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Biological Services Program
FWS/OBS-82/25
September 1982
COLLECTION
THE ECOLOGY OF
THE SEAGRASSES OF
SOUTH FLORIDA: A Community Profile
R' "'^au of Land Management
a|4 and Wildlife Service
\5^ )epartment of the Interior
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FWS/OBS-82/25
September 1982
THE ECOLOGY OF THE SEAGRASSES
OF SOUTH FLORIDA: A COMMUNITY PROFILE
by
Joseph C. Zienan
Department of Environmental Sciences
University of Virginia
Charlottesville, VA 22903
Project Officer
Ken Adams
National Coastal Ecosystems Team
U.S. Fish and Wildlife Service
1010 Gause Boulevard
Slidell, LA 70458
Prepared for
National Coastal Ecosystems Team
Office of Biological Services
U.S. Department of the Interior
Washington, DC 20240
DISCLAIMER
The findings in this report are not to be construed as an official U.S. Fish and
Wildlife Service position unless so designated by other authorized documents.
Library of Contress Card Number 82-600617.
This report should be cited as
Zieman, J.C. 1982. The ecology of the seagrasses of south Florida: a comnunity
profile. U.S. Fish and Wildlife Services, Office of Biological Services,
Washington, D.C. FWS/CBS-82/25. 158 pp.
PREFACE
This profile of the seagrass commun-
ity of south Florida is one in a series of
community profiles that treat coastal and
marine habitats important to humans. Sea-
grass meadows are highly productive habi-
tats which provide living space and pro-
tection from predation for large popula-
tions of invertebrates and fishes, many of
which have commercial value. Seagrass
also provides an important benefit by
stabilizing sediment.
The information in the report can
give a basic understanding of the seagrass
community and its role in the regional
ecosystem of south Florida. The primary
geographic area covered lies along the
coast between Biscayne Bay on the east
and Tampa Bay on the west. References
are provided for those seeking indepth
treatment of a specific facet of seagrass
ecology. The format, style, and level of
presentation make this synthesis report
adaptable to a variety of needs such as
the preparation of environmental assess-
ment reports, supplementary reading in
marine science courses, and the education
of participants in the democratic process
of natural resource management.
Any questions or comments about, or
requests for publications should be di-
rected to:
Information Transfer Specialist
National Coastal Ecosystems Team
U.S. Fish and Wildlife Service
NASA/SI idell Computer Complex
1010 Gause Boulevard
SI idell, Louisiana 70458
m
CONTENTS
Paae
PREFACE iii
FIGURES vi
TABLES vii
ACKNOWLEDGMENTS viii
CHAPTER 1. INTRODUCTION 1
1.1 Seagrass Ecosystems 1
1.2 Climatic Environment 4
1.3 Geologic Environment 6
1.4 Regional Seagrass Distribution 7
1.5 Seagrasses of South Florida 7
CHAPTER 2. AUTECOLOGY OF SEAGRASSES 11
2.1 Growth 11
2.2 Reproductive Strategies 11
2.3 Temperature 12
2.4 Salinity 14
2.5 Sediments 14
2.6 Current Velocity 17
2.7 Oxygen 17
2.8 Solar Radiation 17
2.9 Zonation 18
2.10 Exposure 19
CHAPTER 3. PRODUCTION ECOLOGY 20
3.1 Biomass 20
3.2 Productivity 22
3.3 Productivity Measurement 22
3.4 Nutrient Supply 25
3.5 Seagrass Physiology 26
3.6 Plant Constituents 29
CHAPTER 4. THE SEAGRASS SYSTEM 33
4.1 Functions of Seagrass Ecosystems 33
4.2 Succession and Ecosystem Development 34
4.3 Species Succession 34
4.4 The Central Position of the Seagrasses to the
Seagrass Ecosystem 38
4.5 Structural and Process Succession in Seagrass 39
iv
CONTENTS (continued)
Page
CHAPTER 5. THE SEAGRASS COMMUNITY - COMPONENTS, STRUCTURE, AND FUNCTION .... 41
5.1 Associated Algae 42
Benthic Algae 42
Epiphytic Algae 44
5.2 Invertebrates 45
Composition 45
Structure and Function 46
5.3 Fishes 49
Composition 49
Structure and Function 51
5.4 Reptiles 53
5.5 Birds 54
5.6 Manmals 56
CHAPTER 6. TROPHIC RELATIONSHIPS IN SEAGRASS SYSTEMS 57
6.1 General Trophic Structure 57
6.2 Direct Herbivory 59
6.3 Detrital Processing 69
Physical Breakdown 70
Microbial Colonization and Activities 71
Microflora in Detritivore Nutrition 72
Chemical Changes During Decomposition 73
Chemical Changes as Indicators of Food Value 73
Release of Dissolved Organic Matter 74
Role of the Detrital Food Web 74
CHAPTER 7. INTERFACES WITH OTHER SYSTEMS 75
7.1 Mangrove 75
7.2 Coral Reef 75
7.3 Continental Shelf 78
7.4 Export of Seagrass 78
7.5 Nursery Grounds 80
Shrimp 80
Spiny Lobster 81
Fish 82
CHAPTER 8. HUMAN IMPACTS AND APPLIED ECOLOGY 84
8.1 Dredging and Filling 84
8.2 Eutrophication and Sewage 86
8.3 Oil 87
8.4 Temperature and Salinity 88
8.5 Disturbance and Recolonization 91
8.6 The Lesson of the Wasting Disease 93
8.7 Present, Past, and Future 93
REFERENCES 96
APPENDIX A-1
Key to Fish Surveys in South Florida A-1
List of Fishes and their Diets from Collections in South Florida A-2
V
FIGURES
Number Page
1 Panoramic view of a south Florida turtle grass bed 2
2 Map of south Florida 3
3 Average monthly temperatures in Florida 6
4 Seagrasses of south Florida 9
5 Diagram of a typical Thalassia shoot 12
6 Response of Thalassia production to temperature 13
7 Response of a Thalassia bed to increasing sediment depth 16
8 Depth distribution of four seagrasses 19
9 Blowout disturbance and recovery zones 35
10 Idealized sequence through a seagrass blowout 35
11 Representative calcareous green algae from seagrass bess 35
12 Origin of sedimentary particles in south Florida marine waters ... 36
13 Ecosystem development patterns in south Florida marine waters .... 37
14 Calcareous algae (Udotea sp. ) from the fringes of a
seagrass bed 43
15 Thalassia blades showing tips encrusted with calcareous
epiphytic algae 45
16 Large invertebrates from seagrass beds 47
17 Snail grazing on the tip of an encrusted Thalassia leaf 4?
18 Relative abundance of fishes and invertebrates over
seagrass beds and adjacent habitats 49
19 Small grouper (Serranidae) foraging in seagrass bed 52
20 Seagrass bed following grazing by green sea turtle 53
21 Shallow seagrasses adjacent to red mangrove roots 54
22 Principal energetic pathways in seagrass beds 57
23 Comparative decay rates 71
24 Grunt school over coral reef during daytime 76
25 Seagrass export from south Florida to the eastern
Gulf of Mexico 79
26 Housing development in south Florida 85
27 Scallop on the surface of a shallow Halodule bed 95
VI
TABLES
Number Page
1 Temperature, salinity, and rainfall at Key West 5
2 Seagrasses of south Florida 8
3 Representative seagrass biomass 21
4 Comparison of biomass distribution for three
species of seagrasses 23
5 Representative seagrass productivities 24
6 C values for gulf and Caribbean seagrasses 28
7 Constituents of seagrasses 30
8 A gradient of parameters of seagrass succession 40
9 Birds that use seagrass flats in south Florida 55
10 Direct consumers of seagrass 60
vn
ACKNOWLEDGMENTS
In producing a work such as this pro-
file, it is impossible to catalog fully
and accurately the individuals that have
provided either factual information or
intellectual stimulus. Here much of the
credit goes to the mutual stimulation pro-
vided by my colleagues in the Seagrass
Ecosystem Study of the International
Decade of Ocean Exploration. Special
recognition must be given to the magus of
seagrass idiom during those frantic and
memorable years, Peter McRoy.
At one stage or another in its gesta-
tion, the manuscript was reviewed and com-
ments provided by Gordon Thayer, Richard
Iverson, James Tilmant, Iver Brook, and
Polly Penhale. Other information, advice,
or v.'elcomed criticism was provided by John
Oi^den, Ronald Phillips, Patrick Parker,
r;obin Lewis, Mark Fonseca, Jud Kenworthy,
Brian Fry, Stephen Macko, James Kushlan,
William Odum, and Aaron Mills.
Two of the sections were written by
my students, richael Robblee and Mark
Robertson. To them and other students,
present and past, I must give thanks for
keeping life and work fresh (if occasion-
ally exasperating). The numerous drafts
of this manuscript were typed by Deborah
Coble, who also provided much of the edit-
ing, Marilyn McLane, and Louise Cruden.
OriQinal drafting was done by Rita Zieman,
who" also aided in the production of Chap-
ter 8, and Betsy Blizard. I cannot thank
enough Ken Adams, the project officer, for
his patience and help in the production of
this work, which went on longer than any
of us imagined.
Thanks are also expressed to Gay
Farris, Elizabeth Krebs, Sue Lauritzen,
and Randy Smith of the U.S. Fish and
Wildlife Service for editorial and typ-
ing assistance. Photographs and figures
were by the author unless otherwise noted.
vn ^
CHAPTER 1
INTRODUCTION
1.1 SEAGRASS ECOSYSTEMS
Seagrasses are unique for the marine
environment as they are the only land
plant that has totally returned to the
sea. Salt marsh vegetation and mangroves
are partially submerged in salt water, but
the seagrasses live fully submerged,
carrying out their entire life cycle com-
pletely and obligately in sea water (Fig-
ure 1).
Seagrass meadows are highly produc-
tive, faunally rich, and ecologically
important habitats within south Florida's
estuaries and coastal lagoons (Figure 2)
as well as throughout the world. The com-
plex structure of the meadow represents
living space and protection from predation
for large populations of invertebrates and
fishes. The combination of plentiful shel-
ter and food results in seagrass meadows'
being perhaps the richest nursery and
feeding grounds in south Florida's coastal
waters. As such, many commercially and
ecologically significant species within
mangrove, coral reef, and continental
shelf communities are linked with seagrass
beds.
Although the importance of seagrass
beds to shallow coastal ecosystems was
demonstrated over 60 years ago by the
pioneering work of Petersen (1918) in the
Baltic Sea, it is only in the past 10 to
15 years that seagrasses have become wide-
ly recognized as one of the richest of
ecosystems, rivaling cultivated tropical
agriculture in productivity (Westlake
1963; Wood et al . 1969; McRoy and McMillan
1977; Zieman and Wetzel 1980).
Studies in the south Florida region
over the past 20 years have demonstrated
the importance of the complex coastal
estuarine and lagoon habitats to the pro-
ductivity of the abundant fisheries and
wildlife of the region. Earlier studies
describing the link between estuarine sys-
tems and life cycles of important species
focused on the mangrove regions of the
Everglades (W.E. Odum et al . 1982), al-
though the seagrass beds of Florida Bay
and the Florida Keys have been identified
as habitats for commercially valuable spe-
cies, as well as for organisms that are
important trophic intermediaries. Many
species are dependent on the bays, la-
goons, and tidal creeks for shelter and
food during a critical phase in their life
cycle.
Many organisms that are primarily
characterized by their presence and abun-
dance over coral reefs, such as the enor-
mous and colorful schools of snappers and
grunts, are residents of the reef only by
day for the shelter its complex structure
provides, foraging in adjacent grass beds
at night. These seagrass meadows, often
located adjacent to the back reef areas of
barrier reefs or surrounding patch reefs,
provide a rich feeding ground for diurnal
reef residents; many of these organisms
may feed throughout their life cycle in
the grass bed. The juveniles of many
Pomadasyid species are resident in the
grass beds. As they grow, however, their
increasing size will no longer allow them
to seek shelter in the grass and they move
on to the more complex structure of the
reef for better protection (Ogden and
Zieman 1977).
Figure 1. Panoraiiic view of a south Florida turtle grass bed.
2
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Mangroves and coral reefs are rarely,
if ever, in close proximity because of
their divergent physio-chemical require-
ments, but seagrasses freely intermingle
with both communities. Seagrasses also
form extensive submarine meadows that fre-
quently bridge the distances between reefs
and mangroves. Seagrass beds of the larger
mangrove-lined bays of the Everglades and
Ten Thousand Island region, while being a
small proportion of the total bottom cov-
erage of these bays, are the primary zones
where important juvenile organisms, such
as shrimp, are found.
There are two major internal pathways
along which the energy from seagrasses is
made available to the community in which
they exist: direct herbivory and detrital
food webs. In many areas a significant
amount of material is exported to adjacent
communities.
Direct grazing of seagrasses is con-
fined to a small number of species, al-
though in certain areas, these species may
be quite abundant. Primary herbivores of
seagrasses in south Florida are sea tur-
tles, parrotfish, surgeonfish, sea ur-
chins, and possibly pinfish. In south
Florida the amount of direct grazing
varies greatly, as many of these herbi-
vores are at or near the northern limit of
their distribution. The greatest quandry
concerns the amount of seagrass consumed
by the sea turtles. Today turtles dre
scarce and consume a quantitatively insig-
nificant amount of seagrass. However, in
pre-Columbian times the population was
vast, being 100 to 1,000 times - if not
greater - than the existing population.
Some grazers, such as the queen
conch, appear to graze the leaves, but
primarily scrape the epiphytic algae on
the leaf surface. Parrotfish preferen-
tially graze the eoiphytized tips of sea-
grass leaves, consuming the old portion of
the leaf plus the encrusting epiphytes.
The detritus food web has classically
been considered the main path by which the
energy of seagrasses makes its way through
the food web. Although recent studios
have pointed to increased importance of
grazing in some areas (Ogdon and Zienan
1977), this generalization continues to be
supported.
When assessing the role of seagrass-
es, sediment stabilization is also of key
importance. Although the seagrasses them-
selves are only one, or at most three spe-
cies, in a system that comprises hundreds
or thousands of associated plant and ani-
mal species, their presence is critical
because much, if not all, of the community
exists as a result of the seagrasses. In
their absence most of the regions that
they inhabit would be a seascape of un-
stable shifting sand and mud. Production
and sediment stabilization would then be
due to a few species of rhizophytic green
algae.
1.2 CLIMATIC ENVIRONMENT
South Florida has a mild, semitropi-
cal maritime climate featuring a small
daily range of temperatures. The average
precipitation, air temperature, surface
water temperature, and surface water sa-
linity, for Key West are given in Table 1.
Water temperature and salinity vai'y sea-
sonally and are affected by individual
storms and seasonal events. Winds affect-
ing the area are primarily mild southeast
to easterly winds bringing moist tropical
air. Occasional major storms, usually
hurricanes, affect the region on an aver-
age of every 7 years, producing high winds
and great quantities of rain that lower
the salinity of shallow waters. Puring
the winter, cold fronts often push through
the area causing rapid drops in tempera-
ture and high winds that typically last 4
to 5 days (Warzeski 1977, in Multer 1977).
In general, summer high temperatures are
no higher than elsewhere in the State, but
winter low temperatures arc more moderate
(Figure 3).
Water temperatures are least affected
on the outer reef tract where surface wa-
ters are consistently mixed with those
from the Florida Current. By contrast the
inner regions of Florida Bay are shallow
and circulation is restricted. Thus water
temperatures here change rapidly with sud-
den air temperature variations and rain.
Water temperatures in Pine Channel dropped
from 20° to 12°C (68° to 54°F) in 1 day
foil owing the passage of a major winter
storm (Zieman, personal observation).
These storms cause rapid increases in sus-
pended sediments because of wind-induced
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Figure 3. Average monthly temperatures in Florida, 1965 (McNulty et al . 1972),
and occasionally reduced salin-
of which stress the local shal-
is thought that
type of water
the relatively
turbulence
ities, all
low water communities. It
the rapid influx of this
from Florida Bay through
open passages of the central Keys, when
pushed by strong northwesterly winter
winds, is the major factor in the reduced
abundance of coral reefs in the central
Keys (Marszalek et al. 1977).
Tides are typically about 0.75 m (2.5
ft) at the Miami harbor mouth. This range
is reduced to 0.5 m (1.6 ft) in the embay-
ments such as South Biscayne Bay and to
0.3 m (1 ft) in restricted embayments like
Card Sound (Van de Kreeke 1976). The mean
range decreases to the south and is 0.4 m
(1.3 ft) at Key West Harbor. Tidal heights
and velocities are extremely complex in
south Florida as the Atlantic tides are
semidiurnal, the gulf tides tend to be di-
urnal, and much of this region is between
these two regimes. Neither tidal regime
is particularly strong, however, and winds
frequently overcome the predicted tides.
These factors, coupled with the baffling
effects of mudbanks, channels, and keys,
create an exceedingly complex tidal circu-
lation.
1.3 GEOLOGIC ENVIRONMENT
The south Florida mainland is low-
lying limestone rock known as Miami lime-
stone. For descriptive purposes the region
can be broken into four sections: the
south peninsular mainland (including the
Everglades), the sedimentary barrier
islands, the Florida Keys and reef tract,
and Florida Bay.
The sedimentary barrier islands of
north Biscayne Bay, Miami Beach, Virginia
Key, and Key Biscayne are unique for the
area because they are composed largely of
quartz sand. The islands are the southern
terminus of the longshore transport of
sand that moves down the east coast and
ultimately out to sea south of Key Bis-
cayne. All other sediments of the region
are primarily biogenic carbonate.
The Florida Keys are a narrow chain
of islands extending from tiny Soldier
Key, just south of Key Biscayne, in first
a southerly and then westerly arc 260 km
(163 mi) to Key West and ultimately to the
Marquesas and the Dry Tortugas some 110 km
(69 mi) further west. The upper keys,
from Big Pine northward, are composed of
ancient coral known as Key Largo 1 lime-
stone, whereas the lower keys from Biq
Pine west are composed of oolitic facies
of the Miami linestone. (A note to boaters
and researchers in these shallow waters:
the limestone of the lower keys is much
harder than in the upper keys, and occa-
sional brushes with the bottom, which
would be minor in the upper keys, will
mangle or destroy outboard propellers and
lower drive units. )
The Florida reef tract is a shallow
barrier-type reef and lagoon extending
east and south of the Florida Keys. It
averages 5 to 7 km (4 to 4.4 mi) in width
with an irregular surface and depths vary-
ing fro-^ 0 to 17 m (56 ft). The outer
reef tract is not continuous, but consists
of various reefs, often with wide gaps be-
tween them. The development is greatest
in the upper keys. The patch reefs are
irregular knolls rising from the limestone
platform in the area between the outer
reef and the keys. Behind the outer reef,
the back reef zone or lagoonal area is a
mosaic of oatchreefs, limestone bedrock,
and grass-covered sedimented areas.
Florida Ray is a triangular region
lying west of the upper keys and south of
the Everglades. This large (226,000 ha or
558,220 acres), extremely
reaches a maximum depth of
(7 to 10 ft), but averages
(3.3 ft) over a great area,
ments of fine carbonate mud
inq, anastomosing
filled "lakes" or
islands.
shallow basin
only 2 to 3 m
less than 1 m
Surface sedi-
occjr in wind-
mud banks, seagrass-
basins, and mangrove
1.4 REGIONAL SEAGRASS DISTRIBUTION
this ,ir(;d (Bittaker and Iverson, in
press). In an inventory of the estuaries
of the gulf coast of Florida, McNulty
et al . (1972) estimated that over 45% of
the total area in the region of Florida
Bay v^est of the Keys and landward to the
freshwater line to Cape Sable was sub-
merged vegetation. By comparison, man-
grove vegetation comprised less than 7% of
the araa.
The amount of seagrass coverage drops
off rapidly to the north of this area on
both coasts. On the Atlantic coast, the
shifting sand beaches signal a change to a
high-energy coast that is unprotected from
v/aves and has a relatively unstable sub-
strate, coupled with the littoral drift of
sand from the north. Throughout this area
seagrasses are usually found only in small
pockets in protected inlets and lagoons.
On the Gulf of f^exico coast north of Cape
Sable, seagrasses are virtually eliminated
by drainage from the Everglades with its
increased turbidity and reduced salinity.
Seagrasses are then found only in rela-
tively small beds within bays and estuar-
ies until north of Tarpon Springs, where
an extensive (3,000 km- or l,15Cmi-) bed
exists on the extremely broad shelf of the
northern gulf. Several bays on the gulf
coast, including Tampa Bay and Boca Ciega
Bay, formerly possessed extensive seagrass
resources, but dredge and fill operations
and other human perturbations have greatly
reduced the extent of these beds.
This profile is primarily directed at
the seagrass ecosystem of southern Flor-
ida. It is necessary, however, to draw on
the pertinent work that has been done in
other seagrass systems.
Florida possesses one of the largest
seagrass resources on earth. Of the
10,000 km- (3,860 mi- ) of seagrasses in
the Gulf of Mexico, over 8,500 km^ (3,280
mi) are in Florida waters, primarily in
two major areas (Bittaker and Iverson, in
press). The southern seagrass bed, which
is bounded by Cape Sable, north Biscayne
Bay, and the Dry Tortugas, and includes
the warm, shallow waters of Florida Bay
and the Florida coral reef tract, extends
over 5,500 km- (2,120 mi- ). Although cov-
erage is broken in numerous places, over
80% of the sea bottom contains seagrass in
1.5 SEAGRASSES OF SOUTH FLORIDA
Plants needed five properties to suc-
cessfully colonize the sea, according to
Arber (1920) and den Hartog (1970):
(1)
(2;
(3;
The ability
med i urn .
to 1 ive in a sal ine
The ability to
fully subinerged.
function while
A well-
tern.
developed anchoring sys-
(4]
(5)
The ability
reproductive
submerged.
to complete
cycle while
their
fully
The ability to
other organisms
environment.
compete with
in the marine
Only a small, closely related group of
monocotyledonous angiosperms have evolved
all of these characteristics.
Worldwide there are approximately 45
species of seagrasses that are divided
between 2 families and 12 genera. The
Potamogetonaceae contains 9 genera with 34
species, while the family Hydrocharitaceae
has 3 genera and 11 species (Phillips
1978). In south Florida there are four
genera and six species of seagrasses
(Table 2). The two genera in the family
Potamogetonaceae have been reclassified
comparatively recently and many of the
widely quoted papers on the south Florida
seagrasses show Cymodocea for Syringodium
and Diplanthera for Halodule. Recent dis-
cussion in the literature speculates on
the possibility of several species of
Halodule in south Florida (den Hartog
1964, 1970), but the best current evidence
(Phillips 1967; Phillips et al . 1974) in-
dicates only one highly variable species.
The small species number (six) and
distinctive appearance of south Florida
seagrasses make a standard dichotonous key
generally unnecessary (Figure 4). General
systematic treatments such as den Hartoq
(1970) and Tomlinson (1980) should be con-
sulted, however, when comparing the sea-
grasses of other areas. The best descrip-
tions of the local species are still to be
found in Phillips (1960).
Turtle grass (Thalassia testudinum)
is the largest and most robust of the
south Florida seagrasses. Leaves are rib-
bon-like, typically 4 to 12mm wide with
rounded tips and are 10 to 35cm in length.
There are commonly two to five leaves per
short shoot. Rhizomes are typically 3 to
5 mm wide and may be found as deep as
25 cm (10 inches) in the sediment. Thalas-
sia forms extensive meadows throughout
most of its range.
Manatee grass (Syringodium f il i forme)
is the most unique of the local seagrass-
es, as the leaves are found in cross sec-
tion. There are commonly tv;o to four
leaves per shoot, and these are 1.0 to 1.5
mm in diameter. Length is highly vari-
able, but can exceed 50 cm (20 inches) in
some areas. The rhizome is less robust
than that of Thalassia and more surfici-
ally rooted. Syringodium is commonly
mixed with the other seagrasses, or in
small, dense, monospecific patches. It
rarely forms the extensive meadows like
Thalassia.
Shoal grass (Halodule wriqhti i ) is
extremely important as an early colonizer
of disturbed areas. It is found primarily
Table
Seanrasses of south Florida.
Family and species
Common name
Hydrocharitaceae
Thai as s_i_a tes tudinum Ko n i g
Halophila decipiens Os ten fold
Halophila ongelmanni Ascherson
Halop'hila johnsonii Eiseman
Potanoqetonacca
Turtle Grass
Syringodium f il i forme Kut7
Halodule wriqhti i Ascherson
Manatee grass
Shoal nrass
Halophila engelmonni
Halophila decipiens
Halodule wrightii
Syringodium filiforme
Thalassia testudinum
Figure ■1. Sedgrds-^es of south Florida,
in disturbed areas, and in areas where
Thalassia or Syringodium are excluded
because of the prevailing conditions.
Shoal grass grows connonly in water either
too shallow or too deep for these sea-
grasses. Leaves are flat, typically 1 to
3 mm wide and 10 to 20 cm long, and arise
from erect shoots. The tips of the leaves
are not rounded, but have two or three
points, an important recognition charac-
ter. Halodule is the most tolerant of all
the seagrasses to variations in tempera-
ture and salinity (Phillips 1960; r^lcMillan
and Moseley 1967). In low salinity areas,
care must be taken to avoid confusing it
with Ruppia.
Three species of Halophila, all small
and delicate, are sparsely distributed in
south Florida. Halophila engelmanni is
the most recognizable with a whorl of four
to eight oblong leaves 10 to 30 mm long
borne on the end of a stem 2 to 4 cm long.
This species has been recorded from as
deep as 90 m (295 ft) near the Dry Tortu-
gas. Halophila decipiens has paired
oblong-elliptic leaves 10 to 25mm long
and 3 to 5 mm wide arising directly from
the node of the rhizome. A new species.
\j_. Johnson i i , was described (Eiseman and
McMillan 1980) and could be easily confus-
ed with jH. decipiens. The most obvious
differences are that _H. johnsonii lacks
hairs entirely on the leaf surface and the
veins emerge from the midrib at 45° angles
instead of 60°. The initial description
recorded H^. johnsonii from Indian River to
Biscayne Bay, but its range could ulti-
mately be much wider.
The major problem in positive identi-
fication of seagrasses is between Halodule
and Puppia maritima, commonly known as
widgeongrass. Although typically found
alongside Halodule, primarily in areas of
reduced salinity, Puppia is not a true
seagrass, but rather a freshwater plant
that has a pronounced salinity tolerance.
It is an extremely important food for
waterfowl and is widely distributed.
Where it occurs, it functions similarly to
the seagrasses. In contrast with Halo-
dule, the leaves are expanded at the base
and arise alternately from the sheath, and
the leaf tips are tapered to a long point.
It should be noted, however, that leaf
tips are commonly missing from older
leaves of both species.
10
CHAPTER 2
AIJTECOLOGY OF SEAGRASSES
2,1 GROWTH
A renarkable sirilarity of vegetative
appearance, growth, and morphology exists
among the seagrasses (den Hartog 1970;
Zienan and Wetzel 1980). Of the local
species, turtle grass is the nost abun-
dant; its growth and Morphology provide
a typical scheme for seagrasses of the
area.
Tonlinson and Vargo (19GG) and Tom-
linson (1969a, 1969b, 1972) described in
detail the morphology and anatomy of tur-
tle grass. The round-tipped, strap-like
leaves emanate from vertical short shoots
which branch laterally from the horizontal
rhizomes at regular intervals. Turtle
grass rhizomes are buried in 1 to 25 cm
(0,4 to 10 inches) of sediment, although
they usually occur 3 to 10 cm (1 to 4
inches) below the sediment. In contrast,
rhizomes of shoal grass and Halophila are
near the surface and often exposed, while
manatee grass rhizomes are most typically
found at an intermediate depth. Turtle
grass roots originate at the rhizomes or
less frequently at the short shoots. They
are much smaller in cross section than the
rhizomes, and their length varies with
sediment type, organic matter, and depth
to bedrock.
Cn a turtle grass short shoot, new
leaves grow on alternating sides from a
central neristem which is enclosed by old
leaf sheaths. Short shoots typically
carry two to five leaves at a time; in
south Florida, Zieman (1975b) found an
average of 3.3 leaves per shoot in the
less productive inshore areas of Biscayne
Bay, and 3.7 leaves per shoot at stations
in the denser grass beds east of the Flor-
ida Keys. Short shoots in areas exposed
to heavy v/aves or currents tend to have
fewer leaves.
The growtfi of individual leaves of
turtle grass in Biscayne Pay averages 2.5
mm/day, increasing with leaf width and
robustness. Rates of up to 1 cm/day were
observed for a 15- to 20-day period (Zie-
man 1975b). Leaf growth decreased exponen-
tially with aoe of the leaf (Patrieuin
1973; Zieman lfi75b).
Leaf width increases with short shoot
age and thus with distance from the rhi-
zome I'leristem, reaching the community max-
imum 5 to 7 short shoots back from the
growing tip (Figure 5). The short shoot
has an average life of 2 years (Patriquin
1975) and may reach a length of 10 cn
(Tomlinson and Vargo 1966). A nnv short
shoot first puts out a few small, tap ""red
leaves about 2 cm wide before producing
the regular leaves. New leaves are produc-
ed throughout the year at an average rate
of one new leaf per short shoot every 14
to 16 days, and times as short as 10 days
have been reported. In south Florida the
rate of leaf production depended on temp-
erature, with a rate decrease in the cool-
er winter months (Zieman 1975b). The rate
of leaf production varies less throughout
the year in the tropical waters of Barba-
dos and Jamaica, according to Patriquin
(1973) and Greenway (1974), respectively.
2,2 REPRODUCTIVE STRATEGIES
Seagrasses reproduce vegetatively and
sexually, but the information on sexual
11
8.5
AVERAGE LEAF WIDTH
7.5
LEAF
RHIZOME MERISTEM
BRANCH OR
SHORT SHOOT
RHIZOME
DISTANCE BETWEEN BRANCHES
Figure 5. Diagram of a typical Thalassia
width on the older, vertical short shoots.
reproduction of the south Florida sea-
grasses is sketchy at best. The greatest
amount of information exists for turtle
grass, because of the extensive beds and
because the fruit and seeds are relatively
large and easily identified for seagrass-
es. In south Florida buds develop in Jan-
uary (Moffler et al . 1981); flowers, from
mid-April until August or September (Or-
purt and Boral 1964; Grey and Moffler
1978). In a study of plant parameters in
permanently marked quadrats, Zieman noted
that at Biscayne Bay stations flowers ap-
peared during the third week in May and
fruits appeared from 2 to 4 weeks later.
The fruits persisted until the third week
of July, when they detached and floated
away.
2.3 TEMPERATURE
One of the first mental images to
be conjured up when considering the trop-
ics is that of warm, clear, calm water,
abounding with fish and corals. This image
shoot. Note increasing blade length and
is only partially correct. Tropical
oceanic water in the Caribbean is typi-
cally 26° to 30°C (79° to 86°F), and feels
cooler than one would at first suspect.
In the past, lack of familiarity with
tropical organisms led many otherwise cap-
able scientists to view the tropics and
subtropics as simply warmer versions of
the temperate zone. Compared with their
temperate counterparts, tropical organisms
do not have greatly enhanced thermal tol-
erances; the upper thermal limit of tropi-
cal organisms is generally no greater than
that of organisms from warm temperate re-
gions (Zieman 1975a). In tropical waters,
the range of temperature tolerance is low,
often only half that of organisms from
equivalent temperate waters (Moore 1963a),
This is reflected in the seasonal r9nge of
the surrounding waters. At 40° north lat-
itude, the seasonal temperature range of
oceanic surface water is approximately
10°C (50°F), while at 20° north, the range
is only 3°C, reaching a low of only 1°C
(33.8°F) at about 5° north. However, be-
cause of the extensive winter cooling and
12
summer heating of shallow coastal water,
Moore (1963a) found that the ratio of mean
temperature range (30° to 50° N) to mean
tropical range (20° N to 20° S) to be
2.5:1 for oceanic waters, but increased to
4.2:1 for shallow coastal waters.
Because of thermal tolerance reduc-
tion in the tropics, the biological result
is a loss of cold tolerance; that is, the
range of thermal tolerance of tropical
organisms is about half that of temperate
counterparts, whereas the upper tolerance
limit is similar (Zieman and Wood 1975).
Turtle grass thrives best in tempera-
tures of 20° to 30°C (68° to 86°F) in
south Florida (Phillips 1960). Zieman
(1975a, 1975b) found that the optimum
temperature for net photosynthesis of
turtle grass in Biscayne Bay was 28° to
30°C (82° to 86°F) and that growth rates
declined sharply on either side of this
range (Figure 6). Turtle grass can toler-
ate short term emersion in high tempera-
tures (33° to 35°C or 91° to 95°F), but
growth rapidly falls off if these tempera-
tures are sustained (Zieman 1975a, 1975b).
In a study of the ecology of tidal
flats in Puerto Rico, Glynn (1968) observ-
ed that the leaves of turtle grass were
killed by temperatures of 35° to 40°C (95°
to 104°F), but that the rhizomes of the
plants were apparently unaffected. On
shallow banks and grass plots, tempera-
tures rise rapidly during low spring
tides; high temperatures, coupled with
desiccation, kill vast quantities of
leaves that are later sloughed off. The
process occurs sporadically throughout the
year and seems to pose no long-term prob-
lem for the plants. Wood and Zieman (1969)
warn, however, that prolonged heating of
substrate could destroy the root and rhi-
zome system. In this case, recovery could
take several years even if the stress were
removed.
The most severe mortalities of organ-
isms in the waters of south Florida are
usually caused by severe cold rather than
heat, as extreme cold water temperatures
are more irregular and much wider spaced
phenomena than extreme high temperatures.
McMillan (1979) tested the chill tolerance
of populations of turtle grass, manatee
10
•
•
8
>-
*
< ^
Q 6
.
c^
.
%
•
\
•
*
5 4
• ■
O
•
• • • . .
2
•
• . • • . . .
. • • • •
• ' •;•■ . •;.'•..■••. •.'
'• • *'•
. • ' *••'•*. '
• • .".•.,
■'■*..•
•
20 25 30
TEMPERATURE
35
Figure 6. Response of Thalassia production to temperature in south Florida,
13
grass, and shoal grass in various loca-
tions from Texas to St. Croix and Jamaica,
Populations from south Florida were inter-
mediate in tolerance between plants from
Texas and the northern Florida coast
and those from St. Croix and Jamaica in
the Caribbean. In south Florida, the
most chill -tolerant plants were from the
shallow bays, while the populations that
were least tolerant of cold temperatures
were from coral reef areas, where less
fluctuation and greater buffering would be
expected. During winter, the cold north-
ern winds quickly cool off the shallow
(0.3 to 1 m or 1 to 3.3 ft) waters of
Florida Pay. The deeper waters, however,
in the area below the Keys and the reef
line (up to 15 m or 50 ft) not only have a
much greater mass to be cooled, hut are
also flushed daily with warmer Gulf Stream
water which further tends to buffer the
environmental fluctuations.
The amount of direct evidence for the
temperature ranges of shoal grass and man-
atee grass is far less than for turtle
grass." Phillips (1960) suggested that
shoal grass Generally prefers temperatures
of 20 "to 30°C (68° to 86°F), but that it
is somevvhat more eurythermal than turtle
grass. This fits its ecological role as a
pioneer or colonizing species. Shoal grass
is commonly found in shallower water than
either turtle grass or manatee grass,
where thermal variation would tend to be
greater, ^''c^'illan (1979) found that shoal
grass had a greater chill tolerance than
turtle grass, while manatee grass showed
less resistance to chilling.
embayments with restricted circulation,
such as southwest Biscayne Bay, many
algal species are reduced during summer
high temperatures and some of the more
sensitive types such as Caulerpa, CI adop-
hora and Laurencia may be killed (Zieman
1975a).
2.4 SALINITY
While all of the common south Florida
seagrasses can tolerate considerable sa-
linity fluctuations, all have an optimum
range near, or just below, the concentra-
tion of oceanic water. The dominant sea-
grass, turtle grass, can survive in salin-
ities from 3.5 ppt (Sculthorpe 1967) to 60
ppt (McMillan and Moseley 1957), but can
tolerate these extremes for only short
periods. Even then, severe leaf loss is
common; turtle grass lost leaves when
salinity was reduced below 20 ppt (den
HartOQ 1970). The optimum salinity for
turtle grass ranges from 24 ppt to 35 ppt
(Phillips 1960; McMillan and Moseley 1^67;
Zieman 1975b). Turtle grass showed maximum
photosynthetic activity in full-strength
seawater and a linear decrease in activity
with decreasing salinity (Hammer 196ob).
At 5Q7o strength seawater, the photosynthe-
tic rate was only one-third of that in
full-strength seav>'ater. Following the
passage of a hurricane in south Florida in
1960, Thomas et al . (1961) considered the
damage to the turtle grass by freshwater
runoff to have been more severe than the
physical effects of the high winds and
water surne.
Seagrasses are partially buffered
from temperature extremes in the overlying
water because of the sediinents covering
the roots and rhizomes. Sediments are
poorer conductors of heat than seawater
and they absorb heat more slowly. In a
study by Redfield (1965), changes in the
tei:;perdture of the water column decrease
exponentially with depth in sediments.
The tolerance of local seagrass spe-
cies to salinity variation is similar to
their temperature tolerances. Shoal nrass
is the most broadly euryhaline, turtle
grass is intermediate, and manatee grass
and Halcphila have the narrowest tolerance
ranges, with Halophila being even more
stenohaline than manatee arass (McMillan
1979).
Macroalgao associated with grass bods
exist totally in the water column, and
thus will be affected at a rate that is
dependent upon their individual temper-
ature tolerances. Most algae associated
with tropical seagrass beds are more
sensitive to thermal stress than the
seagrasses (Zieman l'^75a). In shallow
2.5 SEDIMENTS
Seagrasses qrow in a wide variety of
sediments from fine muds to coarse sands,
depending on the type of source material,
the prevailing physical flow regime, and
the density of the seagrass blades. As
14
rooted plants, seagrasses require a suf-
ficient depth of sediment for proper
development. The sediment anchors the
plant against the effects of water surge
and currents, and provides the matrix for
regeneration and nutrient supply. Run-
ners occasionally adhere directly to a
rock surface, with only a thin veneer of
sediment surrounding the roots, but this
happens sporadically and is quantitatively
insignificant. The single most important
sediment characteristic for seagrass
growth and development is sufficient sedi-
ment depth.
Depth requirements also vary with the
different species. Because of its shal-
low, surficial root system, shoal grass
can colonize thin sediments in an area of
minimal hydraulic stability (Fonseca
et al . 1981). Turtle grass is more robust,
requiring 50 cm (20 inches) of sediment to
achieve lush growth, although meadow for-
mation can begin with a lesser sediment
depth (Zieman 1972). In the Bahamas,
Scoffin (1970) found that turtle grass did
not appear until sediment depth reached at
least 7 cm (3 inches) .
The density of turtle grass leaves
greatly affected the concentration of
fine-grained (less than 63u) particles in
sediments. Compared with bare sediment
which showed only 1% to 3°^ fine-grained
material, sparse to medium densities of
turtle grass increased the fine percentage
from 3% to 6% and dense turtle grass
increased this further to over 15%.
The primary effects of the grass
blades are the increasing of sedimentation
rates in the beds; the concentrating of
the finer-sized particles, both inorganic
and organic; and the stabilizing of the
deposited sediments (Fonseca, in press a,
b; Kenworthy 1981). Burrell and Schubel
(1977) described three effects produced by
these mechanisms:
(1) Direct and indirect extraction
and entrapment of fine water-
borne particles by the seagrass
loaves.
(2) Formation and retention of par-
ticles produced within the grass
beds.
(3) Binding and stabilizing of the
substrate by the seagrass root
and rhizome system.
One of the values of the seagrass
system is the ability to create a rela-
tively low energy environment in regions
of higher energy and turbulence. In addi-
tion to the fine particle extraction due
to decreased turbulence, the leaves trap
and consolidate particles of passing sedi-
ment which adhere to the leaf surface or
become enmeshed in the tangle of epiphytes
of older leaves. As the older portion of
the leaves fragment, or as the leaves die
and fall to the sediment surface, the or-
ganic portions of the leaves decay and the
inorganic particles become part of the
sediment. The continued presence of the
growing leaves reduces the water velocity
and increases the retention of these
particles, yielding a net increase in
sediment.
Key elements in a plant's efficiency
of sediment stabilization are plant spe-
cies and density of leaves. From observa-
tional data in Bermuda, researchers found
open sand areas had 0.17, to 0.2« fine par-
ticles (less than 63p). In manatee grass
beds this increased to 1.9/o fines, while
turtle grass beds had a.?% to 5.^% fine
material (Wood et al . 1969). In the same
study organic matter (% dry weight) was
2.57, to 2.6% in open sand areas with simi-
lar values in manatee grass beds; the
organic matter in turtle grass beds was
3.5% to 4.9%, demonstrating the increased
stabilization and retention pov/er of the
more robust turtle grass.
Seagrasses not only affect mean grain
size of particles, but other geologically
important parameters such as sorting,
skewness, and shape (Rurrell and Schubel
1977). Swinchatt (1965) found that the
mean size of sand fraction particles, the
relative abundance of fines, and the stan-
dard dimension all increased with an
increase in blade density near a Florida
reef tract. The nuantitative effect of
the trapping and bonding was discussed in
several studies (Ginsberg and Lov/enstam
1958; Wood et al . 1969; Fonseca in press
a, b) and is shown graphically in Figure 7
(Zieman 1972).
15
Sediment
Elevation
(cm)
Leaf
Density
(leaves /
iOOcm^)
Leaf
Length
(cm)
Sediment
Depth
(cm)
Distance Across Bed (nn)
Figure 7. Response of a Thalassia bed to increasing sediment depth. Note increasing
blade length and density with increasing depth of sediment. The increase in elevation
in the center of the bed is due to the trapping action of the denser blades.
Particles of carbonate are locally
produced in seagrass beds and removed from
the surrounding water. Older leaves are
usually colonized by encrusting coralline
algae such as Melobesia or Fosliella. It
has been estimated that these encrusting
algae produce from 40 to 180 g/m/yr of
calcium carbonate sediment in Jamaica
(Land 1970) and upwards to 2,800 g/m-^/yr
in Barbados (Patriquin 1972a).
The high production of seagrasses can
affect the production of inorganic partic-
ulates also. Cloud (1962) estimated that
75% of aragonitic mud in a region of the
Barbados was due to direct precipitation
of carbonate when the seagrasses had
removed CO from the water during periods
of extremely high primary productivity.
Zieman (1975b) also noted the ability
of seagrasses under calm conditions to
overcome the carbonate buffer capacity of
seawater and drive the pH up to 9.4.
The microbial ly mediated chemical
processes in marine sediments provide a
major source of nutrients for seagrass
growth (Capone and Taylor 1980). Bacte-
rial processes convert organic nitrogen
compounds to ammonia (Capone and Taylor
1980; Smith et al . 1981b), primarily in
the anoxic sediment which usually exists
only a few millimeters beneath the sedi-
ment surface. The ammonia that is not
rapidly utilized will diffuse upward to
the aerobic zone where it can either
escape to the water column or be converted
to nitrate by nitrifying bacteria in the
presence of oxygen. Endobacteria were
found in the roots of the seagrass Zostera
marina (Smith et al . 1981a), and were
associated with nitrogen fixation (Smith
16
et al . 1081b). The amount of nitrate is
usually low or absent in sediments as it
is either rapidly .Tictabol ized or converted
to dinitronen (N ) via denitrifying bac-
teria.
Sulfur bacteria are primarily respon-
sible for maintaining conditions necessary
for the remineral ization of nutrients in
the sediment. By reducing sulfate to sul-
fide, these bacteria maintain the environ-
mental conditions (Eh and pH) at a level
whore the nitrogen mineralization proceeds
at a rate greater than its utilization by
the microbial community. This produces
the available nutrient fractions.
2.6 CURRENT VELOCITY
Little work has been done to deter-
mine the response of seagrass communities
to different current velocities (Fonseca
et al . in press a, b). Seagrass production
and bionass are strongly influenced by
current velocity (Conover 1968). Roth
turtle grass and Zostera showed naximun
standing crops where current velocities
averaged 0.5 m/sec. In south Florida the
densest stands of turtle grass and manatee
grass v.'i th bright, long leaves are observ-
ed in the tidal channels separating the
mangrove islands. Inferential evidence
suggests that the rapid currents break
down diffusion gradients and make more COj
and inorganic nutrients available to the
plants (Conover 1'^6g). In a cruise of
the Alpha Helix to Nicaragua in 1977, sam-
ples taken from a mangrove-lined tidal
channel showed a leaf standing crop of
2G2 q dry weight (dw)/m- and a total bio-
mass of 4,570 gdw/m'. By comparison, sam-
ples from a quiescent lagoon environment
were 185 and 1,033 g/m (McRoy, Zieman and
Ogden, personal communication).
Where currents are strong and persis-
tent, crescentic features known as blow-
outs are often formed. These are cusp-
shaped holes that actually migrate through
grassbeds in the directions of the main
current flow, eroding at one edge and col-
onizing at the other. Their significance
is discussed in the section on succession.
2.7 OKYGEN
Most seagrass meadows have sufficient
oxygen in the water column for survival of
the associated plants and animals. Often
the shallow beds can bo heard to hiss from
the escaping 0 , bubbles in the late after-
noon. Dense beds in shallow water with
restricted circulation can show extremely
reduced 0-^ levels or even anoxia late at
night on a slack tide. This can be a
greater problem if there is a heavy load
of suspended organic sediment that would
also consume oxygen. Generally the wind
required to generate the turbulence neces-
sary to suspend large quantities of sedi-
ment offsets this effect by aerating the
v/ater.
Low Oj levels can also slow plant
respiration; internal concentrations of 0,
decrease rapidly and CO. increases. Respi-
ration then is limited by the ability of
oxygen to diffuse from the water. Plants,
however, are less affected by low oxygen
levels than animals. Although Kikuchi
(1980) recorded a marked decrease in oxy-
gen in Japanese Zostera beds coincident
with blade die-off and increased microbial
activity, apparently it was not lethal.
Productivity studies in Puerto Pico (Odum
et al . 1960), Florida and Texas (Odum and
Wilson 1962) showed nighttime oxygen val-
ues that were typically 4 to 7 mg 0/1;
the lowest reported value of 2 to 3 mg
0, /I occurred on a calm, extremely low
tide in August.
2.8 SOLAR RADIATION
When one considers the overriding
importance of, solar energy as the main
forcing function on any ecosystem, it is
amazing how infrequently values are re-
ported in the scientific literature. His-
torically there has been a consensus (even
without adequate measurement) that sea-
grasses require high light intensity for
photosynthesis (Zieman and Wetzel 1980).
This is based on the observation that ex-
tensive seagrass beds are not found deeper
than 10 m (33 ft). These observations are
complicated by evidence that there is also
17
indication of a limitation on productivity
due to hydrostatic pressure and not nerely
light limitation (Gessner and Haniner
1961).
The naxiinun depth at which seagrasses
are found is definitely correlated with
the available light reqine, provided that
suitable sediments are available. Off the
northwest coast of Cuba, Buesa (1975) re-
ported maximum depths for tropical sea
grasses as follows: turtle grass, 14 n
(46 ft); manatee grass, 16.5 m (54 ft);
Halophil ia decipiens, 24.3 m (80 ft); and
H_. englemanni, 14.4 m (47 ft). As plant
species grow deeper, the quality and quan-
tity of light changes. In clear tropical
water such as that near St. Croix, Cuba,
and portions of southern waters, the light
is relatively enriched in blue wavelengths
with depth. By comparison, in highly tur-
bid conditions as in shallow bays in Texas
and in Florida Bay, blue light is scat-
tered and the enrichment is in the direc-
tion of the green wavelengths. In both
clear and turbid v;aters the longer red
wavelengths are absorbed in the first few
meters of the water column.
Buesa (1975) studied the effects of
specific wavelengths on photosynthesis of
turtle grass and manatee grass in Cuba.
He found that turtle grass responded best
to the red portion of the spectrum (620
nanometers); the blue portion (400 nanome-
ters) was better for manatee grass.
2.9 ZONATION
Although seagrasses have been re-
corded from as deep as 42 m (138 ft), ex-
tensive development of seagrass beds is
confined to depths of 10 to 15 m (33 to 49
ft) or less. Principal factors determin-
ing seagrass distribution are light and
pressure at depth, and exposure at the
shallow end of the gradient. A general
pattern of seagrass distribution in clear
waters of south Florida and the Caribbean
was presented by Ferguson et al . (1980).
Shoal grass usually grows in the shallow-
est water and tolerates exposure better
than other species. The relatively high
flexibility of its leaves allows it to
conform to the damp sediment surface dur-
ing periods of exposure, thus minimizing
the leaf surfaces available for desicca-
tion. Turtle grass grows in waters nearly
as shallow as that of shoal grass. The
shallowest turtle grass flats are commonly
exposed on spring low tides, frequently
with much leaf mortality. Throuqhout the
range of 1 to 10 m (3 to 33 ft), all of
the species may be found, singly or mixed.
Turtle grass is the unquestionable domi-
nant in most areas, however, freouently
forming extensive meadows that stretch for
tens of kilometers. Although the absolute
depth limit of the species is deeper,
iiature meadows of turtle qrass are not
found belo*-,' 10 to 12 m (33 to 39 ft). At
this depth manatee grass replaces turtle
grass and forms meadows down to 15 m (50
ft). Past the maximum depth for manatee
grass development, shoal grass will often
occur, but it rarely develops extensively.
Past the point at which the major species
occur, fine carpets of Halophil a extend
deeper than 40 m (130 ft).
Numerous studies confirmed the pat-
tern described above, or some portion of
it. The relative abundance of four spe-
cies of seagrasses off northv/est Cuba, is
graphed in Figure 8 (Buesa 1974, 1975).
Halophil a decipiens was the least abundant
with a mean density of 0.14 q/m-. Halop-
hil a engelmanni showed a mean density of
0.25 g/m-. F'anatee grass was nearly 10
times denser than Halophil a with an aver-
age density of 3.5 c/m- down to 16.5 r (54
ft). Turtle grass was the most abundant
seagrass, accounting for nearly 97.57 of
the total seagrass biomass, with an aver-
age of 190 g/i-r down to its maximum depth
of 14 m (46 ft). This area is unique in
that there is little or no shoal grass
which normally is either the second or
third most abundant species in a region.
In St. Croix, turtle grass had the
shallowest range, occurring down to 12 m
(39 ft) on the west side of Buck Island
(Wiginton and Mcf'illen 1979). Shoal grass
and manatee grass showed progressively
greater depth, occurring to IS m (59 ft)
and 20 m (65 ft), respectively, while
Halophila decipiens occurred to 42 m (138
ft) . All the species were found in less
than 1 m (3.3 ft) of water in St. Croix.
Because of the variety of rocky and
sedimentary patterns in the lagoons and
18
M 15
Figure 8. Depth distribution of four seagrasses on the northwest coast of Cuba. 1 =
Thalassia testudinuin, 2 = Syringodiun f i1 i forme, 3 = Halophila dec i pi ens, 4 = h[. engel-
manni (from Busea 1975). Although Syringodium is quite abundant in certain localities,
note the preponderance of Thalassia biomass and the absence of Halodule on the Cuban
coast.
bays of south Florida, the turbidity and
therefore the maximum depth for rooted
plants can vary over short distances.
Phillips (1960) recorded turtle grass
ranging from 10 to 13 m (33 to 43 ft) in
depth. In the relatively clear waters of
the back reef areas behind the Florida
Keys, turtle grass is common to 6 or 7 m
(20 or 23 ft) and occurs down to 10 m (33
ft); by contrast, in the relatively turbid
portion of the "lakes" of Florida Bay,
maximum depths of only 2 m (7 ft) are
common.
2.10 EXPOSURE
The seagrasses of south Florida are
all subtidal plants that do not tolerate
exposure well. Exposed leaf surfaces will
lose water constantly until dry, and there
is no constraint to water loss that would
limit drying (Gessner 1968). Although
exposure to the air definitely occurs at
certain low tides on shallow turtle grass
or shoal grass flats, unless it is
extremely brief, the exposed leaf surfaces
will be killed.
Following exposure, the dead leaves
are commonly lost from the plant. Rafts
of dead seagrass leaves may be carried
from the shallow flats following the
spring low tides. Normally the rhizomes
are not damaged and the plants continue to
produce new leaves.
19
CHAPTER 3
PRODUCTION ECOLOGY
The densities of seagrasses can vary
widely; under optinum conditions they form
vast meadows. The literature is becoming
extensive and often bewildering as density
values have been reported in many forms.
For consistency, the terms used here con-
form to those of Zieman and Wetzel
(1980): standing crop refers to above-
ground (above-sediment) material, whereas
biomass refers to the weight of all living
plant material, including roots and rhi-
zomes. Both quantities should be expressed
in terms of mass per unit area. These
measurements both have valid uses, but it
is sometimes difficult to determine which
an author is referring to, because of in-
complete or imprecise descriptions. His-
torically, standing crop has been the pri-
mary measure of comparison because of the
relative ease of sampling compared with
the laborious methods needed to collect
and then sort belowground material.
3.1 BIOMASS
Seagrass biomass varies widely de-
pending on the species involved and the
local conditions. The biomass of the spe-
cies Halophila is always small, whereas
turtle grass Ras been recorded at densi-
ties exceeding 8 kg dry weight/m (Bauers-
feld et al . 1969). Representative ranges
of seagrass biomass in south Florida and
in neighboring regions are given for com-
parison in Table 3. Because of the ex-
treme variations found in nature and re-
flected in the literature, one must be
careful not to place too much value on a
few measurements. Many of these studies
have been summarized by McRoy and McMillan
(1977) and Zieman and Wetzel (1980). Be-
cause these studies represent a variety of
habitats, different sampling times and
seasons, wide variation in sample repli-
cates (if any), as well as the diverse
reasons for which the investigators col-
lected the samples, it becomes difficult
to draw meaningful patterns from these
published results.
While the standing crop of leaves is
significant, the majority of the biomass
of seagrasses is in the sediments, especi-
ally for the larger species. Although the
relative amounts vary, turtle grass typi-
cally has about 15% to 22% of its biomass
in emergent leaves and the rest below the
sediment surface as roots and rhizomes.
The published ranges for turtle grass,
however, vary from 10% to 45/' for leaf
biomass (Zieman 1975h). In central Bis-
cayne Bay, Jones (1968) found a relatively
consistent ratio of 3:2:2 for leaves and
short shoots; rhizomes: roots. Studies
with turtle grass and Zostera have indi-
cated that the ratio of leaves to roots
increased with a shift in substrate from
course sand substrates to fine muds (Ken-
worthy 1981). This can be interpreted to
indicate either the positive effect of the
richer fine muds on more robust plant de-
velopment, or the need for a better devel-
oped nutrient absorptive (root) network in
the coarser sediments that tend to be low-
er in nutrients and organic matter. Thus,
substrate may be an important variable
when determining phonological indices.
Structurally, turtle grass has the
most wel 1 -developed root and rhizome sys-
tem of all the local seaorasses. Table 4
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lists conparativG biomass values from sev-
eral stations in Pine Channel in the Flor-
ida Keys v/here the three major species co-
exist. Shoal grass and manatee grass have
less wel 1 -developed root and rhizome sys-
tems and consequently will generally have
much more of their total biomass in leaves
than does turtle grass. Samples for these
two species where the leaf component is
50% to 60% of total weight are not uncom-
mon. Maximum values for the species also
vary widely. Biomass measurements for
dense stands of shoal grass are typically
several hundred grams per square meter;
manatee grass reaches maximum development
at 1,200 to 1,500 g/m- , while maximum val-
ues for turtle grass are over 8,000 g/m .
3.
PRODUCriVITY
Seagrasses have the potential for
extremely high primary productivity. Re-
corded values for seagrass productivity
vary enormously depending on species, den-
sity, season, and measurement techniques.
Most studies use turtle grass with only a
few scattered values for shoal grass and
manatee grass.
For south Florida, turtle grass pro-
ductivity values of 0.9 to 15 g C/m^/day
have been reported (Table 5). The highest
reported values (e.g. Odum 1963) represent
community metabolism and reflect the pro-
ducts of the seagrasses, epiphytic algae,
and benthic algae. Measurements of sea-
grass production indicate that the net
aboveground production is commonly 1 to
4 g C/m /day, although the maximum rates
can be several times these values (Zieman
and Wetzel 1980). The importance of the
high sustained level of production of sea-
grasses is especially apparent when com-
pared with the production values of the
contiguous offshore waters.
3.3 PRODUCTIVITY MEASUREMENT
From, the earliest seagrass studies,
researchers have continually noted the
high productivity of seagrasses, and their
ultimate value as food for trophically
higher organisms. As a result, much study
has been devoted to methods for determin-
ing the productivity of seagrass beds.
Three basic methods have been used to
study seagrass productivity: marking,
''■^C, and 0: production. (See Zieman and
V'etzel 1980 for a recent review of produc-
tivity measurement techniques.)
Many assumptions dre made when using
the oxygen production method, and all can
lead to large and variable errors, pri-
marily because leaves of aquatic vascular
plants can store gases produced during
photosynthesis for an indefinite period.
The largest potential error, however, is
related to the storage of metabol ical ly
produced oxygen. To use the oxygen produc-
tion technique, one assumes that oxygen
produced in photosynthesis diffuses rap-
idly into the surrounding water where it
can be readily measured. With seagrasses,
as with other submerged macrophytes, how-
ever, this gas cannot diffuse outward at
the rate at which it is produced and so it
accumulates in the interstitial lacunae of
the leaves (Hartman and Brown 1966). Re-
cent work with freshwater macrophytes has
suggested that under well-stirred condi-
tions only a short period is required for
equilibration (V.'estlake 1978; Kelly et al .
1980); however, this has not been verified
for seagrasses. As the gas accumulates,
seagrass leaves swell up to 2507' of their
original volume (Zieman 1975b). Some of
the oxygen produced is used metabol ically,
while the remainder either diffuses out
slowly or, if production is sufficient,
will burst from the leaves in a stream of
bubhles.
f''easurement of seagrass productivity
by radioactive carbon uptake has the ad-
vantage of high sensitivity, brief incuba-
tion periods, and the ability to partition
out the productivity associated with the
different morphological parts of the
plants as well as productivity of the
attendant epiphytes and macroalgae. Al-
though this measurement technique requires
sophisticated and expensive laboratory and
field equipment, and mav have errors asso-
ciated with CO storage, it apparently
yields a value near to net productivity
and produces values comparable to mark and
recovery techniques. The application of
the I'^C technique to seagrasses is dis-
cussed in detail by Penhale (1975), Bit-
taker and Iverson (1"76), and Capone
et al. (1979).
22
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24
Net production ineasurernents for most
seaqrasses can be obtained by marking
blades and measuring their grov/th over
time (Zieman 1974, 1975b). With this
method, the blades in a quadrat are marked
at their base, allowed to grow for several
weeks, and then harvested. As seagrass
leaves have basal growth, the increment
added below the marking plus the newly
emergent leaves represent the net above-
ground production. After collection, the
leaves of most tropical species must be
gently acidified to remove adhered carbon-
ates before drying and weighing.
Bittaker and Iverson (1976) critical-
ly compared the marking method with the
measurement of productivity by radioactive
carbon uptake. When the ^"^C method was
corrected for inorganic losses (13°'),
incubation chamber light energy absorption
(14?.), and difference in light energy re-
sulting from experimental design {8%), the
differences in productivity wore insignif-
icant. These results reinforce the concept
that the i"C method measures a rate near
net productivity. In a study of turtle
grass productivity near Bimini, however,
Capone et al . (1979) found that the ^''C
measurements yielded values nearly double
that of the marking methods.
A method developed by Patriquin
(1973) uses statistical estimates based on
the length and width of the longest 5%
of the leaf population of a given area.
Capone et al . (1979) used this method; it
agreed +/-15': with the staple marking
method. Indications arc that this method
is very useful for a first order estimate,
but more comparative studies are still
needed.
Some form of oxygen measurement v/as
used to attain the highest production
values recorded in the literature for tur-
tle grass and Zostera. Recently Kemp
et al . (1981) surveyed numerous productiv-
ity measurements from the literature and
confirmed that for seagrasses and several
freshwater nacrophytes, the oxygon method
shov/ed highest productivity values; mark-
ing methods, the lowest; and I'+C values
Mere intermediate. Although those compar-
isons required numerous assumptions, the
results show the need for further study.
The marking method probably gives the
least ambiguous answers, showing net
aboveground production quite accurately.
It underestimates net productivity as it
does not account for belowqround produc-
tion, excreted carbon, or herbivory. Mod-
ifications of the marking method for
Zostera marina have been used to estimate
root and rhizome production (Sand-Jensen
1975; Jacobs 1979; Kenworthy 1981) and
could be adopted for tropical seagrasses.
The generalization that emerges from these
various diverse studies is that seagrass
systems are highly productive, no matter
what method is used for measurement, and
under optimum growth conditions production
can be enormous.
3.4 NUTRIENT SUPPLY
Seagrasses along with the rhizophytic
green algae are unique in the marine envi-
ronment because they inhabit both the wa-
ter column and the sediments. There was
previously much controversy whether the
seagrasses took up nutrients through their
roots or their leaves. McRoy and Barsdate
(1970) showed that Zostera was capable of
absorbing nutrients either with the leaves
or roots. McRoy and Barsdate found that
Zostera could take up ammonia and phos-
phate from the sediments through their
roots, translocate the nutrients, and pump
them out the leaves into the surrounding
water. This process could profoundly
affect the productivity of nutrient-poor
waters.
Sediment depth directly affects sea-
grass development (Figure 7). The implica-
tion is that the deeper sediment is re-
quired to allow sufficient root develop-
ment which would in turn increase the
nutrient absorptive capabilities of the
roots. Thus to sustain growth, the plants
would need greater nutrient absorptive
tissue in sediments that contained less
nutrients. While studying turtle grass
in Puerto Rico, Burkholder et al . (1959)
found a change in the leaf to root and
rhizome ratios of the plants as the sed-
iment type changed. The ratio of leaf
to root and rhizome of turtle grass was
1:3 in fine mud, 1:5 in mud, and 1:7 in
coarse sand. Kenworthy (l^Rl) noted a
similar change in Zostera in North Caro-
lina. The plants from sandy areas had
over twice the root tissue per unit leaf
tissue, possibly indicating the need for
25
more nutrient absorptive area or greater
anchoring capacity in the coarser sedi-
ments. Alternatively, the decrease in
root listeria] in fine sediments could
result from a negative effect fron anae-
robiasis or microbial metabolites.
Although seagrasses require a variety
of macro- and micronutrients for nutri-
tion, most research effort has been di-
rected to the source and rate of supply of
nitrogen. While phosphorous is in very
low concentration in tropical waters, it
is relatively abundant in the sediments,
and estimates on turnover time range from
one to two turnovers per year to once
every few years (M.cRoy et al . 1972; Patri-
quin 1972b). Nitrogen, however, is needed
in much greater quantities and its source
is more obscure (McRoy and McMillan 1977).
Patriquin (1972b) estimates that there was
only a 5- to 15-day supply of inorganic
nitrogen available in the sediments. This
estimate did not account for continuous
recycling, however.
Seagrasses have three potential ni-
trogen sources: recycled nitrogen in the
sediments, nitrogen in the water column,
and nitrogen fixation. Nitrogen fixation
can occur either in the rhizosphere or
phyllosphere. Transfers between leaf and
epiphyte have also been demonstrated (Har-
lin 1971; McRoy and Goering 1974), Capone
et al . (1979) concluded that nitrogen
fixed in the phyllosphere contributed pri-
marily to the epiphytic community while
fixation in the rhizosphere contributed
mainly to macrophyte production. Indi-
rectly the contribution of nitrogen-fixing
epiphytes is important because after the
leaves senesce and detach, most of them
decay and become part of the litter; some
will be incorporated in the sediments.
Other sources of nitrogen to the sediments
include excretion by plants and animals,
pjrticulate matter trapped by the dense
loaves, and dead root and rhizome mate-
rial. Capone and Taylor (198C) agreed
with Patriquin (1972b) that the primary
source of nitrogen for leaf production is
recycled material from sediments, but rhi-
zosphere fixation can supply 2C% to 50% of
the plant's requirements. Orth (1977a)
applied commercial fertilizers directly to
a Zostera bed in Chesapeake Bay. After 2
to 3 months the length and density of
leaves had increased, the amount of roots
and rhizomes was 30'? greater than the con-
trols, and the standing crop of loaves had
increased by a factor of three to four.
Seagrasses seem to he extremely efficient
at capturing and utilizina nutrients, and
this is a major factor in their ability to
maintain high productivity even in a rela-
tively low nutrient environment.
3.5 SEAGRASS PHYSIOLOGY
Seagrasses have evolved a physiology
that often distinguishes them from their
terrestrial counterparts. Since water has
rates of gaseous diffusion that are sev-
eral orders of magnitude lower than air,
much of this physiological modification is
a response to the lowered gas coricentra-
tion and the slower rates of diffusion
when compared with the terrestrial envi-
ronment. It is commonly thought that be-
cause of the abundance of inorganic carbon
in seawater in the carbonate buffer sys-
tem, marine plants are not carbon limited.
Turing active photosynthesis, however, in
shallow grass beds when tidal currents are
slow, the pM may rise from the normal sea-
water pM of 8.2 to 8.9, at which point the
free CO is greatly reduced in the water.
PH values of °.4, a point at which biocar-
bonate is hardly present, have been re-
corded over grass beds.
The internal structure of seagrasses
has been modified to minimize the problems
of life in an aquatic environment. Large
internal lacunal spaces have developed,
often comprising over 70% of the total
leaf volume, to facilitate internal gas
transport (Arber 1920; Sculthorpe 1967;
Zieman and Wetzel 1980). Much of the oxy-
gen produced in photosynthesis is appar-
ently retained in the lacunal system and
diffuses throughout the plant to the re-
gions of hiah respiratory demanci in the
roots and rhizomes. Similarly, because of
the general lack of stomata, the diffusion
of COq into the seagrasses is slow com-
pared with terrestrial counterparts. In
addition, the quiescent water layer next
to the leaves does not enhance diffusion
of gases.
At normal seawater pH, bicarbonate is
much more abundant than CO.. Beer et al .
(1977) showed that the major source of
carbon for photosynthesis for four species
26
of seagrassos vvas bicarbonate ion, which
could contribute to the calcium carhonate
flock frequently observed on seagrass
leaves (Zieran and Wetzel 1980). At normal
seawater pH, CO. concentrations were so
low that the high photosynthetic potential
uds linited by bicarbonate uptake (Beer
and Waisel 1979). Increasing the [iropor-
tion of CO; by lowering pH greatly in-
creased photosynthetic rates in Cymodocea
nodosa, a large seagrass with high poten-
tial production.
Much recent controversy has concerned
whether the nietabolic pathway of seagrass
photosyntliesis utilizes the conventional
Calvin cycle (called C3 as the initial
fixed sugars are 3 carbon chains) or the
C,, B-carhoxylative pathway. C^ plants
refix CO', efficiently and little respired
CO is lost in the light (Hough 1974;
Moffler et al . 1981). C^ plants are dif-
ficult to saturate with light and have
high temperature optimums. This photosyn-
thetic system vjould seem to be of benefit
in regions of high temperature and lioht
intensities, as well as marine waters
(Hatch et al . 1971). Seagrasses, hovjever,
are exposed to lower relative tempera-
tures, light levels, and oxygen concentra-
tions than are terrestrial counterparts;
and as the diffusion capacity of CO2 from
leaves is much slower, metabolic CO is
available for refixation regardless of the
photosynthetic pathway. After much lit-
erary controversy, recent evidence has
shown that most seagrasses, including tur-
tle grass, manatee grass, and shoal grass
are C3 plants (Andrews and Abel i979;
Benedict et al . 1980).
What makes the photosynthetic pathway
0^ interest to those other than the plant
physiologist is that during photosynthesis
plants do not use the ^*'C and "' ^'C isotopes
in the ratios found in nature, but tend to
differentiate in favor of the ^^C isotope
which is lighter and more mobile. All
plants and photosynthetic cycles are not
alike, hov;ever, and those using the con-
ventional C. Calvin cycle are relatively
poor in the ^ "C isotope, while C^ plants
have high ratios of I'C/^'^C. The ratios
of i3c/i C (called 6 1'C or del i-C) gener-
ally varies between -24 to -36 ppt for C4
plants (Bender 1971). Seagrasses have rel-
atively high^^^C values. McMillan et al .
(1980) surveyed 47 species of seagrasses
fro;i 1? genera and found that 45 species
were within the range of -3 to -19 ppt,
with only two species of Halophila being
lower. The mean values and range for the
local species are shown in Table 6. Turtle
grass shows a mean value of -10.4 ppt and
a total range from -8.3 to -12.5. This
va'^iation included samples from Florida,
Texas, the Virgin Islands, and Mexico.
The inean values and ranges for shoal grass
and Halophila from the Gulf of ^'exico and
Caribbean are also very similar with mean
values ranging from -10.2 to -12.6 ppt,
respectively. Manatee grass is the only
local seagrass of significantly different
value with a more dilated mean of -5 ppt
and a range of -3.0 to -9.5 ppt. In
general, tropical species had higher f^^^
values than species from temperate re-
gions. There also appears to be little
seasonal difference in ,-,-^'C values, at
least for Zostcra I'arina (Thayer et al .
1978a).
The.s^^C ratio has attracted much at-
tention recently because of its utility as
a natural food chain tracer (Fry and Park-
er 1979). The seagrasses possess a unioue
iS^t ratio for marine plants, and thus or-
ganisms that consume significant portions
of seagrass in their diet will reflect
this reduced ratio. The carbon in animals
has been shown to be generally isotopical-
ly similar to the carbon in their diet to
within +/-2 ppt (DeNiro and Epstein 1978;
Fry et al . I078). Careful utilization of
this method can distinguish between carbon
originating froin seagrasses (-3 to -15
ppt), marine algae (12 to -20 ppt), par-
ticulate organic carbon and phytoplankton
(-18 to -25 ppt), and manqrove (-24 to
-27) (Fry and Parker 1979). In Texas, or-
ganic matter from sediments of bays that
have seagrasses display a significantly
reduced 6^ "C ratio v,'hen compared with adja-
cent bays lacking seagrass meadows (Fry
et al . 1977). The same trends were re-
ported for the animals collected from
these bays (Fry 19S1). The {>'C value for
one species of worm, Diopatra cuprea,
shifted from an average of -13.3 to -IS. 4
ppt between seagrass- and phytoplankton-
dominated systems (Fry and Parker 1979).
The average values for fish and shrimp
show a similar trend in that the 6^^C
ratios are reduced in organisms from the
seagrass meadows.
27
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Currently the main limitations of the
carbon isotope method are equipment and
interpretation. It requires use of a mass
spectrometer which is extremely costly,
although today a number of labs will pro-
cess samples for a reasonable fee. The
interpretation can become difficult when
an organism has a 5^^C value in the middle
ranges. If the f/^'C value is at one ex-
treme or another, then interpretation is
straightforward. However, a mid-range
value can mean that the animal is feeding
on a source that has this6^^C value or
that it is using a mixed food source which
averages to this value. Recent studies
utilizing both isotopes of carbon and sul-
fur (Fry and Parker 1982) and nitrogen
(Macko 1981) show much promise in deter-
mining the origin of detrital material as
well as the organic matter of higher
organisms. Knowledge of the feeding ecol-
ogy and natural history of the organism is
needed, as is an alternate indicator.
3.6 PLANT CONSTITUENTS
Recognition of the high productivity
of seagrasses and the relatively low level
of direct grazing has led to questions
regarding their value as food sources.
Proximate analyses of seagrasses in south
Florida, particularly turtle grass, have
been performed by many authors (Burkholder
et al. 1959; Pauersfeld et al . 1969; Walsh
and Grow 1972; Lowe and Lawrence 1976;
Vicente et al . 1978; Bjorndal 1980; Dawes
and Lawrence 1980); their results are
summarized in Table 7. As noted by Dawes
and Lawrence (1980), differences in the
preparation and analysis of samples, as
v/ell as low numbers of samples used in
some studies, make data comparison dif-
Mcul t.
The reported ash content of turtle
grass leaves ranges from 45" dry weight
for unwashed samples down to around 25?
for samples washed with fresh water.
Leaves washed in seawater contained 29''
+/- 3.6" to 44% +/- 6.7?o ash (Dawes and
Lawrence 1980).
Values for the protein content of
leaves vary from a low of 37 of dry weight
for unwashed turtle grass leaves with
epiphytes (Dawes et al . 1979) to 29.7% for
leaves washed in distilled water (Walsh
and Grow 1972), although numbers typically
fall in the range of 10% to 15% of dry
weight. Protein values may be suspect if
not measured directly, but calculated by
extrapolating from percent nitrogen. In
grass beds north of Tampa Pay, Dawes and
Lawrence (1980) found that protein levels
of turtle grass and manatee grass leaves
varied seasonally, ranging from 8% to 11%
and 8% to 13%, respectively, with the
higher levels occurring in the summer and
fall. The protein content of shoal grass
ranged from a low of 14% in the fall up to
19% in the winter and summer. Tropical
seagrasses, particularly turtle grass,
have been compared to other plants as
sources of nutrition. The protein content
of turtle grass leaves roughly equaled
that of phytoplankton and Bermuda grass
(Burkholder et al . 1959) and was two to
three times higher than 10 species of
tropical foraae grasses (Vicente et al .
1078). Walsh and Grow (1972) compared
turtle grass to grain crops, citing stud-
ies in v/hich 114 varieties of corn con-
tained 9.8% to 16% protein; grain sorghum
contained between 8.6% and 16.5%; and
wheat was lowest at 8.3% to 12%. Although
several studies have included measurements
of carbohydrates (Table 7), it is imprac-
tical to compare much of the data because
various analytical methods were employed.
Studies using neutral detergent fiber
(NDF) analyses found that cell wall carbo-
hydrates (cellulose, hemicel lulose, and
lignin)"made up about 45% to 60% of the
total dry weight of turtle grass leaves
(Vicente et al . 1978; Bjorndal 1980).
Dawes and Lawrence (1980) reported that
insoluble carbohydrate content in the
leaves of turtle grass, manatee grass, and
shoal grass was 34% to 46%. The rhizomes
of seagrasses &rG generally higher in
carbohydrates than ^.tq the leaves. Dawes
and Lawrence (1980) found that soluble
carbohydrates in turtle grass and manatee
grass rhizomes varied seasonally, indicat-
ing the production and storage of starch
in summer and fall. These authors, how-
ever, were working in an ^rab. north of
Tampa Bay, where such seasonal changes
would be more pronounced than in the
southern part of Florida and the Keys.
29
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32
CHAPTER 4
THE SEAGRASS SYSTEM
4.1 FUNCTIONS OF SEAGRASS ECOSYSTEMS
In addition to being high in net pri-
mary production and contributing large
quantities of detritus to an ecosystem,
seagrasses perform other functions. Be-
cause of their roots and rhizomes, they
can modify their physical environment to
an extent not equaled by any other fully
submerged organism. Phillips (1978) stated
that, "by their presence on a landscape of
relatively uniform relief, seagrasses
create a diversity of habitats and sub-
strates, providing a structured habitat
from a structureless one." Thus seagrasses
also function to enhance environmental
stability and provide shelter.
Seagrass ecosystems have numerous im-
portant functions in the nearshore marine
environment. Wood et al . (1969) originally
classified the functions of the seagrass
ecosystem. The following is an updated
version of the earlier classification
scheme.
(1) High production and growth
The ability of seagrasses to exert a
major influence on the marine seacape
is due in large part to their ex-
tremely rapid growth and high net
productivity. The leaves grow at
rates typically 5 mm/day, but growth
rates of over 10 mm/day are not
uncommon under favorable circum-
stances.
{?.) Food and feeding pathways
The photosynthetically fixed energy
from the seagrasses may follow two
general pathways: direct grazing of
organisms on the living plant mate-
rial or utilization of detritus from
decaying seagrass material, primarily
leaves. The export of seagrass mate-
rial, both living and detrital, to a
location some distance from the sea-
grass bed allows for further distri-
bution of energy away from its orig-
inal source.
(3) Shelter
Seagrass beds serve as a nursery
ground, that is a place of both food
and shelter, for the juveniles of a
variety of finfish and shellfish of
commercial and sportfishing impor-
tance.
[i] Habitat stabilization
Seagrasses stabilize the sediments in
two ways: the leaves slow and retard
current flow to reduce water velocity
near the sediment-water interface, a
process which promotes sedimentation
of particles as well as inhibiting
resuspension of both organic and
inorganic material. The roots and
rhizomes form a complex, interlocking
matrix with which to bond the sedi-
ment and retard erosion.
(5) Nutrient effects
The production of detritus and the
promotion of sedimentation by the
leaves of seagrasses provide organic
matter for the sediments and maintain
an active environment for nutrient
recycling. Epiphytic algae on the
33
leaves of seagrasses have been shown
to fix nitrogen, thus adding to the
nutrient pool of the region. In add-
ition, seagrasses have been shown to
pick up nutrients fron the sediments,
transporting then through the plant
and releasing the nutrients into the
water column through the leaves, thus
acting as a nutrient pump fron the
sediment.
4.2 SUCCESSION AND ECOSYSTEM DEVELOPMENT
In conventional usage, succession
refers to the orderly development of a
series of communities, or serai stages,
which result in a climax stage that is in
equilibrium with the prevailing environ-
mental conditions. In more contemporary
usage, however, succession is more broadly
used to mean the succession of species,
structure, and functions within an ecosys-
tem. Odum (1969) stated the contemporary
concept as follows:
(1) Succession is an orderly process
of community development that in-
volves changes in species structure
and community processes with time; it
is reasonable, directional, and
therefore predictable.
(2) Succession results from modifi-
cation of the physical environment by
the community; that is, succession is
community-controlled even though the
physical environment determines the
pattern and the role of change, and
often sets limits as to how far
development can go.
(3) Succession culminates in a sta-
bilized ecosystem in which maximum
biomass (or high information content)
and symbiotic function between organ-
isms are maintained per unit of
available energy flow.
Species succession has received by
far the most attention as it is most
obvious and easily measured. The study of
succession of processes or functions is
just beginning, hov/ever. It may well prove
to be the most important avenue for under-
standing ecosystem development. Defining
these processes is of much greater impor-
tance than mere scientific curiosity. It
is also the key
denuded systems.
to res tori no damaned or
4.3 SPECIES SUCCESSION
Throughout the south Florida rooion,
and most of the Gulf of riexico and Carib-
bean, the species of plants that partici-
pate in the successional sequence of sea-
grasses are remarkably few because there
are so few marine plants that can colonize
unconsolidated sediments. In addition to
the seagrasses, one other group, the rhi-
zophytic green alqae, has this capability.
These algae, however, have only limited
rhizoidal development and never affect an
area greater than a few centimeters from
their base.
The most common illustration of suc-
cession in seagrass systems is the recolo-
nization following a "blowout." This loc-
alized disturbance occurs in seagrass beds
throughout Florida and the Caribbean where
there is sufficient current movement in a
dominant direction (Figure 9). Usually a
disruption, such as a major storm, over-
grazing caused by an outbreak of urchins,
or a major ripping of the beds caused by
dragging a large anchor, is required to
initiate the blowout. Once started, the
holes are enlarged by the strong water
flow which causes erosion on the down cur-
rent side. Slowly a crescentic shape a
few meters wide to tens of meters wide is
formed. A sample cross section in Figure
10 shows a mature turtle grass community
that has been disrupted and is recovering.
The region at the base of the erosion
scarp is highly agitated and contains
large chunks of consolidated sediment and
occasional rhizome fragments. With in-
creasing distance from the face of the
scarp, turbulence decreases and some mate-
rial is deposited. The area has become
colonized with rhizophytic algae; Hal imeda
and Penicil lus are the most abundant, but
Caulerpa, Udotea, Rhipocephalus and
Avrainvillea arc also common. These algae
provide a certain amount of sediment-
binding capability as illustrated in Fig-
ure 11, but they do not stabilize the sur-
face of the sediments very well (Scoff in
1970). A major function of these algae in
the early successional stage is the con-
tribution of sedimentary particles (Wil-
liams 1981), The generalized pattern and
34
Figure 9. Blowout disturbance and recovery zones.
IDEALIZED SEQUENCE THROUGH A SEAGRASS BLOWOUT
RELATIVE BIOMASS
Above Sediment
Below Sediment
D = 10 GM/M^
Figure 10. Idealized sequence through a Figure 11. Representative calcareous
seagrass blowout. Note erosion and recov- green algae from seagrass beds. Note the
ery zones moving into the dominant water binding action of the rhizoids in forming
flow. _^ small consolidated sediment balls.
35
conposition of marine sediments in south
Florida as taken from Ginsburg (1955) are
illustrated in Figure 12. Behind the reef
tract over 40% of the sediment was gener-
ated from calcareous algae, Penicillus
capita tus produced about 6 crops per year
in Florida Bay and 9.6 crops per year on
the inner reef tract (Stockman et al .
1976). Based on the standing crops, this
would produce 3.2 g/m-/yr on the reef
tract which could account for one-third
of the sediment produced in Florida Bay
and nearly all of the back-reef sedi-
ment. Similarly, Neuman and Land (1975)
estimated that Halimeda incrassata pro-
duced enough carbonate to supply all the
sediment in the Bight of Abaco in the
Bahamas.
The pioneer species of the Caribbean
seagrasses is shoal grass, which colonizes
readily either from seed or rapid vegeta-
tive branching. The carpet laid by shoal
grass further stabilizes the sediment sur-
face. The leaves form a better buffer
than the algal communities and protect the
integrity of the sediment surface. In
some sequences manatee grass will appear
next, intermixed with shoal grass at one
edge of its distribution and with turtle
grass at the other, rianatee grass, the
least constant member of this sequence,
is frequently absent, however.
Manatee grass appears more commonly
in this developmental sequence in the Car-
ibbean islands and in the lower Florida
SE
REEF TRACT
FLORIDA BAY
NW
Outer
Reef Arc
CORAL KNOLL
Back Reef
MUD BANK
MAINLAND
Figure 12. Origin of sedimentary particles in south Florida marine waters (modified
from Ginsberg 1956) .
36
Keys waters. Where the continental influ-
ence increases the organic matter in the
sediments, manatee grass appears to occur
less commonly. Lower organic matter in
Caribbean sediments, due to the lack of
continental effect, may slow the develop-
mental process.
As successional development proceeds
in a blowout, turtle grass will begin to
colonize the region. Because of stronger,
strap-like leaves and massive rhizome and
root system of turtle grass, particles are
trapped and retained in the sediments with
much greater efficiency and the organic
matter of the sediment will increase. The
sediment height rises (or conversely the
water depth above the sediment decreases)
until the rate of deposition and erosion
of sediment particles is in balance. This
process is a function of the intensity of
wave action, the current velocity, and the
density of leaves.
The time required for this recovery
will vary depending on, among other fac-
tors, the size of the disturbance and the
intensity of the waves and currents in
the region. In Barbados, blowouts were
SOLID
SUBSTRATE
O
EPILITHIC
ALGAE
SANDY
SUBSTRATE
MUDDY
SUBSTRATE
RHIZOPHYTIC
ALGAE
restabilized within 5 to 15 years (Patri-
quin 1975). During the study of Patriquin
(1975) the average rate of erosion of the
blowout was 3.7 mm/day, while the rate of
colonization of the middle of the recovery
slope by manatee grass was 5 mm/day. Once
recolonization of the rubble layer began,
average sediment accretion averaged 3.9
mm/yr.
With the colonization of turtle
grass, the normal algal epiphyte and
fauna! associates begin to increase in
abundance and diversity. Patriquin (1975)
noted that the most important effect of
the instability caused by the blowouts is
to "limit the serai development of the
community. The change in the region of
the blowouts of a well -developed epi fauna
and flora, which is characteristic of
advanced stages of serai development of
the seagrass community, is evidence of
this phenomenon."
In areas that are subject to contin-
ued or repeated disturbances, the succes-
sional development may be arrested at any
point along the developmental gradient
(Figure 13). Many stands of manatee grass
o
CORALLINE ALGAE
HALIMEDA
THALASSIA
={> HALODULE
SYRINGODIUM
ECOSYSTEM DEVELOPMENT
Stable Environmental Conditions
Disturbance
Figure 13. Ecosystem development patterns in south Florida marine waters,
generalized pattern, and all stages may not be present. Note that in the
disturbance that the tendency is to a Thalassia climax.
37
This is a
absence of
are present because of its ability to tol-
erate aerobic, unstable sediments and to
rapidly extend its rhizone system under
these conditions. This is especially evi-
dent in back-reef areas. Patriquin (1575)
attributes the persistence of nanatee
grass in areas around Barbados to recur-
rent erosion in areas where the bottom was
never stable for a sufficiently long time
to allow turtle grass to colonize. Mana-
tee grass can have half of its biomass as
leaves (Table 4). Thus, while manatee
grass is colonizing aerobic disturbed sed-
iments, which would be areas of low nutri-
ent supply and regeneration, the amount of
its root surface available for nutrient
uptake would be reduced, and correspond-
ingly leaf uptake would become a major
source of nutrients. If this is the case,
the higher agitation of the water column
would be of benefit by reducing the grad-
ients at the leaf surface.
4.4 THE CENTRAL POSITION OF THE SEA-
GRASSES TO THE SEAGRASS ECOSYSTEM
organisms with their widely differing
requirements and interactions functioned
as a highly intricate web structure that
made each individual or each link less
necessary to the maintenance of the total
system. There was much natural redundance
huilt into the system. For certain seg-
iients of the community this may be true.
The problem is that at climax there is one
species for which there is no redundancy :
the seagrass. In some cases, if the sea-
grass disappears, the entire associated
community disappears along with it; there
is no other organism that can sustain and
support the system.
This is shown in a sf^all way when
minor disturbances occur as was described
with the blowouts. As the grass beds in
these areas are eroded away, the entire
seagrass system disappears, including the
top 1 or 2 m of sediment. These features
are small and readily repaired, but give
an indication of what could happen if
there was widespread damage to the sea-
grasses.
Seagrasses are vital to the coastal
ecosystem because they form the basis of a
three-dimensional, structurally complex
habitat. In modern ecology there has been
a shift from the autoecological approach
of studying individual species independ-
ently, to the community or ecosystem ap-
proach where the focus is the larger inte-
grated entity. With that realization, one
could wonder, "Why spend sO much effort on
a few species of marine plants, even if
they are the most abundant, in a system
that has thousands of other species?" The
reason is that these plants are critical
to most other species of the system, both
plant and animal. There are few other
systems which are so dominated and con-
trolled by a single species as in the case
of a climax turtle grass or Zostera mea-
dow. H.T. Odum (1974) classified turtle
grass beds as "natural tropical ecosystems
with high diversity." Taken as a total
system, tropical seagrass beds are regions
of very high diversity, but this can be
misleading. Comparisons between tropical
and temperate systems were made at a time
when high diversity was equated with high
biological stability. The prevailing con-
cept was that the multitude of different
The largest contribution to the di-
versity of the system is commonly made by
the complex communities that are epiphytic
on the seagrass leaves. Hhen defoliation
of the seagrasses occurs, most of this
community disappears, either by being car-
ried out as drifting leaves or becoming
part of the litter layer and ultimately
the surface sediments. With the leaves
gone, the current baffling effect is lost
and the sediment surface begins to erode.
Algal mats that may form have minimal
stabilizing ability; however, the dead
rhizomes and mats will continue to bond
the sediments, in some cases for several
years (Patriquin 1975; Scoffin 1970).
In south Florida the disappearance of
seagrasses would yield a far different
seascape. Much of the region v/ould be
shifting mud and mud banks, while in many
areas the sediments would be eroded to
bedrock. Based on the communities found
in such areas today, primary production
and detrital production would be dramati-
cally decreased to the point that the
support base for the abundant commercial
fisheries and sport fisheries would shrink
if not disappear.
38
4.5 STRUCTURAL
SEAGRASSES
AND PROCESS SUCCESSION IN
As species succession occurs in a
shallow marine system, important struc-
tural changes occur. Because seagrass
systems do not have woody structural com-
ponents and only possess relatively simp-
listic canopy structure, the main struc-
tural features are the leaf area and bio-
mass of the leaves as well as the root and
rhizome material in the sediment. The
most obvious change with community devel-
opment is the increase in leaf area. This
provides an increase in surface area for
the colonization of epiphytic algae and
fauna, with the surface area of the climax
community being many times that of either
the pioneer seagrass, shoal grass, or the
initial algal colonizers. In addition to
providing a substrate, the increasing leaf
area also increases the current baffling
and sediment-trapping effects, thus en-
hancing internal nitrogen cycling.
As organisms grow and reproduce in
the environment, they bring about changes
in their surroundings. In doing so these
organisms frequently modify the environ-
ment in a way that no longer favors their
continual growth. McArthur and Connell
(1966) stated that this process "gives us
a clue to all of the true replacements of
succession: each species alters the envi-
ronment in such a way that it can no
longer grow so successfully as others".
In a shallow water successional se-
quence leading to turtle grass, the early
stages are often characterized by a low
supply of organic matter in the sediment
and open nutrient supply, that is, the
community relies on nutrients being
brought in from adjacent areas by water
movement as opposed to in situ regenera-
tion. With the development from rhizophy-
tic algae to turtle grass, there is a pro-
gressive development in the helowground
biomass of the community as well as the
portion exposed in the water column. With
the progressive increase in leaf area of
the plants, the sediment trapping and par-
ticle retention increase. This material
adds organic matter to further fuel the
sedimentary microbial cycles. Although
various segments of this successional
sequence have been measured by numerous
authors, the most complete set of data has
recently been compiled by Williams (1981)
in St. Croix (Table 8). In St. Croix,
where the data were collected, as on many
low, small islands with little rainfall,
the climax is commonly a mixture of turtle
grass and manatee grass. In south Florida,
with its higher rainfall and runoff, the
climax more commonly is a pure turtle
grass stand. In turtle grass beds in
south Florida, Capone and Taylor (1977,
1980) found that nitrification was highest
on the developing periphery of the bods
and lower in the centers where particulate
trapping and retention were greater. Add-
itionally, mature ecosystems, both marine
and terrestrial, seem to be based primar-
ily on the detrital food web which aids in
conserving both carbon and nitrogen, as
direct grazing is quantitatively low in
these systems.
39
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40
CHAPTER 5
THE SEAGRASS COMMUNITY - COMPONENTS, STRUCTURE, AND FUNCTION
Seagrass-associated comriunities are
doternined by species conposition and den-
sity of seagrass present, as well as abi-
otic variables. These communities range
from monospecific turtle grass beds in the
clear, deep waters behind the reef tract
to the shallow, muddy bottoms of upper
Florida Bay where varying densities of
shoal grass are intermixed with patches of
turtle grass.
Turney and Perkins (1972) divided
Florida Bay into four regions based large-
ly on temperature, salinity, circulation,
and substrate characteristics. Each of
these regions proved to have a distinctive
molluscan asse;nblage.
Studies have also shown that great
diversity in species number and abundance
exists even within communities of similar
seagrass composition and density, and
within comparatively small geographical
regions. Brook (1978) compared the macro-
fauna! abundance in five turtle grass com-
munities in south Florida, where the blade
density was greater than 3,000 blades/m-.
Total taxa represented varied from a low
of 38 to a high of 80, and average abun-
dance of individuals varied from 292 to
10,644 individual s/m-'.
The biota present in the seagrass
ecosystem can be classified in a scheme
that recognizes the central role of the
seagrass canopy in the organization of the
system. The principal groups are (1) epi-
phytic organisms, (2) epibenthic
organisms, (3) infaunal organisms, and (4)
the nektonic organisms.
The term epiphytic organisms is used
here the same as that of Harlin (1980) and
means any organism growing on a plant and
not just a plant living on a plant. Epi-
benthic organisms are those organisms that
live on the surface of the sediment; in
its broadest sense, this includes motile
organisms such as large gastropods and sea
urchins, as well as sessile forms such as
sponges and sea anemones or macroalgae.
Infaunal organisms are those organisms
that live buried in the sediments. Organ-
isms such as penaeid shrimp, however, that
lie buried part of the day or night in the
sediments, but are actively moving on the
sediment surface the rest of the time
would not be included as part of the
infauna. The infauna would include organ-
isms such as the relatively immobile
sedentary polychaetes and the relatively
mobile irregular urchins, Nektonic organ-
isms, the highly mobile organisms living
in or above the plant canopy, are largely
fishes and squids.
Kikuchi (1961, 1962, 1966, 1980)
originally proposed a functional classi-
fication scheme for the utilization of
Japanese seagrass beds by fauna that has
wide utility. This classification, mod-
ified for tropical organisms, would
include (1) permanent residents, (2)
seasonal residents, (3) temporal migrants,
(4) transients, and (5) casual visitors.
The third category is added here to
include the organisms that daily migrate
between seagrass beds and coral reefs.
These were not included in the original
classification which was based on tem-
perate fauna.
41
5.1 ASSOC lAFEP ALOAE
Major sources of priinary production
for coastal and estuarine areas are the
Fol lowing:
(1) Macrophytes (seagrasses, nan-
groves, macroalgae, and marsh
grasses)
(2) Bonthic inicroalgae (benthic and
epiphytic diatons, dinoflagcl-
lates, filanentous green and
bluegreen algae)
(3) Phytoplankton
Although in deep, turbid northern
estuaries, such as the Chesapeake or Dela-
ware Bays, phytoplankton may be the doni-
nant producer, in most areas that have
been investigated the macrophytes are the
most important primary producers, often by
an overwhelming margin.
Productivities of phytoplankton,
marsh grasses, and seagrasses in a North
Carolina estuary were compared by Williams
(1973); areal production values were 53,
249, and 678 g/m"/yr, respectively. Hhen
the total area of the estuarine sound sys-
tem available to phytoplankton and sea-
grass was considered, the seagrass produc-
tion for the entire estuary was still
about 2.5 times the annual contribution of
the phytoplankton. In the clearer waters
of the Florida estuaries and coastal zone,
the difference is considerably greater.
In Boca Ciega Bay, Taylor and Salonan
(1968) estimated that total production,
which was primarily macrophytes, was six
times the annual phytoplankton production.
Thayer and Ustach (1981) have estimated
macrophytes to account for about 75% of
the plant production in the estuarine-
coastal area of the northern Gulf of
Mexico.
Benthic Algae
Algal communities on hard substrates
can consist of hundreds of species from
all of the major macroalgal phyla. The
areas inhabited by seagrasses do not offer
an optimal habitat for most algae, which
require hard substrate for attachment of
their holdfast. Primary substrate for
algae will include (1) the sediments, (2)
the seagrasses themselves, and (3) occa-
sional rocks or outcrops. In addition
many macroalgae in south Florida form
large unattached masses on the sea bottom,
collectively known as drift algae.
Although much of south Florida offers
sufficient hard substrate for algal at-
tachment, notably the reef tracts and the
shallow zones bordering many of the keys,
the dominant substrate type is not solid.
In many areas mangrove prop roots, oyster
bases, and scattered rocks or shells and
to manmade structures such as bridge sup-
ports and canal walls offer the primary
algal substrates.
The only algae able to consistently
use sediments as substrate are (1) the
mat-forming algae and (2) members of the
order Siphonales (Chlorophyta) which
possess creeping rhizoids that provide an
anchor in sediments (Humm 1973). Ainong
the most important genera are Hal imeda,
Penicillus, Caulerpa, Rhipocephalus, and
Udotea (Tigure 14). These algae are
important as primary producers of organic
carbon; of even greater importance, all
but Caulerpa produce calcium carbonate for
their skeleton which, upon death, becomes
incorporated in the sediments.
These algae have limited sediment
stabilizing properties, the main utility
of their rhizoidal holdfasts being to
maintain then in place. Because they do
not have a large investiture of structure
in the sediments, they can more rapidly
accommodate changes in shifting sediments,
while still maintaining some current
buffering capacity. In this capacity
they form a prior successional stage for
seagrasses (Williams 1981).
Production of lime mud by these algae
can be enormous. Hal imeda tends to break
up into characteristic sand-sized plates,
while Penicil lus produces fine-grained
(less than 15i_, ) araoonitic mud. Stockman
et al. (1967) estimated that at the
present rate of production, Penicillus
alone could account for all of the fine
mud behind the Florida reef tract and
one-third of the fine mud in northeastern
Florida Bay. In addition, the combination
42
4k
Figure 14. Calcareous algae (Udotea sp.) fron the fringes of a seagrass bed.
43
of Rhipocephalus, Udotea, and Acetabularia
produced at least as much mud as Penicil-
lus in the same locations.
In the Bight of Abaco, Neumann and
Land (1975) calculated that the growth of
Penicillus. Rhipocephalus, and Hal imeda
produced 1.5 to 3 times the amount of mud
and Hal imeda sand now in the basin and
that in a typical Bahamian Bank lagoon,
calcareous green algae alone produced more
sediment than could be accommodated. Bach
(1979) measured the rates of organic and
inorganic production of calcareous siphon-
ates in Card Sound, Florida, using several
techniques. Organic production was low in
this lagoon, ranging from 8.6 to 38.4 g
ash free dry weight /m-^/yr, and 4.2 to
16.8 g CaCOj/m^/yr for all the species
combined.
In addition to the calcareous algae,
several algae are present in grass beds as
large clumps of detached drift algae; the
most abundant belongs to the genus Lauren-
cia. The areal production of these algae
is low compared with the seagrasses. Jos-
selyn (1975) estimated the production of
Laurencia in Card Sound to average about
8.1 g dry weight /m-^/yr which was less
than 1% of the 1,100 g/m-^/yr estimated by
Thorhaug et al . (1973) for turtle grass
from the same area.
The least studied components of the
algal flora are the benthic nicroalgae.
In studies of benthic production through-
out the Caribbean, Bunt et al . (1972) cal-
culated the production in Caribbean sedi-
ments to average 8.1 mg C/m-/hr (range =
2.5 to 13.8 mg) using I'+C uptake. By com-
parison, sediments from the Florida Keys
yielded 0.3 to 7.4 mg C/m-/hr fixation.
These values were equivalent to the pro-
duction in the water column. Ferguson
et al . (1980) briefly reviewed inicroalgal
production values and indicated that light
and thermal inhibition can occur, particu-
larly in summer.
Epiphytic Algae
One of the main functions for which
seagrasses have been recognized has been
the ability to provide a substrate for the
attachment of epiphytic organisms. Al-
though unifying patterns arc beginning to
emerge, the study of epiphytes has suf-
fered from what Harlin (1980) described as
the "bits and pieces" approach.
An annotated list of 113 species of
algae found epiphytic on turtle grass in
south Florida was compiled by Hunm"(1964).
Of these only a few were specific to sea-
grasses; most were also found on other
plants or solid substrate. Later, Ballan-
tine and Humm (1975) reported 66 species
of benthic algae which were epiphytic on
the seagrasses of the west coast of Flor-
ida. Rhodophyta comprised 45% of the
total, Phaeophytas were only 12%, and
Chlorophytas and Cyanophvtas each repre-
sented 21% of the species. Harlin (19P0)
compiled from 27 published works a species
list of the microalgae, macroalgae, and
animals that have been recorded as epiphy-
tic on seagrasses. The algal lists are
comprehensive, but none of the reports
surveyed by Humm list the epiphytic inver-
tebrates from south Florida.
Harlin (1975) listed the factors
influencing distribution and abundance of
epiphytes as:
(1) Physical substrate
(2) Access to photic zone
(3) A free ride through
waters
(4) Nutrient exchange with
(5) Organic carbon source
moving
host
The availability of a relatively stable
(albeit somewhat swaying) substrate seems
to be the most fundamental role played by
the seagrasses. The majority of the epi-
phytic species is sessile and needs a sur-
face for attachment. The turnover of the
epiphytic community is relatively rapid
since the lifetime of a single leaf is
limited. A typical turtle arass leaf has a
lifetime of 30 to 60 days' (Zieman 1975b).
After a leaf emerges there is a period be-
fore epiphytic organisms appear. This may
be due to the relatively smooth surface or
the production of some antibiotic compound
by the leaf. On tropical seagrasses the
heaviest coatings of epiphytes only occur
after the leaf has been colonized by the
coralline red algae, Fosl iella or Melobe-
sia. The coralline skeleton of these algae
may form a protective barrier as well as a
suitably roughened and adherent surface
for epiphytes (Figure 15).
44
Figure 15. Thalassia blades showing tips encrusted with calcareous epiphytic algae.
Several of the larger blades show the effects of grazing on the leaf tips.
Seagrass leaves are more heavily epi-
phytized at their tips than their bases
for various reasons. For the snail algae,
being on the leaves has the advantage of
raising them higher in the photic zone.
The shading effect produced by epiphytic
organisms on seagrass leaves decreases
photosynthesis by 31% (Sand-Jensen 1975).
In addition, the upper leaf surface exper-
iences much greater water notion than the
lower surface. This not only provides a
much greater volume of water to be swept
by suspension-feeding animals, but also
reduces the gradients for photosynthetic
organisns. Studies have shown that there
is transfer of nutrients from seagrasses
to epiphytes. Harlin (1975) described the
uptake of PO4 translocated up the leaves
of Zostera and Phyllospadix. Epiphytic
blue-green algae have the capacity to fix
molecular nitrogen, and Coering and Parker
(1972) showed that soluble nitrate fixed
in this manner was utilized by seagrasses.
Epiphytes also contribute to the pri-
mary production of the seagrass ecosystem.
In some areas there are few epiphytes and
little contribution, but in places the
amount of production is high. Jones (1968)
estimated that in northern Biscayne Pay
epiphytes contributed from 255^ to 33% of
the community metabolism. Epiphytes con-
tributed 18% of productivity of Zostera
meadows in North Carolina (Penhale 1977).
The trophic structure of these leaf com-
munities can be quite complex and will be
discussed later. Much of the epiphytic
material, both plant and animal, ultimate-
ly becomes part of the litter and detritus
as the leaf senesces and detaches.
5.2 INVERTEBRATES
Composition
The invertebrate fauna of seagrass
beds is exceedingly rich and can only be
characterized in broad terms unless one is
dealing with a specific, defined area.
This is because the fauna of the grass
beds is diverse, with many hundreds of
45
species being represented within a snail
area, and variable, with dramatic changes
occurring in the faunal composition and
density within relatively small changes of
time or distance. If one does not lose
sight of these facts, it is possible to
list various organisms that are represent-
ative of seagrass meadows over large dis-
tances.
The most obvious invertebrates of
many of the seagrass beds of south Florida
are the large epibenthic organisms (Figure
16). The queen conch (Strombus qigas)
feeds primarily on epiphytes it scrapes
from turtle grass blades, while the Baham-
ian starfish (Oreaster reticulata) and the
gastropods Fasciolaria tul ipa and PI euro-
pi oca gigantea prey largely on infauna.
Numerous sea urchins, such as Lytechinus
variegatus and Tripneustes ventricosus,
are found throughout the beds. Juveniles
of the long-spined urchin Diadema antil -
larum are common, but the adults seek the
shelter of rocky ledges or coral reefs.
The deposit-feeding holothurians Actino-
pyga agassizi and Holothuria floridana may
be found on the surface, while the large
sea-hare, the nudibranch Aplysia dactyl o-
mela, may be found gracefully gliding over
the grass canopy. At night pink shrimp
(Penaeus duorarurn) and spiny lobster
(Panulirus argus~y~may be seen foraging in
the seagrass along with the predatory
Octopus briareus.
On shallow turtle grass flats the
corals Manicinia areolata and Porites
furcata are' common, while in somewhat
deeper waters sponges such as Ircinea,
Tethya, and Spongia may be found.
The infauna can
not visually obvious.
(Atrina rigida) is a
in many grass beds,
be diverse, but are
The rigid pen shell
common filter-feeder
along with numerous
bivalve molluscs such as Chione cancel -
lata, Codakia orbicularis, Tel 1 ina radi-
ata, Luc ina pennsyl vanica, and Laevicar-
dium laevigatum. A variety of annelid
worms are in the infauna, notably Areni-
cola cri s_ta ta , Onuphis magna, Terehel 1 ides
stroemi , and Eunice longicerrata.
The abundance and diversity of epi-
phytic ani:rials on seagrass blades are dra-
matic evidence of the effect the seagrass
has on increasing bottom surface aros and
providing a substrate for attachment (Fig-
ure 17). The most prominent of these epi-
faunal organisms in south Florida are the
gastropods. Cerithium nascarum and £.
eburnum, Anachis sp., Astrea spp.. Modulus
modulus, Mitrella lunata, and Bittium
varium
Mitrel la lunata,
characteristic in
are characteristic 1n turtle grass
and shoal grass habitats throughout south
Florida, as is the attached bivalve
Cardita floridana.
Small crustaceans are also common in
seagrass beds where they live in tubes at-
tached to the leaf surface, move freely
along the blades, or swim freely between
the blades, the sediment surface, or the
water column above the blades. Common an-
phipods are Cymadusa compta, Gammarus muc-
ronatus, Mel ita nitida, and Grandidierella
bonnieroides, while the caridean shrimps
Palaemonetes pugio, P_. vulgasis, and P^.
intermedius, Perici imenes longicaudatus,
and £. americanus, Thorfloridanus, Tozeuma
carol inense, Hippolyte pleuracantha,
Alpheus normanni , and A^. heterochaelis are
abundant within the grass beds. Hermit
crabs of the genus Pagurus are numerous
and at night crawl up the blades to graze
on epiphytic material. When they reach
the end of the blades, they simply crawl
off the end, fall to the sediment, scuttle
to another blade, and repeat the process.
Structure and Function
The structure of the grass carpet
with its calm water and shaded microhabi-
tats provides living space for a rich epi-
fauna of both mobile and sessile organisms
(Harlin 1980). It is these organisms which
are of greatest importance to higher con-
sumers within the grass bed, especially
the fishes. When relatively small quanti-
tative samples are used in estimating pop-
ulation sizes, gastropods, amphipods, and
polychaetes are typically most numerous,
while isopods can be important (Nanle
1968; Carter et al . 1973; Marsh 1973; K.i-
kuchi 1974; Brook 1975, 1977, 1978). In a
Card Sound turtle grass bed. Brook (1975,
1977) estimated that amphipods represented
62. 2? of all crustaceans. When the trawl
is employed as a sampling device, deca-
pods, including penaeid and caridean
shrimp and true crabs, as well as gas-
tropods, are generally most abundant
in invertebrate collections (Thorhaug
and Roessler l'^77; Yokel 1975a, 1^75b;
46
Figure 16. Large invertebrates from seagrass beds. A. A juvenile queen conch (Strom-
bus gigas) in a Thalassia bed. (Photo by M.B. Robblee). B. A group of the long-spined
Caribbean urchin, Diadetna antillarum, feeding in a Thalassia bed near a patch reef,
47
Figure 17. Snail grazing on the tip of an encrusted Thalassia leaf. Small snails and
hermit crabs are frequently seen grazing the heavily epiphytized portions of seagrass
leaves.
Roessler and Tabb 1974; Bader and Roessler
1971; Tabb et al . 1962; Tabb and Manning
1961). Faunal differences among studies
reflect sampling gear selectivity, but
typically penaeid and caridean shrimp are
less numerous than the smaller macrocrus-
taceans (i.e. amphipods, isopods), yet
represent a larger biomass within the bed.
For example, data from Brook (1977) for a
Card Sound turtle grass grass bed indi-
cated that amphipods and caridean shrimp
represent respectively 5.8% and 23.3% of
estimated biomass of principal taxa col-
lected and 12.4% and 50,3% of crustacean
biomass. Demonstrating the importance of
the physical structure of the grass car-
pet. Yokel (1975a) reported that the
standing crop of crustaceans (estimated
using a travel) was 3.9 times larger in
mixed seagrass and algal flats than on
nearby unvegetated bottoms (see Figure
18).
It is a long standing assumption that
the grass carpet represents protection
from predation for the animals living in
it. The dense seagrass blades and rhizomes
associated with the grass carpet provide
cover for invertebrates and small fishes
while also interfering with the feeding
efficiency of their potential predators.
Experimental evidence suggests that grass
bed invertebrates actively select vege-
tated habitat rather than bare sand indi-
cating that habitat preference is an
important force contributing to observed
faunal densities in grass beds (Heck and
Orth 1980). Selection appears to be based
on the form or structural characteristics
of the seagrass (Stoner 1980a).
It is speculated from experimental
work using shapes that the caridean
shrimp, Hippolyte cal iforniensis, locates
its host plant, Zostera marina, visually
48
lOOn
c/)
ir
LlI
DD
75-
<
I-
o
50-
i Fish
Invertebrates
25-
HEAVY
SEAGRASS
( Halodule a
Thalossio )
THIN
SEAGRASS
(Halodule)
SAND/
SHELL
MUD/
SAND/
SHELL
Figure 18. Relative abundance of fishes and invertebrates over seagrass beds and adja-
cent habitats (after Yokel 1975a).
by discriminating on the basis of form
(Barry 1974). Stoner (1980a) demonstrated
that common epifaunal amphipods were cap-
able of detecting small differences in the
density of seagrass and actively selected
areas of high blade density. VJhen equal
blade biomass of the three common sea-
grasses (turtle grass, manatee grass, and
shoal grass) were offered in preference
tests, shoal grass was chosen. When equal
surface areas were offered no preferences
were observed, indicating that surface
area was the grass habitat characteristic
chosen.
5.3 FISHES
Composition
Seagrass meadows have traditionally
been known to be inhabited by diverse and
abundant fish faunas. Often the grass bed
serves as a nursery or feeding ground for
fish species that will ultimately be of
commercial or sport fishery value. The
classification created by Kikuchi (1961,
1962, 1966) was largely inspired by the
fish community found in Japanese Zostera
beds and has effectively emphasized the
diverse character of seagrass fish and
major invertebrates, while also serving to
underscore the important ecological func-
tions of seagrass meadows within the estu-
ary as nursery and feeding grounds.
Permanently resident fishes are typi-
cally small, less mobile, more cryptic
species that spend their entire life
within the grass bed. Few, if any, of
these species are of direct commercial
value but are often characteristic of the
seagrass habitat. The emerald clingfish
(Acyrtops beryl! ina) is a tiny epiphytic
species found only living on turtle grass
blades. In south Florida, members of
families Syngnathidae, Gobiidae, and
Clinidae may be included in this group.
49
The pipefishes, Syngnathus scovil 1 i , _S.
floridae, S^. louisianae, and Micrognatus
crinigerus, as well as the seahorses Hip-
pocampus zosterae and U_. erectus are abun-
dant in seagrass throughout south Florida.
The gobies and clinids are diverse groups
and well represented in seagrass fish
assemblages of southern Florida. The most
abundant goby is Gobisona robustum. The
clinids appear to be limited to the clear-
er waters of the Florida Keys and Florida
Bay, where Paracl inus fasciatus and P^.
marmoratus are most abundant.
Other resident fish species are char-
acteristic of seagrass habitat. The
inshore lizardfish (Synodus foetens) is a
conmon epibenthic fish predator. The
small grass bed parrotfishes -- Spari soma
rubripinne, _S. radians, and S^. chrysop-
terum — are found in the clearer waters
of the Florida Keys where they graze di-
rectly on seagrass. Fels, including mem-
bers of families Moringuidae, Xenocongri-
dae, Muraenidae and Ophichtidae (Robblee
and Zieman, in preparation), are diverse
and abundant in grass beds of St. Croix,
U.S. Virgin Islands. These secretive
fishes are typically overlooked in fish
community surveys. In the grass beds of
south Florida, the Ophochtid eels Myrich-
thys acuminatus, the sharptail eel, and M^.
oculatus, the goldspotted eel, can com-
monly be observed moving through the grass
during the day while young moray eels,
Cymnothorax spp., are not uncommon at
night foraging in grass beds for molluscs.
Seasonal residents are animals that
spend their juvenile or subadult stages or
their spavming season in the grass bed.
Sciaenids, sparids, pomadasyids, lutjan-
ids, and gerrids are abundant seasonal
residents in south Florida's seagrass com-
munities. Seasonal residents use the sea-
grass meadow largely as a nursery ground.
At least eight sciaenid species have
been found over grass in the variable
salinity, high turbidity waters of south-
western Florida's estuaries and coastal
lagoons. Not all of these fishes occur
abundantly, and only the spotted seatrout
(Cynoscion nebulosus) , the spot (Leiosto-
mus xanthurus) , and the silver perch
(Bairdiella chrysura) occur commonly over
grass. The pigfish (Orthopristis chrysop-
tera) is the abundant grunt (Pomadasyidae)
of muddy bottoms and turbid water associ-
ated with grass in Florida's variable
salinity regions (Tabb and f'lanning 1961;
Tabb et al." 1062; Yokel 1975a, ' 1975b;
Weinstein et al . 1977; Weinstein and Heck
1979) and is at best rare in the Florida
Keys. Other grunts occur over grass only
rarely in southwestern Florida and Florida
Bay and include Anisotrenus virginicus,
Haemulon scirus, and H^. aurol ineatum.
Lagodon rhomboides, the pinfish, was the
most abundant fish collected in these
waters and has demonstrated a strong af-
finity for seagrass (Gunter 1945; Caldwell
1957; Yokel 1975a, 1975h). Eucinostomus
quia and £. argenteus are seasonally
abundant gerrids also most common over
grass.
With the exception of the pigfish,
the pomadasyids already mentioned are
joined by H^. flavol ineatum, H. parri, and
U_. carbonarium in the clearer waters of
the Florida Keys. Snappers and grunts are
more diverse in the clearer v^aters of the
Florida Keys. Lutjanus griseus and I.
synqaris, which are common throuohout
south Florida, are joined by the school-
master {I. apodus) the mutton snapper (U
anal is) the dog snapper (U jocu), and the
yellowtail snapper (Ocyurus chrysurus).
Thayer et al . (1978b) list several season-
ally resident fishes that are prominent
fishes of sport or commercial fishery
value and include the sea bream (Archosar-
ous rhomboides), the sheepshead (A. pro-
batocephalus), the gap grouper (Tycterop-
erca microlepis), and the redfish (Sciae-
nops ocel lata).
The subtropical seagrass system of
south Florida appears to differ signifi-
cantly from more temperate beds by the
presence of relatively large numbers of
prominent coral reef fishes over grass at
night when the bed is located in the vici-
nity of coral reefs. Fishes from families
Pomadasyidae, Lutjanidae, and Holocentri-
dae find shelter on the reef during the
day and move into adjacent grass beds at
night to feed. This situation is typical
of Caribbean seagrass meadows. All of the
grunts and snappers mentioned above except
6. chrysurus, when of appropriate size,
will live diurnal ly on the reef and feed
in the grass bed at night. Diel visitors
use the grass bed primarily as a feeding
ground.
50
Occasional nigrants, as the naine im-
plies, are only present infrequently and
unpredictably. Representatives include
large carnivores of offshore or oceanic
origin such as carangids and scrombrids.
Organisns of this type represent only a
snail proportion of the biomass present,
but tnay bo important in determining fish
community structure.
This system (Kikuchi 1961, 1962,
1966) aids in classifying the fish fauna,
but is not exact. For example, the king
mackeral could possibly be found over the
back reef grass beds much of the year, but
during winter large schools move through
the region. Thus this fish could be
classified as a seasonal resident and as
an occasional migrant.
Structure and Function
Because fishes that occupy grass beds
are important to commercial fishermen and
because the seagrass habitat is apparently
important in the life histories of those
fishes, it is surprising that relatively
little is known concerning the distribu-
tion of fishes within the grass bed
itself.
Densities of fishes are typically
greater in grass bed habitat within south
Florida's estuaries and coastal lagoons
than in adjacent habitats (Reid 195^; Tabb
et al. 1962; Roessler 1%S; Yokel 1975a,
1975b; Weinstein et al. 1977). Yokel
(1975a, 1975b), using a trawl, reported
greatest densities of fishes in seagrass
meadows as opposed to bare sand and shell
bottoms in the Ten Thousand Island region
of south Florida. In the Rookery Bay Sanc-
tuary, 3.5 times as many fishes were cap-
tured in crass as in other habitats
(Yokel 1975a). Similar results have been
reported in Biscayne Bay (Roessler 1965;
Roessler et al . 1974; Thorhaug and Roes-
sler 1977). As is true for invertebrates,
often highest densities and greatest spe-
cies richness of fishes are associated
with the red algal complex (Roessler
et al. 1974; Thorhaug and Roessler 1977),
although this is not necessarily an exten-
sive habitat. Clark (1970) in Whitewater
Bay observed high densities of fishes as-
sociated with patchy shoal grass and the
calcareous green alga, Udotea congluti-
nata.
Although it is v/el 1 documented that
fishes are abundant over grass within
south Florida's estuaries and coastal
lagoons (Figure 19), knowledge of vithin-
habitat distributional patterns relative
to grass bed characteristics (i.e., struc-
tural complexity, prey densities) is poor
at best. It would seem more often than
not that patterns attributable to inverte-
brates are assumed in principle to also
apply to fishes. Fishes are generally
larger and more mobile than invertebrates
and the extrapolation may not be valid.
In Tague Bay, St. Croix, U.S. Virgin
Islands, abundance of coral reef fishes
feeding over grass at night exhibited a
distributional pattern strongly correlated
with habitat complexity as measured by
plant biomass and bottom topography
(Robblee, in prep.). Fish predators may
be responding to grass bed characteristics
other than just the grass carpet.
Some fish commonly utilize inverte-
brate fauna found among seagrass (Carr and
Adams 1973; Brook 1975, 1977; Adams 1576b;
Robertson and Howard 1978). The results
of experimental manipulations of predation
by exclosure caging have attempted to
evaluate the effect predation has in
structuring invertebrate populations in
seaqrass beds. Exclusion of fish preda-
tors usually causes increases in species
abundance and densitv (Orth 1977b; Young
et al. 1976; Young and Young 1977). If
expected increases fail to appear, the
abundance of decapod predators probably
increased sufficiently to reduce the abun-
dance and composition of the other inver-
tebrates (Young and Young 1977).
Plant biomass and invertebrate abun-
dance relationships observed in Panamanian
grass beds are governed largely by preda-
tion mediated by the structural complexity
of the grasses (Heck and Wetstone 1977).
Numbers of macrobenthic animals increased
noticeably in the fall with emigration of
fishes from grass beds in Apalachee Bay
(Stoner 1980b). Amphipods consumed most
frequently by the pinfish were epifaunal
(Stoner 1979). In studies by Nelson
(1979a) infaunal amphipods were 1.3 times
more abundant than epifaunal tube-dwelling
amphipods and 4 times more abundant than
free-living epifaunal amphipods during
the seasonal influx of pinfish. These
results reiterate the role predators play
51
Figure 19. Small grouper (Serranidae) foraging in seagrass bed.
in controlling abundances and species com-
position within sea grass beds (Nelson
1979a; Stoner 1979).
Little is known about how fishes
respond to the structural complexity of
the grass canopy. Noting the size distri-
bution of fishes typically inhabiting sea-
grass beds, Ogden and Zieman (1977) specu-
lated that large predators, such as bar-
racudas, jacks, and mackerels, may be
responsible for restricting permanent
residents to those small enough to hide
within the grass carpet. For fishes larger
than about 20 cm (8 inches) the grass bed
can be thought of as a two-dimensional
environment; these fishes are too large to
find shelter within the grass carpet.
Mid-sized fishes (20 to 40 cm or 8 to 16
inches) are probably excluded from the
grass bed by occasional large predators.
Mid-size fishes are apparently restricted
to sheltered areas by day and may move
into the beds at night when predation is
less intense (Ogden and Zieman 1977; Ogden
1980). The size of the individuals in
these groups is a function of the length
and density of the grass beds. In Flor-
ida, where the seagrasses are typically
larger and denser, the grass beds offer
shelter for much larger fish than in St.
Croix, where the study of Ogden and Zieman
(1977) was done.
Heck and Orth (1980a) hypothesized
that abundance and diversity of fishes
should increase with increasing structural
complexity until the feeding efficiency of
the fishes is reduced because of interfer-
ence with the grass blades or because
conditions within the grass canopy become
unfavorable (i.e., anoxic conditions at
night). At this point densities should
drop off. Evidence indicates that feeding
efficiency does decline with increasing
structural complexity.
52
The pinfish's predatory efficiency on
amphipods decreases with increasing den-
sity of Zostera marina blades (Nelson
1979a). Coen Tl9T9l found in single-
species experiments (one shrimp species at
a time) that with increasing cover of red
algae ( Pi gen i a simplex, Laurencia spp.,
Gracilaria spp. and others) the pinfish's
foraging efficiency on Palaemon floridanum
and Palaemonetes vulgaris was reduced.
The killifish (Fundulus heterocl itus) fed
less efficiently on the grass shrimp
(Palaemonetes pugio) in areas of densest
artificial seagrass. Virtually nothing is
known about the relation of typical grass
bed fishes and their predators; research
on this topic would be fruitful.
5.4 REPTILES
Although there are several species
of sea turtles in the Gulf of Mexico and
Caribbean, the green sea turtle (Chelonia
mydas) is the only herbivorous sea turtle
(Figure 20). In the Caribbean, the main
food of the green turtles are sea grasses
and the preferred food is Thalassia,
hence the name turtle grass (see section
6.2).
Green turtles were formerly abundant
throughout the region, but were hunted
extensively. Concern over the reduced
populations of green turtles dates back
to the previous century (Munroe 1897).
Although limited nesting occurs on the
small beaches of extreme south Florida,
the region has almost certainly been pri-
marily a feeding rather than a nesting
site. Turtle and manatee feeding behavior
are described in Chapter 6.
The Ainerican crocodile (Crocodylus
acutus) occurs in the shallow water
of Florida Bay and the northern Keys.
y
Figure 20. Seagrass bed following grazing by green sea turtle. Note the short, evenly
clipped blades. The scraping on the Thalassia blade in the center is caused by the
small emerald green snail, Smaraqdia viridis.
53
Although crocodiles
shallow grass beds,
their util ization of
5.5 Birds
undoubtedly feed in
little is known of
this habitat.
to sieze
swiiiining
The seagrass beds of south Florida
are used heavily by large numbers of
birds, especially the wading birds, as
feeding grounds. This heavy utilization
is possible because of the relatively high
proportion of very shallow grass bed habi-
tat. There are few studies of the utili-
zation of seagrass beds by birds, al-
though there are extensive lists of birds
using temperate seagrasses and aquatic
plants (McRoy and Helfferich 1980). Birds
known to use the seagrass habitat of south
Florida and their modes of feeding are
listed in Table 9.
Three common methods of feeding in
birds are wading, swimming, and plunging
from some distance in the air
prey. The most common of the
birds is the double-crested cormorant
which pursues fish in the water column.
Cormorants may be found wherever the water
is sufficiently deep for them to swim, and
clear enough for them to spot their prey.
The osprey and the bald eagle sieze prey
on the surface of the water with their
claws, while the brown pelican pluges from
some distance in the air to engulf fishes
with its pouch. The value of the seagrass
meadows to these birds is that prey are
more concentrated in the grass bed than in
the surrounding habitat, thus providing an
abundant food sourer.
The extensi' e shallow grass flats are
excellent foraring grounds for the larger
wading birds 'figure 21). The great white
heron is common on the shallow turtle
grass flats on the gulf side of the lower
Keys. The great blue heron is common
Figure 21. Shallow seagrasses adjacent to red mangrove roots,
ing area of small and medium sized wading birds.
54
This is a common feed-
Table 9. Birds that use seagrass flats in south Florida
(data provided by James A. Kushlan, Evergaldes National Park),
Common name
Species name
Preferred
feeding tide
Waders-primary
Great blue heron
Great white heron
Great egret
Reddish egret
VJaders-secondary
Louisiana heron
Little blue heron
Roseate spoonbill
Willet
Ardoa herodias Low
A^. herodias Low
Casnerodius albus Low
Eqretta rufescens Low
£. tricolor Low
£. caerulea Low
Aj a i a ajaja Low
Catoptrophorus semipalmatus Low
Swimmers
Double-crested
cormorant
White pel ican
(winter only)
Crested grebe
(winter)
Red-breasted merganser
(winter)
Flyers-plungers
Osprey
Bald eagle
Brown pel ican
Phalacrocorax auritus
Pelecanus erythrorhynchos
Mergus serrator
Pandion hali actus
Haliaeetus leucocephalus
Pelecanus occidental is
High
High
Hioh
High
High
55
throughout south Florida, but is sometimes
found in greatest numbers on the shallow
grass flats in Florida Bay. Small egrets
and herons probably all feed occasionally
on the shallowest, exposed flats, but are
generally limited by water too deep for
them to wade. The ecology of wading birds
and their feeding behavior have been re-
viewed by Kushlan (1976, 1978). Odum
et al . (1981) reviewed the extensive avi-
fauna of the mangrove regions of southern
Florida.
5.6 MAMMALS
Some marine mammals also feed in sea-
grass beds. Odell (1979) reported that
although 27 species of marine mammals were
either sighted alive or reported stranded
on beaches in south Florida in recent
years, only 2 were common: the manatee
(Trichechus manatus) and the bottlenose
dolphin (Tursiops truncatus) .
Although the range of the manatee was
formerly much larger, now it seems largely
confined to the protected regions of
Everglades National Park. Odell (1976)
surveyed the manatee distribution in
the Everglades region. Of a total of
302 herds with 772 individuals, 46? were
sighted in Whitewater Bay, 20% in the Gulf
of Mexico, 23% in inland waters, and only
1% in Florida Bay. A later study (Odell
1979) reported no manatee sightings in
Biscayne Bay.
The bottlenose dolphin is the most
common marine mammal in south Florida
waters and feeds over grass flats, even
those less than 1 m (3.3 ft) deep. In the
Everglades National Park region, Odell
(1976) reported that 36% of the animals
seen were in the Gulf of Mexico, 33°i were
in Whitewater Bay, 20% were in inland
waters, and 11% in Florida Bay. The rela-
tively low numbers in Florida Bay were
probably due to the extreme shallowness
which would preclude swimming for this
large mammal. Bottlenose dolphin are
opportunistic feeders, primarily on fish.
Their diets are not well known, but they
consume large quantities of mullet in
Florida Bay.
By comparison with the Everglades
region, Biscayne Bay had a low dolphin
density. Odell (1979) found that in
aerial surveys of the two regions, 11.4
animals were sighed per flight hour in the
Everglades area, while only 1.25 animals
per hour were seen in Biscayne Ray.
56
CHAPTER 6
TROPHIC RELATIONSHIPS IN SEAGRASS SYSTEMS
6.1 GENERAL TROPHIC STRUCTURE
Seagrasses and associated epiphytes
provide food for trophically higher organ-
isms by (1) direct herbivory, (2) detrital
food webs within grass beds and (3) ex-
ported material that is consumed in other
systems either as macroplant material or
as detritus (Figure 22). Classically the
detrital food web within the grass beds
has been considered the primary pathway,
and in most cases is probably the only
significant trophic pathway. During the
past few years, new information has been
gathered on the relative role of the other
modes of utilization. The picture emerg-
ing is that in many locations both the
direct utilization pathway and the export
of material may be of far more importance
than previously suspected; however, it
still appears that the detrital food web
is the primary pathway of trophic energy
transfer (Zieman et al. 1979; Kikuchi
1980; Ogden 1980).
Studies have attempted to measure the
proportion of daily seagrass production
which is directly grazed, added to the
litter layer, or exported. Greenway
(1976) in Kingston Harbor, Jamaica, esti-
mated that of 42 g/m"^/wk production of
turtle grass, 0.3% was consumed by the
small bucktooth parrotfish, Sparisoma rad-
ians; 48.1% was consumed by the urchin,
Lytechinus ariegatus; and 42.1% deposited
on the bottom and available to detriti-
vores. The rest of the production was
exported from the system. This study may
PLANT CANOPY
STRUCTURE
Figure 22. Principal energetic pathways in seagrass beds,
57
overenphasize the quantity of seagrass
naterial entering the grazing food chain
since urchins ^re not typically found at
densities of 20 urchins/n- as was the case
in Kingston Harbor (Ogden li^eO). In St.
Croix, it has been estimated that typi-
cally between 5% and 10% of daily produc-
tion of turtle grass is directly consuned,
primarily by Sparisoina radians and second-
arily by the urchins Oiadena antillarun
and Tripneustes ventricosus. Averaged over
the day, turtle 'grass production was
2.7 g dw/m /day of which only about 1%
was exported, while 60? to 100% of the
0.3 g dv//m /day production of manatee
grass was exported (Zienan et al. 1979).
From these figures it is conservatively
estimated that about 70« oF the daily
production of seagrasses was available to
the detrital system.
Many of the small organisms in grass
beds use algal epiphytes and detritus as
their food sources. The gastropods are
the most prominent organisms feeding on
epiphytic algae in seagrass beds. Arphi-
pods, isopods, crabs, and other crusta-
ceans ingest a mixture of epiphytic and
benthic algae as well as detritus (Odum
and Heald 1972). As research continues,
it is becoming apparent that the utiliza-
tion of this combination of nicroalgae and
detritus represents one of the major
energy transfer pathways to hiaher oroan-
isms.
Notable by their absence are the
large flocks of ducks and related water-
fowl found on temperate Zostera beds and
especially the freshwater Ruppia beds
(Jacobs et al . 1981). ricRoy and Helfferich
(19S0) list 43 bird species that consume
seagrass primarily in the temperate zone.
Relatively few species of birds ingest
seagrass species of the tropics or forage
for prey in the sediments of shallow grass
beds.
Detritus undoubtedly serves as the
base of a major pathway of energy flow in
seagrass meadows. A significant proportion
of net production in the seagrass bed re-
sults in detritus either by dying in place
and being broken down over a period of
months by bacteria, funqi and other organ-
isms (Robertson and Hann 1980) or by being
consumed by large herbivores, fragmented,
and returned as feces (Ogden 1980). In
Piscayne ^ay, turtle grass formed the most
important constituent of the detritus
present (37. 1?,). while other portions
included 2.1% other seagrasses, 4.6%
algae, 0.4% animal remains, 3.3% mangrove
leaves and 2.5% terrestrial material
(Fenchel 1970). The microbial community
living in the detritus collected consisted
mainly of bacteria, small zooflagellates,
diatoms, unicellular algae, and ciliates.
It is these types of organisms which form
the major source of nutrition for detrital
feeders. Bloom et al . (1972), Santos
and Simon (1974), and Young and Young
(1977) provided species lists annotated
witii feeding habits for molluscs and
polychaetes, many of which ingest detri-
tus.
Typically penacid and caridean shrimp
are considered to be o.nnivores. The pink
shrimp (Penaeus duorarum), in addition to
oroanic detritus and
detritus
chaetes, nematodes,
mysids, copepods,
ostracods, molluscs
(Eldred 1958; Eldred
consumers strip the
sand, ingests poly-
caridean shrimp,
isooods, amphipods,
and foraminiferans
et al. 1061). These
bacteria and other
organisms from the detritus, and the fecal
pellets are subsequently roingested fol-
lowing recolonization (Fenchel 1970).
Some fishes, notably the mullet (Mugil
cephalus), are detrital feeders (Odum
1970). Several large invertebrates such
as the gastropod Strombus gigas (Randall
1964) and the asteroid Oreaster reticula-
tus (Schoibling 1980) take detritus as a
part of their food. To emphasize the
importance of detritus to higher trophic
levels within the grass, the work of Carr
and Adams (1973) should be noted. They
found that detritus consumers were of
major importance in at least one feeding
stage of 15 out of 21 species of juvenile
marine fishes studied.
It is well documented that fishes
feed while occupying grass beds (Carr and
Adams 1973; Adams 1976b; Brook 1975, 1977;
Robertson and Howard 1978), as opposed to
simply using them for shelter. Typically,
seagrass-associated fishes are small, gen-
eral ist feeders, tending to prey upon epi-
faunal organisms, primarily crustaceans.
Infaunal animals are under used in propor-
tion to their abundance as few fishes
resident in the grass beds feed on them or
on other fishes (Kikuchi 1980).
58
Numerous fishes ingest sono plant
material, while relatively few of these
species &re strict herbivores; exceptions
are the Scarids and Acanthurids already
mentioned. Host plant and detrital mats-
rial is probably taken incidentally while
feeding on other organisms, Orthopristis
chrysoptera and Lagodon rhonboides are two
Mory abundant grass bed fishes in south
Florida and apparently during so^ne feeding
stages are O'lnivores, ingesting substan-
tial ainounts of epiphytes, detritus and
seagrass (Carr and Adains 1973; Adams
1976a, 1976b; Kinch 1979).
include some filefishes,
nies, and qobies.
Other oiiimvores
porgies, blen-
Castropods are fed upon by a variety
of fishes including wrasses, porcupine
fishes, eagle rays, and the permit Trach-
notus folcatus. Randall (1967) listed 71
species of fishes that feed on gastropods,
25 ingesting 10/c or more by volume. Most
species crush the shell while ingesting,
but a few swallow the gastropod whole.
The white grunt (Hacnulon plumeri ) appears
to snap off the extended head of Cerith-
iuni, ignoring the shell. The southern
stingray (Dasyatus americana) has been
observed turning over the queen conch
(Strombus gigas)' and wrenching off the
conch's extended foot with its jaws as
the conch tries to right itself (Randal
1964). The spiny lobster (Panul irus
argus) is an active predator on seagrass
molluscs.
epi fauna, the impact of blue crab pre-
dation may be greatest on epibenthic
fauna.
The majority of fishes within the
grass bed feeds on small, mobile epi fauna
including copepods, cumaceans, amphipods,
isopods, and shrimp. f^ishes feeding in
this manner include all the seasonally
resident fishes of the south Florida grass
beds, such as the Sciaenids, Pomadasyids,
Lutjanids, and Cerrids, as well as many of
the permanent residents, like Syngnatbids,
and Clinids. As such, they are deriving
inuch of their nutrition indirectly from
seagrass epiphytes and the detrital com-
munity present in the grass bed rather
than the grasses theriselves. Many of these
fishes, as adults, will feed on other
fishes; however, as juvenile residents in
the grass beds, their snail size limits
them to eating epi fauna.
Important piscivores are present in
south Florida grass flats. These include
the lemon shark (Negaprion brevirostires)
and the bonnethead shark (Sphyrna tiburo),
the tarpon (Megalops atlanticaTT the liz-
ardfish (Synodon foetensTJ the coronet
fish (Fistularia tobacaria), the barracuda
(Sphyraena barracuda"), carangids, the grey
snapper (Lutjanus oriesus), and the spot-
ted seatrout TC>'noscion nehulosus).
U.2 DIRECT HERBIVORY
The southern stingray and the spotted
eagle ray (Aetobatis narinari) are tv/u of
a relatively few number of fishes that
feed on infauna within the grass bed.
These fishes excavate the sediments.
Other similar feeders are wrasses, goat-
fishes, and mojarras. Adult yellowtail
snapper (Oryhurus chrysurus) have been ob-
served foraging in back reef seagrass sed-
iments (Zieman, personal observation).
That the infauna is not heavily preyed
upon is typical of seagrass beds (Kikuchi
1974, 1980). Apparently the protection
from predation afforded the Infauna of
grass beds is great enough that few fishes
specialize on infauna when feeding (Orth
1977b). The blue crab (Call inectes
sapldus) has been observed to shift its
feeding from Zostera infauna to epibiota
and thus, because of the protective rhl-
zor.e layer and the accessibility of the
Caribbean grass beds may be unique
for the numbers and variety of direct con-
sumers of blade tissue (Ogden 1980) as
relatively few species Ingest green sea-
grass in significant quantities (Table
10). Prominent herbivores Include urchins,
conch, fishes, as well as the green tur-
tle, Chelonia mydas, and Caribbean manatee
(Trichechus manatus). The elucidation of
the role of direct herbivory as a pathv/ay
of energy flow in seagrasses has been
slow In developing. Until recently. It
was assumed that few organisms consumed
seagrasses directly, and that herbivory
had substantially decreased with the
decline of the populations of the green
sea turtle. Direct grazing of seagrasses
In south Florida is probably of greatest
importance in the grass beds of the Flor-
ida Keys and outer margin of Florida Bay
which are relatively close to coral reefs.
59
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66
The hcrbivory of parrotfish and sea ur-
chins may be important in the back reef
areas and in Hawk. Channel; but, with the
exception of sporadic grazing by passing
turtles, herbivory is low or non-existent
in the areas to the west of the Florida
Keys (J.C. Zieman, personal observation).
Parrotfish typically move ofT the
reef and feed during the day (Randall
1965). Sparisopa radians, S^. rubripinnc,
and S^. chrysopterun are known to feed on
seagrass and associated algae (Randall
1967). The bucktooth parrotfish {S_. rad i -
ans) feeds almost exclusively on turtle
grass. Other fishes that dre important
seagrass consumers are surqeonfishes
(Acanthuridae) (Randall 1967;" Clavijo
1974),"" the porcies (Sparidae) (Pandall
1967; Adams 1976b), and the halfbeaks
(Heiniramphidae) ,
Fishes in the Caribbean seagrass beds
tend to be general ist herbivores, select-
ing plants in approximate relation to
their abundance in the field (Ogden 1976;
Ogden and Lobel 1978). Some degree of
selectivity is evident, however. Sparisoma
chrysopterum and S^. radians, when gi"ven a
choice, will select seagrass with epiphy-
tes (Lobel and Ogden, personal communica-
tion). Seagrasses (turtle grass, manatee
grass, and shoal grass) ranked highest in
preference over common algal seagrass
associates.
Urchins that feed on seagrass include
Eucidaris tribuloides, Lytechinus varieaa-
tus, Diadema antil larun
yentricosus (llcPherson
_ and Tripneustes
1964, 1968; Randall
and Grant 1965; Moore
Prim 1973; Abbott
et al. 1973; Moore
The
et al. 1964; Kier
and r'cPherson 1964;
et al. 1974; Ooden
et al. 1963a, lS63b; Greenway 1976),
latter two urchins feed in approximate
proportion to food abundance in the area.
Where present in seagrass beds, J. ventri-
cosus and D. antillarum feed on seagrasses
with epiphytes exclusively (Ogden 1980).
Lytechinus variegatus is largely a detri-
tal feeder (Ogden 1980), but has denuded
large areas in west Florida (Camp et al .
1973).
The queen conch (Strombus giqas),
once a common inhabitant of Caribbean sea-
grass beds, has been dramatically reduced
in many areas because of its high food
value and ease of capture by man. Conchs
are found in a variety of grass beds, from
dense turtle grass to sparse manatee grass
and Halophila. V.'hen in turtle grass beds
conchs primarily feed by rasping the epi-
phytes from the leaves as opposed to eat-
ing the turtle grass. In sparse grass
beds, however, conchs consumed large quan-
tities of manatee orass and Halophila
(Randall 1964). A maximum of 207. of the
stomach contents of conchs at St. John,
U.S. Virgin Islands, was comprised of tur-
tle grass. In manatee grass (Cymodocea)
beds, conchs consumed mostly this seagrass
along with some algae. The maximum quan-
tity of seagrass found v;as 80% Halophila
from the gut of four conchs from Puerto
Pico.
The emerald nerite (Smaragdia v i r i -
d i s ) , a small gastropod, commonly ~5 to
8 mm long, can be numerous in turtle grass
beds although it is difficult to see be-
cause its bright green color matches that
of the lower portion of the turtle grass
blades. It is a direct consumer of turtle
grass where it roams about the lower half
of the green blades; the snail removes a
furrow about 1 mm wide and half the thick-
ness of the blade with its radula (J.C.
Zieman and P.T. Zieman, personal observa-
tion).
^'ost studies (for review, see Law-
rence 1975) indicate that the majority of
seagrass consumers have no enzymes to di-
gest structural carbohydrates and that,
with the exception of turtles and possibly
manatees, they do not have a gut flora
capable of such digestion. Thus, most
macroconsumers of seagrasses depend on the
cell contents of seagrasses and the at-
tached epiphytes for food and must have a
mechanism for the efficient maceration of
the material. The recent work of Weinstein
et al . (in press), however, demonstrated
that the pinfish was capable of digesting
the structural cellulose of detrital mat-
ter or green seagrasses. Feeding rates
are high for urchins and parrotfishes,
while absorption efficiency is around 50*
(Moore and McPherson 1965; Lowe 1974;
Ogden and Lobel 1978). Assimilation effi-
ciencies for ]_. ventricosus and \^. varie-
gatus are relatively low, 3.8% and 3.0%
respectively (Moore et al . 1963a, 1963b).
67
The result of macroherbivore grazing
within the grass bed can be dramatic (Camp
et al . 1973). Of greater overall signifi-
cance, however, is the fragmentation of
living seagrass and production of particu-
late detritus coincident with feeding.
Further, the nature of urchin and parrot-
fish feeding results in the liberation of
living seagrass and its subsequent export
from the bed (Greenway 1976; Zieman et al .
1979). Zieman et al. (1979) observed that
manatee grass blades floated after detach-
ment, whereas turtle grass tended to sink;
the result was that turtle grass was the
primary component of the litter layer
available for subsequent utilization by
detritivores.
Many of the macroconsumers, such as
Acanthurids, S^. rul^ripinne and S^. chrysop-
terum (Randall 1967) , fngesting living
seagrass take in only small amounts, the
majority of their diet consisting of epi-
phytic algae. Species primarily ingesting
seagrass (i.e., S^. radians) typically pre-
fer the epiphytized portion of the sea-
grass blade. These observations suggest
that seagrass epiphytes &re important in
the flow of energy within the grass car-
pet. Many of the small, mobile epifaunal
species that are so abundant in the grass
bed and important as food for fishes feed
at least in part on epiphytes. Typically,
these animals do not feed on living sea-
grass, but often ingest significant quant-
ities of organic detritus with its asso-
ciated flora and fauna. Tozeuma carol in-
ense, a common caridean shrimp, feeds on
epiphytic algae attached to seagrass
blades but undoubtedly consumes coinciden-
tally other animals (Ewald 1969). Three
of the four seagrass-dwel 1 ing amphipods
common in south Florida use seagrass epi-
phytes, seagrass detritus, and drift algae
as food, in this order of importance (Zim-
merman et al . 1979). Epiphytic algae were
the most important plant food sources
tested since they were eaten at a high
rate by Cymadusa compta, Gammarus mucro-
natus, and ^'el ita nitida. Epiphytic algae
were also assimilated more efficiently by
these amphipods (48?, 43?i and 75%, respec-
tively) than other food sources tested,
including macrophytic drift algae, live
seagrass, and seagrass detritus. Live
seagrass had little or no food value to
these amphipods.
There is little doubt that the struc-
ture of many grass beds was profoundly
different in pre-Columbian times when tur-
tle populations were 100 to 1,000 times
greater than those now. Rather than ran-
domly cruising the vast submarine meadows,
grazing as submarine buffalo, turtles
apparently have evolved a distinct feeding
behavior. They are not resident in sea-
grass beds at night, but live in deep
holes or near fringing reefs and surface
about once an hour to breathe. During
morning or evening the turtles will swim
some unknown distance to the seagrass beds
to feed. What is most unioue is that they
return consistently to the same spot and
regraze the previously grazed patches,
maintaining blade lengths of only a few
centimeters (Bjorndal 1980). Thayer and
Engel ("S in preparation) calculated that
an intermediate-sized Chelonia (64 kg or
141 lb) consumes daily a dry v;eight of
blades equivalent to 0.5 m- of an average
turtle grass bed (500 g dw of leaves).
Since the regrazed areas do not contain as
heavy a standing crop as ungrazed grass
beds, it is obvious that their grazing
plots must be considerably larger. The
maximum length of grazing time on one dis-
tinct patch is not known, but J.C. Ogden
(personal communication) observed patches
that persisted for up to 9 months.
The first time turtles graze an area
they do not consume the entire blade but
bite only the lower portion and allow the
epiphytized upper portion to float away.
This behavior was recently described in
some detail by Bjorndal (1980), but the
earliest description was from the Pry
Tortugas where John James Audubon observed
turtles feeding on seagrass, "which they
cut near the roots to procure the most
tender and succulent part" (Audubon 1834).
It was previously thought that there
was an advantage for grazers to consume
the epiphyte complex at the tip of sea-
grass leaves, as this complex was of
higher food value than the plain seagrass
leaf. Although this seems logical, it
appears not to be so, at least not for
nitrogen compounds. While studying the
food of turtles, Mortimer (1976) found
that entire turtle grass leaves collected
at Seashore Key, Florida, averaged 1.7% fl
on an ash free basis, while turtle grass
68
leaves plus their epiphytes averaged 1.4%
N. Bjorndal found that grazed turtle
grass leaves averaged 0.35% N (AFDW)
higher than ungrazed leaves, and Thayer
and Engel (MS, in preparation) found a
nitrogen content of 1.55% (DW) in the
esophagus of Chelonia. Zienan and Iverson
(in preparation) found that there was a
decrease in nitrogen content with age and
epiphytization of seagrass leaves. The
basal portion of turtle grass leaves fron
St. Croix contained 1.6% to 2.0% N on a
dry weight basis, while the brown tips of
these leaves contained 0.6% to 1.1% N,
and the epiphytized tips ranged fron 0.5%
to 1.7% N. Thus the current evidence
would indicate that the green seagrass
leaves contain more nitrogen than either
the senescent leaves or the leaf-epiphyte
copiplex. By successively recropping
leaves from a plot, the turtle main-
tains a diet that is consistently higher
in nitrogen and lower in fiber content
than whole leaves (Bjorndal 1980).
Grazing on seagrasses produces
another effect on sea turtles. In the
Gulf of California (Felger and Moser 1973)
and Nicaragua (Mortimer, as reported by
Bjorndal 1980), witnesses reported that
turtles that had been feeding on sea-
grasses were considered to be good tast-
ing, while those that were caught in areas
where they had fed on algae were consid-
ered to be "stinking" turtles with a defi-
nite inferior taste.
Thayer and Engel (MS. in preparation)
suggested that grazing on seagrasses can
short-circuit the time frame of decomposi-
tion. They showed that an intermediate-
sized green turtle which consumes about
300 g dry weight of leaves and defecates
about 70 g dry weight of feces daily, does
return nitrogen to the environment at a
more rapid rate than occurs for the decom-
position of a similar amount of leaves.
They point out that this very nutrient-
rich and high nutritional quality fecal
matter should be readily available to
detritivores. It is also pointed out that
this matter is probably not produced
entirely at the feeding site and thus
provides aa additional interconnection
between grassbeds and adjacent habitats.
Like the turtles, the Caribbean
manatee (Trichechus manatus) formerly was
common throughout the Caribbean, espec-
ially in the mainland areas, but is now
greatly reduced in range and population.
Manatees live in fresh or marine waters;
and in Florida, most manatee studies have
focused on the manatee's ability to con-
trol aquatic weeds. Manatees, which weigh
up to 500 kg (1,102 lb), can consume up to
20% of their body weight per day in aqua-
tic plants.
When in marine waters, the manatee
apparently feeds much like its fellow
sirenians, the dugongs. The dugongs use
their rough facial bristles to dig into
the sediment and grasp the plants. These
are uprooted and shaken free of adhered
sediment. Husar (1975) stated that feed-
ing patches are typically 30 by 60 cm (12
by 24 inches) and that they form a conspi-
cuous trail in seagrass beds. This author
has observed manatees feeding in Thalassia
beds in much the sam.e manner. The patches
cleared were of a similar size as those
described for the dugongs, and rhizome
removal was nearly complete. The excess
sediments from the hole were mounded on
the side of the holes as if the manatee
had pushed much of it to the side before
attempting to uproot the plants.
Manatees would seem to be more
limited in their feeding range because of
sediment properties, as they reouire a
sediment which is sufficiently unconsoli-
dated that they may either root down to
the rhizome or grasp the short shoot and
pull it out of the sediment. Areas where
manatee feeding and feeding scars were
observed were characterized by soft sedi-
ments and lush growth of turtle grass and
Hal imeda in mounded patches. Nearly all
areas in which sediments were more consol-
idated showed no signs of feeding. In the
areas where the manatees were observed,
the author found that he could readily
shove his fist 30 cm (12 inches) or more
into the sediments, while in the adjacent
ungrazed areas, maximum penetration was
only a few centimeters and it was impos-
sible to remove the rhizomes without a
shovel .
6.3 DETRITAL PPOCESSING
For the majority of animals that
derive all or part of their nutrition from
69
seagrasses, the greatest proportion of
fresh plant material is not readily used
as a food source. For these animals sea-
grass organic natter becomes a food source
of nutritional value only after undergoing
decomposition to particulate organic
detritus, vvhich is defined as dead organic
natter along with its associated nicro-
organisns (Heald 1969).
The nonavailability of fresh seagrass
material to detritus-consuming animals
(detritivores) is due to a complex combi-
nation of factors. For turtle grass
leaves, direct assays of fiber content
have yielded values up to 59% of the dry
weight (Vicente et al . 1978). Many ani-
mals lack the enzymatic capacity to assim-
ilate this fibrous material. The fibrous
components also make fresh seagrass resis-
tant to digestion except by animals (such
as parrotfishes and green turtles) with
specific morphological or physiological
adaptations enabling physical maceration
of plant material. Fresh seagrasses also
contain phenolic compounds that may deter
herbivory by some animals.
During decomposition of seagrasses,
numerous changes occur that result in a
food source of greater value to many con-
sumers. Bacteria, fungi, and other micro-
organisms have the enzymatic capacity to
degrade the refractile seagrass organic
matter that many animals lack. These
microorganisms colonize and degrade the
seagrass detritus, converting a portion of
it to microbial protoplasm and mineraliz-
ing a large fraction. Whereas nitrogen is
typically 11 to 4% dry weight of seagrass-
es (Table 7), microflora contain 5? to ICl'
nitrogen. Microflora incorporate inorganic
nitrogen from the surrounding medium —
either the sediments or the water column--
into their cells during the decomposition
process, enriching the detritus with pro-
teins and other soluble nitrogen com-
pounds. In addition, other carbon com-
pounds of the microflora are much less
resistant to digestion than the fibrous
components of the seagrass matter. Thus,
as decomposition occurs there will be a
gradual mineralization of the highly
resistant fraction of the seagrass organic
matter and corresponding synthesis of
microbial biomass that contains a much
higher proportion of soluble compounds.
Microorganisms, because of their di-
verse enzymatic capabilities, are a neces-
sary trophic intermediary between the sea-
grasses and detritivorous animals. Evi-
dence (Tenore 1977; Ward and Cummins 1979)
suggests that these animals derive the
largest portion of their nutritional re-
quirements from the microbial coinponent of
detritus. Detritivores typically assimi-
late the microflora compounds with effi-
ciencies of 50"J to almost 100%, whereas
plant compound assimilation is less than
5% efficient (Yingst 1976; Lopez et al .
1977; Cammen 1900).
During seagrass decomposition, the
size of the particulate matter is decreas-
ed, making it available as food for a wid-
er variety of animals. The reduced parti-
cle size increases the surface area avail-
able for microbial colonization, thus in-
creasing the deconposition rate. The abun-
dant and trophically important deposit-
feeding fauna of seagrass beds and adja-
cent benthic communities, such as poly-
chaete worms, amphipods and isopods, oohi-
uroids, certain gastropods, and mullet,
derive much of their nutrition from fine
detrital particles.
It is important to note that much of
the contribution of seagrasses to higher
trophic levels through detrital food webs
occurs away from the beds. The more
decomposed, fine detrital particles (less
than 0.5 mm) are easily resuspended and
are widely distributed by currents (Fisher
et al. lf^79). They contribute to the
organic detritus pool in the surrounding
waters and sediments where they continue
to support an active microbial population
and are browsed by deposit feeders.
Physical Breakdown
The physical breakdown and particle
size reduction of seagrasses are important
for several reasons. First, particle size
is an important variable in food selection
for a wide range of organisms. Filter
feeders and deposit feeders (polychaetes,
zooplankton, gastropods) are only able to
ingest fine particles (less than 0.5 mm
diameter). Second, as the seagrass mate-
rial is broken up, it has a higher surface
area to volume ratio which allows more
microbial colonization. This increases
70
the rate of biological breakdown of the
seagrass carbon. Physical decomposition
rate is an approximate indication of the
rate at which the plant material becomes
available to the various groups of detri-
tivores and how rapidly it will be sub-
jected to microbial degradation.
Evidence indicates that turtle grass
detritus is physically decomposed at a
rate faster than the marsh grass, Spartina
al terniflora, and mangrove leaves. Zieman
(1975b) found a 50% loss of original dry
weight for turtle grass leaves after 4
weeks using sample bags of l-mm mesh size
(Figure 23).
Seagrass leaves are often transported
away from the beds. Large quantities are
found among the mangroves, in wrack lines
along beaches, floating in large mats, and
collected in depressions on unvegetated
areas of the bottom. Studies have shown
that the differences in the physical and
biological conditions in these environ-
ments resulted in different rates of phys-
ical decomposition (Zieman 1975b). Turtle
grass leaves exposed to alternate wet-
ting and drying or wave action breakdown
rapidly, although this may inhibit micro-
bial growth (Josselyn and Mathieson 1980).
Biological factors also affect the
rate of physical decompositon. Animals
grazing on the microflora of detritus dis-
rupt and shred the plant substrate, accel-
erating its physical breakdown. Fenchel
(1970) found that the feeding activities
of the amphipod Parahyel la whelpyi dramat-
ically decreased the particle size of
turtle grass detritus.
Microbial Colonization and Activities
Feeding studies performed with vari-
ous omnivores and detritivores have shown
that the nutritional value of macrophyte
detritus is limited by the quantity and
quality of microbial biomass associated
with it. (See Cammen 1980 for other stud-
ies of detrital consumption.) The micro-
organisms' roles in enhancing the food
value of seagrass detritus can be divided
into two functions. First, they enzymati-
cally convert the fibrous components of
the plant material that is not assimilable
by many detritivores into microbial bio-
mass which can be assimilated. Second,
100
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o
90
W^ %"■•■•.
z
\\ ^ '••■■ " ■•
z
80
1 \ ■ ^ '•••'■•■
<
70
1 \'- ^
?
1 V ^-^
LU
60
tt
50
1—
I
40
o
30
Thala>sia(d)
UJ
^
20
«
10
■ ~ ~ ^ Sportino
Distichlif
Jun<ui
Sportino
"'•^. Rhiiophoro
Tholossio (w)
Burkholder t Bornside
De la Crui
Heold
Zieman
Salicof nio
O
1 2 3 4 5 6 7
TIME IN MONTHS
Figure 23. Comparative decay rates showing the rapid decomposition of seagrasses com-
pared with other marine and estuarine plants (references: Burkholder and Bornside 1957;
de la Cruz 1965; Heald 1969; Zieman 1975b).
71
the microorganisms incorporate constitu-
ents such as nitrogen, phosphorous, and
dissolved organic carbon compounds from
the surrounding medium into their cells
and thus enrich the detrital complex. The
microorganisms also secrete large quanti-
ties of extracellular materials that
change the chemical nature of detritus and
may be nutritionally available to detriti-
vores. After initial leaching and decay,
those processes make microorganisms the
primary agents in the chemical changes of
detritus.
The microbial component of macrophyte
detritus is highly complex and contains
organisms from many phyla. These various
components interact and influence each
other to such a high degree that they are
best thought of as a "decomposer commun-
ity" (Lee 1980). The structure and activ-
ities of this community are influenced by
the feeding activities of detritivorous
animals and environmental conditions.
Microflora in Petri tivore Nutrition
Microbial carbon constitutes only 10%
of the total organic carbon of a typical
detrital particle, and microbial nitrogen
constitutes no more than 10% of the total
nitrogen (Rublee et al. 1978; Lee et al .
1980). Thus, most of the organic compo-
nents of the detritus are of plant origin
and are limited in their availabil ity "to
detritivores.
Carbon uptake from a macroalga,
Gracilaria, and the seagrass Zostera
marina by the deposit-feeding polychaete,
Captella capitata, was measured by Tenore
(1977). Uptake of carbon by the worms was
directly proportional to the microbial
activity of the detritus (measured as
oxidation rate). The maximum oxidation
rate occurred after 1*^ days for Gracilaria
detritus and after ICO days for Zostera
detritus. This indicates that the charac-
teristics of the original plant matter
affect its availability to the microbes,
which in turn limits the assimilation of
the detritus by consumers.
Most of the published evidence shows
that detritivores do not assimilate
significant portions of the non-microbial
component of macrophytic detritus. For
example, Newell (1965) found that deposit-
feeding molluscs removed the nitrogen from
sediment particles by removal of the
microorganisms but did not measurably
reduce the total organic carbon content of
the sediments which was presumably domi-
nated by detrital plant carbon. When the
nitrogen-poor, carbon-rich feces were
incubated in seawater, their nitrogen con-
tent increased because of the growth of
attached microorganisms. A new cycle of
ingestion by the animals again reduced the
nitrogen content as the fresh crop of
microorganisms was digested. In a study
of detrital leaf material, Morrison and
White (1980) found that the detritivorous
amphipod Mucrogammarus sp. ingested the
microbial component of live oak (Cliercus
virginica) detritus without altering or
consuming the leaf matter.
While the importance of the microbial
components of detritus to detritivores is
established, some results have indicated
that consumers may be capable of assimi-
latina the plant carbon also. Cammen
(1980) found that only 26% of the carbon
requirements of a population of the
deposit-feeding polychaete Nereis succinea
would be met by ingested microbial bio-
mass. The microbial biomass of the in-
gested sediments could supply 90% of the
nitrogen requirements of the studied poly-
chaete population. The mysid Mysis steno-
lepsis, commonly found in Zostera beds,
was capable of digesting cell-wall com-
pounds of plants (Foulds and Mann 1978).
These studies raise the possibility that
while microbial biomass is assimilated at
high efficiencies of 50% to 100% (Yingst
1976; Lopez et al. 1977) and supplies
proteins and essential growth factors,
the large quantities of plant material
that are ingested may be assimilated at
low efficiencies (less than 5%) to supply
carbon requirements. Assimilation at this
low efficiency would not be readily quan-
tified in most feedinq studies (Cammen
1980).
The microbial degradation of seagrass
organic matter is greatly accelerated by
the feedinq activities of detritivores and
microfauna, although the exact nature of
the effect is not clear. Microbial res-
piration rates associated with turtle
grass and Zostera detritus were stimulated
by the feedinq activities of animals,
apparently as a result of physical frag-
mentation of the detritus (Fenchel 1970;
Harrison and Mann 1975a).
72
Chemical Changes During Decomposition
The two general processes that occur
during decomposition, loss of plant com-
pounds and synthesis of microbial biomass,
can be incorporated into a generalized
model of chemical changes. Initially, the
leaves of turtle grass, manatee grass, and
shoal grass contain 9% to 22% protein, 6%
to 31% soluble carbohydrates, and 25? to
44% ash (dry weight basis), depending on
species and season (Dawes and Lawrence
1980). Direct assays of crude fiber by
Vicente et al . (1978) yielded values of
59% for turtle grass leaves; Dawes and
Lawrence (1980) classified this material
as "insoluble carbohydrates" and calcu-
lated values of 34% to 41% for this spe-
cies by difference. Initially, losses
through translocation and leaching will
lead to a decrease in certain components.
Thus, the organic carbon and nitrogen con-
tent will be decreased, and the remaining
material will consist primarily of the
highly refractive cell wall compounds
(cellulose, hemicellulose, and liqnin) and
ash (Harrison and Kann 1975b; Thayer
et al. 1977).
As microbial degradation progresses,
the nitrogen content will increase through
two processes: oxidation of the remaining
nitrogen-poor seagrass compounds and syn-
thesis of protein-rich microbial cells
(typically 30% to 50% protein) (Thayer
et al. 1977; Knauer and Ayers 1977). The
accumulation of microbial debris, such as
the chitin-containing hyphal walls of fun-
gi, may also contribute to the increased
nitrogen content (Suberkropp et al . 1976;
Thayer et al. 1977). Nitrogen for this
process is provided by adsorption of inor-
ganic and organic nitrogen from the sur-
rounding medium, and fixation of atmos-
pheric N . For tropical seagrasses, in
particular, there is an increase in ash
content during decomposition because of
deposition of carbonates during microbial
respiration and growth of encrusting algal
species, and organic carbon usually con-
tinues to decrease (Harrison and t'ann
1975a; Knauer and Ayers 1977; Thayer
et al. 1977).
Chemical Changes as Indicators of Food
Value
Nitrogen content has long been con-
sidered a good indicator of the food value
of detritus and has been assumed to repre-
sent protein content (Odum and de la Cruz
1967). Subsequent analyses of detritus
from many vascular plant species, however,
have shown that up to 30% of the nitrogen
is not in the protein fraction (Harrison
and Mann 1975b; Suberkropp et al . 1976;
Odum et al . 1979). As decomposition pro-
gresses, the non-protein nitrogen fraction
as a proportion of the total nitrogen can
increase as the result of several process-
es: complexing of proteins in the lignin
fraction (Suberkropp et al . 1976); produc-
tion of chitin, a major cell wall compound
of fungi (Odum et al . 1979b); and decompo-
sition of bacterial exudates (Lee et al .
1980). As a result, actual protein con-
tent may be a better indicator of food
value. Thayer et al . (1977) found that
the protein content of Zostera leaves
increased from standing dead to detrital
fractions, presumably due to microbial
enrichment. The role of the non-protein
and protein nitrogen compounds in detriti-
vore nutrition is not presently well
understood.
Like many higher plants, tropical
seagrasses contain phenolic acids known as
allelochemicals. These compounds are known
to deter herbivory in many plant groups
(Feeny 1976). Six phenolic acids have
been detected in the leaves, roots, and
rhizomes of turtle grass, manatee grass,
and shoal grass (Zapata and McN'illan
1979). In laboratory studies two of these
compounds, ferulic acid and p-coumaric
acid, when present at concentrations found
in fresh leaves, inhibited the feeding
activities of detritivorous amphipods and
snails grazing on S^. al terniflora detri-
tus. During decompositon the concentra-
tions of these compounds decreased to
levels that did not significantly inhibit
the feedinq activities of the animals
(Valiela et al . 1979).
Seagrass leaves may also contain com-
pounds that inhibit the growth of microor-
ganisms; this in turn would decrease the
usable nutritional value of the detritus.
Water soluble extracts of fresh or re-
cently detached Z^. marina leaves inhibited
the growth of diatoms, phytoflagellates,
and bacteria (Harrison and Chan 19B0).
The inhibitory compounds are not found in
older detrital leaves or ones that have
been partially desiccated.
73
Release of Dissolved Organic l^atter
Seagrasses release substantial
amounts of dissolved organic carbon (DOC)
during growth and decomposition. The DOC
fraction is the most readily used fraction
of the seagrass organic matter for micro-
organisms and contains much of the soluble
carbohydrates and proteins of the plants.
It is quickly assimilated by microorgan-
isms, and is available to consumers as
food in significant quantities only after
this conversion to microbial biomass.
Thus, the utilization of seagrass DOC is
functionally similar to detrital food webs
based on the particulate fraction of sea-
grass carbon. Both epiphytes and leaves
of Zostera are capable of taking up label-
led organic compounds (Smith and Penhale
1980).
Experiments designed to quantify the
release of DOC from growing seagrasses
have yielded a wide range of values. The
short-term release of recently synthesized
photosynthate from blades of turtle grass
was found to be 2% to 10%, using radio-
labelled carbon (Wetzel and Penhale 1979;
Brylinsky 1977). Losses to the water col-
umn from the entire community, including
belowground biomass and decomposing por-
tions, may be much higher. Kirkman and
P.eid (1979) found that 50X of the annual
loss of organic carbon from the Posidonia
austral is seagrass community was in the
form of DOC.
Release of DOC from detrital leaves
may also be substantial. In freshwater
macrophytes, leaching and autolysis of DOC
load to a rapid 50% loss of weight (Otsuki
and Wetzel 1974). In laboratory experi-
ments dried turtle grass and manatee grass
leaves released 13% and 20%, respectively,
of their organic carbon content during
leachino under sterile conditions (Robert-
son et al . 19C2).
The carbon released as DOC is ex-
tremely labile and is rapidly assimilated
by microorganisms (Otsuki and k'etzel 1974;
Brylinsky 1977), which leads to its immed-
iate availability as food for secondary
consumers. In 14-day laboratory incuba-
tions, the HOC released by turtle grass
and manatee grass leaves supported 10
times more mfcrobial bionass per unit
carbon than did the particulate carbon
fraction (Robertson et al. 1982).
DOC may also become available to con-
sumers through incorporation into particu-
late aggregates. Microorganisms attached
to particles will assimilate DOC from the
water column, incorporating it into their
cells or secreting it into the extracellu-
lar materials associated with the parti-
cles (Paerl 1974, 1975). This microbial ly
mediated mechanism also makes seagrass DOC
available for consumers.
In most marine systems the DOC pool
contains 100 times more carbon than the
particulate organic carbon pool (Parsons
et al. 1977; references therein). The
cycling of DOC and its utilization in de-
trital food webs are complex. The highly
labile nature of seagrass DOC suggests
that it may play a significant role in
supporting secondary productivity.
Role of the Detrital Food Web
The detrital food web theory repre-
sents our best understanding of how the
major portion of seagrass organic carbon
contributes to secondary productivity. The
organic matter of fresh seagrasses is not
commonly utilized by many animals because
of various factors, including their low
concentrations of readily available nitro-
gen, high concentrations of fiber, and the
presence of inhibitory compounds. The par-
ticulate and dissolved fractions of sea-
grass carbon seem to become potential food
for animals primarily after colonization
by microorganisms. During decomposition
the chemical nature of the detritus is
changed by two processes: loss of plant
compounds and synthesis of microbial pro-
ducts.
The decomposer community also has the
enzymatic mechanisms and ability to assim-
ilate nutrients from the surrounding med-
ium, leading to the enrichment of the de-
tritus as a food source. As a result, the
decomposer community represents a readily
usable trophic level between the produc-
ers and most animal consumers. In this
food web, the consumers derive nutrition
largely from the microbial components of
the detritus. This decomposer community
is influenced by environmental conditions
and biological interactions, including the
feeding activities of consumers.
74
CHAPTER 7
INTERFACES WITH OTHER SYSTEMS
7. 1 MANGROVE
7.2 CORAL REEF
Mangroves and seagrass beds occur
close to one another within the estuaries
and coastal lagoons of south Florida,
especially in the clear waters of the
Florida Keys. While the importance of
nangrove habitat to the estuary has been
established (Odun and Heald 1972, 1Q75;
Odum et al . 1982), its faunal interactions
with adjacent seagrass beds are poorly
understood.
Like the seagrass neadow, the man-
grove fringe represents shelter; fishes
and invertebrates congregate within the
protection of mangrove prop roots. Game
fish found in mangroves include tarpon
(Megalops atlanticuj^), snook (Centropomus
unde£i_mal_iJT; Tidy fish (Flops saurus).
crevalle jack (Caranx hippos), gafftonsail
catfish (Bagre marinus), and jewfish
(Epinephelus itajara) (Heald and Odum
1970). Undoubtedly, when mangroves and
seagrass meadows are in proximity, these
fishes will forage over grass. Grey
snapper (Lutjanus griseus)^ sheepshead
(Arcliosarmis^ probatocephalus). spotted
seatrout (Cynoscion nebulosus), and the
red drum (Sciaenops ocellota) recruit into
seagrass habitat initiallv, but with
growth move into the mangrove habitat for
the next several years (Heald and Odum
1970). All of these fishes have
lected over grass. Little work
done, however, to explore the
interactions between mangroves
grass beds. For a detailed review of the
nangrove ecosystems of south Florida see
Odum et al . (1982).
been col-
has been
possible
and sea-
Coral reefs occur adjacent to exten-
sive turtle grass-dominated grass beds
along the full extent of the oceanic mar-
gin of the Florida Keys. The most promi-
nent interaction involves nocturnal ly
active coral reef fishes of several fami-
lies feeding over nrass beds at nioht.
Randall (1963) noted "that grunts and snap-
pers were so abundant on some isolated
patch reefs in the Florida Keys that it
was obvious that the reefs could not pro-
vide food, nor possibly even shelter, for
all of them. Longley and Hildebrand
(19-11) also noted the dependence (for
food) of pomadasyids and lutjanids on
areas adjacent to reefs in the Tortugas.
Typically, both juveniles and adults
form large heterotypic resting schools
(Ehrlich and Ehrlich 1973) over prominent
coral heads or find shelter in caves and
crevices of the reef (Figure 24). At dusk
these fishes migrate (Ogden and Ehrlich
1977; MacFarland et al. 1979) into adja-
cent seagrass beds and sand flats where
they feed on available invertebrates
(Randall 1967, 1968), returning to the
reef at dawn. Starck and Davis (1966)
list species of the Holocentridae, Lutjan-
idae, and Pomadasyidae families as occur-
ring diurnal ly on Alligator Reef off Mate-
cumbe Key in the Florida Keys, and feeding
nocturnal ly in adjacent grass beds and
sand flats. As such, these fishes epito-
mize what Kikuchi and Peres (1977) defined
as temporal visitors to the grass bed,
which serves as a feeding ciround (Hobson
1973). Starck (1968) discussed further
75
Figure 24. Grunt school over coral reef during daytine. At night these schools will
disperse over seagrass beds and adjacent sand flats to feed.
the fishes of Alligator Reef with brief
notes on their ecology, while Davis (1967)
described the pomadasyids found on this
reef and their ecology.
Little is known about the ecology of
these nocturnal coral reef fishes while on
the feeding ground. These fishes poten-
tially can range far fron their diurnal
resting sites. Lutjanus griseus and
Haemulon flayol ineatun range as far as
1.6 km (1 ni) from Alligator Reef (Starck
and Davis 1966). Haemulon plumeri and H^.
flavol ineatun typically migrate distances
of 300 m (984 ft) to greater than 1 km
(0.6 mi) over the grass beds in Tague Bay,
St. Croix (Ogden and Ehrlich 1977; Ogden
and Zienan 1977). Tagged H_. plumeri were
repeatedly captured on the same reef and
when transplanted exhibited a tendency to
home (Springer and McErloan 1962a). Some
H^. plumeri and H. flavol ineatum success-
fully home to original patch reefs over
distances as great as 2.7 km (1.7 mi) in
the U.S. Virgin Islands (Ogden and Ehrlich
1977).
It is interesting to speculate on the
possible role that habitat partitioning
plays in reducing competition for food
over the feeding ground. Competition is
important in structuring other fish com-
munities, such as Centrachidae (Werner and
Hall 1977), Embiotocidae (Hixon 1980) and
Scorpaenidae (Larson 1980). Starck and
Davis (1966) reported that 11 of 13 pom-
adasyids found in durnal resting schools
on Alligator Reef disperse at night to
feed. The lighter colored grunts (seven
species) move off the reef and generally
distribute themselves along a sand flat-
grass bed back reef continuum. Snappers
(Lutjanidae) follow a similar pattern with
l^. griseus and U synagris moving into
mixed sand, grass and rubble back reef
habitat. The nocturnal distribution of
76
grunts over the grass beds of Tague Bay,
St. Croix, is similar to those reported in
the Florida Keys. The French grunt,
Haenulon flavol ineatum, was most abundant
over sparse grass or bare sand bottom,
while the v/hite grunt H. plumeri was usu-
ally observed in dense grass. Numbers of
coral reef fishes (grunts and squirrel-
fishes) feeding nocturnally over seagrass
were positively correlated with a measure
of habitat complexity. This correlation
implies organization of the fish assem-
blage while feeding (M.B. Robblee, in pre-
paration). Lutjanids were not found in
significant numbers either on the reef or
in the grass beds.
These observations on the distribu-
tion of fishes over the feeding ground
suggest that the nature and quality of
grass bed and sand flat habitat adjacent
to a coral reef nay influence both the
composition and abundance of these noctur-
nal fishes on a reef. Randall (1963)
stated that whenever well-developed reefs
lie adjacent to flats and these flats are
not shared by many other nearby reefs, the
grunts and snappers on the reef may be
expected to be abundant. Starck and Davis
(1966) and Robins (1971) also noted that
it is understandable, given the require-
ment of most pomadasyids and several
lutjanid species for back-reef forage
area, that these fishes are almost com-
pletely absent from certain islands in the
Caribbean which have fringing reefs with
only narrow shelf and very limited back-
reef habitat. Conversely, grunts and
snappers form resting schools over char-
acteristic coral heads, most commonly
Acropora palamata and Porites porities
(Ehrl ich and Ehrlich 1973; Ogden and
Ehrlich 1977), which also influences their
population size. Starck and Davis (196G)
commented that these species are excluded
from many suitable forage areas by the
absence of sheltered locations for diurnal
resting sites. When artificial reefs were
established in the Virgin Islands (Randall
1963; Ogden, personal communication),
rapid colonization by juvenile grunts
occurred, indicating the importance of
shelter to these fishes near their poten-
tial feeding grounds.
Much of the interpretation given
above is speculative, but in light of
current hypotheses, the structuring of
coral reef fish communities is probably
largely controlled by their physical
requirements for living space. Sale
(1978) speaks of a lottery for living
space among coral reef fish communities
composed of groups of fishes with similar
requirements (the representatives on any
one particular reef being determined by
chance recruitment). Alternatively, Smith
(1978) advocated the ordered view that
recource-sharing adaptations determine
which species can live together. Resources
external to the reef influence the species
composition and abundances of at least
nocturnally feeding, supra-benthic species
(grunts and snappers), and perhaps several
of the holocentrids.
It has been hypothesized that the
die! activity patterns exhibited by these
fishes contribute to the energy budqet of
the coral reef. Billings and Munro |1974)
and Ogden and Zieman (1977) suggested, as
originally proposed by Johannes (personal
communication), that migrating pomadasyids
may import significant quantities of
organic matter (feces) to the reef.
Thayer and Engel (in preparation) have
also postulated a similar mechanism for
green turtles, whose contribution to reef
nutrient budgets may also be important.
These assertions are open to investiga-
tion.
Temporary visitors from the coral
reefs are not limited to fishes. The
urchin Diadema antillarum moves off patch
reefs at night into the turtle grass-
dominated grass bed immediately adjacent
in Tague Bay, St. Croix (Ogden et al .
1973). The prominent halo feature asso-
ciated with many patch reefs is attributed
to the nocturnal feeding forays of these
longspine urchins. Of areater signifi-
cance, the spiny lobster (Panulirus
arqus) , is known to move onto offshore
reefs as adults in the Florida Keys, seek-
ing shelter in caves and crevices (Simmons
1980). Lobsters remain in their dens dur-
ing daylight; at or after sunset they move
onto adjacent grass beds to feed solitar-
ily, returning to the reef before dawn
(Hernkind et al . 1975). While farther
from the reef, the spiny lobster ranges
over considerable distances, typically
several hundred meters.
77
Use of adjacent grass and sand flats
by coral reef creatures is not strictly a
nocturnal phenomenon, but seems to be the
dominant pattern. Only large herbivores
(e.g., Chelonia nydas, Scarus quacamaia)
venture far into the grass bed away from
the shelter of the reef. Mid-sized herbi-
vores are apparently excluded by predators
and feed only near the reef (Ogen and Zie-
man 1977). Randall (1965) reported parrot-
fishes (Scarus and Spar i soma) and surgeon-
fishes (Acanthurus) feeding on seagrasses
(Thalassia and manatee grass) closely
adjacent to patch reefs in the Vircin
Islands during the day. He attributed the
formation of halos around patch reefs in
St. John to this grazing.
7.3 CONTINENTAL SHELF
Recently interest has been sparked in
estuarine-Continental Shelf interactions
(Darnell and Sbniat 1979). The seaarass
meadow represents a highly productive,
faunally rich habitat within south Flor-
ida's estuaries and coastal lagoons. Many
species are dependent on the seagrass bed
and estuary. The pink shrimp' Penaeus
duorarum, the lobster Panulirus arqus.
and the grey snapper Lutjanus griseus
may serve as examples of estuarine or
lagocnal dependent fauna which at one life
stage or another are found in seagrass
meadows.
In south Florida, pink shrimp spawn
in the vicinity of the Tortugas Bank, the
pelagic larvae returning to the estuary
and perhaps the seagrass bed (Yokel
1975a). Eventually mature individuals re-
turn to the spawning grounds. Similarly,
the lobster natures in inshore seacrass
nursery grounds and as a sub-adult resides
on inshore reefs while continuing to feed
within the grass bed at night. As'^sexually
mature adults, female lobsters move to
deep offshore reefs and spawn. The grey
snapper initially recruits into grassland
with growth moves into mangrove^ habitat
and eventually on to coral reefs and deep-
er shelf waters. Coming or going, these
organisms and others like then serve to
transfer energy from the seagrass bed to
offshore -waters (see section 7.5), as has
been shov/n by Fry (19C1) for brown shrimp
(f.- iztecus) in Texas waters.
7.4 EXPORT OF SEAGRASS
The most recently recognized function
of seagrass beds is their ability to
export "large quantities of organic matter
from the seagrass meadows for utilization
at some distant location (Zieman et al .
1979; Wolff 1980). This exported material
is both a carbon and nitrogen source for
benthic, mid-water, and surface-feeding
organisms at considerable distances from
the original source of its formation. The
abundance of drifting seagrass off the
west Florida shelf is illustrated in
Figure 25 (Zieman et al., in preparation).
This material originates on the shallow
grass flats and is transported westward by
the prevailing winds and tides.
Leaves and fragments of turtle grass
were collected by Menzies et al . (1967)
off the North Carolina coast in 3,160 m
(10,368 ft) of water. Although the near-
est source of turtle grass was probably
1,000 km (625 mi) away, blades were found
at densities up to 48 blades per photo-
graph. Roper and Brundage (1972) surveyed
the Virgin Islands basin photographically
and found seagrass blades in most of some
5,000 photographs taken at depths averag-
ing 3,500 m (11, 48^ ft). Most were clearly
recognizable as turtle grass or manatee
grass. Seagrasses v/ere collected by trav;l-
ing in three Caribbean trenches and sea-
grass material v/as found in all the
trenches sampled (Wolff 1976). Most of
the material collected was turtle grass,
and there was evidence of consumption by
deep-water organisms. Interestingly,
some grass blades collected from 6,740 m
(22,113 ft) in the Cayman Trench showed
the distinctive bite marks of parrot-
fish which are found only in shallow
waters.
The primary causes of detachment are
grazing by herbivores, mortality on shal-
low banks caused by low tides, and wave-
induced severing of leaves that are becom-
ing senescent. In addition, major storms
will tear out living leaves and rhizomes
(Thomas et al . 1961). Which mode of
detachment will be most important in a
particular area will be largely deter-
mined by physical conditions such as
depth and wave exposure. Peduced salin-
ity or extreme temperature variation will
78
83'
26°
TOTAL SEAGRASS. G M'
0000 - 0009
OOIO - 0090
0100 - .0900
I
,1000- .9000
> .9000
W^^i^tv
'^'^^.
5^.
. .<
D .■ ^^
26°
25°
83°
82°
Figure 25. Seagrass export from south Florida to the eastern Gulf of Mexico. In cer-
tain areas there is a substantial subsidy to the local carbon and nitrogen budgets by
material exported from nearby seagrass beds.
limit the herbivores responsible for de-
tachment (primarily parrotfish, urchins,
and turtles).
Freshly detached, healthy blades of
all species float better than senescent
ones. Because of the difference in size
and shape of turtle grass and manatee
grass blades, the effect of direct herbi-
vory on the two species is quite differ-
ent. When a parrotfish or urchin bites a
turtle grass blade,
only a portion of the
attached. However, a
is typically only 1
one bite severs it,
portion to float
1979). Similarly,
it usually removes
blade, which remains
manatee grass blade
to 1.5 mm wide and
allowing the upper
away (Zieman et al .
green turtles sever
whole turtle grass blades during initial
grazing.
Because of this difference in re-
sponse to grazing, Zieman et al . (1979)
found that in Tague Ray 60% to 100% of the
daily production of manatee grass was de-
tached and exported, whereas only 1% of
turtle grass was exported, and this was
primarily as bedload. This also indicates
the relative successional status of these
species. Turtle grass retains more of its
leaves within the bed, which thus become
part of the litter layer, promoting carbon
and nitrogen recycling in the seagrass bed
and enhancing its performance as a climax
species. By contrast, relatively little
of the leaf production of manatee grass is
79
retained in the bed to contribute to fur-
ther development of the little layer
(Zieman 1981),
It is possible that in certain re-
gions, exported seagrass could be an
important food source. Sediment collected
from the bottom of the Tongue of the Ocean
that was not associated with turtle grass
patches had carbon and nitrogen contents
of 0.66% and 0.07?, respectively (Wolff
1980). Turtle grass blade and rhizome
samples had a carbon content of 20% and a
nitrogen content of 0.77%.
Many species of fishes and inverte-
brates use south Florida grass beds as
nurseries. Approximately one-third of
the species collected at Matecumbe Key,
including all grunts, snappers, file-
fishes, and parrotfishes, occurred only as
young, indicating that the grass-dominated
shore area was a nursery ground (Springer
and r^cErlean 1962b). In" Tampa Bay, 23
species of finfish, crab, and shrimp of
major importance in Gulf of Mexico fisher-
ies were found as immature forms (Sykes
and Finucane 1966). Comparatively little
is known concerning invertebrates other
than those of commercial value.
7.5 NURSERY GROUNDS
Grass beds serve as nursery grounds
where post larval stages of fishes and
invertebrates concentrate and develop and
also as spawning grounds for adult breed-
ing populations of some species. To be of
significance as a nursery, a habitat must
provide protection from predators, a sub-
strate for attachment of sessile stages,
or a plentiful food source (Thayer et al .
1978b). Seagrass habitats fulfill all of
these criteria with their high productiv-
ity, surface areas, and blade densities,
as well as a rich and varied fauna and
flora. Seagrass provides abundant nursery
habitat and is often preferred, based on
abundance and size data, over available
alternatives, in the estuaries and coastal
lagoons, by many commercially or ecologi-
cally important species (Yokel 1975a).
The importance of grass bed habitat
as a nursery has been historically demon-
strated and should not be minimized. Fol-
lowing the decline of Zostera marina along
the east coast of the United States in the
early 1930's, the sea brant, a variety of
goose dependent on eel grass for food (as
are many waterfowl; McRoy and Helffrich
1980), v/as reduced in numbers to one-fifth
its former levels (Moffitt and Cottam
1941). Pronounced decreases in abundance
of bay scallops (Argopecton irradians)
were also noted following the disappear-
ance of oelorass (Stauffer 1937; Dreyer
and Castle 1941; Marshall 1947). The
post-veligor larval stage of the scallop
depends on eelgrass to provide cir\ above-
sediment surface for attachment. Disrup-
tion of eelgrass beds resulted in lowered
numbers of bay scallops (Thayer and Stuart
1974).
Shrimp
Pink shrimp (Penaeus duorarum) occupy
south Florida grass beds as juveniles
(Tabb etal. 1962; Costello and Allen
1966). Penaeus aztecus and P_. brasil ien-
sis are also present, but never as abun-
dantly as the pink shrimp (Tabb and Man-
ning 1^61; Saloman et al . 1968; Bader and
Roessler 1971). Shrimp spawn on the Tor-
tugas grounds, probably throughout the
year (Tabb et al . 1962; Munro et al .
1968). Roessler and Rehrer (1971) found
postlarval pink shrimp entering the estu-
aries of Everglades National Park in all
months of the year.
Pink shrimp were distributed through-
out Rookery Bay Sanctuary in southwestern
Florida, but were most abundant at sta-
tions with grass-covered bottoms (shoal
grass and turtle grass), and within these
stations were most abundant where benthic
vegetation was dense (Yokel 1975a). Pink
shrimp were also abundant in grass habitat
at Marco Island and Fakahatchee Bay, also
in southwestern Florida (Yokel 1975b).
Postlarval pink shrimp with carapace
length less than 3 mm were taken only at
stations where shoal grass and turtle
grass were present in Rookery Bay Sanc-
tuary, while other stations without grass
alv/ays had larger mean sizes. These ob-
servations are in accordance with Hilde-
brand (1955) and Williams (1965), who
noted that ^ery small pink shrimp prefer
grassy areas and with increasing size are
found in deeper water. In terms of the
functioning of the grass bed as a nursery
ground, it is interesting to speculate
whether this distributional pattern repre-
sents a preference on the part of pink
shrimp postlarvae for grass bed habitat
80
(associated characteristics) or is the
result of differential mortality within
the estuary.
Spiny Lobster
Juvenile spiny lobsters (Panul irus
argus) are coimnonly found in nearshore
seagrass nursery areas of Biscayne Bay,
Florida (Eldred at al. 1P72); the Carib-
bean (Olsen et al . 1975; Peacock 1974);
and Brazil (Moura and Costa 1966; Costa
etal. 1969). In south Florida these
inshore nursery areas are largely limited
to clear, near-normal oceanic salinity
waters of the outer margin of Florida Bay,
the Florida reef tract, and the coastal
lagoons. Tabb and Manning (1961) noted
that the spiny lobster is rare on the
muddy botto-ns in northern Florida Bay.
Residence time in shallow grassy
areas is estimated at about 9 to 12 months
(Eldred et ,il . 1972; Costa et al . 1969)
after which time the small lobsters (cara-
pace length typically less than 60 mm)
take up residence on small shallow water
patch reefs. On the reefs, the lobsters
live gregariously during the day while
foraging at night over adjacent grass and
sand flats. With maturity (1.5 to 2.0
years. Peacock 1974; up to 3 years in
Florida, Simmons 1980) mating occurs and
females migrate to deeper offshore reefs
to release larvae (Little 1977; Cooper
et al . 1975) and then return. Reproduc-
tive activity occurs throughout the year
in Florida waters, but is concentrated
during March through July (Menzies and
Kerrigan 1980).
Theories differ about where the lar-
vae which recruit into south Florida
inshore nurseries originate. The question
is of great importance to the management
of this fishery. Once released alono
Florida's offshore reefs, the larvae
(phyllosomes) drift with the current dur-
ing a planktonic stage of undetennined
length; estimates range from 3 months to 1
year (Simmons 1980). Controlled vertical
movements in the water column may allow
the larvae to remain in the area of hatch-
ing via eddies, layered countercurrents
or other localized irregularities in the
movements of the v/ater (Simmons 1980). Al-
ternatively, larger scale countercurrents
and gyres may allow for larval development
v^hile still returning the larvae to south
Florida waters (Menzies and Kerrigan
1980). It has also been suggested by <^ims
and Ingle (1966) that larvae recruited to
south Florida nursery areas may have been
spawned in locations south of the Yucatan
Channel, perhaps as distant as the Leser
Antilles or Brazil, and deposited ready
for settlement by oceanic currents in
south Florida waters. Ongoing studies of
protein variation as a reflection of gene-
tic variation between adult populations
and puerili postlarvae are designed to
determine if Florida spiny lobsters origi-
nate within Florida's waters or are re-
cruited from adult population centers
elsewhere (Menzies and Kerrioan 1<^70,
197^, 1980).
Phyllosomes that survive their plank-
tonic existence recruit into the nursery
areas as puerulus lobsters (postlarvae)
that resemble adults in form, but are
transparent. The postlarvae swim toward
shore at night and burrow in the bottom by
day until they reach inshore seagrass nur-
series, where they gradually become pig-
mented (Johnson 1C!74; Serflino and Ford
1975; Simmons 1980). Recruitment takes
place throughout the year in south Florida
with peak influxes usually between Febru-
ary and June and between September and
December (Eldred et al. 1972; Witham
et al. 1968; Sweat 1968). This pattern
may be less pronounced in the lower Flor-
ida Keys where high summer influxes have
also been noted (Little 1977). A summer
peak in abundance was also noted in the
Less Antilles (Peacock 1974). Greatest
monthly recruitment takes place between
new and first quarter moon (Little 1977).
There is some evidence to suggest
that pueruli first settle temporarily
above the bottom on algal mats, mangrove
prop roots, or on floating algal rafts
(Smith etal. 1950; Lewis 'etal. 1952;
Witham et al . 1953; Sweat 1968; Little
1977). Peacock (1974), working in Antiqua
and Barbados, noted that no pueruli were
collected from within the grass bed in
the lagoon where juveniles were present,
but were collected commonly from the
prop roots of mangroves lining its en-
trance. After the puerulus molts, the
body of the young lobster is heavily pig-
mented. At this time it assumes a demer-
sal behavior in the nursery (Eldred et al .
81
Sinilar habitat use by juvenile £.
has been reported in Cuba (Buesa
the Virgin Islands (01 sen
the Lesser Antilles (Peacock
Brazil (Costa et al . lf^69).
et al.
1974),
Deora-
of this habitat v/ould certainlv
1972).
argus
vm,
1975),
and in
da t ion
threaten lobster productivity (Little
1977).
Fish
In south Florida it appears that con-
tinental fish faunas and insular fish
faunas mix. Continental species reauire
changing environments, seasonally shifting
estuarine conditions, high turbidities,
and muddy bottoms (Robins" 1971). South-
western Florida and northern Florida Bay
typify these conditions and their fish
assemblages are characterized by many
sciaonid species (drums) and the prominent
scarid, Lagodon rhomboides, which is also
the most abundant fish in Clearwater sea-
grass areas of Biscayne Bay and Card Sound
(I. Brook, personal communication). Insu-
lar species require clear water, buffered
environmental conditions, and bottom sedi-
ments composed largely of calcium carbon-
ate (Robins 1971). These conditions are
found within the grass beds of the Florida
Keys and outer margins of Florida Bay.
Representative species of families Poma-
dasyidae, Lutjanidae, and Scaridae are
most numerous in these waters. This pat-
tern is most evident among the seasonally
resident fishes using soaqrass meadows as
nurseries.
At least eight sciaenid species (see
Appendix) have been associated with the
seagrass beds in southwestern Florida
coastal lagoons and estuaries. Not all of
these fishes occur abundantly, and only
the spotted seatrout (Cynoscion nebulo-
sus), the spot (Leiostomus xanthurus), and
the silver perch (Bairdiella chrysura)
occur commonly over grass as juveniles.
The spotted seatrout is one of the
few larger carnivorous fishes present in
south Florida waters that spawns within
the estuary (Tabb 1961, 1966a, 1966b).
Eggs sink to the bottom and hatching takes
place in bottoi^i vegetation or debris (Tabb
1966a, 1966b). The spotted seatrout and
another sciaenid, the red drum (Sciaenops
oscellata), spend the first few weeks of
their lives in the grass beds of Florida
and I'hitewator Bays and then move into the
mangrove habitat for the next several
years (Heald and Odum 1970).
The pinfish (Lagodon rhomboides) was
the most abundant fish collected and was
taken throughout the year in the turtle
grass beds of Florida Pay (Tabb et al .
1962), as is generally true for southwest-
ern Florida (Weinstein and Heck 1979;
Weinstein et al. 1977; Yokel 1975a,
P75b). Yokel (1975a) in Rookery Bay and
Yokel (1975h) in Fakahatchee Bay, both of
the Ten Thousand Island region of south
Florida, noted a strong preference of
juvenile pinfish for vegetated areas. The
sheepshead ( Arc hos argus prohatocephalus) ,
another sparid, initially "recruits into
grass beds but quickly moves into mangrove
habitats (Heald and Odum 1970) or rocks
and pilings (Hildebrand and Cable 193R).
The snappers, Lutjanus griseus and j^.
synaqris, are common throughout south
luvenile gray snapper (L^. gris-
Florida
eus), are
Northern
including
f'anning
side red
often the most common snapper in
Florida and Whitewater Pays,
freshwater regions (Tabb and
1961). The gray snapper is con-
to recruit into grass beds and
then after several weeks move into man-
grove habitat (Heald and Odum 1970). The
lane snapper {\^. synagris), never reaches
sufficient size within the bay to enter
the fishery significantly. Young lane
snappers were abundant in turtle grass
habitat when salinities were above 30 ppt
(Tabb et al. 1962) in Northern Florida
Bay, and wore the most abundant snapper
taken commonly within grass habitat of the
Ten Thousand Island region of the south-
western Florida coast (Weinstein and Heck
1979; Weinstein et al . 1977; Yokel 1975a,
1975b). In Whitewater Bay, L_. griseus and
L. synagris were most abundant when asso-
ciated with henthic vegetation (primarily
the calcareous green algae Udotea flabel-
lum, but also with some shoal grass. )
TTfark 1970).
On the reefs fringing the Florida
Keys alono their oceanic margin, lane and
grey snappers are joined by up to 10
additional lutjanid species (Starck and
Davis 1966; Starck 1968; Longley and
Hildebrand 1941; U.S. Pept. of Commerce
1980). Of these, the schoolmaster (L.
apodus), the mutton snapper (1^. analisT,
82
the dog snapper (]^. jocu), and the yellow-
tail snapper (Ocyurus chrysurus) all occur
in low numbers, relative to the grey snap-
per, as juveniles near shore over grass in
the Florida Keys (Springer and McErlean
1962b; Bader and Roessler 1971; Roessler
1965).
Of the Ponadasyidae, juvenile pigfish
(Orthopristic chrysoptera) are abundant on
muddy bottoms and turbid water in Flor-
ida's variable salinity regions; adults
and juveniles were collected throughout
the year in Florida Bay (Tabb and Manning
1961; Tabb et al . 1962) and Rookery Bay
(Yokel 1975a). The white grunt (Haemulon
plumeri ) is common throughout south Flor-
ida, occurring most often over turtle
grass beds in clear water as juveniles
(Tabb and Manning 1961; Roessler 1965;
Bader and Roessler 1971; Weinstein and
Heck 1979). Adults were not found over
grass during the day, but were abundant
diurnally on coral reefs and at night over
grass and sand flats adjacent to coral
reefs (Starck and Davis 1966; Davis 1967),
Tabb et al , (1962) lists the pigfish and
the white grunt as typical residents of
the turtle grass community of Florida Bay.
Other grunts, including Anisotremus vir-
qinicus, Haenulon sciurus, and H^. aurol in-
eatum, occur over grass only rarely
southwestern
(Tabb and
1979).
Manning
over grass only rarely in
Florida and Florida Bay,
1961; V/einstein and Heck
Clearer water, higher and less vari-
able oceanic salinities, and the proximity
of coral reefs may account for the in-
creased species richness of juvenile
pomadasyids in Florida Keys inshore grass
beds. In addition to the species already
mentioned (except 0. chrysoptera), Haemu-
lon flavol ineatum, H_. parrai and H^. car-
bonarium are also present as juveniles in
these waters (Springer and McErlean 1962b;
Roessler 1965; Bader and Roessler 1971;
Brook 1975).
In addition to lutjanids and pomada-
syids, other coral reef fishes use sea-
grass beds as nurseries. Surgeon fishes
are found as juveniles in grass beds: most
commonly the ocean surgeon (Acanthurus
bahianus) and the doctorfish (A^. chirur-
gus). The spotted goatfish (Pseudupeneus
maculatus) and the yellow goatfish (Mul-
loidicthys martinicus) occur as juveniles
in grass beds (Munro 1976; Randall 1968).
The spotted goatfish was taken at Mate-
cumbe Key (Springer and McErlean 1962b).
Parrotfish (Scaridae) are often the most
abundant fishes on reefs (Randall 1968).
Springer and McErlean
seines on Matecumbe Key,
cies of scarids in turtle
of these were juveniles;
soma
(1962b), using
found eight spe-
grass beds. All
however, Spari-
radians and S^. chrysopterum are also
small fishes which continually reside in
seagrasses. The latter is also found on
reefs (Randall 1967, 1968). The emerald
parrotfish (Nichol sina usta), which is
most common in seagrass (l^andall 1968),
was taken on Matecumbe Key, as well as in
Biscayne Bay (Bader and Roessler 1971).
The remaining species of parrotfishes,
Sparisoma viride and S^. rubripine and
Scarus croicensis, S^. quacamaia, and S^.
coeruleus, are present on reefs as adults,
are less common in Biscayne Bay (Roessler
1965; Bader and Roessler 1971), and are
absent in Card Sound (Bader and Roessler
1971; Brook 1975).
83
CHAPTER 8
HUMAN IMPACTS AND APPLIED ECOLOGY
Since the days when Henry Flagler's
railway first exposed the lush subtropical
environment of south Florida to an influx
of people from outside the region, the
area has been subjected to great change at
the hands of man. Through the 1950's,
booming development precipitated the
destruction of many acres of submerged
lands as demands for industrial, residen-
tial, and recreational uses in this unique
part of the Nation increased. While sea-
grass beds generally have experienced less
direct damage than have the mangrove
shorelines, seagrasses have not been
totally spared the impact of development.
Environmental agencies receive permit
requests regularly, many of which would
directly or indirectly impact seagrass
beds. Because of the concern for these
biologically important habitats several
articles have been published which docu-
ment their importance and man's impact
(e.g. Thayer et al . 1975b; Zieman 1975b,
1975c, 1976; Phillips 1973; Ferguson
et al. 1980).
8.1 DREDGING AND FILLING
Probably the greatest amount of
destruction of seagrasses in south Flor-
ida has resulted from dredging practices.
Whether the objective is landfill for
causeway and waterfront property con-
struction, or deepening of waters for
channels and canals, dredging operations
typically involve the burial of portions
of an estuary with materials from nearby
locations. Such projects therefore can
involve the direct destruction of not
only the construction site, but also many
acres of adjacent habitats. The impact of
dredging can be long-lasting since such
disturbance creates sediment conditions
unsuitable for seagrass recolonization for
a protracted period (Zieman 1975c).
Of the Gulf Coast States, Florida
ranks third, behind Texas and Louisiana,
in amount of submeraed land that has been
filled by dredge spo'il (9,520 ha or 23,524
acres). In Texas and Louisiana, however,
most of the spoil created came from
dredged navigation channels, while in
Florida this accounts for less than 5? of
the State total. Not surprisingly, the
majority of filling of land in Florida,
about 7,500 ha (18,525 acres), has been to
create land for residential and industrial
development (Figure 26), In addition to
the direct effect of burial, secondary
effects from turbidity may have serious
consequences by restricting nearby produc-
tivity, choking filter feeders by exces-
sive suspended matter, and depleting oxy-
gen because of rapid utilization of sus-
pended organic matter. The dredged sedi-
ments are unconsolidated and readily sus-
pended. Thus a spoil bank can serve as a
source of excess suspended matter for a
protracted time after deposition. Zieman
'(1975b) noted that in the Caribbean
dredged areas were not recolonized by tur-
tle grass for many years after operations
ceased. Working in estuaries near Tampa
and Tarpon Springs, Godcharles (1971)
found no recovery of either turtle grass
or manatee grass in areas where commercial
hydraulic clam dredges had severed rhi-
zomes or uprooted the plants, although at
one station recolonization of shoal orass
was observed.
84
<^
Figure 26. Housing development in south Florida . Portions of this development were
built over a dredged and filled seagrass bed. This has historically been the most
common form of nan-induced disturbance to submerged seagrass meadows.
Van Eepoel and Grigg (1970) found
that a decrease in the distribution and
abundance of seagrasses in Lindbergh Bay,
St. Thomas, U.S. Virgin Islands, was re-
lated to turbidity caused by dredging. In
1953 lush growths of turtle grass had been
recorded at depths up to 10 m (33 ft), but
by 1971 this species was restricted to
sparse patches usually occurring in water
2.5 m (8 ft) deep or less. A similar pat-
was observed by Grigg
Brewers Bay, St. Thomas.
Harbor, St. Croix, U.S.
removal of material
tern of decline
et al. (1971) in
In Christiansted
Virgin Islands,
dredging of a ship channel
landfill projects increased
volume by 14% from 1962 to
tion in areas adjacent to
caused extensive suffocation;
deeper water resulted, sediment
conditions
growth.
for
combined with
the harbor's
1971. Silta-
the channel
and v;here
and light
became unsuitable for seaarass
Reduced light penetration was obser-
ved in grassflats adjacent to the dredging
site of an intracoastal waterway in Red-
fish Bay, Texas (Odum 1963). Odum sug-
gested that subsequent decreases in pro-
ductivity of turtle grass reflected the
stress caused by suspended silts. Growth
increased the following year and Odum
attributed this to nutrients released from
the dredge material. While dredging
altered the 38-m (125-ft) long channel and
a 400 m (1300 ft) zone of spoil island and
adjacent beds, no permanent damage occur-
red to the seagrasses beyond this region.
Studies of Boca Ciega Bay, Florida,
reveal the long-term impact of dredging
activities. Between 1950 and 1968 an
estimated 1,400 ha (3,453 acres) of the
bay were filled during projects involving
the construction of causeways and the
creation of new waterfront homesites.
85
Taylor and Salo"ian (1968) contrasted
undisturbed areas of the bay, where luxu-
riant grass grew in sediments averaging
94™ sand and shell, with the bottom of
dredge canals, where unvegetated sedinents
averaged 927. silt and clay. While several
studies of Boca Ciega Ray collectively
described nearly 700 species of plants and
animals occurring there, Taylor and Salo-
pian (1968) found only 20% of those same
species in the canals. Most of those were
fish that are highly motile and thus not
restricted to the canals during extreme
conditions. Interestingly, while species
numbers were higher in undisturbed areas,
30% more fish were found in the canals,
the most abundant of which were the bay
anchovy, the Cuban anchovy, and the scaled
sardine. The authors noted that in the
few years since the initial disturbance,
colonization v.'as negligible at the bottom
of the canals and concluded that the sedi-
ments there were unsuitable for most of
the bay's benthic invertebrates. Light
transmission values were highest in the
open bay away from landfills, lowest near
the filled areas, and increased somewhat
in the quiescent waters of the canals.
Because of the depth of the canals, how-
ever, light at the bottom was insufficient
for seagrass growth. Taylor and Saloman
(19G8), using conservative and incomplete
figures, estimated that fill operations in
the bay resulted in an annual loss of 1.4
million dollars for fisheries and recrea-
tion.
If seagrasses are only lightly
covered and the rhizome system is not
changed, regrowth through the sediment is
sometimes possible. Thorhaug et al .
(1973) found that construction of a canal
in Card Sound temporarily covered turtle
grass in an area of 2 to 3 ha (5 to 7
acres) with up to 10 cm (4 inches) of
sediment, killing the leaves, hut not the
rhizome system. Regrowth occurred when
the dredging operations ceased and cur-
rents carried the sediment away.
the roots, a moderate amount of enrichnient
may actually enhance productivity, under
certain conditions where waters are well-
mixed, as observed by this author in the
rich growth of turtle grass and associated
epiphytes in the vicinity (within 1 km or
0.6 mi) of Miami's Virginia Key sewage
plant. This discharge is on the side of
the key open to the ocean. In the imme-
diate area where these wastes are dis-
charged, however, water quality is so
reduced that seagrasses cannot grow. Stim-
ulation of excess epiphytic production may
adversely affect the seagrasses by persis-
tent light reduction. Often the effects
of sewage discharge in such areas are com-
pounded by turbidity from dredging. In
Christiansted Harbor, St. Croix, where
turtle grass beds were subjected to both
forms of pollution, the seagrasses declin-
ed and were replaced by the green alga,
Enteromorpha. In a 17-year period, the
arassbeds in the embayment were reduced by
662 (Dong et al . 1972).
Phytoplankton productivity increased
in Hillsborough Bay, near Tampa because of
nutrient enrichment for domestic sev/age
and phosphate mining discharges (Taylor
et al . 1973). Phytoplankton blooms con-
tributed to the problem of turbidity,
which was increased to such a level that
seagrasses persisted only in small sparse
patches. The only important macrophyte
found in the bay was the red alga, Gracil-
laria. Soft sediments in combination with
low oxygen levels limited diversity and
abundance of benthic invertebrates.
Few seagrasses grow in waters of
Biscayne Bay that v;ere polluted by sewage
discharge in 1955 (McNulty 1970). Only
shoal grass and Halophila grew sporadi-
cally in small patches within 1 km (0.6
mi) of the outfall. Post-abatement stud-
ies in 1960 showed seagrasses in the area
had actually declined, probably because of
the persistent resuspension of dredge
materials resulting from the construction
of a causeway.
8.2 EUTROPHICATION AND SEWAGE
Seagrass communities are sensitive to
additions of nutrients from sewage out-
falls or industrial wastes. Because
seagrasses have the ability to take up
nutrients through the leaves as well as
Physiological studies reveal that
seagrasses are not only affected by low
levels of light, but also suffer when dis-
solved oxygen levels are persistently low,
a situation encountered where sewage addi-
tions cause increased microbial respira-
tion. Hammer (1968a) compared the effects
86
of dtiaerobiosis on photosynthetic rates of
turtle grass and Halophila decipiens.
I'hile photosynthesis was depressed in both
species, Halophila did not recover after a
24-hour exposure, whereas the recovery of
turtle grass was complete, possibly be-
cause of its greater ability to store oxy-
gen in the internal lacunar spaces. Such
an oxygen reduction, however, will have a
far greater impact on the faunal co;npo-
nents than on the plants.
8.3 OIL
With the Nation's continued energy
deciands, the transport of petroleum and
the possibility of new offshore drilling
operations threaten the coastal zone of
south Florida. The impact on marine and
estuarine communities of several large-
scale oil spills has been investigated;
laboratory studies have assessed the tox-
icity of oil to specific organisms. The
effects of oil spills, cleanup procedures,
and restoration on seagrass ecosystems
have recently been reviewed by Zienan
et al . (in press) .
Tatem ot al . (1978) studied the tox-
icity of two crude oils and two refined
oils on several life stages of estuarine
shrimp. Refined Bunker C and number 2
fuel oil were more toxic to all forms than
were crude oils from south Louisiana and
Kuwait. The larval stages of the grass
shrimp (Palaemonetes pugio) were slightly
more resistant to the oil than the adults,
while all forms of the oils were toxic to
the larval and juvenile stages of the
white shrimp (Penaeus setiferus) and the
brown shrimp (Penaeus aztecus), both com-
mon grass bed inhabitants. Changes in
temperature and salinity, which are rou-
tine in estuaries, enhanced the toxic
effects of the petroleum hydrocarbons.
The greatest danger to aquatic organisms
seems to be the aromatic hydrocarbons as
opposed to the paraffins or alkanes. The
bicyclic and polycyclic aromatics, espe-
cially napthalene, are major sources of
the observed mortalities (Tatem et al .
1978). The best indicator of an oil's
toxicity is probably its aromatic hydro-
carbon content (Anderson et al . 1974;
Tatem et al . 1978)).
The effects of oil-in-water disper-
sions and soluble fractions of crude and
refined oils were evaluated for six spe-
cies of estuarine Crustacea and fishes
from Galveston Bay, Texas (Anderson et al .
1974). The refined oils were consist-
ently more toxic than the crude oils, and
the three invertebrate species studied
were more sensitive than were the three
fishes.
The effects on seagrass photosynthe-
sis of exposure to sublethal levels of
hydrocarbons were studied by NicRoy and
Williams (1977). Plants exposed to low
levels of water suspensions of kerosene
and toluene shov;ed significantly reduced
rates of carbon uptake. Plants probably
are not the most susceptible portion of
the community; in boat harbors where sea-
grasses occur, the associated fauna are
often severely affected.
In the vicinity of Roscoff, France,
den Hartog and Jacobs (1980) studied the
impact of the 1978 "Amoco Cadiz" oil spill
on the Zostera marina beds. For a few
weeks after the spill, the eelgrass suf-
fered leaf damage, but no long-term effect
on the plants was observed. Among the
grass bed fauna, filter-feeding amphipods
and polychaetes were most effected. The
eelgrass leaves were a physical harrier
protecting the sediments and infauna from
direct contact with the oil, and the rhi-
zome system's sediment-binding capabili-
ties prevented the mixing of oil with the
sediment. Diaz-Pi ferrer (1962) found that
turtle grass beds near Guanica, Puerto
Rico, suffered greatly when 10,000 tons of
crude oil were released into the waters on
an incoming tide. Mass mortalities of
marine animals occurred, including species
commonly found in grass beds, fany months
after the incident turtle grass beds con-
tinued to decline.
In March of 1973, the tanker Zee
Colocotronis released 37,000 barrels of
Venezuelan crude oil in an attempt to free
itself from a shoal off the south coast of
Puerto Ric®. The easterly trade winds
moved the oil into Bahia Sucia and contam-
inated the beaches, seagrasses, and man-
groves. Observations v/ere made and sam-
ples collected shortly after the spill.
By the third day following the release,
dead and dying animals were abundant in
the turtle grass beds; and large numbers
of sea urchins, conchs, polychaetes,
prawns, and holothurians were washed up
87
on the beach (Nadeau and Berquist 1977).
Although the spilled Venezuelan crude oil
is considered to have low toxicity, the
strong winds and the wave action in shal-
low waters combined to produce dissolution
and droplet entrainment that yielded an
acutely toxic effect. This wave entrain-
nent carried oil down into the turtle
grass, killing the vegetation. Lacking
the stabilizing influence of the seagrass,
extensive areas were eroded, some down to
the rhizome layer. Some turtle grass
rejuvenation was noted in January 1974,
and by 1976 renewed seagrass growth and
sedinent development were observed. Sur-
veys of the epibenthic communities showed
a general decline following the spill, but
infaunal sample size proved too small
(Nadeau and Berquist 1977) to yield defin-
itive results.
In July 1975 a tanker discharged an
estimated 1,500 to 3,000 barrels of an
emulsion of crude oil and water into the
edge of the Florida current about 40 km
(25 mi) south-southwest of the Marquesa
Keys. The prevailing winds drove the oil
inshore along a 50-km (31-ni) section of
the Florida Keys from Boca Chica to Little
Pine Key. Chan (1977) observed no direct
damage to turtle grass, manatee grass or
shoal grass. The natural seagrass drift
material apparently acts as an absorbent
and concentrator of the oil. This mate-
rial was deposited in the intertidal zone
where the oily deposits persisted at least
1 month longer than the normal seagrass
beachwrack, and Chan thought that this
reduced detrital input into the dependent
ecosystems. The amphipods and crabs typi-
cal of this zone did not occur in the pol-
luted material. The author attributed
mass mortalities of the pearl oyster
(Pinctada radiata) a grass bed inhabitant,
to some soluble fraction of petroleum.
The severest impacts were in the adjacent
mangrove and marsh communities where
plants and animals were extensively dam-
aged. Among the effects noted was the
increase in temperature above the lethal
limit of most intertidal organisms caused
by the dark oil coating.
From various studies it is obvious,
then, that even when the seagrasses them-
selves apparently suffer little permanent
damage, the associated fauna can be quite
sensitive to both the soluble and insol-
uble fractions of petroleum (Figure 25).
Considering the vast amount of ship
traffic that passes through the Florida
Straits, it is somewhat surprising that
there have not been more reported oil
spills. Sampling of beaches throughout
the State has shown that a considerable
amount of tar washes up on Florida
beaches, and that the beaches of the
Florida Keys are the most contaminated
(Romero et al . 1981). In this study, 26
beaches throughout the State were sampled
for recently deposited tar. The density
of ship traffic and the prevailing south-
easterly winds, result in no tar accumula-
tion on many beaches on the gulf coast,
while the largest amounts are found
between Elliot Key and Key West. Of the
26 sample stations, 6 were in the Keys be-
tween Elliot Key and Key West, and there
were 10 on each coast north of this
region. The average for the six Keys
stations was 17.2 gm tar/m^ of beach
sampled, with the station on Sugarloaf Key
showing the highest mean annual amount of
40.5 gm/m . By comparison, the average
annual amount for the 10 east coast
beaches north of f^iami was 2.5 gm/m"^, and
the average for the west coast beaches
north of Cape Sabel was only 0.3 gm/n-.
The implication of this study is quite
frightening, for as damaging and unsightly
as an oil spill can be on a beach, the
potential for damage is inestimably higher
in a region such as the Florida Keys with
its living, biotic interfaces of mangrove,
barely subtidal seagrass flats, and shal-
low coral reefs.
8.4 TEMPEPATUPE AND SALINITY
Tropical estuaries are particularly
susceptible to damage by increased temper-
atures since most of the community's
organisms normally grow close to their
upper thermal limits (Mayer 1914, 1918),
The Committee on Inshore and Estuarine
Pollution (1969) observed that a wide
variety of tropical marine organisms could
survive temperatures of 28°C (32°F) but
began dying at 33° to 34°C (91° to 93°F).
In Puerto Rico, Glynn (1968) reported high
mortalities of turtle grass and inverte-
brates on shallow flats when temperatures
88
reached 35° to 40°C (95° to 104°F).
Planktonic species are probably less
affected by high temperatures than are
sessile populations since larvae can
readily be imported from unaffected areas.
Time of exposure is critical in
assessing the effect of thermal stress.
Many organisms tolerate extreme short-term
temperature change, but do not survive
chronic exposure to smaller elevation in
temperature. For seagrasses that have
buried rhizome systems, the poor thermal
conductivity of the sediments effectively
serves as a buffer against short-term
temperature increases. As a result, the
seagrasses tend to be more resistant to
periodic acute temperature increase than
the algae. Continued heating, however,
can raise the sediment temperature to
levels lethal to plants (Zieman and Wood
1975). The animal components of the sea-
grass systems show the same ranges of
thermal tolerances as the plants. Sessile
forms are more affected as they are unable
to escape either short-term acute effects
or long-tem chronic stresses.
The main source of man-induced ther-
mal stress in tropical estuaries probably
has been the use of natural waters in
cooling systems of power plants. Damage
to the communities involved has been
reported at various study sites. In Guam
characteristic fish and invertebrates of
the reef flat community disappeared when
heated effluents were discharged in the
area (Jones and Randall 1973). Virnstein
(1977) found a decrease in density and
diversity of benthic infauna in Tampa Bay
in the vicinity of a power plant, where
temperatures of 34° to 37°C (93° to 99°F)
were recorded.
The most thorough investigations of
thermal pollution in tropical or semitrop-
ical environments have centered around the
Miami Turkey Point power plant of Florida
Power and Light (see review by Zieman and
Wood 1975). Zieman and Wood (1975) found
that turtle grass productivity decreased
as temperatures rose and showed the rela-
tionship between the pattern of turtle
grass leaf death and the effluent plume,
reporting by late September 1968, that
14 ha (35 acres) of grass beds had been
destroyed. Purkerson (1973) estimated
that by the fall of 1968, the barren area
had increased to 40 ha (99 acres) with a
zone of lesser damaae extending to include
about 120 ha (297 acres). In 1971 the
effluents were further diluted by using
greater volumes of ambient- temperature bay
waters. The net effect, however, was to
expand the zone of thermal stress. One
station 1,372 m (4500 ft) off the canal
had temperatures of 32.2°C (90°F) only 2%
of the time in July 1970, but this in-
creased to 78% of the time in July 1971
(Purkerson 1973).
Temperatures of 4°C or more above
ambient killed nearly all fauna and flora
present (Roessler and Zieman 1969). A
rise of 3°C above ambient damaged algae;
species numbers and diversity were de-
creased. The optimum temperature range
for maximal species diversity and numbers
of individuals was between 26° and 30°C
(79° and 86°F) (Roessler 1971). Tempera-
tures between 30° and 34°C (86° and 93°F)
excluded 50% of the invertebrates and
fishes, and temperatures between 35° and
37°C (95° and 99°F) excluded 75%.
The effects recorded above resulted
from operation of two conventional power
generators which produced about 12 mVsec
of cooling water heated about 5°C (41°F).
Using this cooling system, the full plant,
which was two conventional and tv/o nuclear
generators, would produce 40 m-^/sec of
water heated 6° to 8°C above ambient. The
plant had begun operations in spring 1967
with a single conventional unit, and a
year later a second unit was added. Stud-
ies at the site began in May 1968 when the
area was still relatively undisturbed.
Except for a few hectares directly out
from the effluent canal, the communities
in the vicinity were the same as in adja-
cent areas to the north and south. As
temperatures increased throughout the sum-
mer, however, damage to the benthic com-
munity expanded rapidly.
Because of the anticipated impact of
the nuclear powered units, a new 9-km
(5.6-mi) canal emptying to the south in
Card Sound v;as dredged. Fears that this
body of water also would be damaged per-
sisted, and as a final solution to the
problem a network of 270 km (169 mi) of
cooling canals 60 m (197 ft) wide was con-
structed. Heated water was discharged
into Card Sound until the completion of
89
the closed system, however. Thorhaug
et al . (1973) found little evidence of
damage to the biota of Card Sound, partly
because effluent temperatures there were
lower than those experienced in Biscayne
Bay, and even before the thernal addi-
tions, the benthic community of the af-
fected portion of Card Sound was rela-
tively depauperate compared to Biscayne
Bay.
The temperatures and salinities of
the bays and lagoons of south Florida show
iiiuch variation, and the fauna and flora
must have adequate adaptive capacity to
survive. Although the heated brine ef-
fluent from the Key West desalination
plant caused marked reduction in the
diversity in the vicinity of the outfall,
nearly all beds of turtle grass were unaf-
fected (Chesher 1975). Shoal grass is the
most euryhaline of the local seagrasses
(^lc^!^llan and Moseley 1967). Turtle grass
and manatee grass show a decrease in
photosynthetic rate as salinity drops
below full strength seawater. The season-
ality of seagrasses in south Florida is
largely explained by temperature and
salinity effects (Zieman 1974). The
greatest decline in plant populations was
found when combinations of high tempera-
ture and low salinity occurred sii;iultan-
eously. Tabb et al . (1962) stated: "Most
of the effects of man-made changes on
plant and animal populations in Florida
estuaries (and in many particulars in
estuaries in adjacent regions of the Gulf
of Mexico and south Atlantic) are a result
of alterations in salinity and turbidity.
High salinities (30-40 ppt) favor the sur-
vival of certain species like sea trout,
redfish and other marine fishes, and
therefore improve angling for these spe-
cies. On the other hand these higher
salinities reduce survival of the young
stages of such important species as com-
mercial penaeid shrimp, menhaden, oysters
and others. It seems clear that the
balance favors the low to moderate salin-
ity situation over the high salinity.
Therefore, control in southern estuaries
should be in the direction of maintaining
the supply of sufficient quantities of
fresh water which would result in estua-
rine salinities of 18 to 30 ppt."
Perhaps reduced v/ater flow in the
Everglades has had unexpected impacts in
seagrass beds. The eastern regions of
Florida Bay were formerly characterised by
low salinity, muddy bays with sparse
growths of shoal grass. Fishing here was
often excellent as a variety of species
such as mullet and sea trout foraged in
the heterogenous bottom. One of the main-
stays of the fishing guides of this area
was the spectacular and consistent fishing
for redfish. In recent years the guides
have complained that this fish population
has become reduced, and it is not worth
the effort to bring clients to this area.
In January 1979 this author took a trip
through this region and found that much of
the formerly mud and shoal grass bottom
that he had worked on 10 to 12 years prior
was now lush, productive turtle grass
beds. Where the waters were once muddy,
they were now, according to the guide,
much clearer and shallower, but provided
less sea trout and redfish. Why? The
following hypothetical scenario is one
explanation.
In the late sixties the infamous
C-111 or Aerojet-General canal was built
in south Dade County, on which Aerojet
hoped to barge rocket motors to a test
site in south Dade. The contracts failed
to materialize and the canal, although
completed, was left plugged and never
opened to the sea. Its effect, however,
was to intercept a large part of the over-
land freshwater flow to the eastern Ever-
glades and ultimately to eastern Florida
Bay.
The interception of this water is
thought to have created pronounced changes
in the salinity of eastern Florida Bay,
allowing for much greater saltwater pene-
tration. As the salinity increased, tur-
tle grass, which had been held in check by
lowered salinity, may have had a competi-
tive advantage over shoal grass and
increased its range. The thick anastomos-
ing rhizome mat of turtle grass stabilized
sediments and may have made foraging dif-
ficult for species that normally grub
about in loose mud substrate. Also the
greater sediment stabilizing capacity of
turtle grass may have caused rapid filling
in an environment of high sediment supply
and low wave energy.
This scenario has not been proven;
thus it is hypothesis and not fact. It
90
points out, hov.'Gvor, the conceivabil ity of
how a manmade nodification at some dis-
tance nay have pronounced effects on the
life history and abundance of organisms.
It is interesting to note that the
fishing guides regarded the lush, produc-
tive turtle grass beds as a pest and much
desired the muddy, sparse shoal grass.
What this really illustrates is that quite
different habitats may be of vital impor-
tance to certain species at specific
points in their life cycle. Those fea-
tures that make the turtle grass beds good
nurseries and important to these same car-
nivores when they are juveniles restrict
their foraging ability as adults. It
should be noted in passing that while
lamenting the encroachment of turtle grass
into this area, the guides still hailed
the shallow turtle grass beds to be super-
ior bonefish habitat.
8.
The rate at which a disturbed tropi-
cal grass bed may recolonize is still
largely unknown. Fuss and Kelly (1969)
found that at least 10 months were re-
quired for a turtle grass rhizome to
develop a new apex.
The most common form of disturbance
to seagrass beds in south Florida involves
cuts from boat propellers. Although it
would seem that these relatively small-
scale disturbances would heal rapidly,
typically it takes 2 to 5 years to recolo-
nize a turtle grass bed (Zieman (197G).
Although the scarred areas rapidly fill in
with sediment from the surrounding beds,
the sediment is slightly coarser and has a
lower pH and Eh.
In some regions, disturbances become
nearly permanent features. Off the coast
of Belize aerial photographs show features
in the water that appear as strings of
beads. These are holes resulting from
seismic detonation; some have persisted
for over 17 years (J.C. Ogden, personal
communication) with no recolonization.
This is not just due to problems associ-
ated with explosions, as Zieman has obser-
ved blast holes from bombs on a naval
bombing range in Puerto Rico where some
recolonization occurred within 5 years.
Most cases nf restoration in south
Florida involve turtle grass because of
its value to the ecosystem and its spatial
dominance as well as its truculence at
recolonizing a disturbed area. Recoloni-
zation by shoal grass is not frequently a
problem. The plant has a surficial root
and rhizome system that spreads rapidly.
It grows from remaining fragments or from
seed and can recolonize an area in a short
time.
Ry comparison, turtle grass is much
slower". Fuss and Kelly (1969) found 10
months were required for turtle grass to
show new short shoot development. The
short shoots seem to be sensitive to envi-
ronmental conditions also. Kelly et al .
(1971) found that after 13 months 40°^ of
the transplants back into a central area
had initiated new rhizome growth, while
only 15°^ to 18% of the plants showed new
growth initiation v/hen transplanted to
disturbed sediments. Thorhaug (1974)
reported success with regeneration from
turtle grass seedlings, but unfortunately
seeding of turtle grass in quantity is a
sporadic event in south Florida.
If one accepts the concept of ecolog-
ical succession, there are two basic ways
to restore a mature community: (1) estab-
lish the pioneer species and allow succes-
sion to take its course, and (2) create
the environmental conditions necessary for
the survival and growth of the climax spe-
cies. Van Breedveld (1975) noted that
survival of seagrass transplants was
greatly enhanced by using a "ball" of sed-
iment, similar to techniques in the ter-
restrial transplantation of garden plants.
He also noted that transplantation should
be done when the plants are in a semidor-
mant state (as in winter) to give the
plants time to stabilize, again a logical
outgrowth of terrestrial technique.
Although numerous seagrass trans-
plantings have been performed in south
Florida, the recent study by Lewis et al .
(1981) is the first to use all major sea-
grass species in a comprehensive experi-
mental design that tests each of the tech-
niques previously described in the litera-
ture. The study site was a 10-ha (25-acre)
borrow pit on the southeast side of Craig
Key in the central Florida Keys, which was
studied from February 1979 to February
91
1981. The pit was created over 30 years
ago as a source of fill material for the
overseas highway. The dredged site is 1.3
to 1.7 m (4.3 to 5.6 ft) deep and is cov-
ered with fine calcareous sand and silt.
The surrounding area is 0.3 to 0.7 n (1 to
2 ft) deep and is well vegetated, primar-
ily with turtle grass, and portions of the
borrow pit were gradually being revege-
tated.
The experimental design used a total
of 22 combinations of plant species and
transplantation techniques. Bare single
short shoots and plugs of seagrass plus
sediment (22 x 22 x 10 cm) were used for
turtle grass, manatee grass, and shoal
grass. Seeds and seedlings of laboratory-
raised and field-collected turtle grass
were planted, but seeds and seedlings of
the other species proved impossible to
find in sufficient quantity. Short shoots
were attached to small concrete anchors
with rubber bands and placed in hand-dug
holes 1 to 3 cm deep, which were then
filled with sediment. Seeds and seedlings
were planted by hand without anchors after
it was determined that anchors were
detrimental to the survival of the seed-
lings. The large sediment plugs with
seagrass were placed in similar sized
holes made with another plugging device.
Plugs and short shoots of all species were
planted with both 1- and 2-m spacing,
while the seeds and seedlings of turtle
grass were planted using 0.3-, 1-, and 2-m
spacings.
Of the 20 manipulations of species,
planting techniques, and spacings, only
three groups survived in significant num-
bers for the full 2 years: manatee grass
plugs with 1-m spacing, and turtle grass
plugs with both 1- and 2-m spacing. Tur-
tle grass plugs showed the hiqhest sur-
vival rate (90% to 98%), but did not
expand much, increasing their coverage by
a factor of only 1.6 during the 2 years.
Manatee grass spread rapidly from plugs
under the prevailing conditions and had
increased its area by a factor of 11.4 in
the 2-year period. The initial planting
of shoal grass, however, died out com-
pletely after only a few months, and a
second planting was made with larger, more
robust plants from a different site. This
planting survived sufficiently to increase
its area by a factor of 3.4 after 1 year.
The transplants using short shoots of
the various species were not nearly as suc-
cessful. Although some of the treatments
showed short-term growth and survival,
none of the treatments using short shoots
survived in significant quantitites. Sim-
ilarly, the freshly collected seeds and
seedlings of turtle grass showed no long-
term survival at the barren transplant
site, and showed only 4% survival when
planted into an existing shoal grass bed.
Seeds and seedlings that had been raised
in the laboratory showed a modest survival
of 29% when transplanted to the field, but
even the survivors did not spread signifi-
cantly.
Although several of the restoration
techniques used by Lewis et al . (1981)
proved to be technologically feasible,
there are still major logistic and eco-
nomic problems remaining. The plug tech-
nique showed the highest survival rate,
but the cost estimates ranged from $27,000
to 86,500/ha. Because of the large volume
and weight of the plugs, this method
requires that large source beds be close
to the transplantation site. The removal
of large quantities of plugs can represent
a major source of disturbance for the
source bed, as the plug holes are as slow
to recolonize naturally as propeller cuts
and other similar disturbances. Despite
the spreading recorded at the transplant
site, the source holes for the plugs did
not show any recolonization at the end of
the 2-year period. If source material was
required for a large scale revegetation
project, the disturbance caused by the
acquisition of the plugs could be a major
impact itself. For this reason Lewis
et al . (1981) suggested that this method
be mainly used where there are source beds
that are slated for destruction because of
some developmental activity.
The only other technioue that showed
any significant survival was the utili-
zation of laboratory cultivated seeds
and seedlings. This method was prohibi-
tively expensive with costs estimated
at $182,900/ha, largely due to cultiva-
tion costs; survival was still below
30%. Seeds and seedlings are also suit-
able only in areas where the water motion
is relatively quiescent, as their abil-
ity to remain rooted at this stage is
minimal .
92
Transplants of tropical seagrasses
may ultimately be a useful restoration
technique to reclaim damaged areas, but at
this time the results are not consistent
or dependable, and the costs seem prohibi-
tive for any effort other than an experi-
mental revegetation, especially when the
relative survival of the plants is consid-
ered. Sufficient work has not been done
to indicate whether tropical plants are
really more recalcitrant than temperate
ones. It is likely that continued re-
search will yield more successful and
cost-effective techniques.
8,6 THE LESSON OF THE WASTING DISEASE
The information overload that we are
subjected to daily as members of modern
society has rendered us immune to many of
the predictions of doom, destruction, and
catastrophe with which we are constantly
bombarded. On a global scale, marine
scientists recently feared the destruction
of a major portion of the reefs and atolls
of the Pacific by an unprecedented out-
break of the crown-of-thorns starfish
(Acanthaster planci). The outbreak spread
rapidly and the devastation was intense in
the regions in which it occurred. Yet,
within a few years Acanthaster populations
declined. The enormous reef destruction
that was feared did not occur and recovery
commenced .
In south Florida in 1972-73 there
appeared to be an outbreak of the isopod,
Sphaeroma terebrans, which it was feared
would devastate the Florida mangroves.
This devastation never materialized, and
it now appears that the episode repre-
sented a minor population excursion (see
Odum et al . 1981 for complete treatment).
These episodic events proved to he
short tenr, and probably of little long-
range consequence, yet the oceans arc not
nearly as immune to perturbations as many
have come to think. We witness climatic
changes having major effects and causing
large-scale famine on land, but few think
this can happen in the seemingly infinite
seas. However, one such catastrophic dis-
turbance has occurred in the seas, and it
was in this century and induced by a
natural process.
In the early 1930's, Zostera marina,
a widespread northern temperate seagrass
disappeared from a large part of its
range. In North America, it virtually van-
ished from Newfoundland to North Carolina,
and in Europe from Norway and Penmark
south to Spain and Portugal. The outbreak
began on the open marine coasts and spread
to the estuarine regions.
Many changes accompanied this distur-
bance. Sandy beaches eroded and were re-
placed with rocky rubble. The protective
effects of the grass beds were removed.
The fisheries changed, although slowly at
first, as their detrital base disappeared.
Noticeable improvement did not become
widespread until after 1945 (Rasmussen
1977), and full recovery required 30 to
40 years. It should be emphasized that
this was a large-scale event. In Denmark
alone over 6,300 km- (2,430mi-) of eel-
grass beds disappeared (Rasmussen 1977),
By comparison, south Florida possesses
about 5,000 km- (1,930 mi^) of submerged
marine vegetation (Bittaker and Iverson,
in press). Originally the wasting disease
was attributed to a parasite, Labyrithula.
but now it is felt that the cause was
likely a climatic temperature fluctuation
(Rasmussen 1973). As man's role shifts
from that of passive observer to one of
responsibility for large-scale environ-
mental change, basic understanding of the
fundamental processes of ecosystems is
necessary to avoid his becoming the cause
of associated large-scale disturbance com-
parable to the wasting disease.
8.7 PRESENT, PAST, AND FUTURE
Increasingly, studies have shown the
importance of submerged vegetation to
major commercial and forage organisms
(Lindall and Saloman 1977; Thayer and
Ustach 1981; Peters et al . 1979; Thayer
et al. 1978b). Peters et al . (1979) found
that in the Gulf States the value of the
recreational salt water fish catch exceed-
ed $168 million in 1973, which represents
about 30% of the total U.S. recreational
fishery (Lindall and Saloman 1977). Of
this, 59'^ of the organisms caught were
dependent on wetlands at some stage of
their life cycle. Lindall and Saloman
(1977) estimated an even higher dependency
93
with over 70% of gulf recreational fish-
eries of the region being estuarine
dependent.
The value of the estuarine regions to
important commercial fisheries is even
riore striking. The Gulf of Mexico is the
leading region of the United States in
terms of both landings (35% of the U.S.
total catch) and value {11% of U.S. total
fishery value), according to Lindall and
SaloiT^an (1977), who also determined that
about 90% of the total Gulf of Mexico and
south Atlantic fishery catch is estuarine
dependent.
The pink shrimp fishery, largest in
the State of Florida, is centered around
the Tortugas grounds where 75% of the
shrimp caught in Florida waters ^ltq taken.
Kutkuhn (1966) estimated the annual con-
tribution of the Tortugas grounds to be
10?^ of the total gulf shrimp fishery,
which in 1979 was worth $378 million
(Thompson 1931). The vast seagrass and
mangrove regions of south Florida are the
nursery ground for this vitally important
com.mercial fishery.
In the United States, 98% of the com-
Kiercial catch of spiny lobsters cone from
habitats associated with the Florida Keys
(Williams and Prochaska 1977; Prochaska
and Cato 1980). In terms of ex-vessel
value, the spiny lobster fishery is second
only to the pink shrimp in the State of
Florida (Prochaska 1976). Labisky et al .
(ISCO) reported that the high in lobster
landings, 11.4 million lb, was reached in
1572, and the maximum ex-vessel value of
$13.4 million recorded in 1974. These
figures include lobsters taken by Florida
fishermen from international waters which
encompass the Cahamian fishing grounds.
Since 1975 the Bahamian fishing grounds
have been closed to foreign fishing, plac-
ing qreater pressure on domestic stocks
(Labisky et al . 1^80).
There is an increasing need for more
precise information to first understand
and then to manage these resources intel-
ligently. Although south Florida has
been late in developing compared with
most other regions of the United states,
the pressures atq. becoming overwhelming.
The fishery pressure on the two leading
cominercial species--pink shrimp and
lobster--al ready intense, will inevitably
increase. The Bahamian waters, formerly
open to U.S. lobstermen, are now closed
putting more pressure on the already
depleted stocks. In the past about 12%
of the shrimp landed on the Florida gulf
coast was caught in I'exican waters. Re-
cently the Mexican government announced
that the enabling treaty would not be
renewed. These actions will put increas-
ing pressure on domestic stocks. As this
is happening, development in the region is
dramatically escalating. In the eyes of
many, the main limitations to further
development in the Florida Keys were fresh
water availability and deteriorating
access highways. All of the bridges in the
Keys are now being rebuilt and a referen-
dum was recently passed to construct a
36-inch watsr pipeline to replace the old
Navy line. The price of building lots
took a 30% to 50% jump the day after the
water referendum passed and in many areas
had doubled 6 months after the passage.
It is depressing to read, "Today the
mackerel and kingfish are so depleted that
they have almost ceased to be an issue
with the professional fisherman," or "The
luscious crawfish, howevei^, is now in a
crucial stage in its career. Largely gone
from its more accessible haunts, it has
been preserved so far on the reef.... Eco-
nomic pressure and growing demand however,
have developed more intensive and success-
ful methods of catching them, and though a
closed season has been put on them, in the
open months uncalculahle thousands are
shipped to market and they are rapidly
disappearing." Today we find little sur-
prise in these statements, having come to
expect this sort of natural decline with
increasing development. What is surprising
is that this statement is taken from a
chapter entitled, "Botany and Fishing;
1885-6," froin the story of the founder of
Coconut Grove, Ralph M. Monroe (Munroe and
Gilpin 1930).
Today we see south Florida as a tan-
talizing portion of the lush tropics,
tucked away on the far southeast coast
of the United States. It is not insignif-
icant in size, and its natural produc-
tivity is enormous. Although the waters
still abound with fish and shellfish, in
94
quantities that often amaze visitors, it
is useful to think back to how productive
these waters must have been.
that win not be
ever-increasing
A catastrophic
possible; merely
Their future productivity remains to
be determined. Present productivity can
be maintained, although
easy considering the
developmental pressures,
decline is certainly
maintaining the current economic and
development growth rates will provide that
effect. This point was well made by one
of the reviewers of this manuscript whose
comments I paraphrase here: Insidious
gradual change is the greatest enemy,
since the observer is never aware of the
magnitude of change over time. A turbid-
ity study in Biscayne Bay showed no sig-
nificant differences in turbidity between
consecutive years during 1972 and 1977,
but significant change between 1972 and
1975 (or between 1973"and 1976). In other
words, south Biscayne Bay was signifi-
cantly more turbid in 1977 than 1972, but
a 2-year study would not have uncovered it
(J. Tilmant, National Park Service, Home-
stead, Florida; personal communication).
To properly manage the region, we must
understand how it functions. Decades ago
it would have been possible to maintain
productivity just by preserving the area
and restricting human influence. Now
water management decisions a 100 miles
away have profound changes on the fisher-
ies. Enlightened multi-use management
will require a greater knowledge of the
complex ecological interactions than we
possess today.
Figure 27. Scallop on the surface of a shallow Halodule bed in Western Florida Bay.
95
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APPENDIX
KEY TO FISH SURVEYS IN SOl'TH FLORIPA
Survey
Location
number
1
North Biscayne Bay
2
South Biscayne Bay
3
Card Sound
4
Metecumbe Key
5
Porpoise Lake
6
Whitewater Bay
7
Fakahatchee Bay
8
Marco Island
9
Rookery Bay
10
Charlotte Harbor
Reference
Key to abundance
r = rare
p = present
c = common
a = abundant
Roessler 1965
Bader and Poessler 1971
Brook 1975
Springer and McErlean 1962b
Hudson et al . 1970
Tabb and Manning 1961
Carter et al . 1973
V.'einsteain et al . 1971
Yokel 1975a
Wang and Raney 1971
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A26
50272 -101
REPORT DOCUMENTATION
PAGE
1. REPORT NO.
FWS/OBS-82/25
3. Recipient's Accession No,
4. Title and Subtitle
THE ECOLOGY OF THE SEAGRASSES OF SOUTH FLORIDA:
A COMMUNITY PROFILE
5. Report Date
September 1982
7. Author(s)
J. C. Zieman
8. Performing Organization Rept. No.
9. Performing Organization Name and Address
Department of Environmental Sciences
University of Virginia
Charlottesville, Virginia 22901
10. Project/Task/Work Unit No.
11. Contract(C) or Grant(G) No.
(C)
(G)
12. Sponsoring Organization Name and Address
Office of Biological Services
Fish and Wildlife Service
U.S. Department of the Interior
Washington. D.C. 20240
13. Type of Report A Period Covered
New Orleans OCS Office
Bureau of Land Management
U.S. Department of the Interi
New Orleans. LA 70130
ot*-
15. Supplementary Notes
16. Abstract (Limit: 200 words)
A detailed description is given of the community structure and ecosystem processes
of the seagrass ecosystems of south Florida. This description is based upon a compila-
tion of information from numerous published and unpublished sources.
The material covered includes distribution, systematics, physiology, and growth
of the plants, as well as succession and community development. The role of seagrass
ecosystems in providing both food and shelter for juveniles as well as foraging grounds
for larger organisms is treated in detail. Emphasis is given to the functional role of
seagrass communities in the overall coastal marine system.
The final section considers the impacts of human development on seagrass eco-
systems and their value to both man and the natural system. Because seagrass systems
are fully submerged and less visually obvious, recognition of their value as a natural
resource has been slower than that of the emergent coastal communities. They must,
however, be treated as a valuable natural resource and preserved from further
degradation.
17. Document Analysis a. Descriptors
Ecology, impacts, management, succession
b. IdentJfiers/Open-Ended Terms
Seagrasses, ecosystem, south Florida
c. COSATI Field/Group
18. Availability Statement
Unl imited
19. Security Class (This Report)
Un classi f j ed
21. No. of Pages
yiii + 150
20. Security Class (This Page)
(See ANSI-Z39.ie)
See Instructions on Reverse
OPTIONAL FORM 272 (4-77)
(Formerly NTIS-35)
Department of Commerce
*U.S. GOVERNMENT PRINTING OFFICE 1983-769 265,129
•■ ^-^ ..f^
Hawaiian Islands ^
^ Headquarters, Division of Biological
Services, Washington, DC
X Eastern Energy and Land Use Team
Leetown WV
♦ National Coastal Ecosystems Team
Slidell, LA
♦ Western Energy and Land Use Team
Ft Collins CO
♦ Locations of Regional Offices
Puerto Rico and
Virgin Islands
REGION 1
Regional Direclor
U.S. Fish and Wildlife Service
Lloyd Five Hundred Building, Suite I6'^)2
500 N.E. Multnomah Street
Portland, Oregon 97232
REGION 2
Regional Director
U.S. Fish and Wildlite Service
P.O.B0.X 1306
Albuquerque, New Mexico H7103
REGION 3
Regional Director
U.S. Fish and Wildlife Service
Federal Building, Fort Snelling
Twin Cities. Minnesota 55111
REGION 4
Regional Director
U.S. Fish and Wildlife Service
Richard B. Russell Building
75 Spring Street, S.W.
Atlanta, Georgia 30303
REGION 5
Regional Director
U.S. Fish and Wildlife Service
One Gateway Center
Newton Corner, Massachusetts 021 5H
REGION 6
Regional Director
U.S. Fish and Wildlife Service
P.O. Box 25486
Denver Federal Center
Denver, Colorado 80225
REGION 7
Regional Director
U.S. Fish and Wildlife Service
1011 b.Tudoi Road
Anchorage, Alaska 99503
Reprinted November 1983
DEPARTMENT OF THE INTERIOR
U.S. FISH AND WILDLIFE SERVICE
As the Nation's principal conservation agency, the Department of the Interior has respon-
sibility for most of our nationally owned public lands and natural resources. This includes
fostering the wisest use of our land and water resources, protecting our fish and wildlife,
preserving th& environmental and cultural values of our national parks and historical places,
and providing for the enjoyment of life through outdoor recreation. The Department as-
sesses our energy and mineral resources and works to assure that their development is in
the best interests of all our people. The Department also has a major responsibility for
American Indian reservation communities and for people who live in island territories under
U.S. administration.
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