I I 1 I M I M M I f I I I I I I I I I I M I I I I I I I I M I I M I II M I I I I I

(b) Borrichia frutescens

(e) Salicornia bigelovii

100

80

w
c

8

a.
o

2 20
o
>
<

40

100
80
60
40
20

***^^-**-i^M*-*-^#-*^f^— I

I I I I I I I M I I I I M I 11 I 1 I 1 I I M I I M I II I I I I I I I I I I I I I I I II I I I I I I I I I I I I

(c) Distichlis spicata

(f) Bare area

100
80
60
40
20

100
80
60
40
20

M»lll»l»»llll»ll>ll»llMlfi 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 N iTiTi n iTi 1 1 wi I i¥iwi 1 1 m m m\\\ iTi ^ 1 1 1 1 1 1 1 1 1 n 1 1 1 1 1 1 ii ii ii i n 1 1 1 ii ii i ii ii ii n 1 1 1 ii 1 1 ii i ii ii 1 1 ii i n
JDJDJDJDJDJ JDJDJDJDJDJ

96 97 98 99 00 96 97 98 99 00

Figure 6-7: Reference Station average total transect percent cover for the five dominant species and bare area on
each sampling date. Arrows indicate the dates of composite hydrographic events that caused measurable changes in the
vegetation. Error bars represent ±SE (n=65). J = June and D = December.

6-16 *♦* V^egetation Communities

100
80
60 j
40
20

(a) Batis maritima

V V V

100
80
60
40
20

(d) Monanthocloe littoralis

I I I I i I I I I I II I I I I I I I I I I I I I II II II I I I I I I I I I I II I I I I I I M I U I I I II II II I I I II I I I I I I IIMI III II I II I II II II II II III Ml II IMIII II III

i i ^

(b) Borrichia frutescens

100

80

o 60
O

0}

e 40

<u
a.

0)

W 20

Q)
>
<

(e) Salicornia bigelovii

100 1

*^**^~**-*^***-^^i-^«^

M I n I I I I I I I 1 I I I M I M I M I 1 I 1 I I I I I 1 I I I I I I 1 I I 1 I I I I M I

(c) Distichlis spicata

(f) Bare Area

100

80

60

40

100

80

60

40

20

1 1 1 1 1 1 1 1 II II 1 1 1 1 1 u 1 1 1 II II II III 1 1 1 1 M 1 1 1 II 1 1 1 1 1 II 1 1 1 HI 1 1 II III 1 1 1 1 1 1 1 1 1 II I M 1 1 III I

JDJDJDJDJDJ JDJDJDJDJDJ

96 97 98 99 00 96 97 98 99 00

Figure 6-8: Station II average total transect percent cover for the five dominant species and bare area on each
sampling date. Arrows indicate the dates of composite hydrographic events that caused measurable changes in the
vegetation. Error bars represent ±SE (n=65). J = June and D = December.

Chapter Six ♦ 6-17

100*

a) Batis maritima

80

1

1 1

60

40

A

20
0

1 1 1 1 1 1 1 1 1 1 1 1 M 1 1 [ 1 1 1 1 1 1 I [ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 i

100

I 80

(b) Borrichia frutescens

u

o

0.

0>

o) 40
CO

i_

0)

>

<

20

100
80
60
40

20
0

100
80
60
40
20

d) Monanthocloe littoralis

lit

^H"H~/^S^#-**"^-i-"*^--^-H

I I I I I I I I I I I I I I I I I I M I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

(e) Salicornia bigelovii

I II I II I I I I I I I I I I I I I I I I I I I II I I M III I I I II I I II I I II I I I I I II M

'iwiiTiTn^i if I

100
80
60
40

(c) Salicornia virginica

100
80
60
40
20

(f) Bare Area

JDJDJDJDJD JDJDJDJDJD

96 97 98 99 96 97 98 99

Figure 6-9: Station III average total transect percent cover for the five dominant species and bare area on each
sampling date. Arrows indicate the dates of composite hydrograplilc events that caused measurable changes in the
vegetation. Error bars represent ±SE {n=75). J = June and D = December.

6-18 *♦* Vegetation Communities

Station II

Batis maritima — Batis maritima percent cover at
Station II exhibited a different pattern than the other
two stations (Figure 6-8a). Cover of this species
peaked at 16% in November 1995 but then decreased
by 11% from November 1995 to May 1996. The
decrease began following three months (August 1995
through November 1995) during which about 42.8 cm
(17 inches) of rain fell. The drop in percent cover was
Kkely the result of direct precipitation, since similar but
smaller scale declines were seen at the other two
stations during this time. On November 7, 1995, all
three transects were flooded with 15.1 to 25.2 cm (6 to
10 inches) of water as a result of about 25.2 cm
(10 inches) of rain that fell in one day (October 26,
1995) approximately two weeks earlier.

Batis maritima cover increased 9.5% from May to
November 1996. During this rime, a positive net flow
of over 308 10' m' (250 acre-ft) passed dirough
Nueces Overflow Channel (Events 11 through 14).
However, the diverted water was derived from
primarily small exchanges, and the Rincon Overflow
Channel was not activated. Furthermore, the majority
of the flow was during late October, which was not
sufficient to account for significant changes in percent
cover. Therefore, the observed increase was more
likely correlated to precipitation which occurred during
the late summer (i?.^., > 22.7 cm (9 inches) fell during
August and September). Cover remained near
constant for close to a year, then declined by 9.5%
firom August 1997 to June 1998. The decline occurred
immediately following the July 1997 event. During the
period between the June and August sampling dates, a
total of 9.0 cm (3.55 inches) of rain fell and a positive
net flow of 1,902 10' m' (1,542 acre-ft) passed into
Rincon Bayou. During Event 16, Rincon Overflow
Channel was activated, and it was suspected that this
was also the case with Event 17 (Chapter 3).

Afterwards, cover leveled off and remained relatively
stable, until it began to gradually increase in December
1999 near the end of the study period. It should be
noted that the major hydrographic events in October
1998 and the precipitation direcdy falling on the station
during the spring and summer of 1999 (> 35 cm, or

14 inches) had no effect on cover. This was most
likely a result of cover already being relatively low. An
increase in cover began following the September 1999
event, which deposited 15 cm (6 inches) of rain in one
day and had a positive net inflow through the Nueces
Overflow Channel of 1,012 10' m' (820 acre-ft)
between the September and December sampling dates
in 1999. During this period. Event 36 was also
suspected to have activated the Rincon Overflow
Channel (Chapter 3).

Borrichia frutescens - A correlation between Borricbia
frutescens and Batis maritima was not seen at Station II
(r^ = 0.16). At Station II, B. frutescens cover declined
slighdy (5%) from the beginning of the study period to
the end (Figure 6-8b).

Distichlis spicata - At Station II, Distichlis spicata
cover decreased gradually from May 1996 (14%) to
June 1997 (< 1%) and remained relatively low (2 to
5%) for the remainder of the study period
(Figure 6-8c). Cover fell to almost 0% following the
October 1998 composite hydrographic event but
increased in cover from 0% to 10% from December
1998 to December 1999.

Monanthodoe littoralis - At Station II, Monanthocloe
littoralis cover remained relatively constant between
June 1995 and November 1996 (about 20%)
(Figure 6-8d). Cover declined sharply from November
1996 (17%) to February 1997 (6%). Cover increased
(12%) until December 1998 when it dropped
continuously to 3% in September 1999. The decrease
during the October 1998 event, and the lowest cover
recorded (September 1999) and coincided with the
flooding of the transect after a tidal surge which
occurred due to the passing of Hurricane Bret. This
decrease corresponded to decreases seen in M. littoralis
cover at the Reference Station. Cover then increased
by 5% in December 1999, after Event 36, which
activated the Rincon Overflow Channel and flooded
the transect with fresh water.

Salicornia bigelovii — The most significant changes
in percent cover were seen in the annual succulent
Salicornia bigelovii At Station II, the species was
basically non-existent until June 1998, when cover

Chapter Six ♦ 6-19

reached near 10% (Figure 6-8e). Cover decreased
during the fall mondis foUowing the plant's annual life
cycle (i.e., annual plants complete their life cycle within
a year). Cover increased dramatically from < 1% to
52% between January and June 1999. The increase in
S. bigelovii occurred the summer following the October
1998 composite hydrographic event [i.e., the period
between the October 1998 and June 1999 sampling
periods), during which a positive net flow of over
4,160 10' m' (3,372 acre-ft) entered Rincon Bayou.
The Rincon Overflow Channel was activated during
Event 25, consequendy flooding the station. The
increase in growth corresponded to a period where
11 out of 14 months received over 5 cm (2 inches) of
rain.

The late fall 1998 event significandy lowered open
water salinity values at Station II from about 45 ppt
(June 1998) to about 11 ppt Pecember 1998). Open
water salinity concentrations were also lowered by
precipitation in the fall 1997 (October), which kept
early winter salinity values below 15 ppt as well. The
only spring without Salicomia bigelovii ^o^^jth was 1997,
most likely because a fall event did not occur in 1996
and winter salinity values were relatively high (46 to
58 ppt) compared to the other years.

An increase in cover at Station II during June 1999
coincided with an increase at the Reference Station.
However, the increase at Station II was almost twice
that of the Reference Station. In June 1 998, the
percent cover of i". bigelovii ^^s almost identical at the
reference and Station II (Figures 6-7e and 6-8e). In
this instance, late winter salinity concentrations were
lowered by rainfall at both stations. Prior to June
1999, winter salinity values at Station II were lowered
as a function of freshwater flow through the channels,
possibly explaining the almost doubling in S. bigelovii
cover at Station II compared to the Reference Station
that summer and the increase compared to the
previous year.

A similar increase in S. bigelovii cover was seen in the
summer 2000. Cover in June 2000 at Station II was
41%, while cover at the Reference Station was only
12%. Once again, the differences are most likely a
result of the hydrographic event that activated the

Rincon Overflow Channel in September 1999
(Event 36). The difference in cover between the June
1999 and June 2000 sampling dates at Station II may
be indicativ^e of the tuning of the flow event. The
1999 event occurred in the late summer, while the 1998
event occurred during the fall and was followed by
several months of consistent precipitation.

Bare Area - At Station II, increases (> 2 %) in
Salicomia bigelovii cover corresponded to decreases in
bare area (r^ = 0.64) (Figures 6-8e and 6-8f). During
June 1999, S. bigelovii cover was about 50% and bare
area decreased about 33%, the largest decrease in bare
area obser\'ed during the study period. Bare area was
highest (81%) in September 1999, which was similar to
the peak found at the Reference Station (69%).

Station III

Batis maritima — Total transect percent cover of Baiif
maritima exhibited a similar pattern to that seen at the
Reference Station (Figure 6-9a). Cover cycled
seasonally, and the greatest increase in cover (9.7% to
37%) occurred between February and August 1997. In
general, percent cover at Station III varied between
10% and 37% and was greater than the other two
stations.

Borrichia frutescens - At Station III, percent total
percent cover oi. Borrichia frutescens vincA between 10%
and 25% (Figure 6-9b). No correlation was seen with
Batis maritima {f — 0.05). Total cover oi B. frutescens
declined gradually from February 1997 (25%) to near
11% in December 1999.

Salicomia virginica - Distichlis spicata was not a
dominant species at Station III, so Salicomia virginica
was analyzed instead because it occurred at Station III
(Figure 6-9c). However, this species was rarely found
at the other two stations. S. virginica cover was greatest
in February 1996 (23%) but decreased to 12% in May
1996 and continued to decline to 6.5% in August 1997.
The low cover occurred following the July 1 997 event.
Cover remained relatively low until October 1998,
when it began to gradually increase to 16% in
December 1999. The increase occurred after the

6-20

V^egetation Communities

October 1998 event and continued throughout 1999,
which had several months of consistent rainfall.

Monanthocloe littoralis - At Station III, Monanthodoe
littoralis cover remained relatively constant (13% to
21%) over the study period (Figure 6-9d). Cover
exhibited only minor decreases in cover in response to
the July 1997 event (7% decline) and a 4% decrease
after the October 1998 event.

Salicornia bigelovii - At Station III, Salicomia higelovii
was practically non-existent throughout the study
period, with only 1% cover in June 1998 and 3% cover
in June 1999 (Figure 6-9e).

Bare Area — Bare area cover at Station III gradually
increased 1 5% from the beginning to the end of the
study period, but changes did not appear to be
mediated by hydrographic events (Figure 6-9f).

large-scale whole transect
Analyses: Leaf Area Index

Leaf area index , a non-destructive means of estimating
total vegetation foliage density, exhibited considerable
temporal and spatial variability within and between
sampling transects. Measurements are reported as total
transect values. Average LAI values for the transects
exhibited seasonal peaks in the summer and declines in
the fall and winter (Figure 6-10) and displayed a similar
trend at all three stations. However, values at the three
stations differed in range and magnitude. At the
Reference Station, values range from 0.60 to 1.99 and
had an average of 1.42 compared to a range of 0.89 to
1.34 and an average of 1.13 at Station II. Values at
Station III were higher than the other two stations,
ranging from 1.64 to 2.43, with an average of 2.02. At
the three stations, peaks in LAI occurred in September
and November 1996, following a two-month period
with over 22.5 cm (9 inches) of rain. LAI then
declined in February 1997, which was a period with
litde rainfall. Values increased and peaked in January
1998, another period following a three-month period
of over 30 cm (12 inches) rain. The third major peak
shared by the three stations was in June 1999, which
followed a four-month period with over 30 cm rain.

This spring also followed the October 1998 event.
The greatest decline in LAI occurred in September
1999 and coincided with a large increase in bare area.
The decline occurred several weeks after a tidal surge
due to Hurricane Bret flooded the transects and
drowned much of the vegetation. In general, large
decreases in LAI coincided with increases in bare area.

Small-Scale Analyses

Percent Cover

Changes in the percent cover of individual species were
e\'ident during sampling periods before and after
composite hydrographic events (Table 6-5).

Spring Cover — Analysis of GIS maps created from
springtime percent cover at the Reference Station
indicated that bare area cover appeared to be greatest
following fall periods with little precipitation
independent of amount of spring precipitation
(Figures 6-1 lb and 6-1 le). Salicomia bigelovii increases
occurred when fall and spring precipitation were high
(Figure 6- lid), and Monanthocloe littoralis cover and bare
area decreased with increases in S. bigelovii. The maps
indicate that S. bigelovii invaded previously bare areas.

At Station II, maps indicate that bare area was greatest
following fall periods with no flow events and/ or littie
precipitation. In June 1997 (Figure 6- 12b), bare area
was greatest compared to other springs. Prior to this
spring, there was litde fall rain and no flow. May 1996
and June 1998, springs foOowing falls with heavy rain
but no flow event, had less bare area than June 1997
but more than either June 1999 or June 2000. In the
falls prior to June 1999 and 2000, there were
freshwater diversions and heavy precipitation.
Salicomia bigelovii cover was greatest in June 1999,
having greater than 50% cover over 59% of the
transect (Table 6-5). Cover was also high in June 2000,
with 49% of the transect having greater than 50%
cover. The maps show that S. bigelovii invasion
occurred in previously bare areas. The maps further
indicate that 'Qatis maritima cover decreased after
flooding events (Figures 6- 12c and 6-1 2d).

Chapter Six ♦ 6-21

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98

99

Figure 6-10: Total transect leaf area index (LAI) for each sampling date at each transect. Error bars
represent ± SE. J = June and D = December.

6-22 *♦* Vegetation Communities

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Chapter Six ♦ 6-23

(a) May 96

(b) June 97

(c) June 98

(d) June 99

tSRN Batis mantima
ggg;^ Monanthocloe littorahs
^3 Bomchia frutescens
i Salicornia bigelovn
Bare Area

Figure 6-1 1 : Reference Station percent cover maps for the five springtime sampling periods. Shaded areas for
each species represent parts of the transect covered by at least 50% of that species. Areas with no shading (white) were
not covered by greater than 50% of any one species. Springtime cover differed dramatically over the years. May 1996
represents cover following a fall with heavy precipitation (~36 cm) but a spring with little precipitation (-5 cm); June 1997
represents fall with little precipitation (~9 cm) but a spring with heavy precipitation (~43 cm); June 1998 represents fail with
heavy precipitation (-33 cm) but a spring with little precipitation (-5 cm); June 1999 represents fall with heavy precipitation
(~36 cm) and a spring with heavy precipitation (~18 cm); and June 2000 represents fall with little precipitation (--5 cm) and
a spring with little precipitation (~1 1 cm). Reference Station maps are shown for comparison because this site was not
affected by flow diverted by the demonstration project

6-24

Vegetation Communities

(a) May 96

(d) June 99

(e) June 2000

F^ Batis maritma
^^ Monanthocloe littorahs
^^ Bornchia frutescens
^la SalKornia bigelovii
^M Bare Area

Figure 6-12: Station II percent cover maps for the five springtime sampling periods. Shaded areas for each species
represent parts of the transect covered by at least 50% of that species. Areas with no shading (white) were not covered by
greater than 50% of any one species. Springtime cover differed dramatically over the years. May 1996 represents cover
following a fall with heavy precipitation and no flow, June 1997 represents fall with little rain and no flow, June 1998
represents mid-summer flow event and fall with heavy precipitation, June 1999 represents fall with flow event and heavy
rains and June 2000 represents late summer flow event and early fall heavy precipitation. Note decreases in bare area
and increase in Salicomia bigelovii cover in (d) and (e) following late summer and fall flow events.

Chapter Six ♦ 6-25

(a) May 96

(b) June 97

(c) June 98

(d) June 99

SSd Balis maritima
^^ Monanlhocloe littoralis
^S Bornchia frulescens
\99^ Salicornia virginica
^M Bare Area

Figure 6-13: Station III percent cover maps for the five springtime sampling periods. Shaded areas for each
species represent parts of the transect covered by at least 50% of that species. Areas with no shading (white) were not
covered by greater than 50% of any one species. Springtime cover differed dramatically over the years. May 1996
represents cover following a fall with heavy precipitation and no flow, June 1997 represents fall with little rain and no flow,
June 1998 represents mid-summer flow event and fall with heavy precipitation and June 1999 represents fall with flow
event and heavy rains. Note that Salicornia bigelovii cover is non-existent in the transect regardless of flow events.

6-26

}/egetation Communities

cover maps indicates that both Batis maritima and
Borrichia frutescens decreased in cover in June 1998 and
1999 compared to other springs, while Monanthocloe
littoralis, Salkomia virginica and bare area had increased.
The most evident vegetation changes appeared have
occurred in the front section of the transect, which was
lower and subjected to more influence from freshwater
diversions. The maps also indicate that M. littoralis was
commonly found in the back section of the transect, an
area that was rarely flooded. Furthermore, the greatest
bare area was found in the back part of the transect, an
area that remained relatively constant throughout the
spring periods.

July 1997 Composite Event - The July 1997 event
resulted in few changes at the Reference Station
(Figures 6-14a and 6-14d). Only 8.5 cm (3.4 inches) of
rain fell in the study area between the June and August
sampling period. The most obvious change was a
temporary increase in Batis maritima cover from 1.7% in
June to 12% in August (Table 6-5). The increase in B.
maritima corresponded to a 4-fold decrease in Borrichia
frutescens. However, B. maritima cover quickly dropped
back to 1.5% in November 1997, and B. frutescens
increased. Bare area also decreased slighdy by 5%
between June and August but increased quickly
afterwards.

At Station II, the vegetation was inundated by water
through the Rincon Overflow Channel and
precipitation during the event. Following the event,
Batis maritima cover increased 3.4% but decreased to its
pre-event cover in November 1997 (Figures 6- 15a
through 6-15c) (Table 6-5). Monanthocloe littoralis cover
increased by ~7% in August but returned to its pre-
flood values in November as well. The increase in
these species direcdy following the event corresponded
to an 11% decrease in bare area. Bare area also
increased quickly thereafter.

At Station III, Batis maritima cover increased 22% in
August following the event but dropped dramatically
by 34% from August to November (Table 6-5). The
increase in B. maritima occurred in the front section of
the transect, and corresponded to a decrease in
Borrichia frutescens and a reduction in bare area to almost
0% in that section of the transect (Figures 6- 16a and

6- 16b). B. maritima cover retreated in November, and
the B. frutescens zone near the back of the front section
of the transect began to recover (Figure 6-16c).

October 1998 Composite Event - At the Reference
Station, few changes were seen in the transect during
the two sampling periods following the event;
however, major changes were seen the following spring
(Figures 6-17a through 6-17d). GIS analyses indicate
that in June 1999, Salicomia higelovii expansion into
previously bare areas had occurred. The increase in
S. higelovii also corresponded with a decrease in
Monanthocloe littoralis and an increase in Batis maritima
cover.

During the October 1998 event, diversions through the
Rincon Overflow Channel were significant enough to
wash out the road crossing at the north end of the
channel, inundating Station II with freshwater. In the
sampling periods following the October 1998 event,
several significant changes in vegetation percent cover
occurred (Figures 6- 18a through 6-18d). Bare area
decreased gradually from 86% in October 1998 to 14%
in June 1999. The decrease in bare area began as an
increase in Monanthocloe littoralis cover. However, by the
following spring, Salicomia higelovii had occupied almost
all previously bare area and several parts of the transect
previously occupied by M. littoralis.

Few changes were seen in the transect at Station III
following the October 1998 event (Figure 6-19).

September 1999 Composite Event - At the

Reference Station, a major increase in bare area
occurred following flooding of the transect due to the
tidal surge of Hurricane Bret in August 1999
(Figures 6- 19a and 6- 19b). Afterwards, 42.5 cm
(17 inches) of rain fell between September and
December, leading to noticeable vegetation increases
and bare area decreases. Monanthocloe littoralis cover
quickly filled in almost half of the bare area by
December 1999 (Figure 6- 19c). Bare area continued to
decrease in June 2000 and both Batis maritima and
Salicomia higelovii covet increased (Figure 6-19d).

Chester Six ♦ 6-27

(a) June 97

(b) August 97

(c) November 97

(d) January 98

EiSJ Bal)s manl)ma
W^ Monanlhocloe litoralis

^^ Bornchia fnitescens
i-^^ Sahcornia virginica
^H Bare Area

Figure 6-14: Reference Station percent cover maps on the sampling date prior and three sampling dates following
the July 1997 composite hydrographic event. Shaded areas for each species represent parts of the transect covered
by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species.
Reference Station maps are shown for comparison because this site was not affected by flow diverted by the
demonstration project.

6-28

Vegetation Commumties

(a) June 97

(b) August 97

(c) November 97

tSiS Batis mantima
BS^ Monanthocloe littoralis
^g Bornchia frutescens
■■ Bare Area

Figure 6-15: Station II percent cover maps on the sampling date prior and three sampling dates following the
July 1997 composite hydrographic event. Shaded areas for each species represent parts of the transect covered by at
least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species.
During the event, the Rincon Overflow Channel was activated and Station II was inundated with freshwater. Note
increases in Batis maritima and Monanthocloe littoralis and decreases in bare area in August 1997 following the event.

Chapter Six ♦ 6-29

(a) June 97

(b) August 97

(c) November 97

(d) January 98

ESSlJj Balis maritima
Egg^ Monanlhocloe littoralis
^^ Bornchia frutescens
V//A Salicornia virginica
■■ Bare Area

Figure 6-16: Station III percent cover maps on the sampling date prior and three sampling dates following the
July 1997 composite hydrographic event. Shaded areas for each species represent parts of the transect covered by at
least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species. Note
increases in Safe maritima in August 1997 following the event. Increases in 6. maritima corresponded to decreases in
Bornchia fmtescens and decreases in bare area.

6-30

Vegetation Communities

(a) October 98

(b) December 98

(c) January 99

(d) June 99

I ~l Bornchia frutescens
^H Batis mantima
■■ Salicorma tigelovii
^M Monanthocloe littoralis
^M Bare Area

Figure 6-17: Reference Station percent cover maps on the sampling date prior and three sampling dates
following the October 1998 composite hydrographic event. Shaded areas for each species represent parts of the
transect covered by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of
any one species. Reference Station maps are shown for comparison because this site was not affected by flow diverted
by the demonstration project.

Chapter Six ♦ 6-31

(a) October 98

(c) January 99

(d) June 99

ISSI Batis mantima

Monanthocloe Moralis
Borrichia frulescens
Salicornia bigelovn
Bare Area

Figure 6-18: Station II percent cover maps on the sampling date prior and three sampling dates following the
October 1998 composite hydrographic event. Shaded areas for each species represent parts of the transect covered
by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species.
During the event, the Rincon Overflow Channel was activated and Station II was inundated with freshwater. Note the
increase in Salicornia bigelovii cover and decreases in bare area in the spring (June 1999) following the event.

6-32

Vegetation Communities

(a) October 98

(b) December 98

(d) June 99

(e) September 99

(f) December 99

^3 Batis marihma
ggg^ Monanthocloe lilloralis
|===| Bornchia fnitescens
I Salicornia virginica
Bare Area

Figure 6-19: Station III percent cover maps on the sampling date prior and three sampling dates following the
October 1998 composite hydrographic event. Shaded areas for each species represent parts of the transect covered
by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species.
Little change was observed at Station III during this event.

Chapter Six ♦ 6-33

(a) June 99

(b) September 99

(c) December 99

(d) June 2000

^a Bat)s mantima
^^ Monanthocloe littoralis
^M Bornchia frutescens
^^BBj Salicorma bigelovii
^B Bare Area

Figure 6-20: Reference Station percent cover maps on the sampling date prior and three sampling dates
following the September 1999 composite hydrographic event. Shaded areas for each species represent parts of the
transect covered by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of
any one species. Reference Station maps are shown for comparison because this site was not affected by flow diverted
by the demonstration project. Note the increase in bare area in September 1999 following the tidal surge of Hurhcane
Bret.

6-34

Vegetation Communities

(a) June 99

(b) September 99

(c) December 99

(d) June 2000

^3 Batis mantima
ES^ Monanlhocloe littoralis
^g Bornchia frutescens
^S Salicornia bigelovii
^B Bare Area

Figure 6-21: Station II percent cover maps on the sampling date prior and three sampling dates following the
September 1999 composite hydrographic event. Shaded areas for each species represent parts of the transect
covered by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one
species. During the event, the Rincon Overflow Channel was activated and Station II was inundated with freshwater.
Note the increase in bare area in September 1999 following the tidal surge of Hurricane Bret. This was not a freshwater
inundation effect.

Chewier Six ♦ 6-35

(a) January 99

(b) June 99

(d) December 99

Ea3 Balis maritma
^g Monanthocloe lilloralis
^^ Borne hia frutescens
K^d Salicornia virginica
^H Bare Area

Figure 6-22: Station III percent cover maps on the two sampling dates prior and two sampling dates following
the September 1999 composite hydrographic event. Shaded areas for each species represent parts of the transect
covered by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one
species.

6-36

Vegetation Communities

The tidal surge in August produced similar decreases in
vegetation cover at Station II (Figures 6-20a and
6-20b). The transect was then flooded by fresh water
through the Rincon Overflow Channel several days
later. Bare area had decreased by ~11% in December
1999, following the event. By June 2000, bare area had
decreased by —40% and Salicomia bigelovii had invaded
~50% of the transect in areas that were previously
bare.

In January 1999, the most significant change was the
appearance of Salicomia virginica in the back section of
the transect (Figure 6-21 c). However, the cover had
decreased to levels below the 50% mapping level in
June 1999. An increase in Batis maritima also occiured
in the spring following the event (Figure 6-21d). No
obvious vegetation changes occurred in the transect at
Station III following the tidal surge or the freshwater
flow through the channels during the September 1999
event.

Leaf Area Index

Vegetation maps showing the LAI distribution within
and between the transects indicate that overall fohage
density varied considerably. At the Reference Station,
LAI values were highest in January and June 1998 and
June and December 1999 (Table 6-6). The high
LAI values appear to coincide with high Monanthodoe
littoralis cover (Figures 6-14d and 6-23e). M. littoralis
also appears to be responsible for high LAI values in
June and October 1998 and December 1999. In June
1999, high LAI values corresponded to high Batis
maritima cover in addition to Monanthodoe cover.

At Station II, LAI values were consistendy lower than
the other stations (Table 6-6). However, increases in
values were seen in August 1997, following the
July 1997 event. The increased values occurred in the
same area of the transect as the increased Batis maritima
growth (Figures 6- 15b and 6-24c). High LAI values in
June 1998 coincide with Monanthodoe littoralis and
Borrichia frutescens cover (Figures 6- 12c and 6-24f). In
June 1999, the greatest LAI values occurred where
Salicomia bigelovii cover was high (Figures 6-1 2d and
6-24J). Low LAI values correspond to high bare areas.

LAI values at Station III were typically higher than the
other stations (Table 6-6). The highest values were in
the spring (Figure 6-25) and were lowest in areas with
little to no vegetation.

BlOMASS
Batis Maritima

Batis maritima biomass showed no obvious seasonal
trends or event-mediated responses (Table 6-7).
Biomass at all three stations ranged between 643 and
3,878 g/m' Lowest values occurred at the Reference
Station m May 1996 (1,016 g/nr) and February 1997
(1,201 g/m"^. The lowest value recorded at Station II
was also in May 1996 (643 g/m^. Low values occurred
on the same dates at Station III as well (1,180 and
805 g/m", respectively). Simultaneous occurrence of
the low values between stations does not appear related
to hydrographic conditions. At the Reference Station,
biomass values peak in June 1997 (3,208 g/m"^, and
gradually declined by more than half to 1,379 g/m' in
June 1999. B. maritima biomass did not exhibit the
same trend at the other two stations.

Borrichia Frutescens

Borrichia frutescens has the greatest range of biomass
values and the greatest total biomass of the four
species sampled (Table 6-7). Values between the
stations were within the range of 1,948 to 10,412 g/m^.
At all three stations, the highest values were seen at the
beginning of the study period. At the Reference
Station and Station II, high values of 8,053 and
8,575 g/m*, respectively, were measured on May 1996.
At Station III, the highest values measured were in
February 1996 (10,412 g/m") and 1997 (9,848 g/m").
Biomass gradually decreased over the study period for
all three stations.

Chapter Six ♦ 6-37

Table 6-6: Summary of LAI results from small scale GIS analyses. Values represent the percent of the transect
covered by each LAI range. Only results from sampling dates prior or following composite hydrographic events causing
measurable vegetation changes are included. May 96 data were included for comparison with other springtime values

LAI

Leaf Area Index Distribution (%)

Range

5/96

6/97

8/97

11/97

1/98

6/98

10/98

12/98

1/99

6/99

9/99

12/99

Reference Station

0-1

47.1

40.4

39.8

38.9

2.0

3.1

40.3

44.5

29.6

1.7

87.0

32.8

1-2

39.2

34.1

46.0

49.6

77.1

78.4

45.4

47.7

67.9

80.0

12.7

55.6

2-3

11.8

23.6

13.1

10.6

32.6

15.3

14.3

7.3

2.5

11.3

0.3

11.1

3-4

1.5

1.8

1.0

0.7

6.4

2.7

0

0.5

0

4.1

0

0.6

4-5

0.5

0.1

0.1

0.1

0

0.6

0

0

0

2.9

0

0

5-6

0

0

0

0

0

0

0

0

0

0

0

0

6-7

0

0

0

0

0

0

0

0

0

0

0

0

Station II

0-1

46.0

58.9

55.2

59.9

52.9

45.4

50.5

81.5

72.0

18.5

91.3

28.5

1-2

41.1

37.0

39.5

38.2

39.3

46.0

45.9

17.2

26.9

75.5

8.7

61.9

2-3

11.6

4.1

5.3

1.6

7.6

7.6

3.7

1.3

0.9

6.0

0

8.5

3-4

1.4

0

0

0.4

1.0

1.0

0

0

0.2

0

0

1.1

4-5

0

0

0

0

0

0

0

0

0

0

0

0

5-6

0

0

0

0

0

0

0

0

0

0

0

0

6-7

0

0

0

0

0

0

0

0

0

0

0

0

Station III

0-1

9.0

16.0

15.0

20.1

18.1

16.9

27.4

26.6

25.7

21.6

26.4

26.9

1-2

19.7

28.2

21.7

28.1

29.3

15.1

37.4

51.8

30.9

22.8

42.0

32.6

2-3

38.1

36.8

45.6

37.7

28.5

45.6

29.3

18.3

37.7

34.1

24.1

25.9

3-4

25.2

17.3

16.7

13.9

18.8

21.4

5.6

3.2

5.3

19.8

7.0

12.0

4-5

6.0

1.7

1.1

0.2

4.6

0.7

0.4

0.1

0.4

1.6

0.3

2.5

5-6

1.4

0

0

0

0.7

0.2

0

0

0

0

0.1

0.1

6-7

0.5

0

0

0

0

0

0

0

0

0

0

0

6-38

Vegetation Communities

.^

(a) May 96 ^V

#

(b) June 97

if^

(c) August 97 -'#

(d) November 97

^

I 10-1
f— ]1-2

■■2-3
■|3-4
^■4-5
^■5-6
^■B-7
I I No Data

\
(g) October 98 "p^

J\

^

(e) January 98 S^m

c^

~lr

^^

(h) December98

O

9

^

(0 June 98

r

(i) January 99

^

.r^

J

(i) June 99 ^"^

Ir

i>-

(k) September 99

(I) December 99

e-

JT

Figure 6-23: Reference Station leaf area index (LAI) from GIS analyses. Results from sampling dates prior to and
following the three composite hydrographic events that caused measurable changes in the vegetation are shown. May
1996 is shown for comparison with other springtime sampling dates. Reference Station maps are shown for
comparison, because this site was not affected by flow diverted by the demonstration project.

Chapter Six ♦ 6-39

(a) May 96

^

"C

<fi5

(b) June 97 ^

uk

(c) August 97

\r

(d) Novemb

3r97 r
0

(e) January 98

^

|0- 1

1 1 -2

0

§■2-3
m3-4
■■4-5
^■5-6
^■6-7
I I No Data

(g) October 98 o

0

x^

n.

(h) December98

^

(f) June 98

.>

x-

(0 January 99

O

o

^

<^"

(i) June 99

(k) September 99

\

(I) December 99

^

-^^nr!^

Figure 6-24: Station II leaf area index (LAI) from GIS analyses. Results from sampling dates prior to and following
the three composite hydrographic events that caused measurable changes in the vegetation are shown. May 1996 is
shown for comparison with other springtime sampling dates. Note increases in LAI in August 1997 following the July
1997 event and increases in June 1999 following the October 1998 event.

6-40 V Vegetation Communities

Monanthocloe Littoralis

<cwe Ltnoraits ^^

% ^ ^

(a) May 96 -^V^ (b) June 97 ''^^ (c) August 97

'^ ^ %

(d) November97

V % ^

^ ^Ty (e) January 98 '*'§*/ (f) June 98 ^^

^

I ^0- 1 -5-^

I 11-2

^3-4
^4-5
■15-6
^6-7
I I No Data

V ^" ■%

^^%>

"^^ ♦^^

-M*

> ^

(g) October 98 ^IR> (h) December 98 i^ (i) January 99 a,

V/ ^» "^^c

(j) June 99

'.^^ -ni^ ^1^

^■k (k) September 99 i^ (I) December 99 ^^

%, % %

Figure 6-25: Station III leaf area index (U^l) from GIS analyses. Results from sampling dates prior to and following
the three composite hydrographic events that caused measurable changes in the vegetation are shown. May 1996 is
shown for comparison with other springtime sampling dates. Note increases in LAI in June 1999 following the
October 1998 event.

Chapter Six ♦ 6-41

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6-42

Vegetation Communities

Monanthocloe Littoralis

Monanthodoe littoralis biomass values were between 883
and 3,706 g/m" for all three stations (Table 6-7). Low
values were seen at all three stations in February 1997
(< 1,772 g/m^. Biomass increased to near 3,000 g/m"
in June 1997 at the Reference Station and Station II
and declined to values between 1,885 and 2,213 g/m'
in June 1999. At Station III, values increased by
1,205 g/m^ from February 1997 to June 1998 and
decreased again by 1,653 g/m* to the lowest value
recorded (883 g/m^ in June 1999.

Distichlis Spicata

Distichlis spicata exhibited the lowest biomass range of
all four species sampled (314 to 3,557 g/m^
(Table 6-7). Values ranged between 1,047 and
2,199 g/m" at the Reference Station. At Station II,
values were similar to the Reference Station with the
exception of one high value recorded in January 1998
(3,557 g/m'^. Values at Station III were considerably
lower than the other two stations (314 to 666 g/m"^,
and on three sampling dates, coverage of D. spicata was
so sparse that monospecific stands could not be found
for sampling.

ROOT: Shoot Ratios

Batis Maritima

RS ratios of Batis maritima were lowest in May 1996 at
all three stations (0.16 to 0.22) (Table 6-8). The ratios
gradually increased until January 1998 at the Reference
Station (0.83) and Station III (1.72) and continued to
increase until June 1998 at Station II (1-27). Values
decreased gradually at the Reference Station and
Station II to values of 0.36 and 0.54, respectively. The
ratio peaked again in January 1999 at Station III but
then declined to a final recorded value of 0.61.
Twenty-two of the twenty-four (98%) ratios measured
were between 0.1 and 1.0, indicating that for most of
the demonstration period, the plant biomass above the

ground was proportionately larger that below the
ground.

Borrichia Frutescens

At the Reference Station, R:S ratios were greatest in the
summer months and lowest in the winter (Table 6-8).
This station also exhibited the greatest range in ratios
(0.2 to 0.9). At Station II, values peaked in June 1997
(0.83) and then gradually decreased by 64% to 0.30 at
the end of the study period. Values at Station III
showed litde variation, with the range in values being
limited to 0.2 to 0.4. The lowest values measured at all
three stations were in February 1997. All values
recorded were between 0.2 and 0.9.

Monanthocloe Littoralis

No apparent seasonal trend in the R;S ratios of
Monanthocloe littoralis could be seen at the three stations,
and ratios between stations were variable (Table 6-8).
At the Reference Station, values remain relatively
constant between 0.1 and 0.6. A gradual decrease in
ratios occurred from June 1998 to June 1999. A peak
ratio was measured in June 1997 at Station II (1.3), and
in June 1998 at Station III (1.1). The peaks did not
correspond to changes in freshwater input. Values for
all three stations ranged between 0.1 and 1.3.

Distichlis Spicata

R:S ratios at the Reference Station and Station II
exhibited similar patterns for Distichlis spicata, although
the range of values was greatest at Station II
(Table 6-8). Peak ratios were recorded in June 1997
and June 1999, foUowing periods of hea\'7 rainfall.
Lowest values were measured in June 1998 during a
period of Litde to no rainfall.

Chapter Six ♦ 6-43

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6-44

Vegetation Communities

DISCUSSION

Inorganic Nitrogen

Salinity

In general, open water salinity values appeared to be
influenced by precipitation and flow-mediated
freshwater inputs. Typically, salinity values were
decreased by freshwater inputs and increased by
drought periods. The greatest peaks in salinity
occurred during dry periods. The extent and duration
of the freshwater event determined the degree to which
the salinity fell, and the time span over which the
salinity remained low. For example, a summer event in
1997 reduced open water salinity levels by 40 ppt at the
Reference Station. However, the salinity had increased
by 30 ppt only two months following the event. The
magnitude of the increase was most likely due to the
timing of the event. The event occurred during the
summer, and, after the precipitation ceased, air
temperatures were high and resulted in rapid
evaporation. In contrast, the October 1998 event
reduced open water salinity values at the Reference
Station from 33 ppt in June 1998 to 11 ppt ia
December 1998. Salinity levels remained low
throughout the winter and spring 1999, most likely
because the event occurred during the cooler part of
the year, and because there was steady rainfall during
the beginning of the year.

Pore water salinity concentrations were almost always
higher than open water values. The periods when
values were similar were because the transects or part
of the transects were flooded with open water. This
trend was similar to that described by Hackney and
De la Cruz (1978) for a Mississippi tidal marsh. The
increased pore water values appear to be due
evaporation of water from the soil, as well as lack of
dilution from fresh water, as the highest pore water
values were seen during drought periods (e.^., 85 to
92 ppt in December 1999, which had 0.25 cm of rain).
Changes in pore water salinity concentrations were
usually reflected by changes in open water salimty
values.

Open water NH4'^ and NO^" + NO,' values showed
only small responses to freshwater inundation. A
problem with evaluating the impact of the project on
these values was that it must be assumed that
instantaneous nitrogen levels measured on different
sampling dates were representative of the same body of
water. However, in reality, the channel water was not
stagnant, and moved continually based on tidal
oscillations.

These problems were probably minor in the analysis of
vegetation responses as the nitrogen available for plant
uptake was found in the soils, which, unlike channel
water, did not move. The role of pore water nitrogen
in determining the biomass and productivity of salt
marsh vegetation has been investigated repeatedly. In
general, it has been demonstrated that nitrogen
fertilization of marsh plants has significandy increased
plant biomass, indicating nitrogen as a limiting nutrient
in estuaries. Several investigators have concluded that
nitrogen scarcity is responsible for stunting the growth
oi Spartina altemiflora (Broome et al. 1975; Gallagher
1975; VaHela and Teal 1974; VaUela et al. 1978).
Mendelssohn (1979) demonstrated that nitrogen
fertilization increased the aerial living standing crop of
the short form oi S. altemiflora from 49% to 172%.
De La Cruz et al. (1979) found significant increases in
the annual aboveground net primary productivity of
four tidal marsh communities seven months after
ammonium nitrate fertilization.

In this study, pore water NH4* levels increased
dramatically from the beginning to the end of the study
period. Most increases were seen after the October
1998 composite hydrographic event. The change does
not appear to be flow related, as similar increases were
measured at all three stations. However, the
concentrations were most likely too high to be purely
rainfall meditated. The change may have been a result
of cyanobacterial mats that formed on the soil surface
following the event. Many cyanobacteria species are
capable of taking atmospheric nitrogen and fixing it
into a biologically available form NH4*. These mats
remained on the soil surface for over a year (they were
at the stations through the December 1 999 sampling

ChapUrSix ♦ 6-45

date but were dried up by June 2000). In general, 1999
was a relatively wet year, and the moisture might have
attributed to the survival of the mats. The persistence
of the mats for an extended time could explain the
unusually high soil NH^^ levels. Unfortunately, the
high levels were not seen until the end of the study
period, and any subsequent positive effects on the
vegetation would hav^e taken several months to
mamfest.

Vegetation

The most significant vegetation changes seen as a
result of hydrographic events through the channels
were the germination and establishment of the annual
succulent Salicomia bigelovii. The relative success of this
plant appears to provide relevant information
regarding the timing and quantity of fresh water
needed to promote sexual (seed) coloni2ation in
hypersahne salt marshes. Because the species is an
armual and reproduces primarily by seeds, successful
establishment can only occur if soil salinity
concentrations are reduced to a level that alleviates the
osmotically induced seed dormancy.

In salt marsh habitats, the ability of halophytes to
reproduce sexually (seeds) is critical for the success of
plant populations (Ungar 1991 and Allison 1996).
While vegetative growth can expand plant cover in a
particular area, seeds are necessary for establishment at
locations distant from neighbors. Seeds also allow for
reestabUshment following disturbance events like
flooding, drought or burial that oftentimes result in
adult plant mortality. Quick recovery is only possible
through seeds, which can withstand the disturbance by
remaining dormant and then germinating afterwards.

In this study, the timing of soil sahnity reduction was
found to be critical. Hydrographic events which
lowered soil salinity values during the late fall/early
winter allowed the successful germination and
establishment oi Salicomia bigelovii during the spring and
summer, independent of whether or not the
intermediate seasons were wet or dry. The degree of
survival was most likely a fiinction of air temperatures
and evaporation rates, which were relatively low in the

winter compared to summer and did not increase as
rapidly. VCTien summer approached and soil salinity
increased, the seedlings were large enough to withstand
the increasing salinity concentrations. If an event
occurred in the late spring or summer, the seeds might
have germinated, but unless the soU salinity was kept
suppressed by sequential diluting events, the seedlings
would not sunave and establishment likely failed.

Flooding events through the Rincon Overflow
Channel occurring in the late fall significandy increased
the number of established plants seen the following
spring compared to the number seen after only a
precipitation event. In June 1999, Salicomia bigelovii
cover at Station II (52%) was 26% greater than that
seen at the Reference Station (26%), while cover in
June 1998 was approximately the same at the two
stations (11 to 13%). Heavy rains occurred during fall

1997 with no flow event, hence the approximately
equal cover at the two stations. While m fall 1998,
heavy rains and a flow event occurred through the
Rincon Overflow Channel, possibly leading to a
doubling in cover at Station II compared to the
Reference Station. The flooding of Station II was
reflected in the soil salinity levels, which were 6 to
20 ppt. These values are 1 to 11 ppt lower than the
Reference Station and Station III, respectively,
immediately following the flow event.

The lack of a significant difference between Salicomia
bigelovii covet at Station III between June 1998 and
June 1999 indicated that diverted fresh water in the
channels might not have affected more than just the
lower (adjacent) portions of the transect at Station III.
Most likely, during a major event such as the October

1998 event, most flow diverted through the Nueces
Overflow Channel backed up at the intersection of the
upper and central segments of Rincon Bayou by the
private road crossing, and a majority of this volume
likely passed through the Rincon Overflow Channel.
Some amount of fresh water reached Station III, as
open water salinity was 8 to 1 1 ppt lower than the
other two stations following the October 1998 event,
but this water likely remained channelized. However,
open water salinity values were 8 to 9 ppt and 5 to

6 ppt higher following the July 1997 and September

1999 events, respectively. The higher salinity values.

6-46

Vegetation Communities

even after major hydrographic events, may be
indicative of increased tidal influence, as this station
was closer to Nueces Bay. The smaller responses seen
in S. bigelovii cover at the Reference Station and Station
III compared to the response seen at Station II after
the 1998 event appear to be precipitation-mediated and
limited by a lack of freshwater inundation.

Small scale GIS analyses indicated that the
establishment of Salicomia bigelovii corresponded to a
decrease in bare area at the Reference Station and
Station II. At Station II, decreases in bare area were
proportional to increases in cover. LAI maps
produced from the small scale analyses further
supported the increase in vegetation at both the
Reference Station and Station II during spring 1998
and 1999. Weilhoefer (1998) and Dunton et al. (2000)
have previously dociimented establishment and
expansion of vegetation cover and subsequent decrease
in bare area after freshwater inundation in the lower
Nueces Delta. Decreases in bare area could have direct
affects on the functionality of the marsh.

Other studies have noted that halophyte species, which
can occupy bare areas following disturbance events
through sexual (seed) reproduction, can act to reduce
soil salinity concentrations by shading the soil surface
and by actively uptaking salts. The reduction in soil
salinity eventually allows the establishment of
dominant and persistent perennial species (Bertness
etal. 1992). The establishment of these species is
necessary as they pro\ade stable habitat for many for
many small organisms, including crabs, molluscs and
small terrestrial animals such as rats. The marsh
vegetation also serves as habitat and food for a variety
of permanent and migratory birds (Henley and
Rauschbauer 1981). Successful long-term
establishment provides large amounts of plant biomass
to the detrital food-web, which can then be utilized by
microorganisms and other small animals such as snails
(Marples 1966). This is an especially important role of
annual plants because all of their aimual biomass
production eventually becomes detritus. Marsh
vegetation contribution to the detrital food-web is
essential as it serves as a critical link between primary
and secondary trophic levels (Burkholder and
Burkholder 1956; Odum and Wilson 1962; Teal 1962).

Vegetative occupation of bare space is critical as the
vegetation acts to stabilize marsh sediments, thereby
sheltering the metropolitan area of Corpus Christi
from extensive flood damage and erosion. Without
vegetation expansion into bare space, the functionality
of the marsh would be compromised.

hatis maritima cover at the Reference Station and
Station III changed seasonally, with peak cover in the
summer and declines in the winter. This seasonal
trend appears to be due to natural variation and
corresponds to observations made by Weilhoefer
(1998) in the lower Nueces Delta. B. maritima is a
highly salt tolerant perennial succulent, which may
explain its high cover during the summer. The plant
expanded in cover during the spring and early summer
when soil salinity levels were usually low and was then
able to maintain a high cover during the summer
months, although active growth may not have occurred
during the hot, dry season. The greatest percent cover
observed at the three stations, however, was during the
summer of 1997, following a spring with over 52.1 cm
(20.7 inches) of precipitation. The rain was steady over
a five-month period and was not due to a major
hydrographic event. No other spring during the study
period experienced a similar rainfall pattern. Although
B. maritima is capable of surviving at elevated salinity
concentrations, increases in cover occur when gradual
freshwater inputs alleviate soil salinity levels.

After a major composite hydrographic event in
July 1997, percent cover of hatis maritima temporarily
increased in August but then declined at all three
stations. After the summer 1997 event, the following
fall, winter and spring were relatively dry, potentially
keeping cover low at all three stations during June
1998. Cover increased again in June 1999 at die
Reference Station and Station III; cover remained low
for over two years at Station II. The continual
suppression of B. maritima growth at Station II might
reflect this species intolerance to waterlogging and
anaerobic soils. Freshwater-mediated decreases have
been noted in a species with similar morpholog}',
Salicomia virginica (Zedler and Beare 1986; Weilhoefer
1998). Continual flooding occurred during the
fall 1998 and diroughout 1999. At Station II, the
vegetation began at the mean water line, so, after

Chapter Six ♦ 6-47

flooding, fresh water drained slowly. This effect was
magnified at Station II after an event activating the
Rincon Overflow Channel, as a larger amount of water
affected this station as compared to the other stations.
It is important to note that B. maritima cover began to
increase following the composite hydrographic event
during the summer of 1999, as the winter months were
dry and the waterlogged conditions potentially
alleviated. At Station III, the vegetation zone began
above the mean water line and so was not flooded as
easily by channel water. Furthermore, water from
precipitation did not produce standing puddles that
could waterlog the soils.

The decrease in cover oi Monanthocloe littoralis at the
Reference Station and Station II following the summer
1999 composite hydrographic event was most likely
due to flooding of the transects. During the time of
the floods, the transect soils were covered by thick,
green cyanobacterial mats. When the waters flooded
the transects, the mats were lifted from the soils; when
the waters retreated, the mats were left lying on top of
most of the vegetation. The thick mats most likely
caused decreases in plant cover by passively shading
the vegetation, making interception of light necessary
for photosynthesis almost impossible. Fortunately,
cover of both Batis maritima and M. littoralis began to
quickly recover after the event. In this instance,
although flooding initially served as a disturbance
resulting in decreases in adult plant cover, the
UkeHhood of long-term successful establishment might
not have been affected by the event. However, as
noted by AUison (1996), the recovery may depend
upon the presence of occasional freshwater inxmdation.

Biomass values and R:S ratios did not show obvious
freshwater mediated responses. Previous studies have

suggested that plants living in hypersaline soils invest
more energy into below-groimd tissues in order to
cover a larger area of soil to obtain water (BrugnoU and
Bjorkman 1992; Kuhn and Zedler 1997). However,
the physiological capacities of the plants that control
their salinity tolerance can vary with each species, and
some species may allocate different amounts of
biomass to the below-ground tissues depending on
their individual ability. Additionally, waterlogging of
soils may cause below-ground material to die in some
plants and not in others, thereby decreasing the
R:S ratio. The analyses were further complicated as a
need for increased nutrient uptake might also have
resulted in the plants increasing their below- ground
tissues, especially when increases in nutrients occurred
after precipitation or flooding events.

SUMMARY

The increase of fresh water into the channels and tidal
flats of the upper Nueces Delta positively impacted the
emergent vegetation by decreasing salinity, allowing for
annual seed reproduction, decreasing bare area and
increasing vegetative expansion of plant cover.
Although similar responses were seen at the Reference
Station following heavy precipitation, the effects were
accentuated at Station II, which was direcdy impacted
by flow through the Rincon Overflow Channel. The
project demonstrated the sensitivity of vegetation to
salinity levels and indicated that periodic freshwater
inundation could alleviate the stressful condition
imposed by hypersalinity. Without fresh water
flooding, soil salinity concentrations at several
locations in the study area would have likely increased
to toxic levels and resulted in plant mortality.

6-48

Vegetation Communities

CHAPTER SEVEN

Synthesis and
Conclusions

"Then what of this river that having arisen
Must find where to pour itself into and
empty?"

♦♦♦ Robert Frost, Too Anxious for Rivers

The Nueces Delta is one of the most extensive marshes
on the Texas Gulf Coast. It is an integral component
of the Nueces Estuary, providing economically and
ecologically valuable habitat and food for many
estuarine and marine plant and animal species. In
1997, the worldwide average economic value of
17 ecological services provide by an estuary was
$9,000 per acre per year (Costanza e( a/. 1997). In
Texas, the total of only two recognized estuary
functions (commercial and recreational value of
fisheries) was about $5 billion dollars for the one
million acres of estuarine area in the State (Robinson
et al. 1995), or about $5,000 per acre per year.

The flora and fauna of the Nueces Delta depend upon
periodic freshwater inundation events to maintain their
ecological functions. However, over the past century,
increases in the human population in the Coastal Bend
region of Texas has intensified the demand for fresh
water to meet agricultural, municipal and industrial
needs. From the combined effects of reservoir
construction, changes in land use patterns, increased
ground water withdrawals and other human activities,
the average annual volume of fresh water diverted into
the upper Nueces Delta since 1982 has been reduced
by over 99% from that before 1958 (Irlbeck and Ward
2000). Over time, this decrease in freshwater inflow
has created a non-functioning estuarine ecosystem in
the Nueces Delta. The natural freshwater deficit
imposed by evaporation has been magnified by
decreased riverine inflow, resulting in hypersaline
channel waters and soils. As a result, a "reverse
estuary" condition has developed where the lowest
salinitjr values are near Nueces Bay and the highest are
in the upper delta. While many estuarine species
tolerate this hypersaUne environment, prolonged

Chapter Seeen ^ 7-1

periods of salinity stress have limited active growth and
reproduction in the Nueces Delta, leading to lower
biological productivity and less species diversity.

This demonstration project was designed to increase
the opportunity for freshwater flow events &om the
Nueces River into the upper Nueces Delta assuming
that resultant hydrographic changes would beneficially
affect the ecology of the upper delta. Reclamation
initiated the demonstration project in 1993 with the
excavation of two overflow channels and then
monitored the changes in hydrography, the water
column, benthic communities and vegetation
communities.

CHANGES IN HYDROGRAPHY

During the 50-month period when the overflow
channels were open, over 8,810 10' m' (7,142 acre-ft)
of water was diverted from the Nueces River into
Rincon Bayou. Of this total amount, only about
1,204 10' m' (976 acre-ft) wovdd have been diverted
without the demonstration project features. Therefore,
the total volume of freshwater inflow to the upper
Nueces Delta was increased by about 732% from what
would have occxirred without the project.

In addition to increased inflow, the demonstration
project features also increased the distribution of firesh
water within the tidal flats of the upper delta. On five
different occasions during the demonstration period,
the tidal flats of the upper marsh were supplied with
diverted fresh water. Without the project features, no
natural inflow event would have direcdy freshened
these areas.

These changes occurred because the demonstration
project features lowered the minimum flooding
threshold for the Nueces River firom 1.64 m (5.4 ft msl)
to about 0 m mean sea level. This change not only
allowed more frequent river diversions into Rincon
Bayou and the upper delta, but also provided the
opportunity for other non-riverine elements Uke wind
and tide to force frequent exchanges between the upper
delta and the Nueces River. As a result, near continual
(daily) exchange between the river and delta was

observed during the demonstration period. Before the
project, such interactions were limited to only
extremely infrequent river inflow events.

EFFECTS ON SALINITY

The demonstration project gready lowered short-term
salinity concentrations in the upper and central
segments of Rincon Bayou, as indicated by results of
the hydrographic analysis (Chapter 3) and subsequent
biological response analyses (Chapters 4 through 6).
The demonstration project also affected the overall {i.e.,
long-term) salinity gradient of Rincon Bayou.
Unpublished long-term saUnity data from selected
stations in the Nueces Delta and Nueces Bay were
obtained from Dr. Terry VCTiidedge (University of
Alaska Fairbanks) and Dr. Paul Montagna (University
of Texas Marine Science Institute) (Figure 7-1).).
These data covered the period of January 1992 through
December 1999 and were made available from projects
sponsored by the Texas Water Development Board,
City of Corpus Christi, Corpus Christi Bay National
Estuary Program and Texas Sea Grant Program. This
8-year interval was divided into two nearly equal
periods before and after the Nueces Overflow Channel
was completed (October 26, 1995). The before-project
period was about 3.8 years long, and the after-project
period was about 4.2 years long.

The average salinity at each selected station was
calculated for each month. Monthly averages at each
station were then averaged for each period before and
after the demonstration project to smooth the year-to-
year variation and compare means. These values were
then plotted along their respective channel segment
lengths, beginning at where the Interstate Highway 37
(IH 37) bridge crosses the Nueces River to a point in
Nueces Bay just west of White Point (Figure 7-2).

In the upper and central segments of Rincon Bayou,
the average salinity gradient changed dramatically after
the opening of the Nueces Overflow Channel. During
the period before the demonstration project, there was
no regular interaction with the Nueces River and
salinity concentrations in the delta were highest in
upper Rincon Bayou (Figure 7-2a). This condition may

7-2 ^ Synthesis and Conclusions

DEMONSTRATION PROJECT
STUDY AREA

Approximate Scale
Kilometers

^^"■^^

Figure 7-1 : Selected stations used for analysis of long-tetTn salinity changes in Rincon Bayou.

be termed a "reverse estuary," because the gradient was
opposite what would be expected from a natural
(undisturbed) system in which salinity concentrations
would be lowest in the upper delta {e.g., at Station 66).
For the period after the demonstration project, the
salinity gradient reverted to a more natural pattern for
much of it's reach, particularly within the upper and
middle Rincon Bayou segments (or the first 5 km from
the IH 37 bridge) (Figure 7-2b). This change was due
to increased diversions of fresh water through the
demonstration project features. Without the
demonstration project, average salinity concentrations
in upper Rincon channel during the second period
would have remained strongly hypersaline (likely
greater than 50 parts per thousand (ppt) instead of the
observed range of 21 to 28 ppt.

As expected, the demonstration project had no obvious
effect on the salinity pattern of the Nueces River, nor
was it designed to (the project was found to only divert
approximately 2% of total river flow during a given

hydrographic event (Ward 2000)). During both
periods, salinity concentrations in the river channel
increased with distance downstream as riverine fresh
water (e.g., at Station 68) mixed with salt water from
Nueces Bay (e.g., at Station 7) (Figure 7-2).

From comparison of the actual average salinity values
in the river and bayou between both periods (i.e..
Figures 7-2a and 7-2b), it is apparent that salinity
concentrations were generally higher during the after-
project period than before the project. For example, in
the Nueces River, this difference was about 3 parts-
per-thousand (ppt) in the middle of the river
(Station 4b) and about 8 ppt in the open bay
(Station 7). This observation does not indicate that the
demonstration project increased average salinity
concentrations in the river but more likely is
attributable to a greater freshwater influences on the
river from both river flow and precipitation during the
period before the project than after (Figure 7-3).

Chapter Seven ^ 7-3

(a) Before the demonstration project (1992 through 1995)

60

50

Q.
Q.

40

CD
CO

Q)
O)
TO

i_
(U
>
<

30

20

10

RR

Rincon Bayou

Nueces River

1

. '

%

5.^

RR

' r

^S4-

^ \

\

1 Nueces Bay

IH'^7

"^1

cr\ -1

^
^

Brie

Ige
RR

RR

V

6

8

48--'"

1

.^AT-

1 Nueces Bay

1 1 1

6 8 10 12 14

Distance in channel (km)

16

18

20

22

(b) After the demonstration project (1996 through 1999)

60

50

S 40
>.
g
TO 30

0)
O)
TO

\ 20

10 -

Rincon Bayou

Nueces River

1

RR

\\

.^

/\

\

Nuec

as Bay

7^

V

\

Nil

1

IS ■:

a.4^

^

/

IH3"
Bridg

RR /

;7

RR

J

AN'"

^, — '

A

ueces Ba

y

6

1

48—^

» ^

1

6 8 10 12 14

Distance in channel (km)

16 18 20

22

Figure 7-2: Long-term average salinity values for selected stations in the Nueces River and Rincon Bayou before
(a) and after (b) implementation of the demonstration project. Station numbers coincide with those in Figure 7-1 .
Salinity values for the upper-most reach of Rincon Bayou (near Station 67) during the before-project period (a) were
unavailable but were estimated by extrapolation to be approximately 45 ppt. RR indicates the point at which each channel
is crossed by a railroad.

7-4 V Synthesis and Conclusions

(a) Total annual flow
in the Nueces River
(at Calallen gauge)

600000

(b) Total annual precipitation
(at Corpus Christi International
Airport)

120

E

*«—

c

CD

o

100
80 i
60
40
20
0 J-ls,

(c) Total annual flow
into Rincon Bayou
(at Rincon gauge)

7000

i^ 6000

^ 5000
o

-™. 4000 -

O 3000

TO 2000

° 1000
0

^E^

XI

a

1992 1994 1996 1998

1992 1994 1996 1998

T T

1992 1994 1996 1998

Bef ore-project annual mean
After-project annual mean

Figure 7-3: Total annual cumulations of selected freshwater sources affecting the Nueces Delta during the period
1992 through 1999. Flow in Rincon Bayou for the before-project period was estimated using dally stage data from the
Calallen gauge and the methodology described by Iribeck and Ward (2000). Total flow into Rincon Bayou for the after-
project interval of October 26, 1995, through May 15, 1996, was unavailable, but was estimated to be approximately
123 10^ m^ (100 acre-ft). The average annual flow In the Nueces River for the first period was 224,581 10' m'
(182,053 acre-ft) but only 127,678 10' m' (103,500 acre-ft) for the second. The average annual precipitation for the before-
project period was 94.2 cm (37.1 in) but only 74.7 cm (29.4 In) for the after-project period. Finally, the annual mean
freshwater flow into Rincon Bayou for the first period (1 ,931 10' m', or 1 ,565 acre-ft) was nearly equal to that for the period
after (1 ,854 10' m', or 1 ,503 acre-ft).

Note; 1 acre-fl = 1.2336 10' m'

In summary, the effects of the demonstration project
on the long-term salinity gradient in Rincon Bayou
were measurably significant. In a relatively short period
of time (only 4.2 years after the opening of the Nueces
Overflow Channel), the "reverse estuary" salinity
gradient in the upper delta before the demonstration
project reverted to a more natural form, with average
salinity concentrations in upper Rincon Bayou
becoming the lowest in Nueces Delta.

BIOLOGICAL RESPONSES

The influence of fresh water on the salinity
concentrations of the water and soils of the upper
Nueces Delta appeared to be the most important
parameter affecting the biological response of estuary
organisms in the delta.

Water Column Productivity

The inflow of river water into the upper Nueces Delta
imported vital nutrients (nitrogen, phosphorus and
silicon) required for plant growth into the channels and
ponds of the marsh ecosystem. During the
demonstration period, phytoplankton in the water
column and on the surface of the sediments rapidly
responded to the inputs of riverine nutrients with
increased growth rates and accumulation of biomass.
In addition, the freshwater inflow also reduced salinity
concentrations and lowered the osmotic stress on these
organisms. The increased primary production rates
were especially prominent during periods when salinity
was less than 50 ppt. The assimilation index {i.e., the
relative amount of growth per cell) was also generally
higher during periods of low salinity, indicating that
inherent growth rates were also increased by project
diversions.

The species composition of phytoplankton apparently
remained dominated by primarily small diatoms during

Chapter Seven ^ 7-5

most of the monitoong period. However, several
observations of blooms of other phytoplankton were
noted immediately after freshwater inflow events.
These blooms were typically comprised of single celled
blue-green algae (not of the filamentous cyanobacteria
of algal mats) normally present in fresh water or very
low saUnity environments. These blooms were also
short-lived, persisting no more than a few days. The
presence of these blooms did not occur in the study
area prior to the demonstration project except xmder
natural freshening events that occurred every several
years. The more firequent presence of these blooms in
the upper and central Rincon Bayou were an indication
that the water column ecosystem was showing a more
typical response to freshwater inflow.

An additional indication of improved conditions for
phytoplankton growth during and after freshwater
inflow events was shown by an increase in the
N;P ratio (dissolved inorganic nitrogen to phosphate
ratio), which would provide a relative increase of
nitrogen needed for plant growth. These increases
following inflow events indicate that relatively smaller
amounts of denitrification probably occurred during
and following the inflow events.

Benthic Communities

Benthos consists of two main types of infauna, which
have different ecological roles in marine ecosystems
and respond to inflow at different spatial and temporal
scales. These are the larger macrofauna (organisms
greater than 0.5 mm in length) and smaller meio fauna
(between 0.063 and 0.05 mm in length). Macrofauna
have planktonic larval dispersal and indicate effects of
the demonstration project over larger spatial scales and
longer temporal scales. Meiofauna have direct benthic
development and generation times as short as one
month, thus indicate effects over smaller spatial scales
and shorter temporal scales.

Diverted fresh water lowered salinity concentrations
which triggered bursts of benthic iavertebrate
productivity as indicated by increases in density and
biomass. Salinity levels between 20 and 45 ppt
appeared to correspond with high macrofauna biomass

(> 2 g/m^. Macrofaunal abundance also increased
with increasing biomass. Meiofaunal biomass and
abundance increased when salinity values were between
10 and 40 ppt, with the greatest numbers being seen in
the salinity range of 1 8 to 22 ppt. Biodiversity
increased several months (3 to 6) following inflow
events, indicating more species were able to utilize the
marsh habitat. The macrofauna and meiofauna
responded to inflow with similar patterns, indicating
both trophic levels of benthos were responding
positively to inflow events. Most importantiy, the
lowest biomass and abundance values occurred during
times when there were no flow events. Additionally,
strong seasonal increases of biomass occurred during
the spring, when salinity concentrations were lowest.
In contrast, during summer when salinity values were
highest, density and biomass were always lowest.

Vegetation Communities

The most significant changes in the emergent
vegetation also coincided with changing salinity regimes
caused by flow through the demonstration features or
by direct precipitation. During sampling periods with
no hydrographic events, soil salinity levels were as high
as 80 to 90 ppt, and water column salinity was upwards
of 40 to 60 ppt. Following a large hydrographic event,
surface and soil salinity concentrations were reduced,
with the degree and duration of salinity suppression
being a function of the timing and duration of the
hydrographic event. Major hydrographic events
lowered open water salinity values by over 40 ppt at
some stations, and smaller, precipitation-mediated
events exhibited smaller decreases.

Hydrographic events that alleviated soil salinity levels
during the late faU/early winter allowed the successfiil
germination and establishment of Salicomia higelovii
during the spring and summer, independent of whether
or not the seasons were wet or dry in regards to
precipitation. Because this plant species is an annual
succulent that reproduces primarily by seeds, successfiil
establishment seemed to occur only if soil salinity
concentrations were reduced at the appropriate time of
year to a level that covdd alleviate osmoticaUy induced
seed dormancy.

7-6 V Synthesis and Conclusions

Most importantly, the establishment of Salicomia higelovii
corresponded to a decrease in bare area at all three
vegetation sampling stations, with the decreases in bare
area being proportional to the increase in S. higelovii
cover. Decreases in bare area coxild have direct affects
on the functionality of the marsh. Other studies have
noted that halophyte species, which can occupy bare
areas following disturbance events through sexual
(seed) reproduction, can act to reduce soil salinity
concentrations by shading the soil surface and by
actively up-taking salts. The reduction in soil salinity
can eventually allow the establishment of dominant and
persistent perennial species. These plants can then
provide stable habitat and food for many small
organisms and for a variety of permanent and
migratory birds, as well as provide large amounts of
plant biomass to the detrital food-web, which boosts
productivity of higher trophic levels. Without the
expansion of vegetation into bare space, the
functionality of that marsh area would be
compromised.

The response of Salicomia higelovii also differed after
freshwater inflow events when compared to those that
were mainly precipitation-mediated. In the late fall
1998, a major flow event flooded the Rincon Overflow
Channel and significandy increased the number of
plants that emerged the following spring compared to
the number seen after a precipitation only event. In
June 1999, total Salicomia higelovii cover at Station II was
52%, which was 26% greater than that seen at the
Reference Station. In June 1998, prior to the October
1998 composite hydrographic event {i.e.. Events 21
through 27), percent cover at the two stations was
approximately equal (11% and 13%). The similar
amount of cover during the spring 1998 could be
explained by heavy rains (> 23 cm, or 9 inches) which
occurred during fall of 1997, presumably affecting both
stations the same amount, although Station II was also
affected by fresh water diverted through the Rincon
Overflow Channel.

Peaks in the percent cover of the perennial succulent
^atis maritima occurred after gradual precipitation
inputs, but decreased after major hydrographic events
which flooded the station. The decline was most likely
due to the species' inability to tolerate waterlogged and

anaerobic soils for extended periods. Increases in cover
were noted several months following a major event if
the soils had time to dry {i.e., no other flooding events
occurred).

In summary, freshwater inputs via precipitation or
project diversions reduced salinity concentrations in the
upper Nueces Delta, and vegetation cover increased as
a result. Although large flooding events initially served
as a disturbance resulting in decreased adult plant
cover, it appears that the likelihood of long-term
successful establishment might be enhanced by the
increased opportunity of freshwater inflow.

INTEGRATION OF PROJECT
EFFECTS

The overall effects of the demonstration project on the
ecology of Rincon Bayou and the upper Nueces Delta
were positive to the environment (Table 7-1), and were
attributable to the re-introduction of fresh water. Prior
to the demonstration project, Rincon Bayou was not
direcdy connected with the river {i.e., it was a dead-end
channel). Average salinity concentrations were
consistentiy higher in the upper delta than in Nueces
Bay. These conditions were alleviated in the upper and
central segments of Rincon Bayou by increased
freshwater flow through the Nueces Overflow
Channel.

The increased opportunity for freshwater inflow also
changed nutrient cycling and primary productivity in
the upper delta. Generally, nitrogen occurs in the water
column in three main forms: ammonia, nitrate and
nitrite. Without inflow, the dominant form of nitrogen
in the upper delta was ammonia, which was likely
derived from the recycling of (decaying) organic matter.
Also, water levels were generally low, as shallow as only
a few centimeters, and the substrate was covered by
mats of filamentous blue-green algae {i.e.,
cyanobacteria). Although productivity (per unit
volume) from these mats was high, the total volume of
production was low because the total amount of water
was small. Furthermore, cyanobactena are not food

Chapter Seven ^ 1-1

Table 7-1 : Summary of the effects of the demonstration project on the upper Nueces Delta.

ATTRIBUTE

Before

After

Geomorphology
Salinity gradient
Nutrient cycling

Dead-end Flow-through with free exchange

Higher in the upper delta than in the bay Lower in the upper delta than in the bay
Recycled nitrogen New and recycled nitrogen

Primary production Low In marsh

Secondary production Constrained by dry conditions

Habitat utilization Constrained by dry conditions

Higher in marsh
Increased by flow events
Increased during spring and fall

sources for higher trophic levels, indicating that
cyanobacteria production was a sink, not a link, in the
food chain.

With the re-introduction of fresh water, more oxidized
forms of nitrogen were introduced into the delta.
Because nitrate and nitrite are the preferred forms of
nitrogen by diatoms, these organisms contributed a
substantial proportion of the productivity. Flow events
resulted in moderate levels of primary production (per
unit volume), but the total amount of fixed carbon was
very high because the large volumes of water involved
when elevations approach a meter or so in depth.
Because diatoms were significandy contributing to the
total productivity, there was a link with higher trophic
levels, and the grazing food chain was stimulated.
Increased freshwater inflow lowered salinity
concentrations in the soils and waters of the marsh,
stimulating plant production as well. Marsh
productivity ultimately drives the detrital food chain
when the plant material dies and decomposes.

The increased primary production, in both the water
column and marsh, resulted in increased secondary
productivity in several ways. Grazing organisms could
direcdy utilize the benthic and plankton diatom
production. Increased carbon fixation led to higher
amounts of detritus and higher amounts of food
available to detritivores. A multiplier effect was present
because increased marsh vegetation also increased
habitat quality and complexity. Marsh areas are
important because they provide nursery habitats and
for many commercially and recreationally important
species, {e.g., shrimp, red fish, and sea trout). Without

freshwater flow and concomitant increases in the
marsh system, the habitat does not support these
estuarine-dependent species. Consequendy, the
combination of increased food and increased habitat
quality and area likely resulted in geometric increases in
secondary production.

A CONCEPTUAL MODEL

The data collected during the present study allowed the
creation of the conceptual model presented in
Figure 7-4 on how Rincon Bayou fimctions, and,
perhaps more importandy, how the upper delta
functions with and without freshwater inflow. Rincon
Bayou's connection with Nueces Bay allows the
exchange of materials and energy flow between the two
bodies of water. In addition, nekton (fish and
epibenthic shrimp) utilize Rincon Bayou as a nursery
habitat when water elevations and salinity conditions
are suitable.

The water levels and salinity concentrations in Rincon
Bayou are governed by the interactions between tide,
rain, evaporation and freshwater inflow (Figure 7-4).
These outside forcing elements also drive nutrient
concentrations, primarily through tides and inflow.
Nutrient concentrations are also governed by
biogeochemical processes associated with
decomposition of marsh grass, excretion by organisms
and recycling of dead organic matter. The nutrients,
with sunlight, drive primary production in the water
column (phytoplankton), which in turn drives a grazing

7-8 ^ Synthesis and Conclusions

Figure 7-4: Conceptual model of the Nueces Delta ecosystem. Parts of the Nueces Estuary are bounded by shaded
rectangles. Circles are forcing functions from outside the system. Rounded rectangles represent standing stocks or
energy storage. Arrows represent flow rates of energy and materials. The diamond represents on/off switch. Pentagons
represent multipliers.

food chain. Some benthic invertebrates then utilize
this marsh and water column detritus, contributing
themselves to the detrital food chain. Marsh
production is a function of sunlight, nutrients and soil
salinity. The soil salinity is affected by water column
salinity and elevation. All benthos are strongly affected
by salinity because of physiological tolerance to a
specific sahnity range, or by using salinity gradients as
cues for migration and reproduction. Of these
elements, fresh water is the most critical for
maintaining ecological functions in the upper Nueces
Delta. In natural systems, inflow acts as a continuous
forcing function. In this conceptual model, a diversion

project acts as a "switch" that, when activated, restores
some freshwater inflow to the system.

This conceptual model demonstrates an improved
understanding of the mechanisms controlling
production in the upper Nueces Delta. These
mechanisms indicate the importance of the
demonstration project in restoring fiinctionality to the
marsh-dominated ecosystem. Without fresh water
from river diversions, salinity concentrations increase,
average water levels decrease, and production of
primary and secondary producers declines. A loss of
fiinctionality in the Nueces Delta affects Nueces Bay

Chapter Seven ^ 7-9

because of the reduced transport of exported materials
and the loss of habitat needed by migratory and
nursery-dependent species that utili2e the delta.
Therefore, without freshwater inflow, the delta is not a
functioning component of the greater Nueces Estuary
ecosystem.

SUMMARY

Since 1982, the average annual amount of freshwater
inflow to the upper delta has decreased by over 99%
compared to the period before 1958 (Irlbeck and Ward
2000). This dramatic change indicates the large degree
to which human activity has altered the delta ecosystem
in a relatively short period of time {i.e., less than a
quarter century). Nevertheless, the demonstration
project successfully increased the amount of fresh
water diverted into the upper Nueces Delta by six or
seven times that which would have occurred without

the project. Although this amount of restored inflow
was relatively small (only about 2% of the annual
average before 1958) when compared to historical
volumes, it returned a significant degree of ecological
function to the Nueces Delta and Nueces Estuary
ecosystems. Prior to the demonstration project,
persistendy high salinity concentrations severely
inhibited the function of the delta, and its natural
contribution to the greater estuary ecosystem was
limited to infrequent periods when natural flow events
occurred. With the restored regular interaction
between the river and Rincon Bayou, fresh water and
nutrients were more consistendy introduced into the
upper delta, stimulating critical chemical and biological
processes. As a result, habitat in the delta component
of the Nueces Estuary improved in both quality and
quantity, and foraging opportunities for many estuarine
species were increased.

7-10 ♦♦♦ Synthesis and Conclusions

CHAPTER EIGHT

Future
Opportunities

"In nature there are neither rewards nor
punishments - there are consequences."

♦♦♦ R.G. Ingersoll

The Rincon Bayou Demonstration Project was a short-
term initiative undertaken to answer two fundamental
questions about the Nueces Delta ecosystem. First,
"Was there an opportunity to meaningfully increase the
amount of fresh water diverted into the upper delta?",
and second, "Would the biological resources in the
delta measurably respond to increased fresh water in a
favorable way?" The demonstration project answered
the first question with a definitive "Yes." Data indicate
that project features increased the total amoiont of
diverted fresh water by 732% during the demonstration
period (Chapter 3). In the long-term, had the
demonstration project features been in place since
1982, the average annual amount of freshwater
diversion into the upper delta would have been
increased by 633% (Irlbeck and Ward 2000). Project
results also answered the second question with an
equally definitive "Yes". Because of the freshwater
diversions made by the demonstration project, the
"reverse esmary" salinity gradient that had previously
characterized Rincon Bayou and the upper delta
reverted to a more natural pattern (Chapter 7). Primary
productivity in the water column was stimulated
(Chapter 4), and the abundance, diversity and
distribution of both benthic and emergent vegetation
communities increased as a result of additional fresh
water (Chapters 5 and 6). In summary, the
demonstration project was more successful than had
been anticipated, both in the amount of fresh water
diverted and in the biological responses observed.

Because of limitations in program authority and
landowner agreements, the Rincon Bayou
Demonstration Project was conducted as only an
investigation of the potential for restoration of natural
estuary function. Based upon project results, actual

Chapter Eight ♦ 8-1

function can be restored. The next step in restoring
freshwater flow to the upper Nueces Delta should be
the implementation of a long-term (permanent)
diversion project. In the context of this future
scenario, the authors of this report present several
opportunities for enhanced project design and future
ecological studies. These recommendations were
developed based upon observ^ations gained from the
results of the demonstration project.

OPPORTUNITIES FOR A
PERMANENT DIVERSION PROJECT

From analysis of the hydrographic data collected during
the demonstration period, a permanent diversion
project could be designed that would produce
hydrographic benefits exceeding those of the
demonstration project. During the demonstration
period, the primary limitations of the total volume of
freshwater diverted during a given hydrographic event
were restrictions imposed by channel capacity and
channel obstructions. Widening some reaches of
Rincon Bayou in the upper delta that have significant
restrictions in channel capacity {e.g., the reach between
the Nueces Overflow Channel and the low water
crossing at the head of Rincon Bayou) would result in a
greater available cross-sectional area and lower
factional resistance.

Also, if existing channel obstructions {e.g., the private
road crossing separating the upper and central Rincon
Bayou segments, and the remaining fill material in the
north end of the Rincon Overflow Channel) were
removed, diversion rates during events would improve
(Bureau of Reclamation 2000). Also, the amount of
water passing out of the delta back into the river
through the diversion channel(s) at the end of a given
hydrographic event would likely be reduced. Each of
these two recommendations would increase the
potential for freshwater diversions beyond that
provided by the demonstration project design.

As with the short-term demonstration, a long-term
diversion project would have to address the issue of
voluntary landowner participation. This factor
prevented the continuation of the demonstration

project. Although key landowners were wiUing to
consider easements that would have allowed project
features to remain in perpetuity, a price agreeable to all
parties was not able to be negotiated.

OPPORTUNITIES FOR FURTHER
ECOLOGICAL STUDY

Selection and Monitoring of
Indicator Species

The use of one (or a few individual) species to reflect
the overall condition of an ecosystem is not a new
concept. For such indicator species to be useful in
applied research, they should have the following
characteristics: 1) they should direct attention to
qualities of their environment, 2) they should give an
indication that some environmental characteristic is
present, 3) they should express a generalization about
their environment, 4) their study should suggest a
cause, outcome or remedy, and 5) they should show a
need for action (Soule 1988). During the
demonstration period, benthic organisms were useful as
indicators of ecosystem productivity, particularly in
regards to the effects of freshwater inflow.

Benthos

Future monitoring of delta productivity should
consider the use of indicator benthic invertebrates.
These organisms are useful indicators because they are
relatively long-lived and sessile, so they integrate the
effects of freshwater inflow over appropriate temporal
and spatial scales. Changes in benthic biomass and
abundance indicate changes in secondary productivity,
and changes in benthic biodiversity are an important
indicator of habitat quality. Although the
demonstration project focused primarily on infauna
{i.e., animals living within sediments), an important
aspect of improving inflow conditions in the Nueces
Delta is restoring habitat functionality. This issue could
be assessed using epifauna {i.e., animals living on or
near the sediment surface) as well. Abundances of
epifaunal organisms (i?.^., shrimp, crabs, moUusks and
benthic feeding fish) would indicate habitat utilization.

8-2 V Future Opportunities

Because these organisms are highly seasonal, monthly
sampling of epifauna would likely be required.

Vegetation

Certain vegetation species have also proved to be
useful indicators of the timing and quantity of
freshwater inundation needed to promote sexual
reproduction and plant expansion in hypersaline
marshes. Annual species like Salicomia bigelovii, for
which successful establishment can only occur if soil
salinity concentrations are reduced to a level that
alleviates the osmoticaUy induced seed dormancy, could
provide relevant information regarding the timing and
quantity of fresh water needed to promote sexual (seed)
colonization in hypersaline salt marshes. Because this
species occurs only after significant freshwater
inundation events during the fall and early winter,
spring-time biomass samples may be useful in
indicating relative plant productivity between the
stations.

During the demonstration project, seasonal changes in
emergent vegetation cover and biomass were measured
and correlated to overall delta productivity in response
to freshwater inflow. However, several changes in the
sampling procedure could significandy contribute to
future monitoring. These changes include the addition
of shorter and more closely spaced transects, more
detailed sampling of pore water salinity concentrations
and focus on the colonization of opportunistic species
following major precipitation and inflow events.
Primarily, additional vegetation transects should be
established in several places located direcdy on Rincon
Bayou. While sampling in the tidal flats near the
Rincon Overflow Channel {i.e.. Station II) indicated
changes after major flow events, vegetational changes
direcdy along the upper portions of Rincon Bayou
likely occurred after freshwater flow through the
Nueces Overflow Channel during relatively smaller
positive-flow events. A useful approach would be to
sample four transects all on Rincon Bayou, with the
first transect being located close to the Nueces
Overflow Channel and the fourth transect being near
Nueces Bay. Shorter transects with closer sampling
lines may provide a more detailed picture of vegetation
changes {e.g., 50-m long transects with sampling lines

spaced 2-m apart). Most importandy, every effort
should be taken to ensure that salinity measurements
are acquired on each sampling date. Ideally, pore water
salinity measurements would be taken at 10-m intervals
(if a shorter transect were used) rather than
50-m intervals, as accurate and complete salinity
measurements are key to understanding the effects of
firesh water and consequent changes in ■vegetation.

Modeling

The demonstration monitoring program documented
changes in biological productivity and species
composition in relation to the alteration of the
fireshwater inflow regime. There is an opportunity to
integrate the various data components of this study to
determine; 1) how the marsh would have responded
during the demonstration period without project
diversions, and 2) how the marsh ecology would
respond to different fireshwater inflow conditions
Considerably more data would need to be collected to
provide these answers through field studies.

A numerical model could be developed to calculate
productivity changes in response to prescribed inflow
events. One such modeling concept {e.g., the
conceptual model presented in Chapter 7) has already
been outlined. Once developed, this model could be
used to stmidate productivity in Nueces Delta with and
without the demonstration project by using the existing
monitoring data to calibrate the model. The change in
productivity with and without freshwater inflow would
allow calculation of the percent change due to the
observed restored flow volume. This change would be
a direct estimate of the benefits of the demonstration
project. Furthermore, the modeling of other fresh
water input scenarios could be used to estimate the
benefits of particular permanent diversion project
designs.

A numerical model would also improve the
understanding of how the marsh functions under
various conditions. Model sensitivity studies could
determine transition points within the ecosystem, as
well as how to maximize benefits with adjustments in
the timing and amount of freshwater inflow.

Chapter Eight ♦ 8-3

Additionally, this model, with adequate built-in
generality, could be applied to other estuary systems
with similar characteristics and freshwater allocation

OPPORTUNITIES FOR
INTEGRATION WITH BAY AND
ESTUARY RELEASE SCHEDULES

At present, the City of Corpus Christi is required to
make pass-through releases of water from the reservoir
system on a monthly basis for bay and estuary needs.
Because of the high flooding threshold of north bank
of the Nueces River, none of this water direcdy reaches
the upper delta. Only with the demonstration project,
which allowed for a regular {i.e., daily) exchange of
small volumes of water between the river and Rincon
Bayou, was released water able to (occasionally) freshen
the upper delta. Information gained from the
demonstration project indicates that &esh water passing
through Rincon Bayou provides a more direct benefit
to the estuary ecosystem than water by-passing the
Nueces Delta and flowing direcdy into Nueces Bay.
Therefore, this finding suggests an opportunity for
integrating a permanent diversion project with reservoir
operations.

Demonstration data suggest that the Nueces Estuary
would benefit more if freshwater releases could be
made in such a way as to trigger positive- flow events
into Rincon Bayou and the upper delta. It was
observed during the demonstration period that flow
events coincident with elevated water levels in Nueces
Bay caused a greater proportion of fresh water to be
diverted into the upper delta (Ward 2000). Ward
(1997) and Chew (1964) have indicated that seasonal
secular excursions in the Gulf of Mexico, which are
well reflected in water level variations in the upper
delta, are Likely during the spring and autumn, although
with varying magnitudes and durations. There was also
an observed seasonality to the ecology of the Nueces
Delta. Animal recruitment and marsh plant growth
occurred in spring, and nursery habitat utilization in
fall. Therefore, given the combined probability of
higher water levels in Nueces Bay and increased
ecological benefits to living resources during the spring

and fall seasons of the year, larger, quarterly (or
possibly semi-annual) releases from the reservoir
system could be more direcdy beneficial to the delta
ecosystem than smaller, monthly releases.

There are also theoretical reasons why bigger, less
frequent inflow events would be more beneficial. An
emerging paradigm suggests that large "pulsed," or
punctuated, events will favor large phytoplankton that
can out-compete small phytoplankton for nutrients
when they are present at elevated concentrations (Sutde
et al. 1988). Therefore, a pulsed nutrient supply wiU
select for larger phytoplankton, which can out-compete
smaller phytoplankton (Turpin and Harrison 1980;
Sutde et al. 1987). This results in a food-web based on
large-size phytoplankton, which is much more efficient
in transferring nutrients and energy to higher trophic
levels than is a food-web which is based on pico- or
nano-plankton. For example, a simple model based on
empirical data indicates that distributing nitrogen in
pulses rather than at a low homogeneous concentration
results ia 1.5 times more carbon in large zooplankton
than would occur if the nutrients were present at a low
homogeneous concentration (Sutde et al. 1990). The
results for phosphate are even more dramatic, where
there would be 3.6 times more carbon in large
zooplankton under a pulsed delivery regime. These
results suggest that releasing water ia large pulses rather
than in a continuous manner may deliver more
necessary resources to fish and other larger consumers
in Nueces Estuary.

OPPORTUNITIES FOR ADAPTIVE
MANAGEMENT

Incorporating demonstration project features into a
permanent diversion project, modifying reservoir
operations, and continuiag to smdy resultant biological
responses in the Nueces Delta would present a unique
opportunity for one of the most comprehensive studies
of ecological benefits accrued by adaptive management.
Were such an endeavor to be undertaken, four
fundamental questions should be addressed:

8-4 ♦♦♦ Future Opportunities

1) Which delivery schediile provides more benefit to
estuary productivity: pulsed or continuous?
Included in this question would be a determination
of the release volume necessary to trigger a delta
diversion event.

2) How far downstream in Rincon Bayou do
beneficial effects accrue? That is, the idea that
there is a functional linkage between the marsh,
delta, and bay should be tested explicidy.

3) Does export from the marsh benefit the bay, and
how much water is necessary for this benefit to
accrue? There is Utde doubt of an existing linkage
between the marsh and the bay, but it is not clear
what volume of fresh water is necessary to
maintain a functional linkage.

4) What are the specific trophic linkages between
marsh, planktonic, and benthic production, and
how do these resources affect production of
commercially and recreationally important species
{e.g., shrimp, fish and wildlife).

The minimum freshwater flow necessary for
maintaining the ecological integrity of bay and estuary
ecosystems is an emerging issue in water resources
management, nation-wide. The complexity of this issue
is further magnified in estuary systems that are
supported by a semi-arid watershed and located
adjacent to a large metropolitan area. Therefore, the
Nueces Delta and Estuary is an ideal place to develop
definidve answers to the question of how to most
effectively allocate limited freshwater resources.

Figure 8-1 : View of the lower Nueces Delta with the City of Corpus Christi in the background.

Photo courtesy of the Bureau of Reclamation.

Chapter Eight ♦ 8-5

CHAPTER NINE

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CHAPTER 1: INTRODUCTION

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Chapter Nine ♦ 9-1

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CHAPTER 2: STUDY AREA

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Howell, C.W. 1 879. Survey of Aransas Pass and Bay
up to Rockport and Corpus Christi, Texas and
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House of Representatives, 46th Congress, 2nd
Session, pp. 928-950.

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CHAPTERS: HYDROGRAPHY

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CHAPTER 4: WATER COLUMN
PRODUCTIVITY

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CHAPTERS: BENTHIC
COMMUNITIES

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Chapter Nine ♦ 9-5

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Ritter, M.C. and P.A. Montagna. 1999. Seasonal

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CHAPTER 6: VEGETATION
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44:445-446.
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Chapter Nine ♦ 9-7

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Teal, J. M. 1962. Energy flow in the salt marsh
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CHAPTER?: SYNTHESIS AND
CONCLUSIONS

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CHAPTERS: FUTURE
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Ward, G.H. 2000. Hydrography of the Nueces Delta
and Estuary: 1992-1999. Unpublished. In
Concluding Report: Rincon Bayou Demonstration
Project, Appendix B. United States Department of
the Interior, Bureau of Reclamation, Austin, Texas.

Chapter Nine ♦ 9-9

•«i«$gM«p»-

Appendices

<

Contents

Page

A-1 A Technical Notes ON THE RiNCON Gauge AND Data

B-1 B Hydrography OF THE Nueces Delta AND ESTUARY:
1992-1999

C-1 C ANALYSIS OF THE Historic Flow REGIME OF THE Nueces

RrVER INTO THE UPPER NUECES DELTA AND OF THE
POTENTLY. RESTORATION VALUE OF THE RiNCON BAYOU
DEMONSTRATION PROJECT

D-1 D RECENT Trends IN Precipitation Occurring ON THE
Nueces River Watershed of South Texas

E-1 E utilization OF EsTUARiNE Organic Matter During
Growth and Migration by Juvenile Brown Shrimp

PENAEUS AZTECUS IN A SOUTH TEXAS ESTUARY
F-1 F EFFECTS OF TEMPORALTTY, DISTURBANCE FREQUENCY AND

Water Flow on an Upper Estaurine Macroinfauna
Community

G-l G FIELD Notes AND Observations FROM Benthic Sampling
Trips: October 1994 -December 1999

APPENDIX A

Technical Notes on the Rincon
Gauge and Data

Michael J. Irlbeck
Darwin Ockennan

U.S. Bxireau of Reclamation
United States Geological Survey

INTRODUCTION

On May 16, 1996, as part of the Bureau of Reclamation's (Reclamation) Rincon Bayou Demonstration Project,
the U.S. Geological Survey (USGS) installed a stream flow gauging station on the head-water channel of Rincon
Bayou (Station 08211503, Rincon Bayou Channel Near CalaUen, Texas) located just downstream from the
Nueces River Overflow Channel (Figure 1). The purpose of this gauge was to record daily stage and discharge
through the overflow channel, and direct precipitation at the site during the demonstration period. Data from
the Rincon Gauge was available from the date of its installation (May 16, 1996) through December 31, 1999.

Figure 1: Location of the Rincon gauge.

AppendixA ♦ A-1

Instrumentation

All data ftom the gauge was measured in
1 5-minute intervals, stored by a data collection
platform (DCP), and transmitted to USGS offices
via GOES (Geosyn-chronous Operational
Environmental Satellite) in near real time
(Figure 2). Water level ia the channel was
measured by a pressure transducer, and the
gauged water level {i.e., gauge height) was
referenced to mean sea level. Datum of the gauge
is at mean sea level. Row velocity was measured
by an acoustic velocity meter deployed near the
center of the channel (Figure 3). The acoustic
meter had a resolution of 0.01 feet per second and
measured both positive and negative flow in the
channel. Rainfall was measured by a tipping
bucket rain gauge. Discharge in the channel was
computed as the product of the flow cross-
sectional area and the mean chatmel velocity.

Figure 2: View of the
Data Collection Platform,
Rincon gauge.

Figure 3: View of the
gauging instrumentation,
Rincon gauge.

At this site, there is no well-defined relation between water level (stage) and discharge due to the effect of tide.
The cross-sectional area of flow in the channel is a function of water level in the chaimel. The relation between
water level and cross-sectional area was therefore determined from a cross-section elevation survey.
Furthermore, the acoustic velocity meter measures flow velocity at a single point in the channel. Because the
measured velocity is not necessarily the mean velocity in the channel, manual discharge measurements were
made to determine discharge and mean channel velocity. These measurements of mean channel velocity were
related to acoustic velocity measurements by a rating developed from the manual discharge measurements.
When actual measured discharge (during calibration measurements) were compared with discharge values
determined from calibrated gauge readings, the potential error for flows above 1 cfs were usually within about
10 percent.

SUMMARY OF DATA

Origirjally, the Nueces Overflow Channel was designed to have a controlling bottom elevation of 2.0 ft msl.
However, days after construction of the channel was completed in late October 1995, the Corpus Christi area
received 10-12 inches of local ratafaU in a 2-day period. The resulting runoff and river discharge scoured the
newly cut channel to a new controlling bottom elevation at about mean sea level. The effect of this change was
that, in addition to freshwater discharge events through the overflow channel, there was also now the
opportunity for regular tidal exchange between the river and the upper delta, even when there was litde or no
flow coming down the Nueces River. As a result, flow in the channel regularly occurred in both directions
during the study period. Positive flow from the Nueces River into Rincon Bayou typically occurred during
periods of high discharge in the Nueces River or during rising tide events which pushed water up the river and
through the overflow channel. Negative flow from Rincon Bayou into the Nueces River typically occurred
when the water level in the upper delta was relatively high immediately after river discharge events or during
falling tide conditions.

Stage

As discussed above, the Rincon gauge is tidally influenced. The stage data recorded by the gauge therefore
indicate influences by both freshwater flow events {i.e., stage events driven by discharge in the Nueces River)
and saltwater inundation events {i.e., stage events driven by tidal or other hydro-meteorological activity). This
dual relationship can be best shown by comparing stage values between the Rincon gauge and the Calallen
gauge (Station 08211500, Nueces River at Calallen) (Figure 4). Although there is a strong correlation between
the two gauges, the variance in daily stage at the Rincon gauge is about 1.5 to 2.0 ft for any given stage value at
Calallen. This variance is the result of tidal mfluences on the upper delta and the Nueces River below Calallen,
which is present regardless of flow in the river.

A-2 ^ Technical Notes on the Rincon Gauge and Data

The tnaviimiiTi stage recorded at the Rincon gauge during a large discharge event in the Nueces River was
7.38 ft msl on October 21, 1998. The maximum stage recorded during a tidal event not influenced by the
Nueces River was 5.35 ft msl on August 23, 1999, which was associated with the initial storm surge of
Hurricane Bret. A summary of the stage data collected during the study period is displayed in Figure 5
(monthly) and Table 1 (daily).

Table 1: Summary of Daily Stage Data (ft msl), Rincon Gauge. May 16, 1996 - December 31, 1999.

Stage

Jan Feb

Mar

Apr May

Jun

Jul

Aug Sep

Oct

Nov Dec

Maximum 1.62 2.31 2.89 3.30

Minimum 1.03 0.97 0.97 1.11

Average 1.25 1.43 1.65 1.89

3.10 5.63 5.34 4.52 5.72 7.25 3.49 1.96
1.32 1.25 1.00 1.08 1.25 1.18 1.05 0.97
1.80 1.83 1.90 1.59 2.18 2.55 1.79 1.26

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Daily Stage at Calallen (ft)

Figure 4: Plot of daily stage values from the Calallen and Rincon gauges. May 16, 1996,
through December 31, 1999.

Appendix A ♦ A-3

n r

Jan Feb Mar Apr May Jun

Aug Sep Oct Nov Dec

Figure 5: Average monthly stage, Rincon Gauge. May 16, 1996 - December 31, 1999.

Discharge

As with stage, discharge past the Rincon gauge (either positive or negative) was determined by either freshwater
flow in the Nueces River or tidal events in the bays, or both. Freshwater flow events were typically infrequent,
of high magnitude and positive (i.e. into the delta), while tidally-driven discharge events were more frequent, of
lesser magnitude and both positive and negative in direction. However, such a clear distinction between factors
affecting discharge through the overflow channel is over simplified. This is because tidal conditions in the
lower river and delta consistentiy exerted a strong influence on the rate of discharge, even during moderate flow
events in the Nueces River. For example, daily flow in the Nueces River did not meaningfully contribute to
discharge through the Nueces River Overflow Channel when below about 650 cfs, and did not become the
dominant factor until river flow exceeded approximately 1,400 cfs (Figure 6). For flow values in the Nueces
River below 1,400 cfs, the tide condition at the point of diversion was the dominant factor in determining both
the direction and rate of discharge through the overflow channel.

The largest daily discharge event associated with freshwater flow in the river was 274 cfs on October 21, 1998.
The largest daily discharge event associated with a tidal event essentially independent of the Nueces River
occurred on August 23, 1999, and was approximately 90 cfs. This discharge event was also associated with the
initial storm surge of Hurricane Bret. A summary of the discharge data collected during the study period is
displayed in Figure 7 (monthly) and Table 2 (daily).

A-4 V Technical Notes on the Rincon Gau^ and Data

Table 2: Summary of Daily Discharge Data (cfs), Rincon Gauge. May 16, 1996 - December 31, 1999.

Discharge

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Maximum

2

9

21

36

29

121

99

90

177

274

22

3

Minimum

0

-2

-9

-13

-14

-12

-69

-42

-60

-66

-16

-1

Average

0.1

0.2

0.4

0.6

-0.4

2.8

3.6

0.0

6.2

11.2

1.0

0.0

Precipitation

Precipitation in the study area was sporadic, but generally coincided with the spring (March through May) and
late summer/ fall (August through November) seasons. The summer month of July and the winter months of
December and January were consistently dry. A summary of the precipitation data collected during the study
period is displayed in Figure 8 (monthly) and Table 3 (daily). Specific precipitation events are considered in
context with the discussions of significant discharge events.

Table 3:

Summary of Daily Rainfall Data (inches), Rincon Gauge.

May 16,

1996-

December 31, 1999.

Rainfall

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Maximum 0.35
Average 002

1.49
0.06

2.79
0.10

2.44
0.08

1.65
0.06

1.70
0.09

0.81
0.03

3.38
0.15

2.15
0.10

4.72
0.17

1.39
0.06

0.89
0.02

Appendix A ♦ A-5

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500 1000 1500

Daily Discharge at Calallen (cfs)

2000

2500

Figure 6: Plot of daily discharge values from the Calallen and Rincon gauges. May 16, 1996,
through December 31 , 1999. Flow in the Nueces River did not become the dominant factor
determining the rate of discharge through the Nueces River Overflow Channel until it exceeded
approximately 1 ,400 cfs.

A-6 ^* Technical Notes on the Rincon Gauge and Data

2500

2000

0

1—
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x:
o

1500

1000

S 500 - -■

- 199
■ 199

- 199

6

7
8
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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 7: Total monthly discharge, Rincon Gauge. May 16, 1996 ■
December 31, 1999.

AppendixA ♦ A-7

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11
10

9

8

7 -I

6

5

4

3

2

1

- 1996

-• 1997

— 1998

• 1999

-I — 1 1 1 I 1 1 r-

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 8: Total monthly rainfall, Rincon Gauge. May 16, 1996 - December 31, 1999.

A-8 ♦♦♦ Technical Notes on the Rin(on Gauge and Data

APPENDIX B

Hydrography of the Nueces Delta
and Estuary: 1992-1999

George H. Watd Center for Research in Water Resources, University of Texas, Austin

BACKGROUND

Three time scales are of pertinence to the evaluation of hydrographic data in the Nueces Estuary: intratidal,
intertidal, and event-duration. The intratidal (or intradiumal) time scale represents short-term behavior at an
hourly resolution, the intertidal (or interdiumal) time scale represents day-to-day variations of hydrographic
parameters averaged over 24 hours, and the event-duration scale extends over the time period encompassing all
responses to a specific hydrographic event, and can range from several days to many weeks.

Data Sources

Hydrographic data for this analysis were obtained from a variety of sources, including the Texas Coastal Ocean
Observing Network (TCOON) marine monitoring system of Texas A&M University-Corpus Christi Conrad
Blucher Institute (CBI), the weather station network administered by the National Weather Service (NWS), and
the national stream flow gauging program conducted by the United States Geological Survey (USGS) (Table 1).

Table 1: Summary of hydrographic data sources.

DATA SOURCE

Parameter

Measurement Locatior)

Data Type

Nueces River at Calallen,
USGS

TCOON system, CBI

Corpus Christi Bay, NWS

Rincon Bayou near Calallen,
USGS

Estimated daily flow

Water level

Wind direction and velocity

Salinity

Daily precipitation

Water level
Current velocity
Calculated flow

Daily precipitation

Nueces River

Nueces and Corpus Christi
bays

Corpus Christi
International Airport

upper Rincon Bayou

Data recorded at
15-minute intervals

Data recorded as
6-minute averaged
values

Data archived as daily
values

Data recorded at
15-minute intervals

Data archived as daily
values

Data obtained from the TCOON system included salinity and water level. For each of these parameters, hourly
measurements were obtained from the CBI data archive, and for the analysis reported here, subjected to
24-hour averaging to obtain daily mean values. The salinity data used were from the CBI SALT03 gauge, which
is situated due south of White Point in the center of the bay, about equidistant from the mouth of Rincon
Bayou and the mouth of the Nueces River, and therefore responds to flow from both conveyances. Salinity
concentrations are measured by robot conductivity sensor and converted to salinity, reported in parts per
thousand (ppt).

Appendix B ♦ B-1

Water level in Nueces Bay was that measured at CBI's White Point gauge, nearer the mouth of Rincon Bayou.
While there is no doubt some slope to the water surface in Nueces Bay is in response to meteorology, tides and
river hydrographs, this is negligible in comparison to the temporal excursions in water level in the river and
marsh. Therefore, stage data from the White Point gauge was regarded as an acceptable indication of the
general coincident elevation of Nueces Bay.

The USGS gauges from which data were obtained included the Nueces River at Calallen (Station 0821 1 500)
(Calallen gauge) and Rincon Bayou near Calallen (Station 08211503) (Rincon gauge). Flow data were obtained
from both gauges, and stage and precipitation data were also obtained from the Rincon gauge. There are
significant limitations to the accuracy of the flow data from the Calallen gauge at higher values for two primary
reasons. First of all, the gauge ceases to represent the total flow of the Nueces River above about 56.63 m'/s
(2,000 cfs) due to activation of additional flow channels in the floodplain. Second, no reliable field
observations of discharge values are available above 77.87 m'/s (2,750 cfs), so all daily flow values in excess of
77.87 m'/s (of which there were 3 in the record under review) were estimated by extrapolation.

INTRATIDAL (hour to hour) ANALYSIS

The Nueces Estuary and Delta systems are greatly affected by intratidal processes, including diurnal hearing and
cooling, tidal inflows and outflows, and short term responses to meteorological forcing, such as sea breezes,
convecrive storms, and frontal passages. Analysis of such short-term behavior provides insight into the
dynamics of the study area and the cause-and-effect relations between hydro-meteorological events and the
hydrographic response of the Nueces Delta. Most of the variation on this time scale is oscillatory, and
therefore obsciues the longer period behavior. Intratidal-scale data are best depicted through continuous
animated display.

Time Series Display of Daily-Mean Hydrologicaland Hydrographic Data

In the process of compiling data on the hydrographic elements of the Rincon Bayou Demonstration Project to
support analysis of the various ecological aspects, it was noted that several of the Principal Investigators have
data available from the Nueces Delta area dating back to the early 1990's, which might be of potential value as
baseline information in assessing the response of the region to the diversion project. Therefore, the compiled
data set included as much information that was available for the past decade.

Prior to late- 1993, virtually the only extant data is salinity and inflow measured at Calallen. In 1993, rehable
records from Conrad Blucher Institute (CBI) tide gauges and anemometers become available. By the date of
the breaching of the Nueces Overflow Channel on 26 October 1995, fairly continuous data from the region is
available. Only the USGS gauge in the Rincon chaimel itself is lacking, this not becoming operational until
15 May 1996.

The Nueces Delta in general is subjected to intradaily variations, in response to tides, wind events, water mass
replacement, convecrive storms and other such short-term phenomena. Analysis of such short-term behavior
provides insight into the dynamics of the region and the cause-and-effect relations between hydro-
meteorological events and the hydrographic response of the Nueces marsh area. It is unlikely, however, that
the ecological components respond on such short time scales. In any event, the biological and chemical
observations that have been made in this project have been performed on longer time intervals. It is desirable
to similarly depict the longer term, daily to weekly variations in the hydrographic environment to facilitate
interpretation of the biological response.

For this purpose, daily means have been computed for all of the hydrographic variables. For purposes of
effective presentation, a computer display of this daily data has been constructed. Operation of the program
"BAION," which should be installed and operated on a PC-type machine, is described below.

Program Operation

Two files were developed:

BAION.exe
IRLBECK.dai

B-2 ♦ Hydroffophy of the Nueces Delta and Bstuaty: T 992-1 999

These should be copied into the same directoty on your hard drive. The first is the actual program (i.e.,
executable), and the second is a flat-ASCII data file read by the program. Double-click on the BAION icon (or
cUck on the "START" button on your WINDOWS toolbar, then "RUN...", then enter die path name for
BAION in the dialog box). The MS-DOS window will appear displaying the BAION starting banner.

At this point, the program will display a figure showing a circle embedded in a square. You may see an ellipse
embedded in a rectangle. If so, you will need to adjust the display of your monitor, by using the controls on the
rim of the monitor frame. (Many machines will retain these adjustments in memory, and automatically
implement them whenever the same program is activated.) Program operation will resume when you press any
key.

The computer will ask for a starting date for the display. For now, you can simply press ENTER, and the
display will start at the beginning of the record. Later, you can enter a specific date by using the format
YYDDD. You will be prompted for any changes to the plotting scales for the display (answer "no" for now)
and then you'll be prompted for assurance that the display should continue (answer "yes"). (The program is
rather insecure, and needs frequent encouragement.)

Now the data display wiU (hopefully) begin. AH controls on the program are effected by the keyboard. The
bottom line of the display panel summarizes the controls available to the user:

S - slows the rate of the display by inserting a delay between plotted points, can be pressed successive

times to further slow the display (but see also "P")
A - accelerates the rate of display by decreasing the time delay between plotted points
P - "pause" or "point" display, holds the present display; each time "P" is pressed an additional point is

displayed
R - resumes the default display rate, can be used to cancel the effect of "P"
X - refreshes the axes on the wind panel
L - allows the user to re-scale displays while in progress
Q - terminates the present display

There are three panels in which various data are shown (Figure 1). The panel at the lower left indicates
meteorological and astronomical controls. The daily-averaged vector-mean wind velocity is shown as a line
element terminated by a small circle. (Arrowheads are too hard to plot.) The length of the line segment is
proportional to the speed of the wind (the circle indicates 10 m/s) and the orientation of the segment is the
direction to which the wind is flowing. In the example of Figure 1, the wind blows from SE to NW.

Data from two locations in the region are shown. The yellow vector is representative of the north shore of
Corpus Christi Bay. This is primarily the record from the CBI Ingleside anemometer, which is the earliest such
record from the project region, beginning in late Jime 1992. Unfortimately, the data record terminates in
December 1996. For the remainder of the period of display (through 31 December 1999), data from the
Port Aransas anemometer is used. The combined record is therefore referred to as "North Bay." In July 1994,
a red vector is added to the wind display: this is the data record from the Naval Air Station, characterizing the
south shore of Corpus Christi Bay. There are frequent gaps in both anemometer records, so the redundancy of
two data sources is useful. fThese also display the differing responses of the wind depending upon whether it
blows over land or water.)

The lower left panel also shows the lunar controls, depicting lunar aspect as a moving icon. The appearance of
the icon shows phase of the moon (in Figure 1, the crescent). The vertical (y-) component of the position of
the icon is the declination of the moon, and the horizontal (x-) component is an index of the proximit)' of the
moon to the earth, the line marked "apog" corresponding to greatest distance ("apogee") and the line marked
"perig" to smallest distance ("perigee"). The 12.4 and 24.8-hr tidal components are virtually eliminated by the
24-hr averaging to which this data has been subjected. The lunar controls on ride are not, therefore, so obvious
as when intradaily data are shown, but the lunar declination does account for some of the 15-day oscillation.

Two other panels are shown on the right two-thirds of the screen. The lower of these displays water levels.
There are three sites displayed: the CBI White Point tide gauge, the water level of Corpus Christi Bay, and the
water level at the Rincon diversion USGS gauge. Corpus Christi Bay is the tide record from the CBI Aquarium
gauge, with older records filled in from the CBI Ingleside gauges, vit^

Ingleside 92119-93268

State Aquarium 93269 - 99365

Appendix h ♦ B-3

variable names Julian day (YYDDD) & date

DISPtAY OF RINCON PRQ,
'Upper plot: ~ White pt ^al Rincon sup^
ftower plot
X White Point tide
\cCBay tide

Rincon gage stage
Lunar declinat:j.6n
wind I wind below, circle = 10 m/s
vector .^ I DATE 98317 Nov 1

apoq periq

rainfall

Calallen flow

declination
range

Lunar icon

salinity

Flow in
Rincon
channel

lunar
^ ^efclination

tide (stage)
level

pengee
range

tide datum
end-of-month marker

mnemonic line for key control options

Figure 1 : Display Screen for BAION.

All three stage records are referenced to the same ("arbitrary") consistent datum. (Also, the 24-hr mean values
of the Rincon stage had to be re-computed to reference these to GMT, which is the averaging period for all of
the other hydrographic data.) On this same panel, lunar declination is plotted as a faint blue covet. Although
declination is indicated by the vertical position of the lunar icon on the left panel, it can be hard to follow in
relation to the water level, requiring one eyeball to be fixed on the lower left panel while the other follows one
of the traces in the right panel. The vertical lines demarcate months.

The upper panel displays "hydrography", including the salinity variation m upper Nueces Bay, the super-
elevation of Rincon gauge over White Point, and the measured discharge in the Rincon diversion channel
(starting in May 1996), as well as Calallen flow and regional rainfall. The Rincon discharge is generally about
10% of the flow at Calallen, so it is scaled tenfold to be plotted on the same axis (scale on the right side of
panel). The salinity data is from CBI SALT03 gauge, which turns out to be situated due south of White Point,
but in the center of the bay about equidistant from the moutli of the Rincon and the mouth of the Nueces. It
therefore responds to flows in both conveyances.

Daily rainfall is plotted as a light blue bar, positive downward from the top of the panel. This is a composite
data set, consisting of the measiurement at the USGS gauge back to May 1996, and the Corpus Christi airport
measurement prior to this. The Calallen flow is plotted as a bar graph at the "back" of the panel. This slows
down the display, because more time is required to "paint" the record, but this strikes me as a clearer depiction
of flow conditions. (Note the jazzy shadows of the data points when a trace crosses a flood hydrograph.)

One problem with the record of super-elevation is that both the White Point and the USGS Rincon gauge peg
at low water. The Rincon gauges is especially problematic. For these data, any day with more than 25% of the
data (5 measurements) at the low pegged value was deleted from the record. For those days with 5 or less such
pegged values, a daily mean was computed using the peg values. (To omit them would overestimate the mean
stage.) Approximately 16% of the data at the Rincon gauge proved to be pegged values. Fortunately these
were confined to winter or summer low flows, in the absence of significant river flow, so there should be littie

B-4 ♦ Hydrography of the Nueces Delta and Estuaty: 1992-1999

effect on the use of the data in the biological analyses. However, no super-elevation can be computed when
one of the two stage values is missing, so these peg values become gaps in the super-elevation data stream.

Comments on the Data

upon starting the display at the beginning of the data period, vi^ 1 992, one is immediately struck by the
substantial flow hydrographs in the Nueces River. A series of large hydrographs begin in February and
continue through June, holding salinity concentrations to virtually fresh throughout this period and the early
summer. The Interim Order mandated releases begin in September 1992, and are manifested as the small
pvdses of inflow occurring near the end of each month. The 1992 fall high water occtirs in early October.
During this period, the lunar declination is near its maximum attainable value, and only one front of any
consequence occurs in the fall, the remainder of the period being dominated by onshore flow from the Gulf.

In late June 1993, a substantial rainfall event over a three day period occurred. Simultaneously, a spike in water
levels of over 0.5 m is registered synchronously in Nueces and Corpus Christi Bays. The cumulative rainfall in
the event totaled 0.23 m as measured at Corpus Christi. Evidentiy, such a large volume of rainfall can account
for at least part of the observed excursion in water level. A few days later the associated hydrograph on the
Nueces reached Calallen and delivered a cumulative 3.3 Mm^ of inflow. Spread over the 500 km" of combined
surface area of Corpus Christi and Nueces Bays, this would amount to less than a cm of additional water depth.
Indeed, the water-level data from July show no discernible response to the Nueces inflow.

The moral of this comparison is that hydrograph events on the river could be expected to have littie impact on
the elevation of water in the bay, whereas sudden diluvial rainstorms may have, if the rainfall area encompasses
a substantial portion of the bay area. Several such spikes can be seen in the water level histories that
correspond to intense rainfalls. On the other hand, the Calallen flow hydrographs create a greater response in
the salinity than a rainfall event. The former in fact is a water-mass displacement process, while the latter is a
dilution of the rainfall depth throughout the water depth.

The summer 1993 water-level history is a good example of the summer seasonal low water, due to the absence
of other hydrographic factors from late June through August. In this same year, the subsequent fall high water
is a rather minimal event. The 1 994 fall high water is more typical of this annual event

The only significant hydrographic events occurring during the period after opening of the Nueces Overflow
Channel but before the operation of the USGS Rincon gauge are foimd in October 1995, the month during
which the channel was opened. A low but fairly steady flow over Calallen occurred during October but abated
the day before the channel was opened. Iribeck observed that the level of water in the Nueces did not acquire
the threshold to force flow through the overflow channel. On 28 October, two days after the channel was
opened, an intense rainfall event (over 20 cm in one day) created sufficient local flow to scour down die
channel (see Chapter 3 in the draft report). Although a spike ki bay water level occurs, the effect on salinity is
negligible. For the next several months, only a few minor rainfall events appear in the record, and the bay
salinity climbs, nearly monotonically, into the hyper-saUne range.

Finally, it should be noted that there are other pathways for flow to enter Rincon marsh, vi2. the series of low
points in the north levee of the Nueces River. Only when stage in the rivet becomes sufficiendy high does flow
begin to pass these other openings. Based upon HEC-2 hydraulic model runs (Bureau of Reclamation 2000), a
relation has been developed between the flow in the Rincon channel and the total flow entering Rincon marsh
by aU of the available routes. The results of this modeling analysis are summarized in the following figure
showing the proportion of total flow into the marsh represented by the Rincon channel. For small river stages,
this is clearly 100% (Figure 2). Then the proportion rolls off in a sigmoid-like shape approaching a level of
about 5%. Considering the sources of error in aU of this, the total flow into Rincon marsh can be
approximately related to that in the Rincon channel as follows:

Rincon Q Rincon Q < 200 cfs

Combined Q (cfs) = 0.8 (Rincon Q - 200) Rincon Q 200 < Rincon Q < 450 cfs

20 (Rincon Q) Rincon Q > 450 cfs

Appendix B <* B-5

# From HEC-2 model runs

by US Bureau of Reclamation

300 400 500 600

Row (cfs) in Rincon channel

Figure 2: Proportion of combined flow into upper Nueces delta carried by Rincon channel. Source of
HEC-2 model run is Bureau of Reclamation (2000).

INTERTIDAL (day to day) ANALYSIS

The depiction of hydrographic time history in the Nueces delta on an intertidal time scale is based upon
compiling the data sources and computing their integrated values for each day (24-hour period UCT) within the
period of record. (The astronomical tidal period is actually 24.8 hrs, but an average over 24 hours eliminates
almost all of the variability it contributes.) The term "integrated" means either averaged or accumulated,
whichever is more meaningful for the parameter under consideration. The depiction is best presented either
graphically or La tabular summary of the hydrographic variables.

EVENT-DURATION ANALYSIS

The intertidal (day to day) analysis covered the period of January 1992 through December 1999. After
inspection of the entire period of record on both intertidal and intratidal scales, criteria were formulated to
identify an "event" based upon the separate hydrographic behaviors of each of the key response parameters.
These response variables included stage (in Nueces Bay, in Rincon Bayou and a super-elevation of the two),
flow (in the Nueces River and in Rincon Bayou), and salinity (in Nueces Bay) (Table 2). It should be
emphasized that these criteria were ultimately arbitrary, but were utilized to ensure an objective selection of
candidate events for analysis.

It is noteworthy that precipitation was not treated as a separate hydrographic variable, though it was certainly an
important hydrographic element in imderstanding the response of the delta ecology. The reason for its
exclusion as a defining parameter was that it provides no infotmMon per se on the response of the Nueces
Estuary or Delta. A similar argument was made for excluding wind as a defining criterion.

Once these criteria were established, the daily data for the period of study was manually inspected. Individual
occiurences within the record which met at least one of the six criteria were identified as an "event". Then, for
each event, the 24-hour mean data for all hydrographic variables during the event were separated and
transferred for individual analysis. The duration period for each event was at least that for which the defining

B-6 ♦ Hydrography of the Nueces Delta and Estuary: 1992-1999

Table 2: Criteria used to define hydrographic events in the data record by response variables.

Location Defining Criteria of a Hydrographic Event

RESPONSE
PARAMETER

Flow

Stage

Salinity

Nueces River A 24-hour nnean (daily) flow^ in the Nueces River at Calallen exceeding

14.2 m^/s (500 cfs).

Rincon Bayou A 24-hour mean (daily) flow In Rincon Bayou, either positive or negative,

exceeding 0.28 m^/s (10 cfs).

Nueces Bay A 24-hour mean (daily) stage in the vtrater elevation of Nueces Bay exceeding

0.30 m (1.0 ft). [Referenced to the consistent CBI datum from Ward (1997),
established by "empirical leveling".]

Rincon Bayou A 24-hour mean (dally) stage in the w^ater elevation of Rincon Bayou

exceeding 0.61 m (2.0 ft). [Relative to Rincon gauge datum, which is 422 cm
above the consistent datum for CBI gauges.]

Super-elevation The sum of Rincon Bayou minus Nueces Bay daily stage values exceeding
0.15 m (0.5 ft). [Referenced to common datum]

Nueces Bay Change in salinity concentrations of Nueces Bay exceeding 5 ppt over a five

day period.

criterion was satisfied, though often a longer event period was chosen to be sure that the complete response of
the bay or delta was included in the analysis. When several hydrographic events overlapped {i.e., when several
variables each satisfied criteria separately and simultaneously), the event duration was at least the period fi:om
the first occurrence of the criterion threshold for the earliest parameter to at least the last such threshold for the
latest parameter.

The greatest difficulty in separating such events was met when a time series of events occurred in which the
response of one parameter overlapped that of the next. For example, a series of river hydrographs might occur,
each of which raises Rincon stage or Calallen flow above the threshold defining an event, and a new surge of
inflow occurs before the recession of the preceding has subsided. Separating these into individual events was
rather arbitrary, and from the estuarine response point of view, such a sequence might acceptably be considered
one long event rather than a sequence of separate events.

The parameters compiled for each event include the following: event number, date, duration, rainfall, flow,
stage and salinity.

Event Parameters

Event Number

For purposes of tracking and reference, each discrete event in the record was assigned an event label. Each
event occurring from October 1, 1994, through December 31, 1999 (the duration of the demonstration
project), was numbered sequentially in time, begirming with 1. Events occurring before the demonstration
period were labeled with sequential letters, beginning with A.

Date and Dutation

The span of each event was specified by its starting date. In some cases, the ending date of one event was the
starting day of the next, which indicates that a subjective separation had been assumed in the record for
purposes of analysis. This arbitrary division may or may not have been capable of differentiation in the actual
environment, which may have responded as if the two events were a single "merged" event.

Rainfall

For the period of October 1, 1994, through May 15, 1996 {i.e., prior to the activation of the Rincon gauge), local
daily precipitation obtained fcom Corpus Christi airport, which evidenced a correlation with the USGS gauge

Appendix B ♦ B-7

on Rincon Bayou near Calallen (Station 0821 1503) (Rincon gauge) of 0.81 for the 3 years of coincident data.
After May 15, 1996, daily precipitation was obtained by the Rincon gauge.

Flow (Nueces River)

Daily values for flow in the Nueces River were obtained from the U.S. Geological Survey (USGS) on the
Nueces River at Calallen (Station 08211500) (Calallen gauge). Because no reliable field observations of
discharge values were available above 77.87 m'/s (2,750 cfs), all daily flow values in excess of 77.87 m'/s were
estimated by extrapolation (Irlbeck and Ockerman 2000).

Flow (Rincon Bayou)

For dates before the opening of the Nueces Overflow Channel (October 26, 1995), daily flow data into Rincon
Bayou from the Nueces River were estimated from daily stage values recorded at the Calallen gauge using the
method described by Irlbeck and Ward (2000). No daily data were available from the period of October 26,
1995, through May 15, 1996, when the Rincon gauge was installed. From that date through the end of the
record, flow data reported for Rincon Bayou was the total net daily flow gauged at the Rincon gauge.

It is important to note that, during the demonstration period, the Nueces River did not exceed the natural
flooding threshold for the delta, which is 1.71 m (5.60 ft) (Bureau of Reclamation 2000), except for on three
occasions. This means that, except for these four events (Events 16, 18, 25 and 36), the only water exchanged
between the Nueces River and Rincon Bayou passed through the Nueces Overflow Channel. During the three
excepted events, an additional amount of water entered Rincon Bayou naturally via the low depressions along
the bank of the river, and was estimated using the hydraulic model developed by Reclamation (2000).

Stage

Water level data for the Nueces Bay and Rincon Bayou were obtained from the Texas Coastal Ocean
Observing Network (TCOON) marine monitoring system of Texas A&M University-Corpus Christi Conrad
Blucher Institute (CBI) and the USGS Rincon gauge, respectively. The super-elevation of water levels,
determined by subtracting the Nueces Bay stage from the Rincon Bayou value, was used to determine the
predominant influence on stage in upper Rincon Bayou. On an instantaneous basis, the super-elevation is the
direct force for discharge through the Nueces Overflow Channel.

Salinity

Salinity data were obtaitted from the CBI SALT03 gauge of the TCOON system.

Observations

Freshwater Flow

A principal objective of this demonstration project was to increase the opportunity for partial diversion of flow
events in the Nueces River through the Nueces Overflow Channel into Rincon Bayou and the upper delta. The
project would thereby periodically increase water levels and inundate regions of the upper marsh, while at the
same time reduce salinity concentrations, all of which were considered to be ecologically beneficial. Since the
purpose of the demonstration project was to divert a portion of a flood hydrograph on the Nueces through the
diversion channel, a logical inquiry was the proportion of such a flood so diverted. The bulk event data can be
used to address this question.

Upon examination of the relation between the total flow voliune in the Nueces River (for events which met the
criteria) and in the Nueces Overflow Channel, it became obvious that there was a general association between
the two. The volume diverted increased generally with the flow in the river, and the actual proportion of the
flow amount diverted was on the order of 2 percent of that in the river (Figure 3). But this relation, such as it
was, evidenced considerable scatter. In further analysis, the data was segregated by water level in Rincon Bayou
at 0.3-m (1-ft) intervals. Within each class of water levels, the volume transported through the Nueces
Overflow Channel proved to be substantially independent of the volume in the Nueces River. This was
somewhat surprising.

B-8 ♦ Hydrograph)/ of the Nueces Delta and Estuary: 1992-1999

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0 -

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10000

20000

30000

40000

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Total event flow in Nueces River (acre-ft)

Figure 3: Total flow volume carried in the Nueces River versus total flow volume diverted

through the Nueces Overflow Channel. Only events that met criteria for a flow event in the

Nueces River were used. The values plotted for Rincon flow volume are the total net exchange, or

the integrated positive values.

Note: 1 ft = 0.3046 m, 1 acre-ft = 1.2336 10' m'

That the relation between Nueces River event volume and the volume transported through the Nueces
Overflow Charmel should depend upon water level was not unexpected, based upon hydrauHc considerations.
Unlike a river channel system in which the head gradient and the water level (stage) are closely related, there is
no direct relation between water level and flow in the Nueces River below Calallen Diversion Dam because of
the corrupting effect of tidal and meteorological water-level variations. The Nueces River hydraulic head is
superposed on whatever water level is present in Nueces Bay. However, this water level does affect how the
liver head can drive flow through the overflow charmel, because the deeper the water, the greater the cross-
section area of the channel and upper delta, and the lower the frictional resistance. Therefore, a given hydraulic
head in the Nueces River drives a greater flow through the diversion channel when die Nueces Bay water levels
are higher.

The surprising aspect of Figure 3 was the apparent constancy of the volume diverted versus river flow volume
for a given class of water levels. For this there are two possible explications. The first is that this observation
was an artifact due to die way that a hydrographic "event" was defined (which includes the entire period in
which all of die variables respond, dien return to their pre-event values or, in die case of salinity, to a stable
value). Thus the diuation over which Nueces River flow was computed is generally longer dian die duration of
the flow event in the diversion channel. The rebuttal to this thought is that there is flow in the Nueces River
that occurs when there is not flow in the overflow channel, so it was legitimate to integrate over die full
hydrograph in die river channel. The second is diat this observation was a manifestation of die phenomenon
of hydraulic capadt}', suggesting diat die Nueces Overflow Channel and upper Rincon Bayou very quickly reach
their hydraulic capacity shordy after a flood event begins. The result is diat die volume diverted through the
overflow channel becomes substantially constant, even as flow in the Nueces River increases. The present
writers are inclined to this second view. If this constancy of volume is a valid inference, it would imply tiiat the
2% proportion of flow in the Nueces River diverted into Rincon Bayou is itself an artifact of data points
corresponding to different Nueces Bay water levels.

Appendix^ ♦ B-9

Water Level

As with river flow, there was also a proportional iacrease in total flow through the Nueces Overflow Channel
with an increase in super-elevation of stage. A similar sorting by water levels also occurred when the
event-duration data for flow volume in Rincon Bayou was plotted against the event mean super-elevation
(Figure 4). In this figure, all 28 events were plotted, not just those events which met the criteria, as was done in
Figure 3 Again it was observed that the larger flow volumes were associated with greater depths, independent
of the magnitude of the super-elevation. Therefore, a given hydraulic head gradient drives a greater flow if the
water is deeper.

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Figure 4: Flow volume diverted through the Nueces Overflow Channel for all events versus
event-mean super-elevation.

Note: 1 ft = 0.3046 m, 1 acre-ft = 1.2336 10' m'

Salinity

Upon examination of the relation between salinity response of Nueces Bay before and after the opening of the
Nueces Overflow Channel, all hydrographic events occurring from 1992 through 1999 were considered, rather
than just those occurring after the initiation of the demonstration project. The incremental percentage salinity
responses (%chg) of each event was plotted against the total event flow volume in the Nueces River (Figure 5).
Data for events during which the starting salinity was less than 5 ppt were not included due to "noise" in the
salinity measurements themselves. Several inferences immediately followed an inspection of this figure.

First, for total event flows greater than 12,336 10^ m^ (10,000 acre-ft), there was no obvious difference between
the fractional salinity response of events occurring before the opening of the overflow chaimel and those after
it. That is, the same general relation of diminishing percent salinity response with increasing event flow in the

B-10 ♦ Hydrography of the Nueces Delta and Estuary: 1992-1999

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Total event flow volume in the Nueces River (acre-ft)

Figure 5: Fractional salinity response versus flow volume in Nueces River (measured at Calallen).

Note: 1 acre-tt= 1.2336x10' I

Nueces River was observed within the scatter of the data. That water diverted through the overflow channel
would not measurably affect the response of bay saUnity was not surprising, given the very small proportion of
flow volume diverted through the overflow channel compared with that in the river.

Second, the rate of decrease of the fractional response (that is, the increase of negative values) with increasing
flow in the Nueces River appeared to asymptotically trend to -100%. Considering that the mean water level
volume of Nueces Bay is about 49,344 10' m' (40,000 acre-ft) (Ward 1997), one might have expected the
fractional responses to equal -100 percent for flow volumes exceeding this 12,336 10' m'. But this was not the
case, probably because the definition of an "event" may encompass some of the period of salimty recovery.
Moreover, there were also hydrographic processes that enabled water from lower in the bay (and containing
higher salinity) to infiltrate back into Nueces Bay during the event period, including tides and wind forcing.
Finally, the adjective effect of the Nueces River depended not only upon its volume, but also the period over
which this volume is delivered.

Finally, for total event flow volumes in the Nueces River of less than 12,336 10' m' (10,000 acre-ft), the
fractional salinity response became extremely noisy, and even became positive for a significant number of the
events. Restating this observation in another way, below a threshold volume in the river, other non-
hydrological factors become equally or more important in affecting the salinity of Nueces Bay, and the apparent
value of this threshold is around 12,336 10' m'. Similar threshold-type controls on salinity were theoretically
expected and have been found to operate when salinity and flow data are adequate to characterize the response,
such as in Trinity Bay in the Galveston system. Although quantification of this threshold was not relevant to
evaluating the effects of the subject demonstration project, it may be important in devising operating strategies
for future such diversions.

Appendix B ♦ B-11

LITERATURE CITED

Bureau of Reclamation. 2000. Hydraulic i\nalysis of Rincon Bayou Using HEC-2, Rincon Bayou-Nueces

Marsh Wetlands Restoration and Enhancement Project. United States Department of the Interior, Bureau
of Reclamation, Great Plains Regional Office, Billings, Montana.

Irlbeck, M.}. and D. Ockerman. 2000. Technical notes on the Rincon gauge and data. Unpublished.

In Concluding Report: Rincon Bayou Demonstration Project, Appendix I-A. United States Department of
the Interior, Bureau of Reclamation, Austin, Texas.

Idbeck, MJ. and G.H. Ward. 2000. Analysis of the historic flow regime of the Nueces River into the upper
Nueces Delta, and of the potential restoration value of the Rincon Bayou Demonstration Project.
UnpubUshed. /« Concluding Report: Rincon Bayou Demonstration Project, Appendix I-B. United States
Department of the Interior, Bureau of Reclamation, Austin, Texas.

Ward, G.H. 1997. Processes and trends of circulation ^vithin the Corpus Christi Bay National Estuary Program
Study Area. Report CCBNEP-21, Corpus Christi Bay National Estuary Program, Corpus Christi, Texas.

B-12 ♦ Hydm^aphy of the Nueces De/ta and Estuary: 1992-1999

APPENDIX C

Analysis of the Historic Flow Regime
of the Nueces River into the Upper
Nueces Delta and of the Potential
Restoration Value of the Rincon Bayou
Demonstration Project

Michael J. Irlbeck U.S. Bixreau of Reclamation, Austin

Dt. George H. Ward Center for Research in Water Resources, University of Texas, Austin

In preparation for publication.

INTRODUCTION

Deltaic ecosystems are typically supported by at least one principal river system, and therefore have adapted to
and are dependant upon the dynamic fluctuations of that river's flow pattern. This natural flow regime includes
the fuU range of a river's flow quantity, timing and variability, which may fluctuate by hours, days, seasons, years
and even decades. Until recendy, the importance of the natural stream flow variabUity in maintaining healthy
aquatic ecosystems has been underappreciated in a water development and management framework (Kart
1991, Voiiet al. 1997). However, it has been shown that the integrity of flowing water systems depends largely
upon the components of their natural dynamic character, which is critical in regulating a variety of ecological
processes within these ecosystems (Poff and Ward 1989, Richter et al. 1996, Walker et al. 1995).

In south Texas, the ecological integrity of the Nueces Delta has been primarily determined by the natural flow
regime of the Nueces River, which includes the magnitude, frequency, duration and timing of flow events in the river.
Occasionally, high flows in the Nueces River cause the water elevation to exceed the diversion threshold (or,
rise above the lowest point along the river bank) and spill into the upper delta. These periodic freshwater
inundation events drive several key biological processes important for estuary productivity (Longley 1994). The
magnitude of a given flow event is simply the amount of water moving past a fixed location per uiut time.
Event duration is the period of time associated with a specific flow condition. The frequency of an event refers
to how often a flow at a given magnitude recurs over some specified period of time. And the timing of events,
or their seasonal predictabiUty, refers to the regularity with which flow events of a specified magnitude occur
within a specified time scale.

In 1993, as part of a broader initiative to restore freshwater flows to the greater Nueces Estuar)', the United
States Bureau of Reclamation (Reclamation) began a multi-year demonstration project designed to increase the
opportunity for natural fireshwater flow events to enter the upper Nueces Delta. A key component of the
Rincon Bayou Demonstration Project was a 900-m long overflow channel which was excavated to coimect the
Nueces River with the extreme upper reach of Rincon Bayou, the dominant hydraulic feature of the upper
delta. A second overflow channel, which connected Rincon Bayou with a broad area of barren tidal flats in the
upper delta, was also excavated. These features effectively lowered the flooding threshold of the upper delta
and improved the distribution of diverted freshwater.

If the historic flow regime characteristics of the upper Nueces Delta could be defined for the recent past, the
impacts of human activities, particularly that of reservoir development and management within the Nueces
River watershed, could be analyzed expUddy. These same components could also be used to evaluate the
potential alterations to the natural flow regime of the delta system resulting from a variety of restoration or
enhancement activities.

Appendix C ♦ C-1

OBJECTIVES

The objectives of this analysis were to characterize, through analysis of the magnitude, frequency, duration and
timing of flow events:

1) changes in the historic freshwater flow regime of the upper Nueces Delta, and

2) the potential for restoration of freshwater flow resulting from the Rincon Bayou Demonstration Project.

BACKGROUND

Description of the Study Area

As the Nueces River flows toward Corpus Christi Bay along the south Texas coast, it passes along the southern
edge of a large delta located in southern San Patricio County (Figvue 1). This delta is an integral part of the
Nueces Estuar)', and is roughly 70 square kilometers (km) in size. The water surface for most of the channels
and ponds in the delta is very near sea level, and the low-l^ing flats are intermittendy inundated from the bay by
tides and storm surges. The delta is crossed from north to south by numerous buried pipelines and two
Missouri-Pacific railroads. One of the most dominant hydraulic features of the upper delta is a broad, tidally-
influenced channel known as Rincon Bayou. Between the railroads, the upper delta is separated by a modest
ridge of high ground along the southern bank of Rincon Bayou. There is some evidence that this crest is what
remains of early diking efforts in the delta, probably for agricultural purposes (US Ejigineer Office 1939).
Downstream of the eastern-most railroad, the elevations are more uniform and tidal interactions make a
separation of the northern and southern delta less distinct.

The Nueces River Basin has a total watershed area of approximately 44,224 km^. Occasionally, larger flood
flows in the river spill over the northern bank and inundate the delta with freshwater. These events usually
occur shortiy after periods of heavy rainfall in the basin, usually coinciding with tropical storm activit)' in early
fall or with the passage of frontal systems in late spring. These sporadic flooding events supply freshwater to
plant communities, transport detrital materials from the vegetation and sediments to the bay, provide a medium
for nutrient exchange and buffer bay salinity. The Nueces Delta marsh is therefore one of the most important
sources of nutrient material for the Nueces Estuary system (Texas Department of Water Resources 1981).

Historical Changes in the Study Area

Physical Changes in the Nueces Delta

From a broad perspective, large-scale physical form of the delta has changed Utde during the period under
review {i.e., 1940-1999). Through comparisons of current and historic maps, each of the two railroads which
cross the delta from north to south were in places by 1940, and most of the larger pools and channels in the
delta resembled their present locations, shapes and sizes (US Engineer Office 1939). The two railroad crossings
were elevated by fill material for most of their span, with the exception of a few bridged crossmgs over the
more significant channels. This construction method undoubtedly changed the fundamental hydraulics of the
delta system by isolating, restricting, and channelizing some it's water courses, but these changes had already
occurred well before the beginning of the period under consideration {i.e., 1940).

The road bridge over the Nueces River near Calallen has been in place since before 1930, but has been rebuilt
on several occasions (Texas State Highway Department 1931). In 1931, a two-lane tresde bridge was
constructed by the State of Texas as part of improvements to what was then called State Highway 9. This
structure was removed and replaced with a standard bridge in 1956, and then improved to a four-lane bridge in
1959 as part of the upgrade to Interstate Highway 37 flexas State Highway Department 1959). Finally, in 1983,
the Interstate Highway 37 (IH 37) bridge was upgraded to its current form.

Presendy, the flooding threshold for the majority of the north bank of the Nueces River in the upper reaches of
the delta is about 2.36 m (7.75 fit) msl, although some lower channels are present. Given the natural effects of
scouring and deposition during large flow events, the Nueces River has likely cut and filled a coundess number
of depressions in its geologic past, continually altering the flooding threshold with each event. In addition to
natural causes, human activity has also contributed to this process of change. As one example, numerous large
pieces of concrete rubble and re-bar have been unearthed along the north bank of the river just downstream of

C-2 ^ Analysis of the Historic Flow Reffme of the Nueces River into the Upper Nueces Delta

Approximate Scale

Figure 1 : The Nueces Delta. Generally depicted are open water (shaded) and tidally influenced areas.
Also shown are the overflow channels that were part of the Rincon Bayou Demonstration Project. Sources
of base map: Salas 1993.

IH 37 (Bureau of Reclamation 2000). It is speculated that this material was intentionally placed in low portions
of the bank to reduce flooding of adjacent pastures, and was probably acquired from the road bridge renovation
during the late 1950's.

Reservoir Consttuction in the Nueces Watershed

Reservoir development in the Nueces Basin has primarily been initiated by the population centers along the
coast in efforts to secure a reliable freshwater supply. The fundamental objective of building such
impoundments was to capture large flood events in reservoirs instead of allowing the water to pass into the
bays. Stored water could then be released slowly for municipal and industrial consumption, thereby providing a
dependable freshwater supply for the community, especially during the periods of frequent drought.

The largest of these communities is the City of Corpus Christi, which was incorporated in 1852. At that time,
its citizens obtained their domestic water from shallow wells, cisterns, and natural depressions existing in an
arroyo that ran through town. However, during drought conditions, the City often suffered from water
shortages when the cisterns and tanks became low or dry from lack of rainfall. Bay water was used to meet any
fire fighting needs. In 1887, a committee appointed by the City investigated the possibihty of obtaining an
adequate water supply from local ground water, but exploration drilling found the source to be brackish and
unsuitable for domestic purposes.

Abandoning the idea of using local groundwater, the City looked to the Nueces River. In 1893, the City
finished construction of a steam-powered pumping station and distribution system that would provide raw
water to the City. The pumping station was located on the south bank of the Nueces River at Calallen, some 16
miles to the northwest of the City. To prevent saltwater from being pumped and distributed during low- flow
periods, the City constructed a temporary wooden dam (Calallen Dam) on the Nueces River just downstream

Appendix C ^ C-3

of the intake. Then in 1898, a pennanent rock-filled dam with a crest elevation of 0.46 m (1.5 ft) msl was
completed at the same location. In 1916, the first treatment plant was added at the Calallen pump station. In
1935, the current concrete structure known as Calallen Diversion Dam was completed with a crest elevation of
approximately 1.36 m (4.52 ft) msl.

During 1917, another drought threatened the municipal water supply of the growing City of Corpus Christi and
the surrounding area. A series of engineering surveys identified a suitable site for a large reservoir on the
Nueces River some 35 miles upstream of Calallen Dam. The purpose of the new dam would be to store water
for release during the dry periods. Mathis Dam was originally constructed by the Cit}' of Corpus Christi early in
1930, but failed in November of that same year. In 1934, a second dam across the Nueces River (La Fruta
Dam) was constructed. This new reservoir had a storage capacity of approximately 67,849 10"* m'
(55,000 acre-ft).

From 1930 to 1950, the population of the Corpus Christi area increased by over 400% (Corpus Christi 1990).
As a result of this increase in demand for municipal and industrial water, and from the loss of reservoir storage
in La Fruta Reservoir due to siltation, a new water source was sought, again from the Nueces River. After
several years of study, the State of Texas financed the design and construction of Wesley Seale Dam (Lake
Corpus Christi), which was completed in 1958. This dam was located about 300 m downstream from La Fruta
dam site and was some 6 m higher, inundating the former structure. During the first six years of operation.
Lake Corpus Christi was maintained at 26.8 m (88.0 ft) msl, or a capacity of 229,355 10^ m' (185,922 acre-ft), to
allow for depletion of oil fields located in the reservoir basin. The crest gates were finally closed on July 1,
1964, bringing the operational lake elevation to 28.6 m (94.0 ft) msl, thereby increasing the storage capacity to
372,550 10' m' (302,000 acre-ft).

Forecasts of future water requirements for the Coastal Bend area indicated that demand would exceed the firm
aimual yield of Lake Corpus Christi during the 1 980's. In response to the Area Development Water
Subcommittee's recommendation, the City of Corpus Christi engaged the Bureau of Reclamation to sur\'ey the
lower Nueces River Basin to determine a feasible location for a new reservoir to supplement Lake Corpus
Christi. Reclamation began construction on Choke Canyon Dam in the summer of 1979, and the project was
declared "substantially complete" on May 18, 1982. Due to a drought affecting the Frio River's 14,323 km"
watershed, flow into the new reservoir was minimal during the first three years, and by May 31, 1987, the
reservoir was only at 48% of capacity. However, record rainfall on the Frio River watershed filled the reservoir
to 100% on June 18, 1987, and water was released as flood control for the first time. At this level. Choke
Canyon Dam impounds approximately 852,584 10' m' (691,130 acre-ft).

Developed for the purposes of providing a reliable and municipal water supply and flood protection, these
dams have contributed to reduced streamflow in the lower Nueces River by their diminutive influence on larger
river hydrographs, and through direct water loss to consumptive uses and evaporation. The present permitted
firm yield of the reservoir system is 139,000 acre-ft, and a portion of the dehvered water returns to the estuary
through treated return flows. Because of the relative shallow depth of the two reser^'oirs and the hot summer
climate, evaporation from these two water bodies can remove a significant amount of water from the river
system. For example, during 1999 alone, over 217,730 10' m' (176,500 acre-ft) were lost to evaporation from
the combined reservoir system (Hilzinger 2000).

Other Changes in the Nueces Watershed

Another possible factor contributing to decreased stream flow in the lower Nueces River are increased non-
reservoir surface water withdrawals in the greater watershed. For example, long-term (1940 to 1990) analysis of
reported surface water withdrawls m the basin upstream of the reservoirs indicates an increase of about 60%
from 1965 to 1990 (Greene and Slade 1995), which includes much of the operational time period of the current
reservoir system.

Also, a decreasing precipitation trend in the Nueces River watershed would be expected to reduce streamflow.
However, after analyzing rainfall data from four south Texas gauges (Cotulla, BeeviUe, Sabinal and Corpus
Christi) reflecting conditions for the Nueces watershed, Medina (2000) found that annual precipitation (using a
base period that consisted of data since 1900) produced no particular trend (Figures 2a through 2c). Using a
baseline that began during the late 1940's (e.g.. Figure 2d), annual precipitation portrayed an increasing trend
(Medina 2000). The most prominent and common feature of the precipitation data at all stations was the
drought of the late 1940's and early 1950's. Similarly, Asquith et a/. (1997) also found Utde evidence for
statistical trends in precipitation along the Coastal Bend from 1968 through 1993.

C-4 ^ Analysis of the Historic Flow B^ffme of the Nueces River into the Upper Nueces De/ta

(a) Cotulla

50| — \ — r-

40-

s 30

a.

20

10

T 1 1 1 1 r

^qp^^<i*^^Q^-v^°,'«^°.^^>*^^'*-\^#^°?'V''

(b) Beeville

SOrJ-r

"1 — I — I — 1 — r

10

J I I I I I I I L

^c»^\<i^^^<»T-\<i'^\<b'>\<b^\q,^V\#^<*'*V''

(c) Sabina
50

40

1 1 1 1 1 r

10

J I \ I I

^q,^^cb^^^<i^^^'^^^<i^^<.«'^^#v^<**^<i<^V''

(d) Corpus Christi

50 1 — I — I — I — I — I — I — I — r

40

30

a.

20

10

I I I I I I I I I

^oP\c,NV\<*'^\^'^V\<^*\<^'^\<**''N<i'*V''

Approximate Scale

Kilomelers

Figure 2: Annual precipitation trends at four gauges about the greater Nueces River watershed
since about 1900.

Source: Medina 2000.

Note: 1 inch = 2.54 cm

Appendix C ♦ C-5

METHODS

As previously discussed, the Nueces Delta has been a dynamic system in the past, experiencing changes from
both natural and man-made causes. During the past 60 years, the major attributes of the delta Hke general
topography, railroad crossings and channel locations have not changed significandy, while smaller
modifications, such as flooding thresholds, have occurred. However, lacking the information necessary to
define more subde changes in the study area or to analyze their effects, the assumption was made for the
purposes of this analysis that the delta's physical characteristics during the entire period under study have
essentially resembled those observed in the field during 1993.

Data Sources

Available data included daily discharge and stage
records for several gaizges on the Nueces River
operated and maintained by the United States
Geological Survey (USGS) (Figure 3).

Mathis Gauge

Station 08211 000, the Nueces River Near Mathis,
Texas (Mathis gauge), is located on the Nueces River
0.96 km downstream of Wesley Seale Dam (Lake
Corpus Christi). The gauge receives flow from
approximately 43,512 km^ or 98.4%, of the
watershed. Daily discharge and stage data were
available from August 1 939, to present. The
maximum daily stage value recorded at the Mathis
gauge was approximately 3,546 cubic meters per
second (mVs) (125,000 cfs) on September 25, 1967.

Calallen Gauge

Station 08211500, the Nueces River at Calallen,

Texas (Calallen gauge), is located on the Nueces

River 0.64 km upstream from Calallen Diversion

Dam, approximately 2.3 km upstream from the

bridge on Interstate Highway 37 and some 54.6 km

downstream from the Mathis gauge. This gauge

receives flow from approximately 44,071 km^, or 99.7% of the watershed. Unpublished daily stage data were

available for the period of April 1920 through July 1950 from the USGS District office in Austin, Texas.

Reliable daily discharge and stage data are published from October 1989, to present. The Calallen gauge is

operated as a low-flow gauge, with daily discharges published only for days when instantaneous maximum

discharge does not exceed 72.8 m^/s (2,570 cfs) (Gandara et al. 1996), which corresponds to a daily stage value

of about 2.48 m (8.14 ft). However, higher daily stage values up to 4.13 m (13.55 ft) were available from the

unpublished record. Datum of the gauge is 0.84 ft above mean sea level.

Rincon Gauge

Station 0821 1503, Rincon Bayou Channel near Calallen, Texas, (Rincon gauge) was installed and operated by
the USGS at the request of the U.S. Bureau of Reclamation as part of their Rincon Bayou Demonstration
Project. The objective of this project was to increase the opportunity for freshwater flow events into the upper
Nueces Delta. The main feature of the demonstration project was a 305-meter (m) overflow channel excavated
from the Nueces River at the point of natural diversion to a small headwater of Rincon Bayou. The Rincon
gauge is located in this headwater channel approximately 310 m downstream from the north bank of the
Nueces River. Daily stage, discharge and precipitation data are available from May 1 996 through December
1999. The maximum daily stage value recorded at the Rincon gauge was 2.21 m (7.25 ft) on October 21, 1998.
Datum of the gauge is at mean sea level.

^^^B SAN PATRICIO

Approiimale Scale
Kilomalafi

Corpus Christi

0 12 3 4 5

LIVE OAK Wesley Seale Dam

MATHIS 1 Nueces
Gauge f R'^^r

^ Hondo Creek

JIM WELLS V*i»<\~~~--.

-^

~\ RINCON
j Gaugfr

NUECES Calallen Div(|

gion

( fA Upper

^V^ Nueces

Dam^ r^.

calallenV*

Gauge

Figure 3: Diagram of the lower Nueces River
showing the location of selected stream flow
gauging stations..

C-6 V Analysis of the Historic Flow Reffme of the Nueces River into the Upper Nueces Delta

Developing a Daily Stage Record for the Nueces River at the Point of Diversion

The analysis of the Nueces River's flow regime and associated delta flooding characteristics over the past 60
years required a daily stage record for the Nueces River at the point of natural diversion, which was not
available. The "point of natural diversion", for the purposes of this investigation, was generaUy defined as the
2,000-m reach of the north bank of the Nueces River from Interstate Highway 37 downstream to where it
sharply bends to the south (Figure 4). In lieu of actual gauge data for this reach of the river, an artificial stage
record was developed from correlations of data collected at other gauges in the lower Nueces watershed. This
simulated daily record was then used to generally represent flow conditions at the point of diversion.

Figure 4: The "point of diversion" along the north banl( of the Nueces River. The location of the five
natural depressions in the river bank which contribute to delta inflow during a flood event are indicated by
arrows (Bureau of Reclamation 2000).

Although measures were taken to minimize the error Ln the manufactured data record, the accuracy of specific
quantitative values estimated by this method is debatable. However, this method does provide a reasonable
estimate of general hydraulic conditions in the Nueces River and upper delta during the period under review,
and therefore was considered to be acceptable for use in analyzing qualitative {i.e., relative) differences in flow
event characteristics. Furthermore, this analysis was exclusively focused on the upper Nueces Delta, or that
northern portion of the delta primarily influenced by Rincon Bayou. No attempt was made to characterize the
flow regime of the lower (or southern) portions of the delta.

The procedure used to create a daily record of discharge into the upper Nueces Delta from daily Mathis
discharge data included two basic steps. First, gauge data was used to correct and correlate Mathis discharge
with stage values at Calallen (the Mathis gauge provided the longest reliable daily flow and stage data for the
longest continuous period of record on the lower Nueces watershed, or from August 1939 to present). Second,
gauge data was used to correlate daily Calallen stage with stage values at the point of diversion.

Appendix C ♦ C-7

Correlating Mathis Discharge to Estimated Stage at Calallen

Data from the concurrent periods shared by the Mathis and Calallen gauges (daily records from January 1940
through July 1950, and October 1989 dirough December 1999) were used to correlate Mathis discharge to Caktikn
stage. Prior to this correlation, several modifications to the original Mathis data set were made.

First, the travel time of flow events was examined for the period of conctirrent data between the Mathis and
Calallen gauges. Based upon this analysis, the average travel time for flow events in the Nueces River to travel
from Mathis to Calallen was determined to be about two days. The entire Mathis data set was therefore
corrected by delaying each daily flow value by 48 hours. When analyzed individually, however, the actual travel
time for the flow peak of any one flow event, which depends greatly upon the characteastics of the individual
flood event, has been estimated to take as many as 5 days (Ward 1985). The average correction was used
because individual correction of each flow event was not possible for the entire data set due to the absence of
daily stage data at Calallen for the period of 1950 through 1989.

Second, the resulting data set was corrected to account for channel losses in the reach between the Mathis and
CalaUen gauges. HDR Engineering, Inc., et al. (1991) calculated an average loss rate of 0.1243% per river
kilometers, or a total of 7.0% for the entire 56.3 km reach between Lake Corpus Christi and Calallen Dam.
This rate was based on field measurements reported by the United States Geological Survey (1968), and is
representative of the loss rate during periods of normal water deliveries with minimal intervening flows. The
distance between the Mathis and Calallen gauges is only 54.6 km, sUghdy shorter than between the two dams,
which resulted in only a 6.8% loss rate. A conservative arbitrary value of 8.5 m'/s (300 cfs) was used to
represent the upper limit of "minimal intervening flow". Daily Mathis values below this limit were corrected by
a loss of 6.8%. Stream flow losses for daily values in excess of 8.5 m'/s were assumed to be constant at
0.58 mVs (20.4 cfs), or 6.8% times 8.5 mVs.

Third, the resulting data set was corrected to account for the estimated total daily municipal and industrial
withdrawals made from the Nueces River at or before Calallen Diversion Dam. Estimated total annual
withdrawals for every tenth year of the period of record were derived from a number of sources (Homer &
Shifrin Consulting Engineers 1951, Bureau of Reclamation 1971, Corpus Christi 1990), and then estimated for
each intervening year assuming a linear relationship between the decade totals. Each total annual withdrawal
amount was then divided by an average monthly percentage of water use for the Coastal Bend area (Bureau of
Reclamation 1971), and the resulting values converted from total volume to average daily flow. These daily
values, which represent the estimated daily municipal and industrial withdrawal for that month and year, were
then subtracted from the modified Mathis data set. The daily correction values range from 0.20 to 0.25 m'/s
(7 to 9 cfs) in 1940, and from 4.36 to 6.20 mVs (154 to 219 cfs) in 1999.

Once the Mathis data set had been thus corrected, the relation between this discharge data and published
CalaUen stage data was determined through linear regression (Figure 5). Daily values greater than 2.44 m (8.0 ft)
from the unpublished Calallen data were also used. In addition, one point was added to the data set as an
estimate of the extreme condition. The largest daily flow value recorded at Mathis was 3,539 m'/s (125,000 cfs)
on September 25, 1967, and was a result of the massive flooding caused by Hurricane Beulah. This event
occurred during the period for which there is no available stage data at Calallen, but other sources have
reported that the Nueces River at Calallen crested at 5.02 m (16.48 ft) the same day (Corpus Christi Times
1967).

This relationship therefore allows the use of Mathis daily discharge data as the independent variable to solve for the corresponding
estimated daily stage at Calallen. The entire corrected Mathis discharge data set was then converted to estimated
stage at Calallen using this relation.

There are three primary limitations to this method of correlating Mathis and Calallen gauge data. First, as
previously discussed, travel times for each flow event vary, which compromises the temporal accuracy of the
estimations. The inability to correct each flow event in the record for travel time requires the acceptance of this
error. Second, although there is very litde additional watershed below Mathis and above Calallen (only 559
km'^, some locally intense storms produce flow events at Calallen that are not recorded at Mathis. This
relational difference between the Mathis and Calallen gauge data sets is especially high for tropical storm events
that move onshore from the Gulf. Finally, stage data for the Nueces River at Calallen is not available for values
in excess of 4.13 m (13.55 ft) (which roughly correspond to a discharge of about 50,000 cfs at Mathis), requiring
extrapolation for higher discharge values.

C-8 ^* Analysis of the Historic Flow Reffme of the Nueces River into the Upper Nueces Delta

c

CO

O

ro

<D
05
TO

CD

Q

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

I I ^*

::=^^» :: -' -

20000 40000 60000 80000 100000

Corrected daily discharge at Mathis (cfs)

120000

Figure 5: Regression analysis of corrected daily Mathis discharge and daily stage at Calallen.

Note; 1 (rfs = 0.0283 m'/s; 1 ft = 0.3046 m

Correlating Calallen Stage to Estimated Stage at the Point of Diversion

Stage or flow data for the Nueces River downstream of Calallen Dam is extremely limited, but were available
from the Rincon gauge for a period of approximately i'A years. This gauge was located in the headwater
channel of Rincon Bayou, approximately 310 m downstream from the point of natural diversion of Nueces
River. Therefore, the concurrent period shared by the Calallen and Rincon gauges (daily flow and stage records
from May 1996 through December 1999) was used to correlate Calallen stage to estimated stage at the point of
diversion. This included one modification to the original Rincon data set prior to the correlation.

At higher stage values, the Rincon gauge experienced a minor head loss during discharge events, which was
estimated to be as much as 0.08 m (0.25 ft) by the Bureau of Reclamation using a hydraulic model (2000). The
Rincon stage data was therefore corrected using this model to represent stage at the point of natural diversion
before being correlated with Calallen gauge data.

Once the Rincon data set had been corrected, the relationship between daily Calallen stage values and corrected
daily stage data at the point of diversion was determined through linear regression (Figure 6). Not used were
data from several anomalous stage events recorded at the Rincon gauge (October 1996, November 1996, April
1997, June 1998, and May 1999) which were not associated with discharge at Calallen. In addition, data from
two previous maximum stage events were used as estimates of the extreme conditions. First, during the flood
of 1919, the maximum stage of the Nueces River recorded at Calallen by the USGS was 4.16 m (13.65 ft), and
at the Interstate Highway bridge (representing point of diversion) was 3.75 m (12.3 ft) (Texas State Highway
Department 1956). Second, during the 1967 flood, the maximum stage recorded at Calallen was 5.02 m (16.48
ft) (Corpus Christi Times 1967), and at the bridge was 4.63 m (15.2 ft) ^exas State Department of Highways
and Public Transportation 1983).

This relationship therefore allows the use of Calallen daily stage data as the independent variable to solve for the corresponding
estimated daily stage at the point of natural diversion. The entire estimated Calallen stage data set was then converted
to estimated stage at the point of diversion using this relation.

Appendix C ♦ C-9

15 -
"^ 14 -
^ 13

^

" 12 -

♦

1 11-

^ 10 -

a 9 -

o 8

-I-'

•i 7-

i 6-
0 5 -
CO 4 -

♦ ^

►

#

♦

ll

r

A

if

iP

r

Daily

m^

w

i

^

P

n .

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Daily stage at Calallen (ft)

Figure 6: Regression analysis of Calallen daily stage values and daily stage at the Point of
Diversion.

Note: 1 ft = 0.3046 m

There are three primary limitations to correlating Calallen stage to stage at the point of diversion. First, under

certain conditions, Calallen gauge data does not represent the total river discharge under the IH 37 and past the

point of diversion. At higher flows, a portion of the Nueces River spills out of the stream channel upstream of

the gauge, and re-enters the river via Hondo Creek below Calallen Dam but above the point of natural

diversion. This portion of the river's flow,

although relatively small, evades detection

at the Calallen gauge (Figures 3 and 7).

Second, a variety of tides, storm surges

and other hydrographic events in Nueces

and Corpus Christi bays also affect the

Rincon stage data. Because the Calallen

gauge is insulated from all but the most

extreme of these events by Calallen

Diversion Dam, correlation between the

two data sets, especially under low-flow

conditions, was "noisy". Finally, the lack

of a long-term record at Rincon limits the

ability to analyze a full range of hydraulic

conditions in the Nueces River.

In summary, the first two steps of this
methodology produce a daily stage record

which represents the approximate water ^'^^'^ ^r View of Hondo Creek under flood conditions

level of the Nueces River at the point of ^""f 26. 1 997. This road crossing is located approximately

... r ^ J f 2 miles upstream of the creek s confluence With the Nueces

natural diversion for the penod of ^^^^ ^^^^^ .^ downstream of the Calallen gauge. The flow is

January 1, 1940, to December 31, 1999. 3 ^^^^^ ^^ ^^^^^ ^^^^ ^^^ ^^^^^ pj^g^ at a point further

upstream. Ptioto courtesy of the Bureau of Reclamation.

C- 1 0 ^ Analysis of the Historic Flow Rtffme of the Nueces River into the Upper Nueces Delta

Estimating Freshwater Inflow into the Upper Nueces Delta

Once a representative daily stage record had been developed for the Nueces River at the point of diversion, two
different sets of stage-discharge rating curves were used to estimate daily discharge into the upper delta. Prior
to this analysis, the Bureau of Reclamation, as part of their Rincon Bayou Demonstration Project, conducted
(1996) and revised (2000) a hydraulic study of the relation between flow in the Nueces River and that in Rincon
Bayou. These modeling efforts, which did not address the southern portion of the delta, produced a series of
rating curves based on field conditions during March of 1993. Among the scenarios developed were the pre-
project (or historical) condition, and the post-project (restored) condition.

Historical Freshwater Inflow into the Upper Nueces Delta

Under the without-project (historical) scenario analyzed by the Bureau of Reclamation (2000), a total of five
natural depressions in the north bank of the Nueces River were identified tliat would naturally contribute to
discharge into the delta at various stages in the river (Figure 4). The lowest of these drainage channels was
along the west side of the Missouri-Pacific railroad bridge, which had an effective bottom elevation of about
1 .64 m (5.4 ft) msl. The hydraulic characteristics of each of these depressions were combined into one
commutative set of rating curves, including both a rising and falling limbs, for daily discharge into the upper
Nueces Delta (Figures 8 and 9). The obvious 'Tsend" in the rating curves reveal the natural flooding threshold
for the greatest part of the river bank, which is about 2.36 m (7.75 ft) msl. Reclamation's without-project rating
curves were used without modification, and estimated daily stage values for the Nueces River at the point of
diversion were converted to estimate daily discharge into the upper Nueces Delta.

Potential for Restored Freshwater Inflow into the Upper Nueces Delta

Base on data obtained from the Rincon gauge, the Bureau of Reclamation (2000) also constructed a set of rating
curves, including both rising and falling Umbs, which estimated daily discharge into the upper Nueces Delta
with the demonstration project features in place (restored condition) (Figures 8 and 9). Because of the
compromising effect of tide on water elevations at the point of diversion, and therefore on discharge estimates,
these curves did not estimate discharge into the upper delta when the stage in the Nueces River was 0.76 m
(2.50 ft) or below.

However, when discharge estimates for individual events using the falling limb curve (which represented an
average of several observed events) were compared with actual discharge data from the Rincon gauge, the
results were unsatisfactorily inconsistent. Upon examination, it was discovered that, although the falling limb
curve of each event expressed the same slope when plotted, the beginning point of each curve depended upon
the maximum water surface elevation attained by the Nueces River during that particular event. Therefore, a
series of falling Umb curves were subsequendy constructed by extrapolation of the average curve in 0.1 -ft
intervals. Discharge estimates for individual events using the modified, event-specific set of rating curves were
then tested for accuracy, this time with acceptable results. For each of the eight (8) major freshwater flow
events recorded at the Rincon gauge, the estimated discharge was within about 10% of the actual for five
(5) events, and within about 25% of the actual for two (2) others (Table 1). The combined accuracy for all
discharge estimates made using the revised rating curves for the restored condition, including all eight events,
was about 14% over the actual gauged discharge value.

Once an acceptable set of rating curves for the "restored" condition was thus developed, the estimated daily
stage values for the Nueces River at the point of diversion were converted to estimated daily discharge into the
upper Nueces Delta.

Definition of Flow Regime Parameters

From the two sets of daily inflow data {i.e., historic and restored conditions), fovir separate flow regime
characteristics were analyzed; including, event magnitude, duration, frequency and timing. First of all, event magnitude
was used to indicate the amoimt of discharge from the Nueces River into the upper Nueces Delta during the
period under consideration, and was determined by separating the estimated daily discharge values by period,
and then averaging these by month. Next, event duration was used to express the cumulative length of flow
events, and was determined by averaging the total number of days in which the stage of the Nueces River
exceeded the flooding threshold (event days), regardless of discharge amounts. Also, e.v&at frequency was used to
estimate the return period of peak daily flow events into the delta, and was determined by summing the number
of events in each period that attained a given peak daily discharge amount, and then dividing this total by the

Appendix C ♦ C-11

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Daily stage in the Nueces River at the Point of Diversion (ft msl)

Figure 8: Stage-discharge rating curves for the Nueces River into the upper
Nueces Delta, including both historic and restored conditions. Full range of
stage values shown.

Note: 1 acre-ft = 1.2335 10=' mM ft = 0.3046 m.

(0 (B

-a 3
-oZ
0) •-

■S <i>
m Q.

E Q.

S|

o

2000

1800

1600

1400

1200

1000

800

600

400

200

0

Historic conditions

Restored conditions

!5 30 35 40 45 50 55 60 65 70 75 80 8.!

Daily stage in the Nueces River at the Point of Diversion (ft msl)

Figure 9: Stage-discharge rating curves for the Nueces River into the upper
Nueces Delta, including both historic and restored conditions. Only lower stage
values shown to emphasize the rising and (average) falling limbs of each cun/e.

Note: 1 acre-ft = 1.2335 10' m^ 1 ft = 0.3046 m.

C-12 ^ Analysis of the Historic Flow Ke^me of the Nueces River into the Upper Nueces Delta

Table 1: Comparison of estimated versus actual
of Reclamation (2000), with modified falling limb

during each event were used.

discharge into the upper Nueces Delta using curves from Bureau
curves. Only daily stage values greater than 0.76 m (2.5 ft) msl

EVENT DATE

Total

Duration

(days)

Actual

Discharge

(acre-ft)

Estimated

Discharge

(acre-ft)

Deviation
from Actual

Maximum Stage
Attained
(ft msl)

1997

1998

1999

Jun 23 -Julio

27

1,657

1,559

-5.9%

5.72

Oct 9-17

9

466

1,232

153.5%

5.74

Mar 30

1

24

26

8.3%

2.6

Sep 5-21

17

614

547

-10.9%

4.43

Sep 25 -Nov 11

48

3,609

3,888

7.7%

7.31

Mar 27 - Apr 7

12

191

201

5.2%

3.36

Jun 30 - Jul 7

8

171

130

-24.0%

3.22

Aug 23 -Sep 13

22

988

1,254

26.9%

5.81

TOTAL

7,740

8,837

14.2%

Note: 1 acre-fl = 1.2336 10' mM ft = 0.3046 m

number of years in the period (annual frequency). Daily peak values greater than 123 10^ m^ (100 acre-ft) were
rounded to the nearest hundred, and those less than this were rounded to the nearest ten. From annual
frequency, cvimulative frequency was then determined by incremental summation. The return period of peak
flow events for a given magnitude in each period was then calcvilated as the inverse of cumulative frequency.
Finally, event timing was used to identify seasonal and annual patterns in flow events, and was determined by
Slimming daily discharge values by week for the entire period vinder review.

RESULTS

For pvuposes of comparison, the 60-year record under investigation was divided into three separate periods,
each corresponding to the construction of a major reservoir in the basin (Table 2).

Period I extends from January 1, 1940 to April 9, 1958 (approximately 18.3 years). During this period the only
major regulating structure in the basin was La Fruta Dam on the Nueces River. The dam's mfluence on larger
flood events in the watershed was limited because the storage capacity of the reservoirs capacity was relatively
small to begin with, and this decreased significandy overtime due to sedimentation (City of Corpus Christi
1990). Given the absence of data prior to this structure. Period I therefore represents approximate "baseline"
conditions in the watershed with, minimal influences on stream flow from reservoir construction.

Period II extends from April 10, 1958, when Wesley E. Seale Dam was closed, to May 17, 1982 (approximately
24.1 years). Wesley Seale Dam was also constructed on the Nueces River just downstream of the La Fruta dam
site, submerging and replacing it as the City of Corpus Christi's primary water supply. Once completed, the
larger size of Wesley Seale Dam enabled it to more significandy affect flood events. Period II therefore
represents an intermediate period of reservoir development in the watershed.

Period III extends from May 18, 1982, when Choke Canyon Dam on the Frio River was declared substantially
complete, to December 31, 1999 (approximately 17.6 years). The addition of Choke Canyon Dam's storage
capacity to that of Lake Corpus Christi increased the total storage capacity in the basin to over 1,221,165 10' m'
(990,000 acre-ft). For die latter part of this period (since June 24, 1997), Lake Corpus Christi was operated at
an elevation of only 27.74 m (91.0 ft) msl (effective storage capacity of approximately 229,431 10' m'
(186,000 acre-ft)) because of safety concerns. Period III therefore represents the climax period (or present
conditions) of reservoir development and operation in the watershed.

For each of these three periods, data from the largest flood event in that period was not considered in the
analysis of flood event magnitude, frequency and duration for two primary reasons. First, these events were
considered extra-ordinary, and therefore were not typical of the more frequent flow events of primary interest

Appendix C ♦ C-13

in this analysis. Second, the dutadon of
each of the three periods was not deemed
adequate to statistically appreciate the full
dimensions of these events. For example,
data for the flood event of 1967 (Period
II), which is the largest on record in the
past century, would have only been
considered within a 24. 1 -year span, and
would therefore have unacceptably skewed
the results of that period, especially during
the months in which the event occurred.
Accordingly, the omitted flow events for
Periods I, II and III were July 1942,
September-October 1967, and June-July
1987, respectively. ^\11 flow events,
however, were included in the analysis of
event timing, which is a more subjective
measure where the fiill dimensions of each
event are relevant for purposes of historical
comparison.

c
o

40

30 -

g.
o

0)

i_
a.

c
c
<

n 20

10

1940-1957 1958-1981 1982-1999

H Sabinal

I I Corpus Christi

19 Beeville 5 NE
□ Cotulla

Figure 10: Mean annual precipitation of available data at
four gauges about the greater Nueces River watershed.

Source: Medina 2000. Note: 1 inch = 2.54 cm

It is interesting to note that Medina (2000)

found the mean annual precipitation in the greater watershed during the first period (1940 through 1957) was
consistendy the smallest when compared with the other two periods (1958 through 1981, and 1982 through
1999) (Figure 10). The distribution of large precipitation events were also found to be less frequent during the
drovight years of the late 1940's and early 1950's than in the latter two periods, which were not significandy
different from each other (Medina 2000). This information is relevant because this first period, which was used
as the baseline for calculating percent changes in delta inflow, likely under-represents, to some degree, the actual
baseline conditions.

Historical Freshwater Inflow into the Upper Nueces Delta

Magnitude

Historical event magnitude during Period I exhibited two primary peaks, one in the spring (May) and one in the
fall (September), each at over 43,173 10^ m' (35,000 acre-fit) per month (Figure 11). Dtxring Period 11, event
magtjitude also peaked twice during the year, but at lower discharge amounts. The spring peak Qune), which was
less defined than in Period I, attained only about 8,635 10^ m^ (7,000 acre-ft), while the fall peak (October)
attained almost 30,838 10^ m^ (25,000 acre-ft). During Period III, the trend of decreasing event magnitude was
observed in extreme. No annual peaks were obvious, and the highest monthly average discharge amount was
less than 370 10' m' (300 acre-ft) (May).

Annual event magnitude represents the sum
of all daily discharge values during the
period divided by the period's duration in
years. During Period II, or after the
construction of Wesley Scale Dam, annual
event magnitude decreased by 39'yo
compared to Period I (Table 3). Since the
construction of Choke Canyon Dam
(Period III), the annual event magnitude of
discharge events into the upper Nueces
Delta decreased by over 99% from Period I.

Table 3: Summary of historic annual event magnitude In the
upper Nueces Delta. Mean discharge values rounded to the nearest
10 acre-ft.

Period

1940-1958
1958-1982
1982-1999

Historic

Mean Total Discharge

per Year

(acre-ft)

Percent Change
from Period I

128,000

78,000

540

-39%
-99.6%

Note: 1 acre-ft = 1 2335 10^ m=

C- 1 4 ^ Anafysis of the Historic Fhw Keffme of the Nueces River into the Upper Nueces Delta

45000

„ 40000

it:

b 35000

O) 30000
ra

« 25000

2

(D
>
01

20000 -

15000

10000
5000 J

1940-1958
1958-1982
1982-1999

^V

^'

IV

k

K

:^^

\.[

V

V

A^

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 1 1 : Historic magnitude of flow events into the upper Nueces Delta. Not

included were data from the largest event in each time period.

Note: 1 acre-ft = 1.2335 10' m'

Jan

Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 12: Historic duration of flow events into the upper Nueces Delta. Not

included were data from the largest event in each time period.

Appendix C ♦ C-15

100

1940-1958
1958-1982
- 1982-1999

^

.yy

^"^

10 100 1000 10000 100000

Peak daily discharge into the upper Nueces Delta (acre-ft)

Figure 13: Historic return frequency (ogive) of flow events into the upper
Nueces Delta. Not included were data from the largest event in each time period.

Note: 1 acre-ft = 1.2335 10^ ml

300000

o
■o

0)

IS

E

LU

100000 -

Figure 14: Historic timing of
flow events into the upper
Nueces Delta. Data from all
events in each time period
included. Scale of the y-axis is
curtailed from 700,000 acre-ft
(September 1967) to improve
resolution of lower values.
Note; 1 acre-ft = 1.2335 10' ml

C- 1 6 ♦ Anatfsis of the Historic Flow Regme of the Nueces River into the Upper Nueces Delta

Table 4: Summary of historic annual event duration in the upper
Nueces Delta.

Diuation

As with event magnitude, event duration under historical conditions also exhibited two seasonal peaks (Figure
12). During Period I, the spring peak (June) and fall peak (September) attained about 5 average event days per
month (5.0 and 4.8, respectively). During Period II, the spring peak (Jvme) was similar to that of Period I, but
attained only 3.0 average event days per month, and the fall peak (October) about 4.2 average event days. In
Period III, event duration also showed dramatic decreases, but unlike event magnitude, seasonal peaks were still
discemable. The spring peak (May) peaked at about 1 . 1 average event days per month, and the fall peak
(October) only about 0.5 average event days. During Period III, an anomalous third peak (Februar)') attained
about 0.6 average event days, which reflects a disproportional influence of an (uncommon) winter event in 1992.

Annual event duration represents the sum of
all event days during the period divided by
the period's duration in years. For Period I,
event duration averaged about 21.1 average
event days per year (Table 4). This mean fell
to about 16.8 event days during the period
after the closure of Wesley Seale Dam, and to
about 4.5 event days during the period after
the closure of Choke Canyon Dam.

Frequency

In general, the historical return period for event peak flows during Period I were slightly shorter than those in
Period II (except for the largest values), but not appreciably (Figure 13). Period III, however, did exhibit a
dramatic increase in the return period of event peak flows.

Period

1940-1958
1958-1982
1982-1999

Historic

Mean Number of Event

Days per Year

Percent Change
from Period I

21.1
16.8
4.5

-20%

-79%

The difference in the return period of a flow
event with a daily peak flow of 123.4 10^ m^
(100 acre-ft) between Periods I and II was
an increase of less than a month (0.06 years)
(Table 5). However, for the same event in
Period III, the return period rose by over
19 months (1.64 years). There was no
return period for events with daily peak
flows of greater than 617 lO'* m^ (500 acre-
ft) during Period III.

Timing

Table 5: Summary of the historic return period for a flow event
into the upper Nueces Delta with a daily peak flow of 100 acre-ft.

Period

I: 1940-1958
II: 1958-1982
III: 1982-1999

Historic

Return Period

(years)

Percent Change
from Period I

0.56
0.62
2.20

11%
293%

Note: 1 acre-tt = 1.2335 10^ m^

In the three-dimensional presentation of event timing (Figure 14), the x-axis represents the calendar year in
weeks, the y-axis represents the year of record and the z-axis represents total event discharge into the upper
Nueces Delta. The seasonal peaks (spring and fall) observed in the magnitude and duration analyses were also
manifest in event timing under historical conditions. As observed from the x-axis, spring flow events (May-June)
were more frequent and smaller than fall flow events (September-October), which were more sporadic but
generally larger during Periods I and II (Figure 13). This seasonal pattern, however, was noticeably absent during
Period III. Winter (December-January) and svimmer (August) flow events for all periods were very rare.

From the perspective of the y-axis, "wet" or "dry" periods in the delta were evident over the past 60 years. The
more significant "wet" years for each period include 1941, 1942, 1946 and 1957, during Period I; 1958, 1967,
1971, 1973 and 1981 during Period II; and 1987 and 1992 during Period III. Significant "dry" years in the delta
include 1943, 1947, 1950, 1952, and 1955-56 in Period I; 1962-63, 1966, 1969, 1972 and 1978 in Period II; and
all of Period III with the exception of 1987 and 1992. Except for an apparent decrease in the size of spring
events during Period II, there was no obvious difference in flow event appearance between Periods I and II.
There was, however, a marked difference in the comparison of flow events in Period III with either of the two
preceding periods. The only discemable events in Period III were the summer event of 1987 (which would have
been a much more significant event had not a large part of the flooding event served to fill and spill a nearly
empty Choke Canyon Reservoir), and the late winter and spring events of 1992. Even so, these two events,
while the largest in Period III, would be considered extremely small events in either of Periods I or II.
Therefore, both the relative number and size of flow events in the delta contrasted starkly with the two
preceding periods.

Appendix C ♦ C-17

Potential for Restored Freshwater Inflow into the Upper Nueces Delta

One way of assessing the potential for how the Rincon Bayou Demonstration Project might restore freshwater
flows to the upper Nueces Delta is to assiime that it had been in place in the past, and compare the results with
what actually occurred. Event magmtude, duration, frequency' and timing were therefore analyzed assuming that
the demonstration project features had been in place since the completion of Choke Canyon Reservoir
(Period III), which was generally assumed to represent realistic future conditions. The results from this
"restored" condition, when compared to those of the historical condition, would then give an indication of the
restorative potential of the project.

Magmtude

The restored event magnitude for Period III, when compared with average monthly volumes that historically
flowed into the upper Nueces Delta during Periods I and II, was almost imperceptible (Figure 15). The largest
average monthly discharge amount (May) was only 1,208 10' m' (980 acre-ft), which represented only a fraction
of the average for the same month during Periods I or II (2.5% and 16.8%, respectively).

However, in the limited context of present
conditions {i.e.. Period III only), the restored
event magnitude compared much more
favorably, increasing the estimated average
monthly discharge by several rimes over
what had occurred without the
demonstration project. When analyzed
annually, the restored event magnitude
increased by over 633% from the historical
Period III amount (Table 6).

Duration

Table 6: Summary of restored annual event magnitude In the
upper Nueces Delta. Mean discharge value rounded to the nearest
10 acre-ft.

Period

1982-1999

Restored

Mean Total Discharge

per Year

(acre-ft)

Percent Change

from Historic

Period III

Conditions

3,420

633%

Note: 1 acre-ft = 1 2335 10^ m'

Restored event duration, xmlike restored magnitude, compared very favorably with the historical duration of
events flowing into the upper Nueces Delta (Figure 16). Both seasonal peaks were strongly evident, and the
spring peak (May) was approximately the same duration as that of the Period I. The restored event duration
even slightly exceeded the historical duration of both Periods I and II for seven (7) of the twelve (12) months of
the average year, and in only two months was it lower than the historical Period II values. The only notable
limitation of the restored event duration was that of the fall peak, which only attained an average of 2.5 days
(October), compared to the historical fall peak of 4.8 days (September) in Period I and 4.2 days (October) in
Period II.

From the perspective of present conditions
(Period III) only, the restored event
duration also greatly surpassed historic
levels (Table 7). The restored annual event
duration increased by over 578% from that
historical (without-project) level to
26.0 days.

Table 7: Summary of restored annual event duration in the upper
Nueces Delta.

Period

1982-1999

Restored

Mean Number of Event

Days per Year

Percent Change

from Historic

Period III

Conditions

26.0

578%

Note: 1 acre-ft = 1 2335 10^ m^

C- 1 8 ^ Analysis of the Historic Flom Re^me of the Nueces River into the Upper Nueces Delta

T3
TO
O
<D

>

to

45000
40000
35000
30000
25000
20000
15000
10000

■4= 5000

1940-1958
1958-1982
1982-1999
1982-1999*

Jan Feb Mar Apr May Jun

Aug Sep Oct Nov Dec

Figure 15: Potential for restored flow event magnitude* into the upper Nueces

Delta during Period III, which assumes the features of the Rincon Bayou

Demonstration Project had been in place since 1982. Not Included were data

from the largest event in each time period.

Note: 1 acre-ft = 1.2335 10^ m^

Dec

Figure 16: Potential for restored flow event duration* into the upper Nueces
Delta during Period III, which assumes the features of the Rincon Bayou
Demonstration Project had been in place since 1982. Not Included were data
from the largest event in each time period.

Appendix C ♦ C-19

100

100 1000 10000 100000

Peak dally discharge Into the upper Nueces Delta (acre-ft)

Figure 17: Potential for restored flow event return frequency (ogive)* into the
upper Nueces Delta during Period III, which assumes the features of the Rincon
Bayou Demonstration Project had been in place since 1982. Not included were
data from the largest event in each time period.

Note: 1 acre-ft = 1 2335 10^ m\

Figure 18: Potential for
restored flow event timing*
Into the upper Nueces Delta
during Period III, which
assumes the features of the
Rincon Bayou Demonstra-
tion Project had been In
place since 1982. Data from
all events in each time period
included. Scale of the y-axis is
curtailed from 700,000 acre-ft
(September 1967) to improve
resolution of lower values.
Note: 1 acre-ft = 1.2335 10' m'.

C-20 'J* Anafysis of the Historic Flow Re^me of the Nueces River into the Upper Nueces Delta

Frequency

The results of restored event return frequency Table 8: Summary of the restored return period for a flow event
varied depending upon the peak flow into the upper Nueces Delta with a daily peak flow of 100 acre-ft.

considered (Figure 17). For example, for Restored Percent Change from

Period Return Period Historic Period III

(years) Conditions

events with peak daily flows below 49 10' m'
(40 acre-ft), the estimated return period

decreased to below historical levels of either III: 1982-1999 068 '324%

Periodlorll. Above about 123 10' m' Note: 1 acre-ft = 12335 10' m'

(100 acre-ft), the return period was greater than

Periods I and II, but substantially less than that

of Period III. If a peak flow event of 123 10' m' were used to illustrate the change in restored event return

frequency from historical Period III conditions, the return period for this event would have decreased by about

18 months, or by 324% (Table 8).

Timing

From inspection of Figure 18, and comparison with Figure 14, it is evident that the restored riming of freshwater
flow events into the upper Nueces Delta, both in seasonal and annual distribution, would improve from Period
III conditions. However, events xmder the restored condition would in no way attain the relative numbers or
size of flow events that occurred during histoncal times. Each of the visible events imder restored conditions,
although noticeably larger and longer, would still be considered only extremely small events in either historical
Periods I or II.

DISCUSSION

Historical Freshwater Inflow

Without exception, a decreasing trend over time was observed in each of the four historical flow regime
characteristics analyzed. Event magnitude, duration, frequency and timing all declined with varying degrees from
Period I through Period III. When compared to Period I, which represents a conservative reservoir scenario,
flow regime characteristics during the period after the construction of Wesley Scale Dam (Period II) generally
exhibited a lesser degree of change than did the period immediately after the completion of Choke Canyon Dam
(Period III), which showed a more dramatic change. This would be expected, as Choke Canyon Dam provided
over twice the amount of storage capacity of Lake Corpus Chtisti and was operated in conjunction with the
latter, adding its effect upon the other.

The magnitude of flow events during Period II decreased substantially from Period I levels, especially in regards
to the spring event. Dxmng Period III, event magnitude was virtually eliminated when compared to previous
periods. Changes in event duration closely reflected those in event magnitude, except that Period III levels did
not decreased as significantly. The differences between the return period of flow events into the upper delta
during Periods I and II were not as pronounced as with event magnitude and duration, but Period III again
showed a significant departure from previous periods. Finally, event timing dramatically changed with the virtual
elimination of meaningfiil seasonal flow events. This change was so distinct, that the entire post-Choke Canyon
Reservoir period (with the exception of 1987) could be likened to a perpetual "dry" year, which occurred only
infrequendy during the previous two periods.

Potential for Restored Freshwater Inflow

When compared with present conditions (represented by the historic Period III), each of the flow regime
characteristics analyzed under the restored Period III conditions were substantially increased. j\nnual event
magnitude was increased by over 633%, and annual event duration by over 578%. Event return frequency was
also improved, and even exceeded that which occurred in Periods I and II for lower peak flow events. Finally,
event timing also showed some improvement in the relative size and length of freshwater flow events.

However, when compared with the historical flow regime of Periods I and II, the restored regime characteristics,
with the exception of event duration, compared less favorably. The restored event magnitude was still only a
fraction of even Period II levels, and event frequency was not affected for peak flow events greater than about

Appendix C ♦ C-21

1^34 10' m' (1,000 acre-ft). Also, improvements in event timing were not sufficient to eliminate the significant
number of seasonal and annual "dry" periods in the Period III historical record.

Comparison of Freshwater Flow into the Upper Nueces Delta Versus
Inflow into Nueces Bay

This analysis was focused exclusively on flow event characteristics into the upper (northern) Nueces Delta, and
not on the total freshwater inflow into Nueces Bay. However, one minor digression worth consideration is the
difference between changes in the historical freshwater inflow pattern of the upper deha compared with that of
Nueces Bay. Clearly, flow into the upper Nueces Delta represents only a fraction of the river's total inflow to
the receiving bays and estuary. Consequendy, the results of a historical analysis of total estuarine inflow, without
consideration of the delta component, would be expected to differ considerably from the results of this
investigation.

And this is indeed the case. Asquith el al. (1997) evaluated the mean and median annual stream flow of the
Nueces River near Mathis for the period 1940-1996, and performed statistical tests for historical trends with
time. Their results indicated that the change in stream flow from 1958 through 1982 (or Period II) was
negligible (an 0.8% decrease in mean annual flow, and an 18.5% decrease in median annual flow), while the
change in stream flow after impoundment of Choke Canyon Reservoir was large (a 55.0% decrease in mean
annual flow, and a 63.4% decrease in median annual flow). This conclusion is in contrast to the results of the
present analysis, which indicate the decrease in annual mean volume of water entering the upper Nueces Delta
during Period II was considerable (about a 39% reduction), and during Period III was extreme (over a 99%
reduction).

This pragmatic explanation of this difference between the historic flow characteristics of the upper Nueces Delta
and those of the greater Nueces Bay Hes in the concept of "flooding threshold" at the point of natural diversion.
Because of the higher threshold imposed by the elevated river bank, freshwater diversions into the delta system
are extremely sensitive to the peak segments of flood events in the Nueces River. Therefore, if the flooding
threshold is not met, the flow event in the river will bypass the delta, providing Nueces Bay with an freshwater
inflow event without the same courtesy for the delta. Hence, changes in river flow patterns which lower the
peak flows of flood events disproportionately affect the upper delta as compared to the bay.

The value of the distinction between delta inflow and total estuarine inflow is fully appreciated when recognizing
the fact that the Nueces Delta is a distinct and critical component of the greater estuary system. Without
consideration of this point, one may erroneously conclude that, for example, the reductions in freshwater inflow
during Period II did not meaningfully alter the freshwater flow regime of the Nueces Estuary ecosystem because
total mean inflow into Nueces Bay was reduced by only one percent.

CONCLUSIONS

The historic flow regime of the Nueces River into the upper Nueces Delta has changed dramatically over the
past 60 years. In each of the flow event characteristics analyzed, a strong declining trend was observed from
Period I through Period III. The Rincon Bayou Demonstration Project would restore a considerable amount of
freshwater to the upper Nueces Delta and significandy improve the flow regime characteristics compared to the
present (historical Period III) conditions. However, the demonstration project would not restore the delta's
inflow patterns to historic (Periods I and II) levels.

Reservoirs in the basin and deltaic inundation events have a imique relation to peak flow events in the lower
Nueces River. On one hand, because of the high flooding threshold of the north bank of the river, the upper
Nueces Delta relies almost exclusively on the peaks of flow events for freshwater inflow. On the other hand,
main-stem reservoirs, by design, significandy attenuate the peaks of flow events in the watershed for purposes of
water storage and flood control. This relation is exemplified by the fact that, during the period after the
construction of Lake Corpus Christi (Period II), although annual mean flow of the river into Nueces Bay was
reduced by about 1% (Asquith et al. 1997), the annual mean flow into the upper Nueces Delta was reduced by
about 39%. Similarly, during the period after the construction of Choke Canyon Dam, while the annual mean
flow of the Nueces River into the bay was reduced by about 55% (Asquith et al. 1997), the annual mean flow into
the upper delta was all but eliminated (reduced by 99%).

C-22 ^* Anatjists of the Historic Flow Eiffme of the Nueces River into the Upper Nueces Delta

Therefore, if the upper deha's historical contribution to the estuary ecosystem is to meaningfully exist in the
future, the multi-component flow regime of the Nueces River into the upper Nueces Delta must be considered
in the context of reservoir operation and management.

LITERATURE CITED

Asquith, W.H., J.G. Mosier, and P.W. Bush. 1997. Status, Trends and Changes in Freshwater Inflows to Bay

Systems in the Corpus Christi Bay National Estuary Program Study Area. CCBNEP-17. Coastal Bend Bays

and Estuaries Program, Corpus Christi, Texas.
Bureau of Reclamation. 1971. Nueces River Project, Texas: Appendixes to Feasibility Report, Volume I-

Hydrology. United States Department of the Interior, Bureau of Reclamation.
Bureau of Reclamation. 1996. Hydrauhc analysis of Rincon Bayou using HEC-2. United States Department of

the Interior, Bureau of Reclamation, Billings, Montana.
Bureau of Reclamation. 2000. Hydrauhc Analysis of Rincon Bayou Using HEC-2, Rincon Bayou-Nueces Marsh

Wetlands Restoration and Enhancement Project. United States Department of the Interior, Bureau of

Reclamation, Great Plains Regional Office, Billings, Montana.
Corpus Christi. 1990. Annual report for fiscal year 1989-1990, City of Corpus Christi Water Division. City of

Corpus Christi, Texas.
Corpus Christi Times. 1967. Article from September 26 edition entided 'TSIueces River Crest Past; Beach is

Safe". Corpus Christi, Texas.
Gandara, S.C., W.J. Gibbons, F.L. Andrews, R.E. Jones and D.L. Barbie. 1997. Water resources data for Texas,

water year 1996. United States Geological Survey, Austin, Texas.
HDR Engineering, Inc., Shiner, Moseley & Associates, Inc. and Naismith Engineering, Inc., in association with

University of Texas, Marine Science Institute. 1991. Regional water supply planning study: Phase I, Nueces

River Basin, Volume II. HDR Engineering, Inc., Austin, Texas.
Hilzioger, L. 2000. Personal communications. City of Corpus Christi, Water Supply Department, Wesley Scale

Dam.
Homer & Shifiin. 1951. Review of the Corpus Christi water supply problem. Homer & Shifrin Consulting

Engineers, under an association with Harland Bartholomew & Associates, St. Louis, Missouri.
Karr,J.R. 1991. Biological integrity: A Long-neglected aspect of water resource management. Ecologcal

Applications 1 : 66-84.
Longley, W. L., ed. 1994. Freshwater inflows to Texas bays and estuaries: ecological relationships and methods

for determination of needs. Texas Water Development Board and Texas Parks and Wildlife Department,

Austin, Texas. Page 386.
Medina, J.G. 2000. Recent trends in precipitation occurring on the Nueces River watershed of south Texas.

Unpubhshed. In Concluding Report: Rincon Bayou Demonstration Project, Appendix D. United States

Department of the Interior, Bureau of Reclamation, Austin, Texas.
Poff, N. LeRoy, J. and J.V. Ward. 1989. ImpUcations of streamflow variabUIity and predictabiUty for lotic

community structure: a regional analysis of streamflow patterns. Canadian Journal of Fisheries and Aquatic

Sciences \(i: 1805-1818.
Poff, N. LeRoy, J. D. Allen, M.B. Bain, J.R. Karr, K.L. Prestegaard, B.D. Richter, R.E. Sparks and J.C.

Stromberg. 1997. The natural flow regime: A paradigm for river conservation and restoration. BioScience

Vo. 47, No. 11, pp. 769-784.
Richter, B.D., J. V. Baumgartner, J. Powell, and D.P. Braun. 1996. A method for assessing hydrologic alteration

within ecosystems. Conservation Bioloff/ 10: 1163-1174.
Salas, D.E. 1993. Vegetation assemblage mapping of the Nueces River delta, Texas. Unpubhshed. United

States Department of the Interior, Bureau of Reclamation, Denver, Colorado.
Texas Department of Water Resources. 1981. Nueces and Mission-Aransas Estuaries: A study of the influence

of freshwater inflows. Texas Department of Water Resources, LP-108, Austin, Texas.
Texas State Highway Department. 1931. Plans for proposed Nueces River and rehef bridges. Highway No. 9,

Nueces and San Patricio Counties. Texas State Highway Department, Corpus Christi, Texas.
Texas State Highway Department. 1956. Plans of proposed State highway improvement. State highways 9 and

77, Nueces and San Patricio Counties. Texas State Highway Department, Corpus Christi, Texas.
Texas State Highway Department. 1959. Plans of proposed State highway improvement. Interstate Highway 37,

San Patricio County. Texas State Highway Department, Corpus Christi, Texas.
Texas State Department of Highways and Public Transportation. 1983. Plans of proposed State highway

improvement. Interstate Highway 37, San Patricio County.
United States Engineer Office. 1939. Report on survey of Nueces River and tributaries, Texas, for flood control

and allied purposes. United States Engineer Office, Galveston, Texas.

Appendix C ♦ C-23

United States Geological Survey. 1968. Water delivery study, lower Nueces River valley, Texas. Report 75.

Texas Water Development Board, Austin, Texas.
Walker, K.F., F. Sheldon and J.T. Puckridge. 1995. A perspective on dryland river ecosystems. Regtlated Rivers:

Research i& Management \\: 85-104.
Ward, G.H. 1985. Marsh enhancement by freshwater diversion, journal of Water Resources Planning and

Management, ASCE, 111 (1), pages 1-23.

C-24 ^ Analysis of the Historic Flow Regme of the Nueces River into the Upper Nueces Delta

APPENDIX D

Recent Trends in Precipitation
Occurring on the Nueces River
Watershed of South Texas

Jonnie G. Medina Meteorologist, U.S. Department of the Interior, Bureau of Reclamation, P. O. Box
25007, Code D-3720, Denver, CO 80225.

ABSTRACT: Precipitation from the Nueces River watershed was analyzed for recent trends as part of the
Rincon Bayou Demonstration Project (Rincon Project) conducted by the United States Bureau of Reclamation.
Of interest were comparisons of precipitation across the three periods, 1940-57, 1958-81, and 1982-99, of
interest to the Rincon Project, a demonstration and study of impacts on the ecology of the upper Nueces delta
Bay from enhanced inundations of the marsh. The role is discussed of the EI Nino-Southem Oscillation
(ENSO) and other climate variables to tropical cyclone occurrence in the Atlantic basin and in particular, the
Gulf of Mexico. The potential modulation by ENSO of watershed precipitation was investigated. A
multivariate comparison was conducted of daily precipitation sampling distributions across project study
periods. The occurrence of large precipitation events was of particular interest because they could lead to
natural inundations of the marsh. Analyses indicated no particular watershed precipitation trend unless the
basis of comparison was the extreme drought period of the 1950s. The La Nina (El Nino) phase does appear
to lead to more (less) tropical cyclone impacts in the western Gulf of Mexico than baseline years. Of 21 years
(since 1948) with large precipitation events (=> 4 inches per day) impacting Corpus Christi, 6 were El Nino
years, 8 were La Nina years and 7 were baseline years. Tropical cyclones occurred in 7 of the 21 large-event
years; 5 were La Nina years and 2 were baseline years. Of 7 years with annual precipitation exceeding
40 inches, 5 years were baseline years, 1 year was La Niiia, and the largest-amount year was El Nino. Two of
the seven highest years did not have a high daily event. The multivariate comparison revealed the sampling
daily precipitation of the drought years differed from the other two study periods, otherwise no differences.

INTRODUCTION

The United States Bureau of Reclamation (Reclamation) is sponsoring and partnering in the Rincon Bayou
Demonstration Project (Rincon Project) aimed at determining marsh response to increased freshwater
inundation caused by construction of an overflow channel on the Nueces that flows into Nueces Bay just
north of Corpus Christi, Texas (see Figure 1). The occurrence of natural over-banking events from the Nueces
River into the Nueces Delta was reduced by the construction in 1958 of Lake Corpus Christi on the Nueces
River and in 1982 by Choke Canyon Reservoir on the Frio River (Irlbeck and Ward 2000). In addition to
regulated flows into the Nueces River, precipitation in the Frio and Nueces watersheds, and locally in the delta,
also impact the delta ecosystem. The question arises as to how much, if any, of the observed decreases in the
magnitude, frequency, duration and timing of delta inundation events since 1958 (Idbeck and Ward 2000) has
been caused by variations in climate.

The weather systems impacting south Texas migrate from the temperate zone to the north or the tropics. Some
disturbances originate in southwestedy or westerly flow over Mexico. Tropical disturbances approach the
Nueces watershed generally from the southeast or east. Except in circumstances of deep continental flow, the
troposphere over south Texas generally contains ample moisture for precipitation, but often lacks a trigger
mechanism to build storm clouds. Perturbations moving toward the Rincon area generally provide the triggers.
For example, a slow-moving front or tropical wave can trigger the development of clouds that can produce
small amounts to several inches of precipitation, often in isolated patterns rather than general coverage. So, a
weather system may produce littie precipitation at a location, but a short distance away can produce three inches

Appendix D ♦ D-1

Figure 1: Nueces River Basin and precipitation gauges.

Trends in precipitation should be described within an a priori selected time period. For example, a short-term
trend following a drought would be increasing precipitation. The duration of such trends is of great interest
and the ability to forecast them is constandy sought. For purposes of the Rincon project, precipitation trends
over 1940-99, a 60-year period, are the main concerns of study. Additionally, Rincon area precipitation is
compared among the periods 1940-57, 1958-81, and 1982-99, that correspond to different construction
influences on Nueces River flows.

On the National basis, the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric
Administration routinely develops temperature and precipitation trend estimates (available on the Internet,
address ftp://ftp.ncep.noaa.gov/charts.htinl/'). CPC uses 102 climate regions of near equal area for the lower
48 states. Each climate region is composed of one or more climate divisions. Monthly data are assembled into
time series for three-month periods and an annual average. Baseline values were developed from data to 1966.
The linear change per decade is computed from the base line value through the current year. Current trends are
based on 1941-1998 data for temperatures and 1931-1998 data for precipitation. The CPC trends indicate
National increases of 0.15 °F and 0.9 inches of precipitation per decade. For south Texas, no trend in mean
annual temperatures is indicated, but a 0.2 to 0.6 inch increase in precipitation per decade is revealed.

The following covers a discussion of precipitation in the Nueces River watershed based on data from several
gauges located there. A summary is given of some study results on the El Nino-Southem Oscillation (ENSO)
and other climate variables, and their relationship to tropical cyclone occurrence in the Atlantic basin and in
particular, the Gulf of Mexico. Nueces River watershed precipitation anomalies stratified by ENSO type are
presented. Finally, daily precipitation distributions for one watershed gauge are developed and discussed in the
context that for different periods there can occur different precipitation distributions causing differing runoff
while maintaining similar annual precipitation.

NUECES RIVER WATERSHED PRECIPITATION SURVEY

The 30-year mean precipitation for gaviges shown in Figure 1 portray differences across the Rincon area. Mean
annual preapitation (1961-1990 record) for Corpus Christi WSO AP, CotuUa, Carrizo Springs, BeeviUe 5 NE,
and Sabinal are the respective 30.14, 22.95, 21.22, 31.97, 25.45 inches. These values support lesser
precipitation occurs in the southwestern Rincon area and higher amounts along the coastline. Figures 2
through 5 present annual precipitation totals for Cotulla, Corpus ChristL, Beeville 5 NE, and Sabmal through
1999, with a Lowess (Cleveland, 1979; 1981) smoothing line (average of weighted values in a moving window

D-2 *♦♦ Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas

that allows adjustable flex by changing the window size) that suggests trends. These precipitation gauges are
distributed across the Rincon Project drainage suppljnng the delta and possess the better quahty data for that
area. Annual precipitation recording began shordy after 1900, except for the Corpus Christi record that began
in 1948. The figures show the prominent drought of the 1950s (and several years of the late 1940s). The
Lowess cixrves show short-term recent rising precipitation at three of the four gauges. The Corpus Christi
Lowess curve portrays an unrealistic initial pattern because of analysis effects from initial data from the
drought. The inclusion of the drought years to the CPC baseline largely caused the positive trend in
precipitation.

Line plots (not given) of the Corpus Christi WSO AP, Sabinal, and Beeville 5 NE gauges produced respective
positive, negative, and positive slopes using the available annual precipitation for each gauge. The Corpus
Christi WSO AP data yields a negative slope if precipitation is restricted to 1965 or more recent toformation.
The Beeville 5 NE precipitation produces a negative slope if restricted to 1960 or more recent.

Asquith et al (1997) conducted a study to determine the ctirrent mean freshwater inflows to bay systems that
included the Corpus Christi Bay. As part of their study, they examined temporal trends in inflows. They
studied trends in precipitation of several precipitation gauges. Asquith et al (1997) applied the Mann-Kendall
test (Helsel and Piirsch, 1992; Hollander and WoLfe, 1973) to time series of seasonal and annual rainfall from
the Corpus Christi WSO AP and Refugio 7 North (located approximately 30 miles east of the Beeville 5 NE
gauge) gauges. The Mann-Kendall test is a rank-based nonparametric test similar to the familiar Wilcoxon-
Mann-Whitoey procedures to test for monotone increasing character in data. Results of the statistical analysis
indicated P- values not significant at the 0.05 level in all seasonal and annual time series of 1968-93 data for
Refugio 7 North, and similarly for Corpus Christi WSO AP data excepting winter (January, February, March)
that produced an upward trend with a P-value of 0.05. The annual Corpus Christi data produced a 0.25
P-value.

Figure 6 presents the mean annual precipitation computed separately for the four gauges, Sabinal, Corpus
Christi, Beeville 5 NE, and Cotulla for each time period of interest to the Rincon Project. This portrayal of
gauge mean precipitation shows the change across time periods and gauges. Because the Corpus Christi record
began in 1948, the first period duration was restricted to 10 years; other period lengths were 24 and 18 years.
The figure shows all gauges displaying the same general pattern over the three periods, least precipitation during
the drought period, greatest amount during the second period, and intermediate amounts during the third and
most recent period.

Precipitation in the Nueces watershed is not monotonic increasing over the three study periods. However,
there could be patterns of rainfall amounts that could cause more inundations of the Nueces Delta than other
patterns with similar annual precipitation. Namely, the annual precipitation could occur in relatively few large
amoimt events. This could occur from increased tropical disturbances numbers, more persistent frontal
patterns that favor precipitation in the watershed, or perturbations in a persistent moist Pacific flow across
northern Mexico and the southern United States (shown in Figure 9).

Figures 7 and 8 present cumulative adjusted annual precipitations over the gauge record periods for the Sabinal
and Corpus Christi WSO AP gauges (through 1998), respectively. Annual precipitation values were adjusted by
removing the long-term annual mean. Difference values were then summed in time and plotted. The
cumulative precipitation differences are related to soil moisture. Though such cumulative difference figures are
impacted by the choice of data starting point, the figures for the Rincon Project study area portray the
substantial annual precipitation deficit created by the severe drought of the late 1940s and 1950s. The longer-
term Sabinal record shows the contrasting conditions prior to and after the 1950s. The plot for Sabinal
suggests that the upper Nueces River basin may have frequendy been in below-normal soil moisture conditions.

ENSO AND TROPICAL CYCLONES IN THE ATLANTIC BASIN

The relative maximum in precipitation that occurs in the fall season in the Nueces River watershed is largely the
result of, (1) mid-latitude perturbations that in fall strengthen, adequate to reach south Texas as the subtropical
and polar jets reestablish in the stronger north/south temperature gradient, and (2) tropical perturbations that
impact the western Gulf of Mexico. Tropical cyclones can substantially impact western Gulf precipitation.
Landsea et al. (1999) studied Atlantic hurricanes. United States (lower 48 States) land-falling hurricanes and U.S.
normalized damage (adjusted for changes in inflation, coastal county population, and wealth) time series for

Appendix D ♦ D-3

50

40

c
o

c5 30

a.
o

0)

20-

"1 — 1 — I — 1 — I — 1 — I — I — r

»• •* • . .

J I I I ■ I L

\ •

10

s<i^^ ^^^° ^<i^° ^.i^ ,9^° ^^^ ^<,^ ^<»A° ,# ^^ ^OCP

Year

Figure 2: Cotulla annual precipitation.

50

40

c
o

ffl 30

20

10

1 I I I I 1 r

J I I I L

J L

^<i^ ^9^° ,<»T-^ ,<i^° ,.»^° ,<i^^ ^<^ ,9^^ ,<i%^ ^^^ ^^

Year

Figure 3: Corpus Christi WSO AP annual
precipitation.

50

40

c
o

« 30

Q.
O

V V V V V V V V \^ V
Year
Figure 4: Beeville 5 NE annual precipitation.

20

10

"I I — I — I — I — I — r

-• ^

J L

J L

^<i^ ^<J^^ ^<iT? ^ci^"" ,<.^° <^^ ^c,^ ^<i^^ ^^«P ^q<^° TpO°

Year
Figure 5: Sabinal annual precipitation.

35

c 30
o

a 25

o.
^ 20

15

M Sabinal

Bl Corpus Christi WSO AP

n Beeville 5 NE

H Cotulla

12 3

PERIOD

Figure 6: Mean annual precipitation per each time period of 1940-57,
1958-81, and 1982-99.

D-4 ♦♦♦ Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas

Cumulative Adjusted Precipitation

Figure 7: Sabinal cumulative annual precipitation that was
adjusted by subtracting the long-term mean. A dotted vertical line
is placed at 1982 indicating the beginning of the Rincon third study
period.

Cumulative Aa^usted Frecipitation
lOi

-90

1940 1950 1960 1970 1980 1990 2000
Year

Figure 8: Same as figure 7 except for Corpus Christi Gauge
information.

Appendix D ♦ D-5

inter- annual trends and multi-decadal variability. They used records on tropical disturbances back to the 1 940s
for basin-wide Atlantic cases, and the turn of the 20* cenfur)' for U.S. land-falling systems. \'arious
environmental factors were considered including Caribbean sea level pressures, 200-niillibar zonal winds,
stratospheric Quasi-Biennial Oscillation (described below). El Nino-Southem Oscillation (ENSO, described
below), African West Sahel rainfall and Atlantic sea surface temperatures. AH variables indicated significant,
concurrent relationships to the frequenc}', intensity and duration of Adantic hurricanes. The ENSO was found
to be significandy linked to changes in damages by tropical cyclones that impact the United States.

Gray et al (1993) found that various relationships can be determined if data are stratified into spatial location
and by disturbance intensity. The linear trend in hurricane activity of the Atlantic basin is vety weak (Landsea
et al., 1999). Multi-decadal variabilitj' is more characteristic of the region. This is suggested in that intense
Atlantic hurricanes (50 m/s or more) were common in the 1940s through the 1960s, but much reduced from
the 1970s through the early 1990s (Landsea, 1993). In 1995 and 1996, there occurred a return to high levels of
tropical cyclone activity similar to earUer active decades. There were similarities in hurricane dilation variations
with longer-lived systems (about 25-40 tropical cyclone days per year) in the 1 940s through the 1 960s and fewer
hurricane days (around 10-25 days per year) in the decades of the 1970s through eady 1990s.

While the Atlantic basin exhibits multi-decadal variation, the United States Gulf Coast from Texas to the
Florida panhandle expresses much weaker intense hurricane strike variability (Landsea et al, 1999). The
variation pattern is quite different as above average acti\'ity occurred only in the 1910s and reduced activity only
in the late 1940s and early 1950s. Thus, some components of the Adantic basin may exhibit markedly different
intense hurricane variation.

ENSO events induce moderate-sized changes in the frequency and intensity of Adantic basin tropical cyclones

(Landsea et al, 1999). The ENSO state can be characterized by the sea surface temperature (SST) anomalies in
the eastern and central equatorial Pacific (Philander, 1989). Warm SSTs in this region are referred to as El Nino
events, and cool SSTs are known as La Niria events. The state of ENSO can also be characterized by the
Southern Oscillation Index (SOI), the standardized difference in sea level pressiure between Tahiti and Darwin,
Australia. High (low) pressure at Darwin and low (high) pressure at Tahiti corresponds to El Nino (La Nina)
events.

Figures 9 and 10 present schematics

of the winter tropospheric jet

streams and major

cyclone/anticyclone features of the

El Niiio and La Nina events,

respectively. The schematics show

the El Nino Pacific jet stream

transporting moisture into the

southern States thus favoring wetter

conditions there, while under the La

Nina structure the storm track is

fiirther north. ENSO events alter

the global atmospheric circulation

patterns and are able to affect

tropical cyclone frequencies. The

mechanisms for the latter are the

alteration of the lower tropospheric

source of vorticity (measure of

rotation in a fluid) and the vertical

wtad shear profiles (Gray, 1968,

1979). ENSO fluctuates on the scale of a few years (Philander 1989)

Figure 9: El Nifio winter atmospheric features.

El Niiio events are associated with fewer numbers of Adantic basin tropical cyclones (recall that El Nino leads
to more nontropical storms particularly during the cool season). La Nina events cause 36 percent more
named/subtropical storms than El Niiio events. Their mean intensities are also 6 percent stronger (Landsea et
al, 1999). Though ENSO events modulate hurricane landings in the Caribbean region and the United States,
the impacts are only weakly significant for the Gulf Coast intense hurricanes (and not significant for weaker
storms).

D-6 V Recent Trends In Precipitation Occurring on the Nueces River Watershed of South Texas

For intense humcane strikes along the Gulf
Coast, the single largest factor of influence
is the phase of the Quasi-Bieimial
Oscillation (QBO; Gray 1984, Shapiro
1989). The QBO is an east-west oscillation
of stratospheric-level winds that encircle
the globe near the equator. In the west
phase, Atlantic hurricane activity is
enhanced, but is diminished in the east-
phase QBO years. The QBO is followed in
importance by the 200-millibar zonal winds
and sea-level pressure anomalies, such that
westedy winds and high pressures in the
Caribbean favor more tropical cyclone
landings (Knaff, 1997). With regard to
interannual variation, normalized United
States' damages are well related to ENSO,
such that significantly less damage occurs
during El Nino events and than dvuing
La Nina events.

JET "^ .

Figure 10: Winter features for La Nina.

Lack of hurricane variation consistency in the Gulf of Mexico compared to East Coast occurrences is likely
caused by the dominance of local conditions over basin-wide factors that can lead to intense hurricane
development (Landsea et al, 1999). An example is tropical storm development along a stationary frontal
boundary of strongly contrasting conditions. This occurred in the case of hurricane Alicia in 1983. The
prominent role of local conditions is likely why there is no distinct multi-decadal variation of intense hurricanes
in the Gulf of Mexico (Landsea et al, 1992).

Tropical cyclone activity in the western Gulf of Mexico during the three periods of interest to the Rincon
Project somewhat followed the causal/correlated patterns discussed previously. Table 1 gives tropical cyclone
occurrences by ENSO type and baseline (non-ENSO) years. Classification of years was according to PieLke and
Landsea (1999). More tropical cyclones impacted the western Gulf of Mexico in La Nina years than El Nifio
years, generally following the script. However, table 1 shows that six tropical cyclones impacting the western
Gulf occurred during the relatively low Adantic basin activity period of the 1970-80s. In the first Rincon study
period (1940-57), only hurricanes' Alice and Audrey affected the western Gulf of Mexico. During the second
study period (1958-81), 11 tropical cyclones impacted the Gulf coast. In the third study period (1982-99), four
hurricanes (Arlene, Charley, Frances and Bret) occurred, three during El Niiio years and the fourth (Bret)
during the La Nina year of 1999.

WATERSHED PRECIPITATION ANOMALIES AND ENSO PHASE

Table 1 shows twice as many tropical cyclones occurring in the western Gulf of Mexico during La Nina years as
El Nino years. The question arises as to whether Nueces River watershed precipitation is sensitive to ENSO
phases. For example, do La Nina months record more precipitation baseline-year months. To investigate this
issue, monthly precipitation for 1950-99 for Corpus Christi WSO AP, BeeviUe 5 NE, and Sabinal were stratified
into baseline (16 years and 4 tropical cyclones). El Nino (18 years and 4 tropical cyclones), and La Niiia years
(16 years and 8 tropical cyclones). All months for a particular category (ENSO, baseline) were used in analyses
to capture the effects on precipitation of nontropical storms as well as tropical cyclones. For each precipitation
gauge, the 75 percentile precipitation was determined in the base years to serve as the benchmark of
comparison. The analysis technique applied, similar to Gershunov and Bamett (1998), determined the
proportion of months of record with a precipitation amount that equaled or exceeded the benchmark. The
proportion determined is compared to 0.25 and the quotient is reduced by 1.0 and converted to percent
anomaly.

Appendix D ♦ D-7

Table 1: Tropical cyclones during ENSO and baseline years.

El Nifio Years

La Nina Years

Baseline Years

1940

1942

1943

1941

1944

1946

1951

1945

1947

1953

1948

1952

1957 (Audrey)

1949

1958

1963

1950

1959

1965

1954 (Alice)

1960 (TS1)

1969

1955

1962

1972

1956

1966

1976

1961 (Caria)

1968 (Candy)

1977

1964

1979

1982

1967 (Beulah)

1980 (Allen, Danielle)

1986

1970(Celia)

1981

1987

1971 (Fern)

1983

1990

1973 (Delia)

1984

1991

1974

1985

1993(Arlene)

1975

1989

1994

1978 (Amelia)

1992

1997

1988

1996

1998 (Charley, Frances)

1995
1999 (Bret)

Table 2 lists the anomalies calculated. It is noted that the baseline months included the effects on precipitation
from four tropical cyclones. Additionally, because proportions developed use counts of months rather than
precipitation amounts, extreme events cannot excessively and unreahsticaUy skew results. The results reveal a
mixture with El Nino monthly precipitation ranging from a (positive) 1.3 percent anomaly in the BeeviUe 5 NE
data to a -30.7 percent anomaly in the Corpus Christi data. La Niiia anomalies ranged from -12.7 to 25.9
percent, also a mixed result. Four of six ENSO anomahes were negative. Both ENSO phases led to negative
anomalies at the Corpus Christi gauge, but a smaller anomaly (-12.7 percent) for La Nina months that included
the effects from 8 tropical cyclones. The inland gauge, Sabinal, yielded the same pattern of a more positive
anomaly from La Nina months, but this pattern was not maintained in the Beeville 5 NE data. NX'hile the
Beeville 5 NE and Corpus Christi WSO AP gauges yielded the same anomaly for La Nifia months, there was a
large difference in the El Nino related anomaly despite being separated by only about 40 miles (however,
Beeville 5 NE is inland about 35 miles). There is a "weak" sviggestion that ENSO events may lead to a
somewhat higher proportion of negative anomalies than expected by randomness (discounting the zero
anomaly, there is a 23 percent probability of obtaining 4 negative anomahes in 6 possibihties). More data from
additional gauges would be needed to further explore this possibility. Generally, these analyses in search of a
precipitation dependence on ENSO do not reveal any well-defined pattern, but rather expressed the high
variability in the precipitation data.

Table 2: Percentage anomaly of monthly ENSO rainfall when compared to base year 75 percentile
benchmark precipitation.

Beeville 5 NE
La Nina: -12.7%
El Nino: +1.3%

Sabinal
La Nina:
El Nino:

+25.9%
-6.8%

Corpus Christi
La Nina: -12.7%
El Nino: -30.7%

DAILY PRECIPITATION DISTRIBUTION COMPARISONS

While there appears litde indication of a trend in the annual and monthly precipitation of the Nueces River
watershed during the 1940 to 1999 period, there remained the possibility that there could have been a
difference in the distribution of daily precipitation across the three periods of interest in the Rincon Project
study. For example, the mean monthly precipitation during the second period could have been similar to that

D-8 ^* Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas

during the third period but a differing daily amount distribution could cause highly differing delta inundations
in one period versus the other period. One approach to study this possibility is the cross-tabulation of daily
precipitation. The Corpus Christi WSO AP daily precipitation was selected for analysis because of better
quality, despite the shorter record period that initiated in 1948.

Given in table 3 are the frequencies from cross-tabulation of the Corpus Christi daily precipitation in
approximately one-inch size categories. Inspection of the frequency patterns over time su^ests some
differences in shorter time periods of about 10 years (particularly the first 10 years). The number of years per
each of the 3 Rincon Project periods in succession was 18, 24 and 18 years. Available Corpus Christi WSO AP
data limited the initial study period to 10 years (1948-99). Cross-tabulations present outcomes such that pattern
comparisons for selected periods can often be made using muUtvariate analysis. A distribution-free
permutation test was selected and applied in exploratory analysis to explore whether the three distribution
patterns differed. The statistical test applied is one of a family of tests known as multi-response permutation
procedures (MRPP) (Mielke et al., 1976, 1982). These tests avoid making the unjustified assumption that the
joint distribution of the response measurements is multivariate normal or some other specific distribution.

Application of the MRPP yielded a P-value of 0.08 in comparing frequency patterns of all three periods. In the
exploratory sense, the first 10 years of frequencies were separately compared with those of each of the 24 and
18 year periods using MRPP, and ia both applications found to differ at better than the 5 percent level.
Application of MRPP in a comparison of frequencies of the 24 versus 1 8 year periods was not significant at the
5 percent level. In this exploratory analysis, no attempt was made to correct for experimentwise error rate
under a partial null hypothesis in which some comparisons are equal but others differ (Miller, 1981). The
results of these comparisons are not surprising given the extreme level of drought in the 1950s. Additionally,
shorter comparison periods, such as the 10-year period in this data, likely increase the probability of finding
significant differences, given the dependence in rime in precipitation data. Multivariate analyses of
crosstabvJated daily precipitation from the Corpus Christi WSO AP gauge express differences in the
precipitation distribution of the 1950s drought years versus distributions from the other study periods of the
60-year record under consideration. The second study period precipitation distribution did not differ from that
of the third and most recent period.

Inspection of event frequencies of large precipitation amounts (=> 4 inches per day) revealed 25 events in the
Corpus Christi WSO AP data. There were 14 years of the record with tropical cyclone occurrences (8-La Niiia,
3-El Nino, 3-baseline). Ten large events (40 percent) occurred in La Niiia years of which seven events (70
percent) were during five tropical cyclone years. Seven large events (28 percent) occurred in El Niiio years but
none in tropical cyclone years. Eight events (32 percent) occurred in baseline years of which three events (38
percent) were in tropical cyclone years. Seven of 21 years with large events involved tropical cyclones. The
three largest category events were La Nina events in years without tropical cyclones. Two of the three largest
events occurred during the drought years of the 1950s. These results suggest somewhat over one-third of large
precipitation events occur during the La Niiia phase and often in years (La Niiia) with tropical cyclones. A
modest number of large events seem to occur during the El Nifio phase but seldom involving years with
tropical cyclones. Large events occur in baseline years, some of which may be tropical cyclone years. These
results are generally consistent with those of the computed precipitation anomalies. An additional multivariate
comparison of interest that was not conducted is the comparison of distributions between ENSO types and
baseline years.

The crosstabulated data revealed 21 (40 percent) of 52 years included large precipitation events. Of the 21
years, 6 were El Niiio, 8 were La Niiia, and 7 were baseline years. Tropical cyclones occurred in seven high-
event years: five were La Niiia years and two were baseline years. Annual precipitation exceeded 40 inches in 7
years of the record. Five years were baseline years, one year was La Niiia, and one year was El Nino and the
greatest amoimt (48.07 inches) of the record. Two years of the high seven years did not have a high daily-
amount event. These results express a role by ENSO but also other mechanisms in causing Nueces River
watershed precipitation.

yippendixD ♦ D-9

Table 3: Frequencies are given from cross-tabulation of Corpus Christi WSO AP daily precipitation.

Column names are coded to indicate the precipitation category. The LT1 code represents precipitation less
than one inch but greater than zero, and 1T02 represents precipitation greater than one inch and less than
or equal to 2 inches, etc.

YEAR

ZERO

LT1

1T02

2T03

3T04

4T05

5T06

6T07

7T08

1948

305

53

4

3

0

0

0

0

0

1949

281

75

6

3

0

0

0

0

0

1950

311

47

5

1

0

0

0

0

0

1951

291

66

3

4

0

1

0

0

0

1952

297

62

5

2

0

0

0

0

0

1953

294

64

6

0

0

0

1

0

0

1954

311

50

3

1

0

0

0

0

0

1955

308

53

2

1

0

0

0

0

1

1956

314

48

2

1

0

0

0

0

1

1957

295

60

8

2

0

0

0

0

0

1958

274

78

6

5

1

1

0

0

0

1959

278

76

7

2

1

1

0

0

0

1960

266

90

6

2

1

0

1

0

0

1961

297

61

4

1

2

0

0

0

0

1962

301

62

2

0

0

0

0

0

0

1963

305

56

3

1

0

0

0

0

0

1964

291

70

4

0

1

0

0

0

0

1965

284

75

5

1

0

0

0

0

0

1966

281

77

2

4

1

0

0

0

0

1967

297

58

6

0

3

0

0

1

0

1968

271

85

3

6

1

0

0

0

0

1969

290

69

5

1

0

0

0

0

0

1970

288

67

6

2

0

1

0

1

0

1971

301

55

4

3

2

0

0

0

0

1972

274

83

4

3

2

0

0

0

0

1973

268

83

10

3

0

1

0

0

0

1974

285

73

7

0

0

0

0

0

0

1975

284

73

7

1

0

0

0

0

0

1976

281

72

10

2

1

0

0

0

0

1977

286

73

4

1

0

1

0

0

0

1978

287

68

5

2

1

2

0

0

0

1979

278

78

6

2

0

0

0

1

0

1980

298

59

6

1

0

0

0

2

0

1981

276

75

9

3

1

1

0

0

0

1982

307

54

1

2

0

1

0

0

0

1983

287

65

7

6

0

0

0

0

0

1984

299

61

3

3

0

0

0

0

0

1985

290

62

12

0

0

0

1

0

0

1986

279

76

8

2

0

0

0

0

0

1987

283

70

10

2

0

0

0

0

0

1988

300

62

2

2

0

0

0

0

0

1989

305

55

5

0

0

0

0

0

0

1990

286

74

4

1

0

0

0

0

0

1991

259

94

7

3

0

1

0

1

0

1992

269

84

8

4

1

0

0

0

0

1993

292

63

3

3

4

0

0

0

0

1994

288

66

5

5

0

0

1

0

0

1995

288

68

5

3

0

0

0

0

1

1996

311

50

4

1

0

0

0

0

0

1997

268

86

6

3

2

0

0

0

0

1998

287

69

7

1

1

0

0

0

0

1999

300

56

5

3

0

1

0

0

0

D-10 4* Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas

SUMMARY

Precipitation from the Nueces River watershed was analyzed for recent trends. Of particular interest were
comparisons of precipitation across the three periods, 1940-57, 1958-81, and 1982-99, selected for study in the
potential long-term effects of the Rincon Project (Irlbeck and Ward 2000). Tropical cyclone occurrences in the
Adantic basin and possible relationships with ENSO and climate variables were investigated. An analysis was
conducted of Nueces River watershed precipitation anomalies stratified by ENSO type, to assess whether
ENSO may affect watershed precipitation. A multivariate comparison was conducted of daily precipitation
sampling distributions for the Rincon three study periods using data from the Corpus Christi WSO AP gauge.
Differences could point to more potential inundations of the Nueces delta marsh, despite possible similar
annual precipitation amounts.

Precipitation analyses indicated highest amounts occiorred along the Nueces Bay coastline of the Rincon Project
study area. Least amounts occurred in the west-central area of the watershed. Precipitation from three gauges
located across the watershed displayed the same general pattern across the three study periods: least annual
amounts during the 1950s' drought, highest amounts during 1958-81 (second study period), and intermediate
amounts during 1982-99. Using a base period that included the 1950s, annual precipitation portrayed an
increasing trend. However, a broader time view using a base period that consisted of pre-1950s data produced
no particular trend in Nueces River watershed precipitation.

The survey of studies of ENSO and some climate variables, and their possible relationships to tropical cyclone
activity in the Adantic basin jrielded results of analyses of interannual trends and multi-decadal variability. The
studies found that only weak Unear trends can be ascribed to the hurricane activity and that multi-decadal
variation is the stronger characteristic present (in particular; Gray, 1979, 1984; Gray et al, 1993; Landsea, 1993;
Landsea et al, 1992, 1999). Various environmental factors including are analyzed for interannual links to the
Adantic hurricane activity. Environmental variables showing significant, concurrent relationships to the
firequency, intensity and duration of Adantic hurricanes include the stratospheric Quasi-Biennial Oscillation,
Adantic sea surface temperatures, 200mb zonal winds, Caribbean sea level pressures, African West Sahel
rainfall, and ENSO. ENSO was linked to changes in tropical cyclone-caused damages. More damage occurred
during La Nina years followed by baseline years and El Nino years.

ENSO related precipitation anomalies in the Nueces River watershed were investigated using monthly data
stratified by ENSO phase to develop proportion of months of record with precipitation that equaled or
exceeded a benchmark determined from the baseline years. Analyses were separately conducted for three
gauges of the Nueces River watershed. Results were variable and did not definitively point to either phase
expressing a consistent anomaly sense (positive or negative). Suggestions in the results include that two of
three gauges yielded more positive anomalies in La Nina months (eight tropical cyclones) than was obtained for
El Nino months (four tropical cyclones). Four of six anomalies were negative. Overall, the results suggest that
high variability in the watershed precipitation can mask whatever ENSO effects occur. Perhaps data for more
watershed gauges would be more revealing.

The sampling distribution of Corpus Christi WSO AP daily precipitation was investigated per each Rincon
Project study period. While annual precipitation changed Utde after the 1 950s drought, possibly the distribution
of daily amounts differed from one Rincon study period to another. Differing distribution could favor more
marsh intmdations in some study period. Multivariate analysis was used to compare the sampling distributions
of the three study periods. Exploratory analyses revealed that the precipitation distribution of the first study
period that included the 1950s drought differed from each of the other two study periods. The second period
data did not differ from the third period data, suggesting that neither period would on average be dominant in
potential natural inundations of the Nueces delta marsh.

The crosstabulated Corpus Christi WSO AP data revealed 21 (40 percent) of 52 years (since 1948) with large
precipitation events (=> 4 inches per day): 6 were El Nino years, 8 were La Nina years and 7 were baseline
years. Tropical cj'clones occurred in seven high-event years: five were La Niiia years and two were baseline
years. Annual precipitation exceeded 40 inches in 7 years of which 5 years were baseline years, 1 year was La
Nina, and 1 year was El Niiio and the greatest amount (48.07 inches) of the record. Of the seven high-annual
years, two years did not have a high daily-amount event.

Appendix D ♦ D-11

This study revealed that the most prominent feature of Nueces River watershed precipitation since 1940 was
the 1 950s drought. No particular precipitation trend was apparent unless the basis of comparison is largely the
period of drought years. While on average, the La Nina ENSO phase leads to more tropical cyclones in the
western Gulf of Mexico, no consistent anomaly across the Nueces River watershed was revealed. This result is
important because the Adantic basin may be returning to a more active tropical cyclone period possibly leading
to more related storms impacting the Nueces delta.

ACKNOWLEDGMENTS

This study was conducted uith funding from the Rincon Project allocation. Suggestions and comments from
Mike Irlbeck of Reclamation were beneficial and gready appreciated. Editorial reviews were conducted by Mike
Irlbeck and Gray Harris of Reclamation.

LITERATURE CITED

Asquith, W. H., J. G. Mosier, and P. W. Bush, 1997. Status, Trends, and Changes in Freshwater Inflows to Bay

Systems in the Corpus Christie Bay National Estuary Program Study Area. CCBNEP-17, Coastal

Bend Bays and Estuanes Program, Corpus Christi, TX,
Cleveland, W. S. 1979. Robust locally weighted regression and smoothing scatterplots. J. Amer. Stat. Assn., 74,

829-836.
Cleveland, W. S., 1981. LOWESS: a program for smoothing scatterplots by robust locally weighted regression.

The American Statistician, 35, 54.
Gershunov, A., and T. Bamett, 1998. ENSO influence on intraseasonal extreme rainfall and temperature

frequencies in the contiguous United States: Observations and model results. /. Climate, 11, 1575-1586.
Gray, W. M., 1968. Global view of the origins of tropical disturbances and storms. Mon. Wea. Rff., %, 669-700.
Gray, W. M., 1979. Hurricanes: Their formation, structure and likely role in the tropical circulation.

Meteorology over the tropical oceans. D. B. Shaw (Ed.), Roy. Meteor. Soc, James Glaisher House,

Grenville Place, Bracknell, Berkshire, RG12 IBX, 155-218.
Gray, W. M., 1984. Adantic seasonal hurricane frequenc)': Part I: El Nino and 30 mb quasi-biennial oscillation

influences. Mon. Wea. ^v., 112, 1649-1668.
Gray, W. M., C. W. Landsea, P. W. Mielke,Jr., and K. J. Berry, 1993. Predicting Adantic basin seasonal tropical

cyclone activity by 1 August. Weather Forecasting 8, 73-86.
Helsel, D. R., and R. M. Hirsch, 1992. Statistical methods in water resources. New York: Elsevier. 522 p.
Hollander, M., and D. A. Wolfe, 1973. Nonparametric statistical methods. New York: John Wiley. 503 p.
Irlbeck, MJ. and G.H. Ward. 2000. j\nalysis of the historic flow regime of the Nueces River into the upper

Nueces Delta, and of the potential restoration value of the Rincon Bayou Demonstration Project. In

Concluding Report: Rincon Bayou Demonstration Project, Appendix I-C. United States Department

of the Interior, Bureau of Reclamation, Austin, Texas.
Knaff, J. A., 1997. ImpUcations of summertime sea level pressure anomahes in the tropical Atlantic region.

Mon. Wea. Rev., 10, 789-804.
Landsea, C. W., W. M. Gray, P. W. Mielke, Jr., and K. J. Berry, 1992. Long-term \'anations of Western

Sahelian Monsoon Rainfall and Intense U.S. Landfalling Hurricanes. J. Climate, 5, 1528-1534.
Landsea, C. W., 1993. A Climatology of Intense (or Major) Adantic Hurricanes. Mow. Wea. Rev., 121, 1703-1713.
Landsea, C. W., R. A. Pielke, Jr., A. M. Mestas-Nunez, and J. A. Knaff, 1999. Adantic Basm Humcanes:

Indices of Climatic Changes. Climatic Change, 42, 89-129.
Mielke, P. W., K. J. Barry, and E. S. Johnson, 1976. Multi-response permutatation procedures for a priori

classifications. Commun. Statist. Theor. Meth., A5, 1409-1424.
Mielke, P. W., K. J. Barry, and J. G. Medina, 1982. Climax I and II: distortion resistant residual analyses.

/ Appl. Meteor, 21, 788-792.
Miller, R. G., Jr., 1981. Simultaneous statistical inference. New York: Springer- Verlag.
Philander, S. G. H., 1989. El Nino, La Niiia, and the Southern Osdllation., Academic Press, New York,

293 pp.
Pielke, Jr., R. A., and C. W. Landsea, 1999. La Nina, El Nino, and Atlantic Hurricane Damages in the United

States. Submitted to Bull Amer. Meteor. Soc.
Shapiro, L. J., 1989, The relationship of the quasi-biennial oscillation to Atlantic tropical storm activit)'. Mon.

Wea. Rev., 117,2598-2614.

D-12 ^* Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas

APPENDIX E

Utilization of Estuarine Organic
Matter During Growth and Migration
by Juvenile Brown Shrimp Penaeus
aztecus in a South Texas Estuary

p. Riera Centre de Recherche en Ecologie Marine et Aquaculture de L'Houmeaii, France

P.A. Montagna University of Texas at Austin, Marine Science Institute

R. D. Kalke University of Texas at Austin, Marine Science Institute

P. Richard University of Texas at Austin, Marine Science Institute

In Press in: Marine Eco/o^-Prvgress Series

ABSTRACT: The trophic dynamic links of migratory juvenile brown shrimp Penaeus as^tecus were investigated
along the South Texas coast from the i\ransas Pass to Corpus Christi and Nueces Bay and to the nursery
ground in the Nueces Delta. Shrimps and their potential food sources were measured for 6'^C and 6'^N ratios
between December 1995 and July 1996. During this period, shrimp length increased from 10-11 mm when the
animals entered Corpus Christi Bay as larvae to 80-90 mm when they returned to Mexico Gulf as subadults.
Brown shrimp exhibited spatial and temporal 6'^C variation (from -25.2 to 12.5%o) indicating a high diversity of
food sources throughout their migration. From o"C values, the main sources used as food sources by juvenile
brown shrimp in the Rincon Bayou marsh, were Spartina altemiflora and Spartina spartinae detritus and benthic
diatoms. o'^C and o'^N values showed that organic matter inputs carried by the river inflow can also contribute
significandy to the feeding of migratory brown shrimp. In these marsh habitats, shrimp isotopic ratios changed
rapidly suggesting high tissue turnover rates. The study showed that coastal marshes after restoration through
the introduction of freshwater inflow may provide feeding habitats favorable for growth and development of
juvenile brown shrimp.

KEY WORDS: Penaeus a^eats, food sources, migration, nursery area, 6"C and 6''N

INTRODUCTION

Many marine species utilize salt marshes, coastal lagoons and estuaries dixring part of their life cycle (Day et al.
1989). The brown shrimp, Penaeus aztecus Ives, is widely distributed along the Texas coastline where it is an
important commercial fishery. Like most penaeids, the life history of brown shrimp is complex (Dall et al.
1990). In the Gulf of Mexico, after offshore spawning, post-larval brown shrimps are carried by on-shore water
movement and enter bays and shallow estuarine waters where they generally find productive areas and are
protected from storms and predators (Day et al. 1989, Fry 1981). Following growth of juvenile shrimp in
nursery areas there is a subsequent offshore migration of subadults to complete their life cycle. More precisely,
throughout the Texas bay systems, most of the larval brown shrimp enter marine bays from late winter through
early spring, spend about 3-4 months in estuarine nursery grounds, and return to the offshore Gulf of Mexico
in eady summer (Moffett 1970). However, the shrimp fishery is an exploited resource in the Gulf of Mexico
(Parker et al. 1988). Restoration of coastal marshes, which act as nurseries, could contribute to increased brown
shrimp populations because the marshes provide habitats for juveniles. Enhancing freshwater inflow may have
at least two beneficial effects for restoring shrimp habitats: terrestrial inputs may be used as food sources

Appendix E ♦ E-1

(Hackney & Haines 1980 Riera & Richard 1996), and freshwater inflow may increase the total primary
production through nutrient inputs (Nixon et al., 1986).

The food sources utili2ed by juvenile brown shrimp during migration are more difficult to determine than that
of adults, which live in deeper offshore environments that are generally phytoplankton-based systems (Frj' &
Parker 1979). As juvenile penaeids migrate, they occupy different habitats, which correspond to different
feeding grounds (Dall et al. 1990). Food availabilit}' can differ gready within and between these habitats (Frj^
1981), and different food sources may be used preferentially by shnmp. For example, stomach content analyses
of small juvenile Penaeus semisculatus vittt composed of a large variety of prey including diatoms, meiofauna,
insect larvae and seagrass (Heales et al 1996). Moreover, Wassenberg & liill (1987) observed large intraspecific
differences in the food ingested by Penaeus esculentus collected from widely separated areas. Using immunological
methods, it was found that the Penaeus ai^ecus and Penaeus setiferus have a diverse diet (Hunter & Feller 1987).

Stable isotope analysis has been used successfully to determine original food sources of marine and estuatine
invertebrates (Harrigan et al. 1989). Stable isotopes assess food sources assimilated over time (Fry & Sherr
1984), so they are valuable for feeding studies when material in gut contents are difficult to identify due to
digestion and trituration. \^ariarion in d -^C values of the migratory brown shrimp along the South Texas coast
has been investigated previously (Fry & Parker 1979, Fry 1981). These studies have pointed out that seagrass
meadows of shallow-water habitats were important feeding grounds for migratory juvenile brown shrimp.
However, littie is known about spatial and temporal variation of food sources encountered by brown shrimp
throughout a complete migration between oceanic waters and upper estuarine reaches. As suggested by Fry
(1981), habitats other than seagrass meadows may contribute significandy to the feeding of migrating shrimps.
Therefore, it is important to know which feeding habitats contribute the most to the growth and development
of the brown shrimp along the Texas coastline.

The aim of the present study was to identify the trophic dynamic links of migratory Penaeus a^ecus with food
sources in various habitats along the South Texas coast. Shrimp migrations were followed from the Aransas
Pass to Corpus Christi and Nueces Bay and to the ultimate nursery ground in the Nueces Delta. A primary
objective was to determine if the primary production in a coastal marsh, which is being currendy restored by
the re -introduction of freshwater inflow, can support the feeding and growth of juvenile brown shrimps.
Shrimp and potential food sources were determined by stable isotope analyses (6"C and 6'*N).

Material and Method
Sampling Area

The study was carried out in the Nueces Estuary (Fig.l). The estuary consists of the Nueces River, the Rincon
Bayou marsh and the Rincon Bayou mouth. The Nueces River empties into Nueces Bay, which is connected to
Corpus Christi Bay, which is connected to the Gulf of Mexico by the Aransas Pass. Historically, the Nueces
River fed into Rincon Bayou, which is in the center of the Nueces Delta. A dam was created early in the 20*
century to contain Nueces River water. Recendy, the dam was lowered to allow flood events from Nueces River
to flow into the Rincon Bayou to restore the Rincon Bayou marsh. Sampling locations into Rincon Bayou
marsh (Fig.l) were distributed from the freshwater entrance into Rincon Bayou marsh (River site) to the Rincon
Bayou mouth (Rincon mouth). Sampling stations also included "Up Marsh" and "Down Marsh" within Rincon
Bayou marsh, and "Aransas Pass" which connected Corpus Christi Bay to the Gulf of Mexico. These sites have
been considered because, as reported by Moffett (1970), they may lie on the migratory route of brown shrimps
that enter Corpus Christi Bay through Aransas Pass.

Collection and Preparation of Samples

Sampling for organic matter sources and brown shrimp was carried out during from December 1995 to July
1996. Shrimp were collected by otter trawls or hand-thrown cast nets. They were sorted by hand and kept in
seawater from the sampling site so that guts would be purged prior to analysis. At the laborator)', shrimp were
identified to the species levels using a magnifying glass and keys, measured (length) and were frozen (-80°C).
For stable isotope analyses shrimp were prepared according to Fry & Parker (1979). White muscle tissue was
dissected from the shrimp abdomen, acidified (10% HCl) to remove any residual carbonates from cuticules,

E-2 ^ Utili:(ation of Estuarine Organic Matter During Gnwth
and Migration by Juvenile Brown Shrimp

tinsed with distilled water, and dried in a oven at 60°C. Then, the white muscle of each individual was ground
to a fine powder with a mortar and pesde to homogenize the sample. Individuals were used as samples except
where shrimp were too small to obtain sufficient tissue for accurate 6"C and 6'^N analyses. For larval shrimp,
6 or 8 individuals (10-11 mm size length) from the same sampling occasion were combined. Some of the brown
shrimp collected and analysed for 6"C were also measured for 6'^N. Zooplankton was collected with a (176
|Jm) mesh net in Nueces Bay near Rincon mouth (Fig.l). Samples of zooplankton were acidified (10% HCl) to
remove any residual carbonates from cuticules, rinsed with distilled water, dried in a oven at 60°C and ground
to a fine powder with a mortar and pesde to homogenize the sample.

Corixidae Corix sp and mysids Mysidopsis almyra were sampled at Up Marsh at different occasions, between April
1995 and February 1996, by using the same procedure as for brown shrimp. Composite samples of corixidae (8
individuals) and mysids were prepared in the same way as for zooplankton and brown shrimp, respectively.

For sampHng particulate organic matter (POM), 15 1 of water were collected, then filtered on precombusted
Whatman GF/F glass fiber filters under moderate vacuum within five hours after collection. Samples were
acidified (10% HCl) to remove carbonates, dried at 60°C and kept frozen (-80°C) vintil analysis. For sedimented
organic matter (SOM) analysis, sediment samples were taken in the Nueces River at "River site" and in the
Rincon Bayou marsh at "Up Marsh" and "Down Marsh" by scraping the upper 1 cm of mud. At the laboratory,
samples were homogeneized, dried at 60°C, ground using a mortar and a pesde, and then acidified (10% HCl)
to remove any inorganic carbon. These samples were not rinsed to prevent any loss of dissolved organics. They
were dried overnight at 50°C under a slight vacuum to evaporate the acid. Once dried, the sediment was mixed
with Milli-Q water, freeze-dried, ground again to a fine powder and kept frozen (-80°C) until analysis.

At "Rincon mouth", leaves and twigs of the two dominant marine phanerogames, Spartina altemlflora and
Salicomia sp, were collected. For samples of terrestrial organic matter, leaves of the dominant vascular plants,
namely Salix sp, Fraxinus sp and the switchgtass Panicum virgatum, were collected along the Nueces River up
"River site". Most samples within Rincon Bayou marsh were the Gulf Cord Grass Spartina spartinae which
dominates along the Rincon Bayou channel. These plant samples were cleaned of epibionts, and prepared
similarly to shrimp muscle tissue. Blue green algal mats were collected in the Rincon Bayou channel and
acidified (10% HCl), rinsed with distilled water, freeze-dried, and firozen until analysis. Benthic diatoms were
also sampled from muddy sediments near the Rincon Bayou channel, and separated using a procedure from
Couch (1989) as modified by Riera & Richard (1996). Briefly, the surficial sediments with dense microalgal mats
was scraped and brought into the laboratory where it was spread on flat trays to a depth of about 1 cm. A
nylon screen (63 p.m mesh) was laid upon the sediment surface and covered with a 4 to 5 mm layer of
combusted silica powder (60-210 p.m). After 12 hours, the top 2 mm of the sihca powder was gendy scraped
and then filtered on previously combusted glass fiber filters, acidified (10% HCl), rinsed with distilled water,
freeze-dried, and frozen (-80''C).

Stable Isotope Analysis

Samples for isotope analyses were combusted at 900°C using CuO as an oxydant in evacuated quartz tubes
(Stump & Frazer 1973). Samples for isotope analyses were prepared as in Boutton (1991). Before the
purification of CO^, N^ was trapped on silica gel granules in a stopcock sample ampule and analyzed immediady
after CO, collection (Mariotti 1982). The carbon and nitrogen isotope ratios were measured using a Sigma 200
(CJS Sciences) double inlet, triple collector isotope ratio mass spectrometer. Data are expressed in the standard
d unit notation where 6 X = [(R,^piyR„fe^cc)-l] x 10\ widi R = '^C/''C for carbon and '*N/"N for nitrogen,
and reported relative to the Pee Dee Belemnite standard (PDB) for carbon and to air N^ for nitrogen. The
typical precision of the overall procedure (i.e., preparation plus analysis) was ± 0.1 %o for carbon and ± 0.2 %o
for nitrogen.

Results
6"C AND 6'^15N OF POM, SOM, Sources and Invertebrates

O'^C and o'*N of POM, SOM, plant sources and invertebrates are presented in Table 1. There was a gradient in
carbon isotope values from the sea to the river. At Aransas Pass, POM 6"C values were 6'^befween -24.8%o

Appendix E ♦ E-3

and -22.0%o while at the Nueces River site, POM 6"C ranged from -28.8 to -26.3 %o. POM 6"C at Rincon
Bayou mouth ranged from -27.2%o to -20.8%o. ^X'ithin Rincon Bayou marsh, POM exhibited a large range for
6"C, from -26.3%o to -17.4%o at Down Marsh, and from -24.1%o to -17.r/oo at Up Marsh. Corresponding 6'*N
values for POM ranged from 3 to 11 %o at Aransas Pass, from 8.8 to 10.8 %o at Nueces River and from 9.2 to
9.4 %o at Rincon Bayou mouth. Within Rincon Bayou marsh, POM 6'^N ranged from 2.6 to 9.3 %o. 6"C for
SOM (i.e., including benthic algae) ranged from -24.0 to -22.2%o at Nueces River and from -21.8 to -20.2 %o in
Rincon Bayou marsh. SOM 6'^N values were between 7.1 to 8.2 %o in Rincon Bayou marsh. At Rincon Bayou
mouth, o'^C values ranged from -27.8 to -26.3 %o for SaUcomia sp and from -16.2 to -13.7 %o for Spartina
altemifhra, tj-pical of 6'^C for C3 and C4 plants, respectively (Fry & Sherr 1984, Currin et al. 1995) and close to
values previously observed for Spartina sp and Salicomia sp by Creach (1997). Within Rincon Bayou marsh, 6"C
for Spartina spartinae ranged from -16.8 to -14.5 %o. Benthic diatoms inhabiting muddy sediments near the
Rincon Bayou channel had 6'^C values from -18.5 to -16.3 %o. Blue green algae had 6'^C from -15.7 to -15.9
%o typical of "C-enriched surfidal blue-green algal mats from Texas (Calder & Parker 1973). Blue green algae
were the most '*N-depleted primary producers with 6'^N from -0.7 to 1.7 %o. 6"C values for leaves of the
most common terrestrial vegetation along Nueces River, from -30.3 to -27.6 %o, for Fraxinus sp and Sa&x sp,
were typical of terrestrial C3 plants (Degens 1969). 6"C oi Panicum virgatum ranged from -15.9 to -14.6 %o
indicating a C4 photosynthetic pathway (Fr)' & Sherr 1984). At Rincon Bayou mouth zooplankton was "C-
depleted (from -26.3 to -25.6 %o) and '^N-enriched (1 1.7 %o). Within Rincon Bayou marsh Corix sp and
Mysidopsis almyra showed 6'^C from -20.8 to -16.2 %o and from -21.6 to -20.5 %o, respectively. Corresponding
6'*N values were -0.2%o for Corix sp and from 10.5 to 10.8 %o for Mysidopsis almyra.

Size Length, 6"C and S'^N of Penaeus aztecus

Early in the study, only small larval shrimp were found in the Aransas Pass during their migration into the bay
(Fig. 2). Shrimp length increased from 10-11 mm (larvae) injanviaiy 1996 to 80-90 mm (subadults) in July 1996
when they migrated seaward, out of the Pass. Shrimp size increased from about 40 mm to 60 mm in the nursery
habitat (Fig. 2). Isotope values of 6"C and 6'*N for Penaeus a^ecus also differed between i\ransas Pass and
Nueces River and within sites over the period Januar)' to July 1996 (Table 2). >\lthough the sampling procedure
was similar at each sampling occasion, the number of shrimp collected was unequal at the different sites and
dates, owing to abundance fluctuations throughout the migration. In particular, on the different sampUng dates,
brown shrimps were not found at every site (Table 2) confirming the migratory pattern of the juvenile brown
shrimp previously observed along the South Texas coast (Moffet 1970, Fry 1981). However, the sampling
procedure did not allow an accurate quantitative evaluation of shrimp abundance at the different sites over the
sampUng period.

Shrimp feeding habitats and assimilated carbon and nitrogen were different in the animals at various stages of
the migratory life cycle. At i\ransas Pass, 6'^C values for brown shrimps varied between -21.7 and -20.7 %o for
larval stages (10-11 mm) in winter and from -16.4 to -12.5 %o for sub-adults (80-90 mm) in summer. 6''C
values for juvenile shrimps ranged from -20.6 to -14.8 %o at Rincon Bayou mouth, from -21.3 to -14.5 %o at
Down Marsh and from -18.4 to -15.3 %o at Up Marsh. At the Nueces River site, brown shrimp were collected
between mid-May and mid-lune and their 6'^C ranged from -25.2 to -20.4 %o. 6"C for Penaeus a^ecus^e.r&
significantly different among the sampling sites (Kruskal-WalHs test=35.3, df=4, p<0,001). At Aransas Pass,
o'^N values were between 5.5 and 7.7 %o for larval shrimp and between 3.3 and 9.7 %o for sub-adults. o'*N
ranged from 4.5 to 13.1 %o at Rincon Bayou mouth, from 1.0 to 9.9 %o at Down Marsh and from 1.2 to 5.1 %o
at Up Marsh. At the Nueces River site, 6"N values ranged between 6.6 and 13.9 %o. 6'^N values were
significandy different among the sampling sites either (Kruskal-Wallis test=36.8, df=4, p<0,001).

DISCUSSION

6"C Variations of Penaeus aztecus Throughout the Migration

spatial and temporal O'^C of migrating brown shrimp exhibited variation throughout migration (Fig.3)
confirming the migratory pattern of juvenile brown shrimp in Texas bays previously observed (Moffet 1970,
Ft)' 1981). The range of 6''C values for Peneaus amicus (from -25.2 to -12.5 %o) is similar to the range of o"C
observed for the main sources of organic matter (from -30.3 to -13.7 %o). This observation is consistent with

E-4 ^ Utilis^ation ofEstuarine Organic Matter During Growth
and Migration by juvenile Broom Shrimp

the hypothesis that a high diversity of food sources is used by brown shrimp throughout the migration.
However, during each sampling date from January 25 to July 29 (Table 2, Fig.3), there was little variation in
shrimp 6'^C values (sd < 2 %o), indicating that the shrimps had a similar diet at specific sites and times (de Niro
& Epstein 1978). Wide ranges of 6"C for invertebrates have been previously observed along salinity gradients
(Incze et al. 1982, Hughes & Sherr 1983). The oyster Crassostna gigas exhibited significandy different 6"C
variations between sites along an estuarine gradient, reflecting the preferential utilization of different food
sources, namely, terrestrial detritus, benthic diatoms and marine phytoplankton (Riera & Richard 1996). The
present study suggests that individual 6''C variation of a migrating species (Penaeus a^ecus) along an estuarine
gradient can be as large as inter-individual o"C variation of a sedentary species {Crassostna gigas).

Contribution of salt marsh sources to brown shrimp diet

Salt marsh habitats appear to be important food sources for young brown shrimp. When entering Corpus
Christi Bay through Aransas Pass, brown shrimp larvae had typically O C values (-21.7 to -20.7 %o)
characteristic of animals feeding primarly on an oceanic planktonic food soiurce (Fry & Parker 1979, Incze et al.
1982). However, as they entered the mouth of Rincon Bayou through the Spartina altemiflora and Salicomia sp
marsh, brown shrimp became more "C -enriched (-19.4 to -16.8 %o) indicating a significant contribution of a
"C-enriched source to shrimp diet. At Rincon mouth, suspended POM, SaUcomia sp and zooplankton were too
"C-depleted (-27.3 to -24.2 %o) to explain the enrichment in "C of migratory brown shrimp (Table 1, Fig.3).
Moreover, assuming phytoplankton is the main food source for zooplankton, and that the trophic O C
enrichment is about 1 %o per trophic level (de Niro & Epstein 1978), phytoplankton O C should be about -27
%o, which is too negative to be a major contribution to brown shrimp diet. The "C-enrichment of brown
shrimp at the mouth of Rincon Bayou can be explained by a significant contribution of carbon derived from
Spartina altemiflora detritus (-16 to -14.5 %o). Consistent with these results, plant detritus derived &om Zostera sp,
Vhragmites sp or Spartina sp were found in gut contents of post-larval penaeids indicating that marsh detritus may
be a food source for these shrimps when they occupy that habitat (see Dall et al., 1990 for a review). Likewise,
mangrove detritus has been shown to contribute to the diet of juvenile Penaeus merguiensis inhabiting tidal creeks
in Peninsular Malaysia (Newell et al. 1995). In contrast, from feeding experiments Gleason & Wellington (1988)
reported that Spartina altemiflora detritus and its epiphytes contributed only a small part of Penaeus a^ecus
assimilated carbon. Finally, Dall et al (1990) concluded that plant detritus itself is not a major food source for
prawns.

The results of the present study indicated that detritus derived from Spartina altemiflora can be an important
carbon source to juvenile brown shrimp. However, Penaeus a^ecus may assimilate carbon that is ultimately
derived from Spartina via several routes other than direct feeding on plants detritus. It is possible that Penaeus
a^^ecus may obtain part of its carbon derived from Spartina altemiflora detritus through microbial mediation. For
example, in '''C labelling experiments, the grass shrimp Palaemonetes puffo could assimilate carbon from detrital
Spartina altemiflora with 38,4 % efficiency via bacterial mediation between non-living organic detritus and shrimp
(Crosby 1985). In fact, bacteria associated with debris of refractory plant material can facilitate the carbon
transfer from plant sources to bivalves (Langdon & Newell 1990, Crosby et al. 1990).

As brown shrimp occupied the down and up marsh, they remained "C-enached (-19 to -17 %o), indicating a
persistence in their utilisation of a relatively heavy ''C source (Fig.3). Although there is no Spartina altemiflora
within the Rincon Bayou marsh, these 6"C values may be explained by utilization of benthic diatoms, blue
green algae and/or detritus derived from Spartina spartinae as carbon source (Fig.3). However, the respective
contributions of these '^C-enriched sources to brown shrimp feeding cannot be established from 6'^C values
alone. It is known that benthic microalgae from mudflats represent one of the dominant food source for
juvenile penaeids within tidal creeks in Peninsular Malaysia (Newell et al. 1995). Also, a positive growth rate of
postlarval brown shrimp can be supported over 16 days by feeding only on the planktonic diatom, Skeletonema
costatum (Gleason & Zimmerman 1984). Although living microalgae can be more readily used than detritus of
vascular plants, as shown for marine bivalves (Bayne et al 1 987, Crosby et al 1 989), a significant contribution of
detritus derived from Spartina spartinae to shrimp diet may also accoimt for the observed carbon isotope values.
These results are in accordance with recent isotopic data of Deegan and Garritt (1997) showing a preferential
utilisation of local sources organic matter in coastal marsh areas by invertebrates.

Appendix E ♦ E-5

Contribution of Riverine Inputs to Brown Shrimp Diet

6"C values also changed as shrimp moved near and into areas that are subject to freshwater inundation. In May
and June 1996, juvenile brown shrimp were collected at the Nueces River site up to Rincon Bayou marsh
(Fig.l). Other shrimp, (e.g. Penaeus merguiensis (Staples 1980) and Penaeus setiferus (Dall et al. 1990)), have also
been observed in lower salinities environments far upstream from the river mouth. As juvenile brown shrimp
occupied Nueces River, their O'^C became significandy more negative (-24 to -21 %o) compared with o"C
measured within Rincon Bayou marsh (Fig.3). This depletion in '^C indicates that a significant part of brown
shrimp carbon at Nueces River is derived from terrestrial detritus and/or riverine phytoplankton carried by
freshwater inflow, because Rincon Bayou marsh lacks a "C-depleted source (Fig.3). Unfortunately, 6"C for
riverine phj'toplankton could not be estimated in the present study. However, previous results showed
phytoplankton 6"C values of -44 to -47 %o in rivers (Rau 1978) and of -40 %o (Hedges et al. 1986) in a lake.
Similarly, 6"C of freshwater phytoplankton in the Charente River (France) ranged between -41.8 and -31.2 %o
(Riera & Richard 1996). Considering this higher depletion in "C for freshwater phytoplankton compared with
terrestrial C3 plants, a primar)' contribution of riverine phytoplankton to the diet of brown shrimp is tinlikely,
but cannot be excluded from these results.

In fact, because the metabolic '^C-enrichment dviring assimilation is close to 1 %o (De Niro & Epstein 1978,
Rau et al. 1983), shrimp 6'^C values at Nueces River are consistent with a significant utilisation of terrestrial
C3 plants (-29 to -28 %o) as food source. Moreover, the similarity of 6"C values of terrestrial C3 plants {Salix
sp, Fraxinus sp) and POM in the Nueces River indicates that detritus firom C3 plants contribute predominandy
to river POM. Therefore, the resiJts of this study can explain the depletion in '^C for brown shrimp observed
in lower salinity bays that are flushed by freshwater inflow or are most influenced by river inputs along the
South Texas coast (Fry 1981). This result is consistent with the hypothesis that terrestrial organic inputs could
be incorporated into estuarine food weebs (Hackney & Haines 1980, Incze et aL 1982). In fact, previous results
showed the significant contribution of terrestrial detritus derived from C3 plants to the diet of oysters
{Crassostrea gigas) in the upper reaches of the Charente Estuary (Riera & Richard 1996), and in the middle
estuarine reaches as a high river discharge period occurred (Riera & Richard 1997). In addition, from o"C
analyses, Stephenson & Lyon (1982) reported that the bivalve Chione /tofrA^w^; inhabiting the Avon-Heathcote
Estuary (New Zealand) could utilise carbon of terrestrial origin depending upon its position in the estuary' and
local hydrolog)'. Freshwater inputs can be an important source of nutrition for juvenile brown shrimp in
habitats lacking salt marsh plants and benthic diatoms. Therefore, during periods of high river discharge,
riverine inputs may support a substantial part of the food webs in South Texas bays and elsewhere as well.

Offshore Migration of Subadult Penaeus aztecus

Subadult Penaeus a:(tecus that are migrating offshore have different diets than the subadults found in marshes. Aa
enrichment in "C for Penaeus a^ecm (-13.8 ±1.5 %o) was observed at a\ransas Pass at the end of July 1996
(Fig.3). Brown shrimp in Aransas Pass were likely returning towards the nearshore Gulf of Mexico because they
were sub-adidt size and offshore migration occurs in summer in Texas bays (Moffet 1970). Similar 6"C values,
between -12.6 and -14.6 %o, were also obser\'ed by Fr}' & Parker (1979) for brown shrimp collected offshore in
the Gulf of Mexico, but more negative 6''C were observed for other shrimps {Penaeus setiferus) from the same
area. This enrichment in "C in shrimp tissues is not Ukely to be a result of a metabolic effect as shrimp grow
due to a variation of carbon fractionation. In fact, the inter-individual 0"C variability among animals having a
similar food source does not usually exeed 2 %o for fishes and invertebrates, these differences being attributed
to size, age or sex (Fry & Parker 1979, Hughes & Sherr 1983). The 8 %o mean enrichment in "C in shrimp
tissues (Fig.3) between the marsh and pass locations in spring and summer may have other interpretations.

At Aransas Pass, phj'toplankton was likely to be the main organic matter source for brown shrimps because
there are no seagrass meadows. However, considering the 6'^C of -22.7 %o for marine phytoplankton in the
Northern Gulf of Mexico given by Thayer et al. (1983), the enrichment in ''C due to metabolic fractionation
between phytoplankton and brown shrimps would be much more than 1 %o. Most likely, less negative 0"C for
brown shrimp, as they enter the offshore area, may be explained by a progressive enrichment in "C as shrimps
returned from Nueces River environments towards offshore waters (Fig.3), as su^ested by Fry & Parker
(1979). In fact, the shrimps collected in late July, at the end of the migration, exhibited 6"C typical of the
feeding habitats recendy encountered where they used "C -enriched food sources, e.g., marsh grass or

E-6 ♦♦♦ Utilisation of Estuarine Organic Matter During Growth
and Migration by Juvenile Brown Shrimp

seagrasses. ParticvJaily, within Corpus Christi and Redfish Bays, seagtasses have the highest 6"C values (-3 to -
13 %o) in the ecosystem as reported by Fry and Parker (1979). High 6"C of -10 %o were also observed recendy
for seagrasses of Laguna Madre in south Texas (Street et al. 1997). Additionally, macroalgae is known to occur
in bay bottoms and along the jetties at Aransas Pass, but we have not sampled this source. As they returned
towards marine waters through Rincon Bayou mouth shrimp may also increase their feeding on Spartina detritus
direcdy or through predation on detritivores. Therefore, as subadult brown shrimp feed offshore, their 6''C
should progressively converge on the 6"C value characteristic of offshore environment, close to -18 %o (Fry
1981).

Temporal 6"C Variation: Importance for Tissue Turnover

Tissue tiunover rates are important to know to interpret temporal 6"C variation. There was about a 3.5 %o
decrease in 6"C values from the up marsh to the Nueces River site (Fig.3) indicating a high tissue turnover rate
for brown shrimp as they migrate. This isotopic 6"C variation occurred over a distance of less than 5 km and
within a period of 9 weeks. From one feeding habitat to another the "old carbon" of shrimp tissue is
progressively diluted due to 1) growth of new tissue using "new carbon" and 2) metabolic loss due to tissue turn
over (Anderson et al. 1987). Therefore, after a variation in food, shrimp 6"C will change isotopically as rapidly
as tissue turnover rate will allow (Fry 1982). In the present study, the 6"C decrease of migratory brown shrimp
is consistent with a high tissue turnover rate, which support the hypothesis of a high growth rate in the nursery
habitat. This suggestion is consistent with previous results based on experimental observations showing that
posdarval shrimp can increase in weight by a factor of 4 within a week or less at 25°C (Zein-Eldin & Aldrich
1965). A 14%o variation for o"C of posdarval brown shrimp has been observed after a 3.9 fold increase in
weight after a change in food source, indicating a high tissue turnover rate (Fry & Arnold 1982). From feeding
experiments Gleason (1 986) showed that the half-life of the initial tissue carbon of Penaeus a^ecus fed with plant
and animal diets was reached before the first doubling of weight. Finally, juvenile shrimp (initially 6"C:-18.6
%o) in an experimental feeding pond with feed at 6'^C: -22.9 %o for 8 weeks attained an eqtiilibrium 6"C at -
21.3 %o after 3 weeks and an increase in weight of 300 % (Parker et al. 1988). High tissue turnover rate for
young shrimp can be related with behaviour and feeding activity. In fact, small juveniles of Penaeus semisculatus
were active and fed both day and night and are thought to feed continuously and to digest most of their food
within only one hour (Heales et al 1996). It is likely that the variation in carbon isotope values of brown shrimp
in the Rincon Bayou Marsh are a result of changed food sources and rapid growth in this nursery habitat.

Food Sources Determination from 6'*N Values

Although only part of the shrimp collected were measured for 6'*N, a dual isotope approach is useful for
identifying food sources. In up and down Rincon Bayou marsh, O'^N for brown shrimp showed a high
variability both between sampling periods and between individuals (Table 2). This is consistent with a higher
range in o'^N values for sources in this habitat (i.e., from -0.7 to 7.6%o) and suggest a high diversity of nitrogen
sources for shrimps (i.e., Spartina spartinea, benthic diatoms, blue green algae). Lower o'^Nvalues that were
observed for some shrimps in Rincon Bayou marsh could be explained by a depletion in '*N during nitrogen
assimilation (Mako et al 1982). In fact, these authors observed a mean 6'^N fractionation of -0.3%o for the
amphipod Amphithoe vaUda fed with fresh and detrital algae. However, this hypothesis is unlikely for Penaeus
species because feeding experiments demonstrated a trophic enrichment in '^N of 2.4%o for Penaeus vannamei
(Parker et al. 1988). These lower 6'*N values for brown shrimp may partly result from a preferential
assimilation of specific chemical components of plant tissues (Mako et al 1982) and/or by the utilisation of '^N-
depleted blue green algae (Table 1). Therefore, in the present study, 6'*N was not as valuable as 6"C to
characterize food sources of brown shrimp. This result is consistent with the suggestion of Fry & Sherr (1984)
that o'^N is not as discriminating as 6"C for food sources determination in coastal ecosystems. However, at
the Nueces River mean 6'^N for shrimp (i.e., 8.2, 10.1 and 1 1.7 %o) are consistent with a significant utilization
of the terrestrial C3 plants Fraxinus sp (i.e., 6.6 %o) and/or Salix sp (8.3 ±1.1 %o) when taking into account the
mean trophic enrichment for 6'^N (i.e., 3.5 %o given by Minagawa & Wada 1984). In this riverine habitat, where
terrestrially-derived organic matter dominated, 6'*N values confirmed 6"C results.

Appendix E ♦ E-7

Importance of Trophic Mediation Through Predation

Predation on animals can also be important for brown shrimp. Like many Penaeus species, Penaeus a^ecus can
feed on different prey taxa including oligochaetes, polychaetes, crustaceans, mysids, moUusks and meiofauna
(Hunter & Feller 1987, Dall et al. 1990). In fact, the utilization of detritus derived from terrestrial and marsh
vascular plants could occur indirecdy through infaunal prey or through bacterial mediation (Gleason &
Zimmerman 1984). For example, juvenile Penaeus merguiensis tany use mangrove leaf detritus as a food source
indirecdy through predation on small detritivorous invertebrates (Newell et al. 1995). Detritus from Spartina
altemiflora salt marshes may be predonunant in the diet of meiofauna when an accumulation of detrital Spartina
is associated with the development of an important microbial biomass (Couch 1989). Then, meiofauna may also
have a role in the trophic mediation between plant detritus and brown shrimp in these habitats. Likewise,
benthic diatoms can be an indirect food source for juvenile shrimp through direct grazing or through
intermediate infaunal prey (Stoner & Zimmerman 1988). In the present study, Corixidae {Corix sp) and mysids
{Leptomis sp and Mysidopsis almyra), which were collected frequendy at the Nueces River and the Rincon Bayou
marsh, could be part of the diet oi Penaeus a^ecus. 6"C values (mean: -17.9%o) and 6'*N (-0.2%o) of Conxsp
may explain partly the enrichment in "C and the depletion in '^N observed for individuals of Peneaus a^ecus in
Rincon Bayou marsh (Table 2). In contrast, isotopic values measured for both for zooplankton near Rincon
Bayou mouth and Mysidopsis almyra in Rincon Bayou marsh were too "C-depIeted and '*N -depleted to
contribute significandy to the feeding of brown shrimp in these two habitats.

Variation in Trophic Level of Brown Shrimp

Brown shrimp may vary their food sources and feed at different trophic levels during their migratory life cycle.
Variations in brown shrimp feeding can be direcdy related with habitats. However, the habitat variations may be
associated with a variation in the trophic level of shrimp. Variations in the diet of Penaeus sp with size and age
were partly attributed to the variation from a herbivorous to a carnivorous feeding mode as shnmp grow
(Chong & Sasekumar 1981). Therefore, shrimp 6'^N should become more positive as size increases. This effect
should be more obvious in animal tissue with 6"N than 6''C values, due to a higher 6'^N fractionation during
assimilation. The relationship between 6'''N and shrimp length varies in each feeding habitat occupied by
migratory brown shrimp, because nitrogen isotopic values at the base of the food chain can vary among the
different habitats (Fig.4). At Aransas Pass, this effect should have been especially clear, because brown shrimp
size range was largest there. However, there was no significant Pearson correlation between 6'*N and shrimp
size at Aransas Pass (r=-0.22, p>0.5), at Rincon Bayou mouth (r=0.12, p>0.5), at Rincon Bayou marsh
(r=0.18, p>0.1) and at Nueces River (r=0.35, p>0.1). These results mdicate that differences in food sources of
Penaeus a^ecus thoughout its growth and migration are not associated with an increase in trophic level with size
but mosdy related to feeding habitats, as indicated by 6"C.

In addition, large ranges of o'^N observed for shrimps within the different habitats (Fig.4) suggest that shrimps
may use the different sources both direcdy and indirecdy through predation. This hypothesis could explain a
higher variability for shrimp 6'^N values compared to corresponding 6"C (Table 2) due to the higher trophic
enrichment during nitrogen assimilation. Therefore, the results of the present study are consistent uith an
omnivore feeding mode for migratory juvenile brown shnmp. This is concordant with gut content analyses,
which indicate that animal prey is part of the food ingested by Penaeus a^ecus, but that there is no variation in
dietary breadth and prey preference as shrimp grow (Hunter & Feller 1987).

In conclusion, 0"C values suggest that the main food sources for juvenile brown shrimp {Penaeus a^ecus) in the
Rincon Bayou marsh are Spartina detritus, benthic diatoms and blue green algae. Tissue turnover rates in these
marsh habitats are apparendy high, because shrimp isotopic signatures change rapidly. Moreover, O'^C and
6'^N results show that terrestrial inputs carried by the freshwater inflow can contribute significandy to the diet
of juvenile brown shrimp in the Nueces River. Finally, these results show that the restoration of coastal
marshes through the introduction of freshwater inflow can provide nursery areas favorable for feeding and
growth of juvenile brown shrimp.

\Jtilit(ation ofEstuarine Organic Matter During Growth
and Migration hy Juvenile Brown Shrimp

ACKNOWLEDGMENTS

Funding for this project was provided in part by the U.S. Bureau of Reclamation (Grant No. 4-FG-60-04370)
and the Texas Water Development Board (Research & Planning Fund Contract No. 94-483-046) to The
University of Texas at Austin, Marine Science Institute.

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Thayer GW, Govoni JJ, ConnaUy DW (1983) Stable carbon isotope ratios of the planktonic food web in the

northern Gulf of Mexico. Bulletin of marine Science 33: 247-256
Wassenberg T], Hill BJ (1987) Natiual diet of the tiger prawns Penaeus esculentus and P. semisulcatus. Australian

Joiotnal of Marine and Freshwater Research 38: 169-182
Zein-Eldin ZP, Aldrich DV (1 965) Growth and survival of postlarval Penaeus a^ecus under controlled conditions

of temperature and salinity. Biological BuUetin, Marine Biological Laboratory, Woods Hole, Mass. 129:

199-216

E-10 ♦♦♦ Utili^tion of 'Estuarine Organic Matter DuringGrowth
and Migration by juvenile Brown Shrimp

Table 1 : Average stable isotope values for food sources from different habitats

Sources

6"C

6'=N

Nueces River

Riverine POM

-27.6 ± 0.9 (n=6)

9.5±1.0(n=3)

Riverine SOM

-23.2 ± 0.9 (n=3)

4.7 (n=1)

Fraxinus sp.

-28.5 ± 0.6 (n=5)

6.6 (n=1)

Salix sp.

-29.8 ± 0.4 (n=6)

8.3 ±1.1 (n=3)

Phragmites sp.

-15.2 ± 0.6 (n=5)

-1.6 (n=1)

Rincon Bayou
Marsh

POM upper marsh

-21.2±2.8(n=18)

5.5±2.2(n=10)

POM lower marsh

-22.8 ± 0.8 (n=2)

6.5 ± 0.9 (n=2)

Marsh SOM

-21.2 ± 0.5 (n=7)

7.6 ± 0.7 (n=2)

Marsh Grass

-15.6 ±1.1 (n=3)

7.6 (n=1)

Benthic diatoms

-17.5 ± 0.9 (n=5)

4.6 ± 1 .9 (n=2)

Rincon Bayou
Mouth

POM marsh

-24.3 ± 2.9 (n=4)

9.3 ±0.1 (n=2)

Spartina sp. (fresh leaves)

-16.0 ± 0.3 (n=2)

Spartina sp. (old leaves,
stems)

-14.5 ± 0.6 (n=6)

2.6 (n=1)

Salicomia sp. (fresh leaves)

-27.3 ± 0.4 (n=4)

Salicomia sp. (old leaves,
stems)

-27.1 ± 0.6 (n=5)

5.4 ± 1 .9 (n=3)

Zooplankton

-26.0 ± 0.3 (n=3)

11.7(n=1)

Aransas Pass Inlet

Ocean POM

-23.5 ± 0.9 (n=8)

8.1±3.0(n=5)

Appendix E ♦ E-11

Table 2:

Spatial and temporal changes in stable

isotope compositions

of Brown Shrimp

Locations

Dates

Aransas Pass Rincon Bayou
Inlet Mouth

Rincon Bayou
Lower Marsh

Rincon Bayou
Upper Marsh

Nueces River

(1996)

5"C 5''N 6"C S'^N

5 "C 6 '^N

(5 "C 6 "N

6 "C 6 ''N

Jan 25

-20.7

7.7

Feb 9

-20.7

5.5

Mar 4

-21.7

5.8

Apr 10

Apr 25

May 14

May 30

-19 3 to 3.4

-18.0

-19.3 to 4.8 to 8.7 -17.9 to 4.5

-14.8 -14.5

-21.3 to 1.0 to 6.6 -18.3 to 1.2 to 5.1 -25.2 to 6.6 to 9.9
-17.0 -15.3 -23.9

-20.3 to 6.5 to 10.2 -20.4 to 5.2 to 9.9
-16.3 -15.5

Jun2 -18.0 to 1.3 to 2.3 -23.2 to 9 5 to 11.0

-18.4 -21.7

Jun 19 -21.8 to 10.1 to

-20.4 13.9

Jun 21 -20.6 to 4.5 to 13.1 -20,1 to 5.6 to 6.4

-18.4 -16.5

Jul 29 -16.4 to 3.3 to 9.7
-12.5

E-12 ^ Utili^tion of Rstuarine Organic Matter DuringGnwih
and Migration by Juvenik Broivn Shrimp

Figure 1 : Locations of sampling sites in various shrimp habitats.

Appendix E ♦ E-13

100
90
80
70

g 60

B

5 50
so

J 40
30 H
20

10

Penaeus aztecus

A

■

•A
D

« Aransas Pass |

Rincon Bayou

■ u

mouin

i Down Marsh

X Up Marsh

• Nueces River

0 T 1 1 1 1 1 1 1 1 1 1 1 1 1 —

16/01 31/01 15/02 01/03 16/03 31/03 15/04 30/04 15/05 30/05 14/06 29/06 14/07 29/07

Sampling dates
Figure 2: Size of brown siirimp Penaeus aztecus caught on sampling dates, in different locations.

E-14 ^* Utilisation ofEstuarine Organic Matter During Growth
and Miff-ation hy Juvenile Brown Shrimp

T Spartina
Panicum virgatum

spartinae

Blue

green

f

algae

( • (living)
j§ (detrital)

-15 -
-16 -

Sources

-17 -
-18 -

(»

Spartina altemiflora

-19 -

Benthic diatoms

-20 -

-21 -

U-22-
CO -23 -

^j POM

-24 -

ocean

-25 -

-26 -

Salicornia sp

-27 -
-28 -
-29 -

( ( Fraxinus sp

(living) (detrital)

-30 -

^

Salix sp

-31 J

1 —

1

1 '

N

lueces River

Rincon Bayou

Marsh

Rincon Bayou Aransas Pass

-n

Mouth

B -

Sub-adults (80-90 mm)

-12-

(July 29)

-13-

Penaeus aztecus

■

■

_ ocean

-14 -

II --^ 1

(April 10-June 21)

4

-15-

/

r ' ^

.

-16-

-I-

u

-17-

X

II

r

-|

r -|

r

-|

11 '

P /

/

CO

-18-

•

-f/^-*^

1

1

/

-19-

^ /

■ ~- 1

1

1

'l

' 1 1 V-

:1 1

II

-20-

/

-^ J

.

.

-21 -

'I ''

^ ~^ ■ * -

' 1

i

1 /

-^ ocean

-22-

1 ,

f. 1
1

-23-

\ 1

\ }(■

(January 25-March 4)

X

Larvae (10- 1 1mm)

-24-

II

-25-

(May 14-June 19)

-26-

, ,

Nueces River Up Marsh Down Marsh

Rincon Bayou Aransas Pass
Mouth

Figure 3: Variation of carbon Isotopic ratios in different feeding habitats. A. Potential food sources. 8.
Juvenile brown shrimp Penaeus aztecus as they migrate through ecosystem as indicated by dotted lines with
arrows.

Appendix E <* E-15

5i^N (%o)
10

6 -

4-

Aransas Pass

515N (%o)
14

— I 1 1 1 1 1 1 1 1 —

0 10 20 30 40 50 60 70 80 90 100

0 10 20 30 40 50 60 70 80 90 100

14-

-♦■

•

Nueces

River

121

•

♦ ♦

10-

♦\

♦

♦

8-

♦

♦

%

) 10

20

30

40 50

60

70 80 90 IC

Length (mm) Length (mm)

Figure 4: Relationship between 6"N and length (mm) of brown shrimp Penaeus aztecus in different
habitats.

E-1 6 V UtiJi^ation o/Esluarine Organic Matter During Growth
and Migration hy Juvenile Broum Shrimp

APPENDIX F

Effects of Temporality, Disturbance
Frequency and Water Flow on an
Upper Estuarine Macroinfauna
Community

Christine Ritter Texas Water Development Board, Austin

Paul A. Montagna University of Texas at Austin, Marine Science Institute (corresponding author)

Submitted for publication to Marine Ecology Progress Series (draft date: January 14, 2000)

ABSTRACT: The effects of disturbance frequency and altered flow on estuarine macrobenthic community
structure and colonization were studied ia Rincon Bayou, a low-inflow, microtidal, shallow habitat in the upper
Nueces Delta, Texas. Three flow treatments were used: increased flow (within a weir), natural flow (control),
and decreased flow (between nets). Four disturbance frequency treatments were used: undisturbed, biweekly,
monthly, and bimonthly disturbance. Disturbance was imitated by placement and replacement of 6.5-cm
diameter trays filled with defaunated sediment. Significant flow velocity differences were found among flow
treatments, but turbidity, abimdance, biomass, and diversity were not different. Significant differences among
disturbance frequencies were found. Abundance and biomass decreased with increasing disturbance frequency
indicating post-disturbance community persistence is important in regulating commimity structure. In the flow
experiment, structvire and tray effects were evident because abundance and biomass were higher near structures
and in trays than in controls. The higher abundance and biomass in defaunated sediments relative to
background sediments indicates disturbance plays an important role in community production of early
succession commimities. Collection date was the most important determinant of community structure, thus
natural variability overwhelmed effects of both experimental manipulations. The temporal changes were driven
by a Streblospio benedicti recruitment event (resulting in densities as high as 1.3 10*^ m^ captured 20 June 1997,
and a flood event that began 22 June 1997. After the flood, S. benedicti density declined rapidly and freshwater
species invaded, leading to three distinct community states over the 14-week period of the study. The
overwhelming significance of "temporality" (i.e., short-term temporal change in commvinity structure) is the
unexplained temporal component of community variation in experimental manipulations. Temporality is
simply a smaller temporal scale than seasonality.

KEY WORDS: benthos, disturbance, estuary, flow, macrofauna, Streblospio benedicti

INTRODUCTION

Flow dynamics in estuaries result from complex interactions among tidal cycles, estuarine circulation,
hydrology, cUmatological forcing, and rate of freshwater inflow. Flow dynamics may vary over the coiu:se of a
day, month, season, or on climatological scales. Characteristic flow patterns of an estuary depend on its size,
shape, and location. Numerous mechanisms exist by which an estuary's flow regime may affect macrobenthic
populations. Changes in flow velocity can affect the sediment flux of ammonium (Asmus et al. 1998), depth of
oxygen penetration (Beminger and Huettel 1997), and benthic aerobic respiration and chemical oxygen demand
(Boynton et al. 1981). Flow velocit)' also plays an important role in larval dispersal and settlement (Butman
1986a, 1986b, 1987, 1989), food availability (Smaal and Haas 1997), and suspension feeder growth rates
(Eckman and Duggins 1993, Hentschell 1999a, 1999b). The pattern of flow dynamics (or frequency of change)

Appendix F ♦ F-1

chaiactensdc of an estuary may play an important role in determining macrobenthic community structure
(Pearson and Rosenberg 1987). At^'pical changes in flow dynamics, such as those occurring during storms, may
also be important determinants of benthic commumt}' structure (Renaud et al. 1996).

Physical disturbances (e.g., dredging, trawling, or major storms) affect macrobenthic communities within the
context of the flow regime and ambient environmental conditions. ITie frequency of disturbance may be a
major determinant of community structure. Connell (1 978) h)'pothesi2ed that more frequent disturbances will
maintain a commumty at an earlier stage of succession, which is characterized by lower diversit)', abundance,
and biomass than reference communities of lower disturbance frequencies. Because flow dynamics can vary on
short temporal scales, it could be an important physical regulator of community change.

The purpose of this study was to determine temporal effects of disturbance frequency and altered water flow
on macroinfauna early succession. Macrobenthic recruitment and community structure were expected to
change for each flow treatment (Rhoads and Young 1970). Increased flow was expected to promote
suspension feeder populations and growth rates (Hentschel 1999a; 1999b). Communities subjected to greater
frequencies of disturbance were expected to have lower diversity, abundance and biomass than those subjected
to less frequent disturbance as predicted by Connell (1978). Early succession was expected to be dynamic and
vary among flow treatments because the dominant organism at the study site, Streblosio benedicti. is an
opportunistic species (Grassle and Grassle 1974), can recruit in excess of 25,000 m ' a month (Levin 1984), and
can increase growth rates with flow speeds between 9 and 18 cm s' (Hentschel 1999b).

METHODS

Site Description

Rincon Bayou is a tidal creek in the Nueces-Corpus Chnsti Bay estuarme ecosystem. Rincon Bayou lies west of
Nueces Bay and north of the Nueces River. It may have once been the primary channel of the Nueces River
but is now connected to the Nueces River via an overflow channel constructed in October 1 995 (Irlbeck and
Ergerl998).

Rincon Bayou is a low-inflow environment, subject to hypersalinity during droughts and freshets during floods.
Monthly discharges from the Nueces River into Rincon Bayou ranged from -1200 to 300,000 m' between May
and December, 1996, but about 60 % of daily discharges were < ± 1200 m' (USGS unpublished data 1997).
Rincon Bayou receives little tidal influence (< 0.2 m) and water movement is predominandy wind driven (Salas
1994), or inflow driven during floods.

The location within Rincon Bayou, station C (27° 53.927' N, 97° 36.250' W), was chosen because it lies
approximately along the axis of prevailing winds, which blow from the southeast and south (Port of Corpus
Christi Authority 1993). In addition, station C is part of a quarterly sampling program begun in 1994 from
which a historical context can be drawn. Surface sediment is approximately 86 % silt and clay, and contains 0.7
% total organic carbon. Sahnity averages 40 %o, but has ranged between 0.2 - 160 %o over a four-year
monitoring period (Montagna unpublished data). Dissolved oxygen averaged 7.36 mg 1 ', but has ranged
between 1.01 - 15.32 mg 1 ' over the same period. Six macrobenthic taxa were commonly found at station C:
the polychaetes Streblospio benedicti Webster and Laeoneris culven (Webster), the crustaceans Ostrocoda and
Hemicyclops sp., and insect larvae of Ceratopogonidae and Chironomidae.

Experimental Design

The goal of the present study was to determine the effects of flow alteration and disturbance frequency on
short-term changes of macrofauna (Fig. 1). Three flow treatment levels were used: increased flow, decreased
flow, and control. Increased flow was effected through the use of weirs parallel to currents. Weirs are used to
increase water pressure, and hence flow, over a specific area (Fig. 2a). Decreased flow was effected through the
use of nets perpendicular to currents. Nets are commonly used to reduce wave energ)' by slowing water
velocity. Control flow was areas perpendicular, but away from structures and represents the natural flow
conditions of the Bayou (Fig. 2b).

F-2 V Effects ofTemporality, Disturbance Frequency and Water Flow
on an Upper Estuarine Macroinfauna Community

Two plots of each flow treatment were constructed (Fig. 1). The plots, thus represent replication at the
treatment level. Two control plots, 1 m x 1 m area, were delineated by PVC comer poles. Two weir plots
(increased flow) were constructed of 1 m x 0.5 m, 3/4" (1.9 cm) plywood attached to 1.5 m PVC poles (Figs. 1
and 2). Net plots (decreased flow) were 2 m long and 1 m wide and made of 2.5 cm mesh attached to 1 .5 m
PVC poles (Fig. 2a). All PVC poles extended 1 m into the sediment. The open faces of the weirs and the long
sides of the net faced southeast, the direction of predominant tidal and wind driven flow (Montagna et al.
1998). All experimental sampling areas were 1 m x 1 m squares within the central region of each structure
(Fig. 2a).

Structures were biult within a 0.25 km^ region (Fig. 2b). Furthest downstream were a weir plot and a net plot.
Fxirthest upstream were a control plot and a net plot. In between were a weir plot and a control plot. Plots
were staggered to Umit possible structure interactions and were all roughly the same water depth. Structure
interactions are unlikely because Rincon Bayou is generally a low-inflow, microtidal environment.

Frequency of disturbance was varied. Four distiubance frequency treatment levels (i.e., undisturbed, biweekly,
monthly, and bimonthly) were nested within each flow treatment plot (Fig. 1). Disturbance was imitated by
emplacement of defaunated sediment in trays. Undisturbed sediment was the control for the disturbance
frequency treatment. Bi-weekly disturbance was the most frequent disturbance investigated and trays remained
in the field for 2 weeks. Monthly disturbance was the moderate frequency level and trays remained in the field
for 4 weeks. Bimonthly disturbance was the least frequent disturbance investigated and trays remained in the
field for 8 weeks. Short-term frequencies of disturbance were used to determine effects on development of
early succession communities.

Disturbance ttays were constructed from 6.5-cm diameter acrylic tubes, 3 cm high, and fused to 7 cm x 7 cm
square bottoms. Trays were filled with sediment collected from station C and defaunated using a microwave.
Approximately 500 ml of sediment were microwaved for 1 5 minutes to bring sediment temperatures to roughly
100 °C. Microscopic investigation of subsampled sediment yielded no live animals following defaunation.
Trays were filled with well-mixed defaunated sediment in the field, and placed at predetermined random
positions within the experimental area of each flow plot. Flagged rods were used for treatment identification
and to avoid tray loss. Only one tray was turned over and lost during the study.

The disturbance frequency experiment was replicated two different ways: by initial placement date (23 April and
04 June), and by ending sampling date (20 June and 01 August) (Fig. 3). This required eight sampling dates over
a 14- week period (Fig. 3). On each sampling date, three replicate tray samples and three undisturbed samples
(the top 3 cm of a 6.5-cm diameter core) were taken in each disturbance frequency level (Fig. 1).

Measurements

Hydrographic data (e.g., salinity, temperature, dissolved oxygen) were collected from each flow treatment plot
on each field date using a Hydrolab 4000 Sonde. Three turbidity samples were collected from each flow
treatment plot and analyzed using a HACH model 2100A turbidimeter. Rainfall, tidal stage, and flow volumes
were collected from a gage placed at the mouth of Rincon Bayou where it connects to the Nueces River
(unpubUshed data collected by the United States Geological Survey at Station 08211503, Rincon Bayou Channel
near Calallen, Texas).

Flow velocity was measured twice for each flow treatment plot and level to determine the effect of the
structures. How velocity was measured only twice because flooding prevented equipment deployment and
retrieval on most dates. Flow was measured on 07 May and 17 July 1997 using a UNIDATA Starflow
Ultrasonic Doppler flow meter. Velocity, depth, and temperature were recorded every two minutes for 6 to
24 minutes per flow plot per date while sediment samples were being taken from the adjacent flow plot.

Macro fauna were extracted from sediment using a 0.5-mm sieve, identified to the lowest taxonomic level
practicable, and counted. Lecithotrophic larvae were assumed to be Streblospio benedicti. which are more
common at Rincon Bayou than the alternative Polydora comuta (Montagna and Ritter unpublished data).
Biomass was determined for each taxonomic group by drying samples for 24 hours in a 55 °C oven and
weighing to the nearest 0.01 mg. Abundance and biomass were converted to a per meter basis and (natural
logarithm) transformed for statistical analysis as follows: In (n m"^-l-l) and In (g m'^+1) respectively. Hill's
diversity (Nl), and Peliou's evenness (El) were calculated to summarize community structure characteristics.

Appendix F ♦ F-3

Diversity was calcvilated as Nl = (exp) , where IT = 2 (p, In p), where p, = n, / n, where a, = abundance of
species i, and n = total abundance (Ludwig and Reynolds 1988). Evenness was calculated as El = ln(Nl) /
ln(NO), where NO = total number of species (Ludwig and Reynolds 1988). Nl is the number of abundant
species, and El is the familiar J .

Statistical Analysis

A two-way analysis of variance (AN OVA) was used to determine turbidity and velocity differences among flow
treatments and sampling dates. More complex ANOVA was used to test for differences in macrobenthic
response among flow, disturbance frequency, and sampling date treatments. All ANOVA models were
calculated using SAS GLM procedures (SAS 1985).

A three-way, incomplete factorial, randomized block design was tised to test for community differences in over
all sampling dates (Fig. 1). Main effects (i.e., treatments) were flow, disturbance frequency, and sample
collection date. The randomized block was flow plot. Blocks are replicates that do not have interactions with
main effects. The design is incomplete factorial because distiorbance frequency treatments were not started or
ended on the same dates. Because some date cells are missing in the biweekly, monthly, and bimonthly
frequency levels, the frequency*date interaction and flow* frequency* date interaction do not exist in the
ANOVA model (Table 1). In this model, date is more like a block (controlling nuisance variation) than a main
effect.

Full rank, three-way ANOVA models do exist for subsets of the data set (Fig. 3). The experiment was
replicated two different times: as starting dates and as ending dates. So, macrobenthic data was analyzed with a
three-way randomized block design twice: Once for trays deployed on two dates: 23 April and 4 June 1997, and
once for trays collected on two dates: 20 June and 1 August 1997. These analyses are referred to as "initiating"
and "ending" sampling dates respectively (Table 1).

Interpretation of results from complex ANOVA designs is often obscured by significant interactions, because
the main effects tests are invalid. To simply interpretation of the present study, two-way, incomplete,
randomized block models were calculated by a treatment level. Examination of the simple main effects allows
testing and interpretation of the first main effect at all levels of the second main effect. The simple main effects
models were calculated by disturbance frequency for flow and sampling date treatments with plot as a
randomized block. Tukey multiple comparison tests were used to determine differences among levels of flow
or distiirbance frequency cell means. The implementation of Tukey uses the harmonic mean of cell sample
sizes when sample sizes are unequal (SAS 1985). \'ariance components analysis was used to estimate the
percent of variation attributable to each main and interaction effect in all ANO\''A models.

Principal component analysis (PCA) was used (SAS 1985) to determine treatment effects on species
composition. The covariance matrix of log transformed species abundance, standardized to a normal
distribution was used for PCA. Using the covariance, ittstead of the correlation matrix, eliminates problems
encountered where many rare species with zero counts exist. The multivariate PCA method is a species
dependent analysis of community structure, unlike the species independent analysis of diversity indices.

RESULTS

Hydrography

Hydrographic conditions at station C varied during the course of the experiment (Fig. 4). The small (about
5 %o) drop in salinity from 23 April - 22 May 1997 is due to local rainfall. Average salinity peaked on 20 June
1997 at 25.6 %o, but dropped to 0 %o on 2 July. The fresh conditions correspond with a flood event that began
on 22 June. The flood resulted primarily from rain in the watershed northwest of the delta. Dissolved oxj'gen
was highest 23 April and 7 May, but lowest 4 June. On 4 June, dissolved oxygen data collected prior to
9:30 a.m. were 2.4 mg 1"' at weir 1, and 2.88 mg 1 ' at net 1, indicating hypoxic conditions probably occurred
during the previous night. Temperature generally increased with onset of summer.

A significant (p = 0.0020) flow*date interaction was present because there were greater differences among flow
rates on 17 July than on 07 May (Fig. 5). On both dates, weir structures had increased flow velodries, and the

F-4 ^ Effects of Temporality. Disturbance Frequency and Water Flow
on an Upper Estuarine Macroinjauna Community

net structiues had decreased flow velocities relative to control plots. Average flow velocities were highest in
between weir structures (132 mm s '), were lower in control plots (102 mm s '), and lowest in between net
structures (75 mm s '). There was no difference in flow treatments between the two replicate plots.

Turbidity did not vary significandy among flow treatments, indicating flow manipulations did not alter the
concentration of suspended sediment. Turbidity differences were found among dates (p = 0.0001). No
interaction effects were detected. Turbidity samples for 23 April were not included in ANOVA because
sediment was resuspended during sample collection.

Treatment Effects

Macroinfauna community structure response over all sampling dates was clear, because there were no
significant treatment interactions for diversity (Nl), and evenness (El). There was no significant differences
among flow treatments for diversity, and evenness. There were significant differences for disturbance
frequency levels for diversity (p = 0.0001), and evenness (p = 0.005). Significant differences (0.0001) among
dates were detected for diversity and evenness. Diversity was highest for biweekly samples (1.82) and lowest
for undisturbed samples (1.31); both of which were significandy different from monthly (159) and bimonthly
(1.62) samples, which were the same fTable 1, Tukey test). Biweekly (0.59) samples had the highest average
evenness, which was different from monthly (0.44), bimonthly (0.37), and undisturbed levels (0.28) (Table 1,
Tukey test). Evenness in monthly and bimonthly samples were the same, and bimonthly and undisturbed
sample means were the same (Tukey test).

Macroinfauna standing stock (i.e., abundance and biomass) response to the experimental treatments was
analyzed three ways: by all sample dates, by initiation dates, and by ending dates (Table 2). Regardless of the
design or analysis used, there were many significant interactions obscuring interpretation of main treatment
effects ^able 1). Total biomass and abundance did not have similar results in regard to which interactions
existed. The two main experimental treatments (flow and disturbance frequency) contributed very litde
variance components regardless of the analysis technique, ranging from 0 % to 12.7 %.

In general, the full-rank analysis for two ending dates yielded similar results to the incomplete factorial design
for aU sampling dates for both abundance and biomass (Table 1). The difference between the contribution of
the date effect is particularly striking. Sampling date contributed the largest percentage of variance for
abundance and biomass for the incomplete factorial analysis and the fiill-rank analysis of ending dates, ranging
from 35.4 % - 86.9 %. In contrast, date contributed no variance to the full-rank analysis of initiating dates, and
nearly all the variance was contributed by the frequency*date interaction for abundance (91.9 %) and biomass
(57.7 %). The similarity between analyses of p-values and variance components for aU dates and ending dates
indicates macrobenthic response is similar among all treatments on given dates. Similarly, sigmficant
frequency*date interactions found in the initial date analysis indicates that commumty change is different for
each experimental treatment due to samples being taken on different dates.

Regardless of the analysis method, the flow*frequency interaction was always significant (Table 2). The nature
of the interaction is different responses to flow variation in frequent disturbances (at biweekly and monthly
scales) compared to less frequent disturbances at longer times scales ( bimonthly and undisturbed) (Fig. 6).
Abundance was very similar in flow treatments during short-term disturbance, but the increased flow treatment
had much higher abundances in the samples that had recovered the longest or were undisturbed (Fig. 6a). The
trend for biomass was similar, except that biomass was higher in the decreased flow treatment than in the
control or increased flow treatment over the short-term disturbances (Fig. 6b).

Simple Main Effects

Analysis of the full rank model yielded many significant interaction effects (Table 2), so simple main effects
models were run (Tables 3 and 4). Flow treatment effects were determined with separate analyses by each
disturbance frequency level (Table 3) and frequency treatment effects were determined with separate analyses
by each flow level (Table 4).

Flow Effects

Appendix F ♦ F-5

The flow tieatment tests are generally valid because of non-significant interactions with date, except for the
undisturbed samples (Table 3). No sigmficant abundance differences were found among flow treatments. The
biomass differences among flow treatments for biweekly and monthly levels (Fig. 6.) were significant (Table 3).
At the biweekly and monthly frequency levels, decreased flow yielded significandy greater biomass than control
and increased flow levels (Tukey test). Abundance changed over collection dates for all frequency levels.
Biomass changed over the long-term, but over short-term (biweekly) time intervals.

The flow*date interaction was significant for abundance and biomass for the undisturbed treatment (Table 3).
This interaction is especially interesting because it represents how undisturbed sediment changed over time as a
result of the flow treatments alone (Fig. 7). In undisturbed sediment, highest abundance and biomass values
were found within structures that altered flow. Except for the first collection date, the abundance and biomass
in the control plots were always lower than the altered flow plots indicating a possible structure effect. So, the
interaction was due to the first collection date (when all flow levels were similar) and the similarity' of flow
alteration structures (which alternated between highest and second highest values).

Frequency Effects

Disturbance frequency tests were affected by significant frequency*date interaction effects, except in decreased
flow structures ^able 4). At the decreased flow level, average biomass of monthly (1.20 g m "), bimonthly
(1.15 g m^, and weekly (0.90 g m^ frequencies were similar to each other, but were significandy different from
undisturbed average biomass (0.69 g m ■^.

Effects of the disturbance frequency experiment on the natural communit)' is represented by changes in the
undisturbed sediment communit)' in control level of flow treatments. There was a significant frequenc)'*date
interaction for control flow levels for both abundance and biomass (Table 4). Average abundance and biomass
was lower in undisturbed sediment than in all disturbance treatment levels except for biweekly samples
collected on 07 May (Fig. 8). The lower abundance and biomass in undisturbed samples may indicate
disturbance enhances community productivity'. On most dates, average abundance and biomass in bimonthly-
frequency, control-flow, samples were greater than other disturbance frequency samples. In general, there was
a trend of increasing abundance and biomass from biweekly, to monthly, to bimonthly distixrbance frequencies,
indicating growth or recruitment with time.

Community Structure

Ten taxa were found during the experiment. Streblospio benedicti and Laeoneris culveri were present on each
sampling date. Chironomid larvae were observed 23 April and 7 May, but appeared in abundance 20 June,
persisting through 1 August. Polydora comuta Bosc was present between 4 June and 2 July, but was most
abundant on 20 June. Polydora comuta was observed only once in 31 observations at the control- flow plot
indicating a possible positive structure effect. Mysidopsis aknyra Bowman was present between 23 April and
22 May, and was found only in undisturbed samples. Nermerteans were found on 20 June and 02 July in
undisturbed and monthly samples. Mulinia laterahs (Say) was foimd infrequendy between 23 April and 20 June
in undisturbed sediment of decreased and control flow treatments. Mediomastus ambiseta (Hartman) was
fotmd infrequendy between 4 June and 1 August. Amphipods and oligochaetes were each present in a single
sample during the course of the study.

Average abundance and biomass were greatest for samples collected on 20 June 1997 (Figs. 7 and 8). Average
abundance was 422,000 m'^ and average biomass was 5.62 g m'^. Streblospio benedicti accounted for 96 % of
abundance and 76 % of biomass in these samples on average. The bimonthly- frequenc}', increased-flow plots
contained the greatest average abundance and biomass of all samples in the study. For example, one sample
contained 4,730 organisms (1,342,000 m ■^, of which 4,667 (1,324,000 m "') were S. benedicti. Many S. benedicti
were very small, presumably lecithotrophic larvae or post larval juveniles, but sizes were not measured.

Principal component analysis (PCA) yielded three distinct collection date groups (Fig. 9a), but no flow or
disturbance frequency effects. PCI separated samples collected 20 June and 02 July from those collected
17 July and 01 August. PC2 separated samples collected 23 April - 04 June from those collected 20 June -
01 August. Chironomid larvae (CL) loaded 0.97 on PC2 (Fig. 9b) indicating their importance to characterizing
communities samipled 20 June and later. Streblospio benedicti and Laeoneris culveri loaded positively for both

F-6 V Effects of Temporality, Disturbance Fnquengi and WaterFbw
on an Upper Estuarine Macroinfauna Community

PCI and PC2 indicating their importance to characterizing samples collected 20 June and 02 July. PC A of each
date group separated in the first PCA also failed to segregate flow treatments or disturbance frequency
treatments.

DISCUSSION

Flow Effects

The structures used to manipulate flow significandy altered flow velocity in the expected manner (Fig. 5), but
they did not significandy affect tvirbidity or macrobenthos overall (Table 2, Fig. 6). Flow treatments accounted
for only a small fraction of the total variance in abundance and biomass (<1.9 %). Significant interactions with
sampling date and disturbance frequency treatments indicate that altered flow affected macrobenthos only in
certain instances (Table 2). The strongest flow effects were found in undisturbed samples only (Table 3). In
undisturbed sediments, average abundance and biomass were greater at increased and decreased flow levels
than in control plots for all sampling dates except the first (Fig. 7). Increased abundance and biomass in
increased and decreased flow plots relative to control plots indicate the physical presence of experimental
structures may have affected macrobenthos. It is possible structure proximity afforded the community a refuge
from predation or increased food supply by increasing sedimentation rate. The difference may also explain the
flow*frequency interaction, because fi-equentiy disturbed (biweekly and monthly) samples had low abundance
and biomass due to recent defaunation.

At the species level, Polydora comuta was found only at increased and decreased flow plots with one exception,
indicating a possible attraction to structures. Between 1994 and 1997, P. comuta was not observed at Rincon
Bayou (Montagna and Ritter unpublished data). The present study is the only report of its presence at
station C.

Failure to detect significant abundance and biomass differences among flow treatments may arise from
inadequacy of the manipulations. Flow treatments did not alter flow velocity enough to significandy affect
resuspended sediment. Average flow velocities of weir treatment plots ranged between 10.1 and 16.4 cm s'.
These velocities may not have reached critical erosion velocities of the natural cohesive sediment of Rincon
Bayou. For example, critical erosion velocities of the Skeffling intertidal mud flat on the Hiunber Estuary, U.K.
ranged between 21.8 cm s ' and 30.8 cm s ' (Widdows et al. 1998). Community change driven by altered
resuspension rates would not be detected by the manipulations in the present study.

Changes in flow velocity affects growth of some species. For example, growth of some macrobenthic
suspension feeders, such as Membranipora membranacea. Balanus crenatus. and Pseudochitinopoma
occidentalis. are inhibited by flow velocities 12-30 cm s ' (Wildish and Kristmanson 1979; Eckman and
Duggtns 1993). In contrast, growth rates of spionid polychaetes common to Rincon Bayou increase with
increasing flow. Polydora comuta and Streblospio benedicti increase growth rates with flow velocities between
9 and 18 cm s ' (Hentschel 1999a; 1999b). For example, P. comuta grew at a rate of 1.4 volumetric doublings
per day, reaching sexual maturity within 1 week at 18 cm s ' velocity (Hentschel 1999a; 1999b). Because the
response by the two spionid polychaetes common to the study area increased over the range of measured flow
velocities, inhibition probably did not occur near structures. It is more likely slight enhancement occurred,
explaining the small increase of abundance and biomass in increased flow plots (Fig. 7).

The increase of abundance and biomass in increased flow (weir structure) plots is due to changes in population
size of Streblospio benedicti. Population growth of S. benedicti appears to have been enhanced at increased
flow plots prior to the flood of 22 June 1997. On 20 June, S. benedicti populations at increased flow plots were
twice that of decreased flow and control plots. Streblospio benedicti grows faster under higher flow conditions
(Hentschel 1999a; 1999b). The unique life history of S. benedicti could also be partly responsible for the rapid
population growth. Colonists could have been adults carrying a full brood (Levin 1984).

Disturbance Frequency Effects

Disturbance frequency treatments appear to demonstrate succession. Longer periods between disturbances
(i.e., biweekly to monthly to bimonthly) were associated with increasing abundance and biomass and decreasing
evenness (Table 1, Fig. 8). The trend demonstrates early colonization and successively greater abundance,
biomass, and less dominance with longer periods between disturbance. Lower evenness and abundance in

Appendix F ♦ F-7

more frequently disturbed communities indicates greater disturbance frequency maintains the community at a
earlier stage of succession (Pearson and Rosenberg 1976; 1978; Dauer 1993; Trueblood et al. 1994; Weisberg et
al. 1997). If a commumt)' is disturbed every two weeks, it cannot proceed beyond the coloiuzation stage. Thus,
the length of time since the last disturbance regulates the communit)' (Connell 1978). In addition, communities
subjected to altered flow disturbance (i.e., increased or decreased) had higher abundance and biomass than
natural (control) communities (Fig. 7) explaining the frequency*flow interaction (Fig. 6). These results indicate
disturbance may play an important role in regulating macrobenthic community dynamics and increase
secondary' production as suggested by Rhoads et al. (1978).

.\11 disturbance frequenc)' levels, including undisturbed ambient samples, were dominated by the polychaete
Streblospio benedicti. Streblospio benedicti is an opportunistic species (Grassle and Grassle 1974).
Dominance by opportunists is a key characteristic mdicating early succession, or highly disturbed, estuanne
macrobenthic communities (Pearson and Rosenberg 1976; 1978; Thistle 1981; Dauer 1993; McCook 1994;
Weisberg et al. 1997). Dominance of S. benedicti in the current study may indicate the community of station C
is highly disturbed by natural environmental variation, e.g., as the broad salinity ranges and flow conditions
foimd during this study (Fig. 4).

Community abundance, biomass, and diversity of undisturbed sediments were lower than that of all disturbance
treatments after the first (7 May) biweekly samples (Fig. ). The undisturbed sediment communit)' is the
ambient, reference community against which coloni2ation of defaunated (disturbed) sediment was compared.
Increasing abundance, biomass, and diversity were expected in defaunated sediment until disturbed and
undisturbed communities were similar. In contrast, abundance of the bimonthly disttirbance frequency level
was 8 times that of the undisturbed community; biomass of bimonthly level was twice that of undisturbed
sediment; and diversity of bimonthly level was higher than that of undisturbed sediment.

There are three possible explanations for the differences between undisturbed and defaunated communities.
The structure of experimental trays may attract organisms as a refuge or alter water flow affecting deposition
and recruitment (Butman 1986b; Snelgrove et al. 1993). The defaunated sediment may attract macrobenthos or
promote macrobenthic reproduction. Macrobenthic succession of defaunated sediment may be mitiated by a
poptJation burst of opportunistic species. The population burst may exceed late succession commumt)'
abundance and biomass, but return to normal levels after a period longer than 8 weeks. One, two, or all three
explanations may be responsible for the observed differences.

Temporality

Temporality, "the quality or state of being temporal (Mish 1985, p. 121 4)", is a property of communities that
arises in all studies from the complexity of ecological interactions in the natural environment. Communit)'
variation through time is not in itself a new finding. Temporalit)', however, is the unexplamed temporal
component of communit)' variation that may exceed that of experimental treatments. Examples of temporality
can be found in benthic communities of the Savin Hill mudflat, Boston Harbor (Trueblood et al. 1994),
microbial communities of the Parker River salt marsh, Rowley, MA (Montagna and Ruber 1980), and epifaunal
communities at Beaufort, NC (Sutherland and Karlson 1977; Holm et al. 1997). In all these cases, natural
commumt)' variation over rime (i.e., temporality) exceeded the effects of experimental mampulations.

In the present study, collection date had the strongest effect on macrobenthic abundance (Figs. 7 and 8;
Tables 2 - 4), biomass (Figs. 7 and 8; Tables 1 - 4), diversity, and community structure (Fig. 9). Thus, natural
temporal variation of salinity, temperature, and dissolved oxygen (Fig. 4), which may be associated with
seasonality and flooding, played a greater role in determining community structure than flow or disturbance
frequency treatments (Tables 2-4). There were three community states through time (Fig. 9). The first group,
representing 23 April - 4 June 1 997, was dominant when average salinity varied between 1 1 and 1 8 %o, average
dissolved oxygen declined from 11.22 mg 1 ' to 3.89 mg 1 ', and temperature varied between 24.5 and 26.5 °C
(Fig. 4). The low abundance and biomass estimates of 4 June, compared with previous dates (Figs. 7 and 8) is
probably related to overnight hypoxia that likely occurred under low-flow conditions observed that day (Fig. 4).
After 4 June, the community state shifted because of increased abundances of chironomid larvae, Streblospio
benedicti and Laeoneris culveri (Fig. 9b). Streblospio benedicti appears to have had a recruitment event
between 4 June and 20 June when total average S. benedicti abundance increased from 1 1,000 to 94,000 m^.
The recruitment event appeared to be greatest under increased flow conditions. This indicates a possible flow
treatment and sampling date interaction whereby flow velocity may have affected active substrate selection,

F-8 V Effects of Temporality, Disturbance Frequency and Water Flom
on an Upper Estuarine Macroinjauna Community

passive organisms deposition (Butman 1986a; 1989), or spionid organismal growth (Hentschel 1999a; 1999b).
The third community state is related to the flood event that began 22 June 1 997. SaUnit)' dropped to 0 %o by 02
July and had increased to only 2 %o by 01 August. The persistence of low salinity over a period of more than
one month probably led to declines in total abundance and biomass following the flood (Figs. 7 and 8). For
example, when salinity was reduced from 19.9 %o to 3.19 %o, S. benedict stzrvived up to 1 1.5 h, whereas P.
comuta expired within 7 hours, and Leptocheirus plumulosus remained active and normal (Sanders et al. 1965).
Differential salinity tolerance explains persistence and species changes, and is one component of temporality.

The strong response of macroinfauna to temporally variable, ambient, environmental conditions rather than to
experimental manipulations indicates experimental effects were less important than temporal community
response to natural environmental fluctuations. This is almost certainly true for flow treatment manipulations
for which no significant effects on macrobenthos were found. Flow effects probably exist, but are barely
detectable due to environmental variation. Though significant distvirbance frequency effects were found,
community structure differences were masked by community composition similarities in all treatments within
collection dates.

Temporality is the controlling feature of the Rincon Bayou benthic community, explaining most of the variance
in abundance and biomass (Table 2). Rincon Bayou is an extreme environment, ranging from hypersalinity in
droughts and freshwater in floods. From 1994 - 1997, salinity at station C ranged between 0 and 160 %o. The
present study unintentionaDy captured a flood event that led to a sudden drop of salinity from 18 to 0 %o,
which persisted for more than a month (Fig. 4). Such a salinity change exceeds the tolerances of most
euryhaline species (Sanders et al. 1965), and led to an increased populations of chironomid larvae and decreased
populations of Streblospio benedicti and Laeoneris culveri (Fig. 9). In Rincon Bayou, community change
occurring in response to physical disturbance appears to be limited by natural environmental variation through
time, which we call temporality. Temporality is simply the short-term analogy to longer-term seasonality.

Summary

Succession is controlled more by the natural tempo of environmental variation (e.g., availability of recruits, and
coincidence of rainfall) than by small-scale events (e.g., patch defaunation and water flow). How appears to be
a less important determinant of community structure for Rincon Bayou station C compared to natural variation
of background environmental characteristics (e.g., salinity), but appears to influence abundance, biomass, and
diversity. Rincon Bayou is generally a low-inflow, microtidal, shallow water habitat subject to broad salimty
fluctuations concomitant with periodic drought and flood events. During the period of the present study,
sample collection date was the most important factor in determining community structure indicating
experimental effects were overwhelmed by natural environmental variation arising from recruitment and flood
events. Within the context of natural temporal variation, significant differences were found among disturbance
frequency treatments. Community abundance, biomass, diversity, and evenness decreased with increasing
disturbance frequency indicating the importance of post-disturbance community persistence in determining
community structure and succession state, and possible tray effects. Disturbance in the form of flow alteration
or defaunated sediment increased community abundance and biomass, indicating disturbance may increase
production of early succession macrobenthic communities in estuaries.

Acknowledgments

Funding for research in Rincon Bayou was provided in part by the U.S. Bureau of Reclamation (Grant No. 4-
FG-60-G4370) and die Texas Water Development Board (Research and Plaiming Fund Contract No. 94-483-
046) to the University of Texas at Austin, Marine Sciences Institute. Partial support, via the Lund Fellowship,
was granted by the University of Texas Marine Science Institute. The authors also acknowledge Rick Kalke and
Robert Biurgess for assistance in the field, and Carol Simanek for data management.

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Appendix F ♦ F-9

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Appendix F ♦ F-11

Table 1 : Summary of mean community characteristics over all sampling dates for disturbance
frequency and flow treatments.

Treatment
Level

Abundance
log(n m ^+1)

Biomass
log(gm^+1)

Diversity
(N1)

Evenness
(El)

Frequency

Biweekly

9.33

0.64

1.82

0.59

Monthly

10.37

1.00

1.59

0.37

Bimonthly

10.98

1.27

1.62

0.28

Undisturbed

9.87

0.66

1.31

0.44

Flow

Increased

10.12

0.85

1.54

0.39

Decreased

10.16

0.90

1.51

0.37

Control

9.78

0.66

1.46

0.38

F-12 ^ Effects of Temporality, Disturbance Frequeng and Water Flow
on an Upper Estuarine Macroinfauna Community

Table 2: Analysis of three replications of the flow-disturbance frequency experiment. Analyses for total
experimental data including all dates (3-way incomplete design), when experiments were initiated on two dates (3-way
factorial design), and when experiments ended on two dates (3-way factorial design) (Fig. 3). Results for a) abundance
and b) biomass. Table finds degrees of freedom (df), probability level (p), and variance component percentage (%) for
each main effect. Freq = disturbance frequency, * = interaction, and - = does not exit.

Treatment

All Dates
Incomplete design

Initiating Dates
3-way factorial design

Ending Dates
3-way factorial design

df

P

%

df

P

%

df

P

%

a) Abundance

Flow

2

0.9548

0.5

2

0.1061

0

2

0.0003

0.2

Freq

3

0.0001

4.3

3

0.0001

0

3

0.0001

5.0

Flow*Freq

6

0.0001

2.2

6

0.0001

1.4

6

0.0005

1.4

Date

7

0.0001

76.5

1

0.0001

0

1

0.0001

86.9

Flow*Date

14

0.0026

2.0

2

0.4542

0

2

0.0033

1.1

Freq*Date

-

-

-

3

0.0001

91.9

3

0.1339

0.2

Flow/*Freq*Date

-

-

-

6

0.0171

1.5

6

0.3368

0.1

Plot

1

0.2443

0

1

0.5136

0

1

0.9747

0

Error

271

-

14.5

118

-

5.3

118

-

5.2

b) Biomass

Flow

2

0.0672

1.2

2

0.0172

1.9

2

0.0872

0

Freq

3

0.0001

12.7

3

0.0001

0

3

0.0001

8.0

Flow*Freq

6

0.0001

4.6

6

0.0001

3.2

6

0.0149

1.8

Date

7

0.0001

35.4

1

0.1490

0

1

0.0001

46.8

Flow*Date

14

0.0132

3.4

2

0.9475

0

2

0.0006

6.6

Freq*Date

-

-

-

3

0.0001

57.7

3

0.0002

7.5

Flow*Freq*Date

-

-

-

6

0.0042

10.2

6

0.1015

3.5

Plot

1

0.1170

0.1

1

0.0275

1.4

1

0.8423

0

Error

271

-

42.7

118

-

25.6

118

-

25.8

Appendix F ♦ F-13

Table 3: Simple main effects for the flow*frequency interaction over all sampling dates. Two-way. randomized
block ANOVA calculated for each disturbance level for a) abundance and b) biomass Abbreviations as in Table 2.

Treatment

Disturbance Frequency Level

Biweekly

Monthly

Bimonthly

Undisturbed

df

P

df

P

df

P

df

P

a) Abundance

Flow

2

0.1373

2

02861

2

0.0511

2

0.0001

Date

2

0.0001

3

0.0001

1

0.0001

7

0.0001

Flow*Date

4

0.4825

6

0.0538

2

0.1522

14

0.0001

Plot

1

0.4240

1

0.9244

1

0.4019

1

00195

Error

b) Biomass

44

59

28

119

Flow

2

0.0238

2

0.0090

2

0.3416

2

00001

Date

2

0.1138

2

0.0001

2

0.0001

2

0.0001

Flow'Date

4

0.3841

4

0.5400

4

0.0168

4

0.0001

Plot

1

0.7145

1

0.3949

1

0.1490

1

0.0645

Error

44

44

44

44

F-14 V Effects ofTemporality, Disturbance Frequency and Water Flow
on an Upper Estuarine Macroinfauna Community

Table 4: Simple main effects for the flow'frequency interaction over all sampling dates. Two-way,
randomized block ANOVA calculated for each flow level for a) abundance and b) biomass. Abbreviations as
in Table 2.

Treatment

Flow Treatment Levels

Control

Increased

Decreased

df

P

P

P

a) Abundance

Frequency

3

0.0001

0.0001

0.0001

Date

4

0.0001

0.0001

0.0001

Freq*Date

6

0.0001

0.0001

0.0001

Plot

1

0.4117

0.5310

0.5807

Error

69

b) Biomass

Frequency

3

0.0001

0.0001

0.0033

Date

4

0.0001

0.0001

0.0006

Freq*Date

6

0.0505

0.0001

0.1453

Plot

1

0.0593

0.9175

0.9739

Error

69

Appendix F ♦ F-15

Flow Treatment

Two plots per flow
treatment

Increased (Decreased or Control)

Disturbance

Frequency

treatments

Collection
Date

Same as for 1

Biweekly Monthly Bimonthly Undisturbed

5 8

12 3 4 5 6 7 8

Sample

Replicates .-, ,

(3 per date) ' " -^ l 2 3 l 2j* I z J

I

1 23

Figure 1 : Experimental desigr^ of the study. The experiment was a three-way, randomized block,
incomplete factorial design to evaluate effects of flow regime and disturbance frequency on macrobenthos
over time. Flow treatment levels were: increased by weirs and decreased by nets relative to controls with no
structure. Disturbance frequency treatment levels were: biweekly = every 2 weeks, monthly = every 4
weeks, bimonthly = every 8 weeks, and undisturbed. Collection dates were: 1 = 23 April 1997, 2 = 07 May
1997, 3 = 22 May 1997, 4 = 04 June 1997, 5 = 20 June 1997, 6 = 02 July 1997, 7 = 17 July 1997, and 8 = 01
August 1997.

Predominant Water Flow

a)

Control 2

Net 2

Direction of

wind and tidal

flow

Weir 2

) ( ^^

Control 1

Weir 1

/ \ Net 1

b)

Figure 2: Flow alteration treatments for a) weir and net structure designs and b) field emplacement.

Trays of defaunated sediment were randomly placed in hatched center regions. Undisturbed core samples
were obtained in the shaded regions. Control plots were defined by corner posts (dotted lines).

F- 1 6 V Effects ofTemporality. Disturbance Frequency and W''ater Flow
on an Upper Estuarine Macroinfauna Community

Biweekly Monthly

Bimonthly

23 April

r 1

u

V

07 May*

22 May

>

04 June

• 2 mi

1

20 June

Y3

Y3 I3

02 July

^ f

17 July*

•

Y4

01 August

\f4

>

Figure 3: Time line for sample collection dates in 1997. Tray placement indicated by dots (#) and
sample collection indicated by arrow heads (>■). Date flow velocity measurements were taken indicated by
asterisk (*). Beginning of the flood event (22 June) indicated by dashed (-) line. Disturbance frequency
treatments were replicated twice, two different ways: by same initial dates (dots 1 and 2) and same ending
dates (dots 3 and 4). Undisturbed samples were collected on every date.

Appendix F ♦ F-17

Figure 4: Hydrographic conditions over sampling period. Average salinity, dissolved oxygen, water
temperature, and turbidity measured each sampling date. Standard error bars are smaller than symbols.
Minor ticks placed every Monday. Daily rain fall and inflow from gage (USGS 1997).

150 -

^

r

iv

^_^

125 -

:

I

i

(

►

1

100 -

5

I

O

75 -

o

^

50 -

o

•

Control

PL,

25 -

o

Decreased

0 -

▼

[ncreased

1^

^

X

07 r

1

1
17 Jul

19

97

Figure 5: Average flow velocity measurements from flow treatments on two dates (7 May and
17 July 1997). Tukey minimum significant difference is 26.45 for flow regimes, and 18.29 for dates.

F-1 8 ♦♦♦ Effects ofTemporality, Disturbance Frequeng and Water Flow
on an Upper Estuarine Maavinfauna Community

14.5

V. 14.0 ]

B

GO

O

? 13.0

I
"^ 12.5

c

5

12.0

3.5

+ 3.0
I

s

^ 2.5

I 2.0

1.5

-•— Control
-O— Decreased
-▼— Increased

b)

.eV\^ <v^^ ^^"^^ x<0^^

Disturbance Frequency
Figure 6: Disturbance frequency*flow interaction for average a) abundance and b) biomass.

Appendix F ♦ F-19

+

13

12

11

^ 10

o

c

-S 9

c

2.5

1/5

E
o

-•— Control
O Decreased
-▼— Increased

-•— Control
O Decreased
-T— Increased

Figure 7: Flow'date interaction for average a) abundance and b) biomass (b) in undisturbed
sediment. Minor ticks placed every Monday. Tukey minimum significant difference is 0.20 for abundance
and 0.12 for biomass.

F-20 V Effects ofTemporality, Disturbance Frequency and Water Flow
on an Upper Hstuartne Macroin/auna Community

+

T3
00

o
o

c

c

3
<

13
12
11
10
9
8

2.5

+

.s

GO

o

«0

I

— • — Undisturbed

V Biweekly
— ■ — Monthly
— O Bimonthly

i

a)

T"

-• — Undisturbed
V Biweekly
-■ — Monthly

Figure 8: Disturbance frequency*date interaction for average a) abundance and b) biomass from
control flow plots. Minor ticks placed every Monday. Tukey minimum significant difference is 0.59 for
abundance and 0.25 for biomass.

Appendix F ♦ F-21

<N

PCI

(N

PCI

Figure 9: Principal component (PC) analysis for all species data. A) Plot of PC scores for dates: A =
23 April 1997, B = 07 May 1997. C = 22 May 1997, D = 04 June 1997, E = 20 June 1997, F = 02 July 1997,
G = 17 July 1997, and H = 01 August 1997. B) Plot of standardized PC loadings for species: CL =
chironomid larvae, LC = Laeoneris culveri, ML = Mulinia lateralis. PC = Polydora cornuta, and SB =
Streblospio benedicti.

F-22 ♦♦♦ Effects of Temporality, Disturbance Frequency and Water Flow
on an Upper Estuarine Macroinjauna Community

APPENDIX G

Field Notes and Observations
from Benthic Sampling Trips:
October 1994 - December 1999

In eady October 1994 Paul Montagna, Rick KaUce, Teny Whitledge, Dean Stockwell and Ken Dunton met with
Mike Irlbeck at Rincon Bayou to discuss station locations. The road and the pasture to Stations A & B were
very wet and the water level was near the top of the culverts and flowing from A to B. The secondary diversion
channel was not finished but a ditch cut through the road in its place restricting travel.

SAMPLING DATES

28 Oct. 94. First benthic sampling trip. Salinity was low from previous inflow event, Rt^ppia was noted in the
reference sites A & B. High D. O. at A & B associated with the Ruppia. Roads were muddy.

Note: Rsference sites A<&B don't appear to he affected by daify tidal movement as an sites. C, D, E and F in Rincon Bayou.

11 Jan 95. Benthic samples. Low tides at stations in Rincon Bayou. SaUnity normal, no signs of increase with
low water.

12 Apt. 95. Benthic samples. Salinity normal. Ruppia very short and sparse at A & B. We used push-net at
Station C for isotope samples. Lot of mysids, grass shrimp, and some brown shrimp. Water level up with
spring high tides.

12 July 95. Benthic samples. Summer low tides, hot dry conditions with high evaporation. Road and pond at
Stations A & B covered with white pelicans. Ibis, Stilts and mottled ducks. Lot of terns skimming surface.
Low oxygen levels. Some dead Cjrprinodon variegatas and a few blue crabs along shoreline. Some live crabs
along shore at air/water interface, fish appear to be gulping for air, and numerous corixids (water boatmen) at
A & B. Salinities high at all stations. Water levels down in Rincon Bayou at Stations C, D, E and F. Dead crab
and Cyprinodon at Stations C & D. Station E, mud wet but no standing water. Approximately 40 yards out
water 0.01 m deep , dug hole for hydros, cores from wet mud. Saw a live Laeoneneis culveri on surface of mud.
80.5%o pore water and same on surface water. Salt crystals on svirface of adjacent algae flat Station F, sure is
hot, water low 15-20 ft. from shoreline. Station G (Railroad Trestle) salinity was 43.5%o.

3 Oct 95. Benthic samples. Water level was high due to Hurricane Opal in Gulf of Mexico. Tidal flushing
lowered salinities. Stations A & B water level up, oxygen OK, salinity down a littie. Stations C, D, E & F
flushed from high tides. Isotope sample from Station G.

31 Oct. 95. Exploratory trip. Dean Stockwell and Rick Kalke. Nueces River Diversion Channel just
completed. On 29 Oct. 95 Corpus Christi received 3"^ highest record rainfall, 10+ inches over-night. We were
able to motor with the jet boat from Nueces River to C-5 the culvert between the river and Station C. Mike
Irlbeck had hiked in from the road and we gave him a ride out. Salinity at the culvert was 0.57%o. We drove to
Stations A & B by vehicle. High water due to run-off and tides. Surface water was flowing from B to A
because of the wind and the salinity was 10.16%o. Large mats of blue -green algae were on the surface and along
the shore. Took slides. Roads too wet to go to other stations.

13 Dec. 95. Isotope samples from the mouth of Rincon Bayou (L=50, C, F, H = 68).

Appendix G ♦ G-1

8 Jan, 96. Benthic samples. Winter low tides evident at Stations E & F. Salinities in Rincon Bayou in the 30's
to 39.0%o at station F.

21 Feb. %. Isotope samples. Water very low. Stations A & B. .1 m deep, salinity 33. 3%o. Rincon Bayou

Station C ~ .05 m deep 81%o, D & E no water. Station F only puddles, refractometer 122%o. Highest
salinities, beginning of 1996 drought?

19 Mar. 96. Isotope samples. No water at Stations A, B, C, D & E. ~ .05 m water at Sta. F, 68.6%o.

9 Apt. 96. Benthic and Isotope samples. At Station G- Railroad Tresde. Water still low, sampled lake east of
railroad tracks ~4" deep, very turbid, collected some post larval brown shrimp for isotopes. Station A- dr)»,
only a puddle in center "- l"deep, algae layer on top of mud. Station B, sample area diy except for water
between cracks in mud (water from rain fall from previous week) 50%o. Station C, dr}', only damp mud, only
water was in cracks and in wash-out area around posts, 98%o. Station D, no standing water, a little water in
cracks and animal tracks, 100%o. Station E, 1/4 inch water from where cores were taken, foot tracks from Jan.
98 still there, 82%o. Station E, removed mud from same depression as Jan. 96 for Hydrolab, very low water 2-
3" deep, 59.3%o. Station H=68 Nueces River Bridge, 6.8%o.

13 May 96. Isotope samples. Water level back up again but salinity levels stiU high. Station C, water ver^' clear,
bottom covered with blue green algae mat (very green) - sampled with push-net - no sign of any shrimp only a
few Cyprinodon, 59.3%o. Sta. F, 51.8%o, water very turbid, lot of small swirls from fish and shnmp, some port-
larval brown shrimp collected. Station G, (Tresde) 51.7%, water turbid, brown shrimp numerous, saw redfish
feeding so shallow its back was out of the water. Station H = 68-river 8.9%o, collected brown shrimp and blue
crabs with push-net.

1 June 96. Isotope samples. Station H=68, 9.9%o collected brown shrimp, spot and Rangia. Station C, 70.4%o,
water from high tide was flowing through upper culverts C-5 & C-4 through diversion channel to the Nueces
River.

12 July 96. Benthic samples. Stations A & B, no water, took cores on dry land, very Uttie moisture in sediment.
Stations C & D. Small amount of water, saltnit)' C=159.2%o, D=120%o. Stations E & F water shallow,
85.86%o, no shrimp, picture of algal mats on slides. Station G=Trestle, 84%o no shrimp. Station H=68 River,
2.1%o, push-net for shrimp, caught 1 brown shrimp, saw a couple of large shrimp avoiding net.

22 Oct. %. Benthic samples. Stations A & B - water back ~ .1 m, 40%o. Stations C, D, E and F - water back,
evidence of lower salinity input from the River Diversion Chaimel 25.1%o at C-* 46.4%o at F.

6 Jan. 97. Benthic samples. Stations A & B, bottom covered with dense bed of Ruppia. 52.4%o, Salinity
gradient from C to F = 25.0%o to 56.4%o. Stations E and F sediment surface with layer of algal mat.
Station H = 68 River 0%o. Low flow coming over dam.

23 Apt. 97. Benthic samples. Stations A & B, low oxygen, water a thick green color, Kuppia seems to be
declining, not as thick as Jan. 97. Stations C, D, E, and F, salinity down to 15-16%o. Station D, lot of medium
to large blue crabs and some reds and drum. Station E, water shallow 2-3 in., slight outgoing current 1045, algal
mat over most of bottom, took benthic samples from bare bottom.

2 July 97. Benthic samples. Stations A & B, salinity down to 4.8%o, water up, Ruppia covering most of bottom,
very healthy, to surface of water. Station C, water ~ 1 ft deep at shoreline, evidence of being higher, Christine's
experimental plot completely under water. Salinity from C to F was 0.2 to 4.4%o. Station H=68 at river 0%o.
Station 62 Whidedge station in flats north of secondary' diversion channel, puddle at culvert was 0%o, water in
flat on way to 62 was 4%o and at 62 stake 2%o Station D, washout in culvert on road.

28 Oct. 97. Benthic samples. Light rain, low salinity at all stations. Stations A & B, 4%o C to F, 0.3 to 4.1%o.
Water level down from the flood event in July 97.

G-2 ^ Field Notes and Observations from Benthic Sampling Trips

16 Jan. 98. Benthic samples. Stations A & B hydrogtaphic conditions good. The area between Stations C & D
seems to pond during low tides. In the summer long term ponding will sometimes evaporate and dry up.
Station C was 0.1 m deep and Station D was 0.04 m deep. Water level was low, only small ditch of water form
culvert between upper and lower Rincon Bayou. Hole in road covered with pile of fill. Station E, no water,
only damp mud with numerous worm casts. Station F, dry, only wet mud with occasional puddles, 35%o.

8 April 98. Benthic samples. Station A, short Ruppia. Station B, surface of sediment with ~ 4cm loose flock
(mud), 16%o. Station D, water level up, C-5- water just to culverts with small channel or trickle of water
moving. Station D, large number of black drum feeding in area aroimd D, water flowing out toward bay.
Salinity from C to F was 12.9 to 30.2%o.

9 July 98. Benthic samples. Salinity, high 104%o. Station A & B, very low water, oxygen low, lot of wading
birds and sea-guUs apparently feeding on Cyprinodon variegatas, etc. Stations C-F, water level normal, not low.
Station D, lot of larger fish, reds and drums. Salinity high C to D, 60%o.

5. Aug. 98. Primary production. Rick KaJke with Kevin Neely. Stations A & B, dry, no water. Station C, salt
deposits on surface, only sheen of water on mud surface. Station D, basically dry, only narrow strip of water.
Stations 67 & 62, dry. Station F=61, 94.93%o, Station 83, 50.2%o, Station 60, 70.24%o.

Starting in September 1998, 1 (Rick Kalke) began sampling with Ijynn Tinnin and Terry Whitkdge 's primary production
processes.

29 Sept. 98. Primary production. Station A & B, water level up, 28.2 - 28.4%o. Stations C & D, salinities
indicate inflow event, 2.3 to 5.6%o, current going out of Rincon through culvert toward bay to Stations E & F,
22.1 and 20.8%o. Station H=68; river 0%o.

28 Oct. 98. Benthic samples and collected water for Lynn Tinnin, not able to get water for primary production
samples. Major flood occurred on 17 Oct. 98. Water level had been high enough to wash out culverts and road
at the secondary diversion channel and washed a large portion of road out at Station D. We had to hike in
from the secondary wash-out to collect benthic and water samples from Stations D, E and F. Stations A & B,
water up, running from A to B, 2 & 1.5%o. Stations C-F , major freshwater flushing 0 to 0.8%o.

TDite to road wash-outs and I or wet conditioru we were unahk to sampk some of the lower Rincon Ba^ou Stations, i.e. Station D,
E, and F until 24 Feb. 99.

18 Nov. 98. Primary Production. Stations A & B, water flowing from A to B, salinity 3.8 to 3.7%o. Station C,
water high, up to grass line along shore, about same in Oct. 98. Road real wet in spots, maybe a Uttie drier than
Oct. Station H=68, river up over sides of boat ramp to shoreline, 0%o.

17 Dec. 98. Primary Production. Stations A & B, current from A to B, 14.6 and 14.7%o. Station C, 11. 7%o,
roads still washed out. Station H=68, river level back down to bottom of boat ramp.

12 Jan. 99. Benthic samples. Stations A & B, 10.1 to 13.8%o, lot of water boatmen (corixidae) Stations C & D,
water in upper Rincon ponding 19.3%o & 19.9%o. Station E & F, had to hike in from Station D, low water in
their area, salinity 25 & 23.8%o.

13 Jan. 99. Primary production. Stations A & B, salinity 10.8%o & 12.3%o. Station C, water golden brown in
color, 19.9%o. Station H=68, river 0%o.

24 Feb. 99. Primary production. Stations A & B, water turbid, 15%o. Station C, salinity, 33.97, higher than
Stations D, E, and F. Water at Station D turbid brown color, flowing into Rincon, 2" deep in ctilvert. Station

E, algal flat dry, Salicomia patches show fresh growth. Station F water to edge of grass and clear. Station H=68
salinity 1 .45, no flow over dam. First Nueces River Diversion Channel flow gage information.

18 Mar. 99. Primary production. Stations A & B, water turbid 20%o. Stations C & D, salinity down to 19.33 &
28.85. Station E, high water line was up to power poles but currently at shore Une. Algal flat inactive. Station

F, water turbid, up to grass line. Station H=68, river 1.25%o, no water over dam.

Appendix G ♦ G-3

14 Apr. 99. Benthic samples. Stations A & B water up, .35m depth, salimty 13.78%o. Station C, salinity 4.84%o
from diversion channel. Station D, water very turbid, 15.78%o, some large fish swirls probably reds, drum and
mullet. Stations E & F water .27 & .35 m, large school of redfish at Station F, tails out of water (see slide).
Station G, tresde, 21.86%o slight out-going tide.

15 Apr 99. Primary production. Conditions similar to 14 Apr. 99 except wind much stronger, NW-20 MPH.
Water flowing over dam at Nueces River. Station H=68, 0.44%o.

24 May 99. Primary production. Stations A & B, dense beds of Bjtppia, salinity, 16-18%o. Stations C & D,
water muddy, lot of fish activity at both sites, redfish at Station D, caught some brown shrimp with push-net at
Station C. Station E & F, lot of small fish, shrimp and redfish activity. Station H=68, river 2.02%o, water
seeping over dam, no flow.

9 June 99. Primary production. Stations A & B, dense beds of Ruppia up to surface, low ox}'gen 1 .95 to 1 .49
ppm (mg/1), salinity 24-25%o. Stations C & D, 20 to 35%o. Station E, 39%o, at least 1 redfish and lot of bait-
fish. A little water in algal flat was 1 10%o, dead Cyprinodon and brown shrimp 8 to 9.5 cm long in algal flat
area. Probably trapped in evaporating pools left by low tides. Station F, lot of mullet and small bait swirls and
a few redfish.

7 July. 99. Benthic samples. Stations A & B, salinity down to 8.3%o, oxygen low 1.59 to 1.7 ppm. (mg/1),
Ruppia present but not as thick as it was in May and June. Freshwater inflow event, salinity down at Stations C-
F, to 1.3 to 6.3%o. Rattiesnake at road right before Station D. Strong flow of freshwater from Station D to E
& F. Station E, algal flat 10%o, algal mat green near shallow edges, gray and floating in deeper areas, no mats
visible at Station E.

21 July 99. Primary productions. Stations A & B, oxygen low 1.6 ppm (mg/1) depth .21 to .27 m. Stations C
to F, salinity still low 4.2 to 13.6%o. Station C oxygen 3.2 ppm. Station H=68, river, 0.6%o.

19 Aug, 99. Primary production. Stations A & B, water diytng up, dead blue crabs in standing water, few live
crabs along edges of bank out of water to get air, lot of shore birds in what litde water left. No water where we
normally sample, water taken by culvert for A & B. Stations C & D, water low, salinity 26 to 45%o, lot of
wading birds, ~ 6 dead black drum carcasses on shore. Station E, no water ("I told you so!") says Ms, Tinnin.
Station F, .05 m, 39%o. Station H=68 river, 0.48%o, barely flowing over dam.

16 Sept. 99. Primary production. Stations A & B, water back as result of rain and tides from hurricane Bret.
Ruppia starting to grow along shorelines. Station C to F salinity 0.93 to 8.51%o. Station D current coming in
from the bay, blue -green algal floating on surface, lot of fish activity, water turbid. Stations E and F, lot of fish
and mullet, reds, shrimp. Algal flat at Station E, 2%o. Blue green algal floating at Stations E & F. Station
H=68, river 0.43%o, flow over dam.

28 Oct. 99. Benthic and primary production samples. Stations A & B, Ruppia growing along shoreline. Station
C, lot of white shrimp, white pelicans and avocets. Station D, water real low, ponding between C & D, white
shrimp jumping by culvert. Stations E & F, 16 & 15%o, Station H=68, river 1.22%o, no flow over dam.

17 Nov. 99. Primary production. Stations A & B water soupy green, 15 to 14%o, Ruppia along shoreline.
Stations C & D, water clear 1 1 to 1 9%o, numerous bird tracks, no shrimp. Station E, water clear 25%o, Station
F, SE wind began blowing and quickly stirred up sediment, now very turbid. Station H=68, river 0.85%o, no
flow over dam.

8 Dec. 99. Primary production. YSI instrument not working. Only refractometer for salinity. Station A & B,
15 to 14%o. Station C, D, E and F, 10, 12 and 25%o. Station H=68 0%o, scattered showers.

Nueces River Diversion Channel Water Exchange

At the February, 1 999 Rincon Bayou Demonstration Project team meeting there was some concern in the
accuracy of the exchange rates of water moving through the diversion channel into and out of upper Rincon
Bayou. During at least six sampling trips to Rincon Bayou we checked the hydrographic conditions, the flow

G-4 ^ Field Notes and Observations from Benthic Sampling Trips

24 Feb. 99

1230

18 Mar. 99

1200

14 Apr. 99

1410

15 Apr. 99

0800

24 May 99

1210

9 June 99

1205

1.1

2.04

1.78

1.54

1.2

1.67

patterns through the diversion channel, at the Rincon Bayou Station, 08211503, at the culverts above station C,
station D and the culvert at this site, stations E and F and when available stations 50 and 51 at the mouth of
Rincon Bayou in Nueces Bay. Observations at Station 08211503 verified with the gage data for this station
indicate that the water exchange reported from this gage is accurate. The following is an example of the staff
readings and the gage read-out for 6 observations.

DATE TIME GAGE STAFF

1.14

2.15

No Data

1.57
1.23
1.69

The daily flow regime through the Nueces River Diversion Channel into Rincon Bayou is now established. The
connection between the channel and upper Rincon Bayou where the ranch road crosses has resulted to a
narrow ditch where water exchanged takes place. The ditch has vegetation growing up in both banks and
seems to be maintained by almost daily tidal exchange into and back out of upper Rincon Bayou. Although no
diel observations were made, the wet banks near low areas suggest tidal inundations moves water onto the flats
occasionally. There is a split of tidal flow in the upper Rincon with tidal movement coming and going from
both Nueces Bay and the Nueces River, resulting in higher salinity estuarine water from the bay meeting with
lower salinity Nueces River water from the diversion chaimel in the upper Rincon Bayou. This exchange has
helped alleviate the stagnated high salinity conditions observed when upper Rincon Bayou was a dead-end.
High salinity conditions can still be expected during droughts and extreme low tides especially dimng the
summer. The extent of water exchange in Rincon Bayou depends on the magnitude of the tidal of freshwater
inflow event. A major event results in complete flushing and mixing of the system all the way to the bay while
normal daily tidal movement may only move enough water to maintain some exchange with upper Rincon
Bayou which is important.

Rincon Bayou Seasonal Observations

Hydrographic and environmental conditions at Rincon Bayou are dependent on annual tidal cycles and seasonal
climatic conditions. Local weather events can dramatically effect these conditions, i.e. droughts, floods and
tropical disturbances.

The typical annual cycle based on annual quarters:

January usually results in low winter tides and cool to cold temperatures. These conditions don't typically result
in high evaporation and high salinities.

April conditions are associated with spring high tides which disperse brown shrimp post larvae and crab larvae
throughout the delta and niu-sery areas. Many drum and redfish migrate into the area to feed on the growing
shrimp population.

July often has periods of very low tides, low rainfall, associated with high temperatures and high evaporation.
Rincon Bayou stations often dry up or the water becomes hypersahne.

October is the transition for summer to fall. The tides may be high especially with tropical storm activity and
flooding may occur modifying typical high salinity conditions from summer. At the control stations A & B
which are often dry in July and August the fall rainfall results in the germination and growth of Ruppia beds
which are an important food source for waterfowl.

Appendix G ^ G-5

United States Department of the Interior
The mission of the Department of the Interior is

to protect and provide access to our Nation's

natural and cuUural heritage and honor our trust

responsibilities to tribes.

Bureau of Reclamation
The mission of the Bureau of Reclamation is to
manage, develop, and protect water and related

resources in an environmentally and
economically sound manner in the interest of

P00001884