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DIVER ASSESSMENT OF THE INSHORE SOUTHEASTERN LAKE MICHIGAN ENVIRONMENT
NEAR THE D. C. COOK NUCLEAR PLANT, 1973-82

John A. Dorr III

and

David J. Jude

Under contract with

American Electric Power Service Corporation
Indiana & Michigan Electric Company

Ronald Rossmann, Project Director

Special Report No. 120

Great Lakes Research Division
The University of Michigan
Ann Arbor, Michigan 48109

1986

CONTENTS

LIST OF FIGURES « ., iv

LIST OF TABLES „ viii

LIST OF APPENDICES x

ACKNOWLEDGMENTS xi

INTRODUCTION 1

METHODS 5

RESULTS AND DISCUSSION • 15

PHYSICAL FEATURES 15

Waves and Cur ren ts 15

Thermal Ef f ec ts 20

Surf icial Fea tures 22

Sediment 26

Transparency 31

Inorganic Debris 35

BIOLOGICAL FEATURES 37

Organic De tr i tus 37

Periphy ton 46

Attached Macroinvertebrates 54

Free-living Macroinvertebrates 58

Fish Spawning 70

Juvenile and Adult Fish 80

ECOLOGY 117

PLANT EFFECTS 125

Physical Presence 125

Opera tional Ef f ec ts 127

SUMMARY 131

REFERENCES 140

APPENDIX 1 145

APPENDIX 2 156

APPENDIX 3 160

111

LIST OF FIGURES

Figure Number Page

1 Scheme of the Cook Plant study area in
southeastern Lake Michigan, 1973-1982,
showing locations of the scuba-monitored
intake, discharge, and reference structures
and stations. Stippled area represents
approximate dimensions of riprap zone.
Depths at intake, discharge, and reference
stations were 9 m, 6 m, and 6 m, respectively 8

Prescribed format in which observations

and measurements were recorded underwater on

water-resistant paper during dives in

southeastern Lake Michigan near the

D. C. Cook Nuclear Plant, 1973-1982 11

Length of periphyton (mm) on top of the south

intake structure (at the 3-m depth stratum) and

on the upper surfaces of riprap (at the 7.4-m depth

stratum) adjacent to the base of the structure.

Measurements were made during dives in

southeastern Lake Michigan near the D. C. Cook

Nuclear Plant, 1973-1982 48

Total number and percent composition by
major groups of periphytic algae collected
by divers from the top of the south intake
structure of the D. C. Cook Nuclear Plant,
located at the 3-m strata of the 9-m contour
in southeastern Lake Michigan.
One sample was collected each month, April-
October, 1974-1981, in most years.
A wet-mounted subsample was qualitatively
analyzed under a microscope, and algae
were identified to lowest recognizable
taxon. Total number of samples analyzed
each year was: 1974 = 1, 1975 = 5, 1976 = 6,
1977 = 4, 1978 = 7, 1979 = 7, 1980 = 7, 1981 = 7 51

Numbers of snails observed by divers in

southeastern Lake Michigan near the

D. C. Cook Nuclear Plant, 1973-1982.

Snails were seen only at stations within

the riprap zone and none was observed after

1978. ND = no diving that month 62

Numbers of crayfish observed by divers
(1973-1982) and impinged on traveling screens
(1975-1981) at the D. C. Cook Nuclear Plant,
southeastern Lake Michigan 65

LIST OF FIGURES
(Continued)

7 Total numbers of crayfish seen by divers
during day and night swims over two adjacent
1 X 10-m transects (20 m^ total area) along
the base of the south intake structure of
the D. C. Cook Nuclear Plant,

southeastern Lake Michigan, 1975-1982. .• 66

8 Chronology of maturation, spawning,

egg incubation, and hatching of alewife,

spottail shiner, yellow perch, johnny darter,

and slimy sculpin, in southeastern Lake Michigan

near the D. C, Cook Nuclear Plant. Spawning

periods were cited from Auer (1982); all other

data were compiled during 1973-1982 studies

at the Cook Plant , 71

9 Comparison of relative ranked abundance of
yellow perch observed by divers during all dives
(1973-1982) and transect swims (1975-1982),
collected in standard series field samples
(1973-1982), and impinged (1975-1982) at the

D. C. Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling 87

10 Comparison of relative ranked abundance of
common carp observed by divers during all dives
(1973-1982) and transect swims (1975-1982),
collected in standard series field samples
(1973-1982), and impinged (1975-1982) at the

D. C. Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling 90

11 Comparison of relative ranked abundance of
alewives observed by divers during all dives
(1973-1982) and transect swims (1975-1982),
collected in standard series field samples
(1973-1982), and impinged (1975-1982) at the
D. C. Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling „ „ 94

LIST OF FIGURES
(Continued)

12 Comparison of relative ranked abundance of
spottail shiners observed by divers during all
dives (1973-1982) and transect swims (1975-1982),
collected in standard series field samples
(1973-1982), and impinged (1975-1982) at the

D, C, Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling 97

13 Comparison of relative ranked abundance of
trout-perch observed by divers during all

dives (1973-1982) and transect swims (1975-1982),

collected in standard series field samples

(1973-1982), and impinged (1975-1982) at the

D. C. Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling 100

14 Comparison of relative ranked abundance of
rainbow smelt observed by divers during all
dives (1973-1982) and transect swims (1975-1982),
collected in standard series field samples
(1973-1982), and impinged (1975-1982) at the

D. C. Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling 102

15 Comparison of relative ranked abundance of
sculpins ( Cottus cognatus or C. bairdi )
observed by divers during all dives (1973-1982)
and transect swims (1975-1982),

collected in standard series field samples

(1973-1982), and impinged (1975-1982) at the

D. C. Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling 105

LIST OF FIGURES
(Continued)

16 Comparison of relative ranked abundance of
burbot observed by divers during all

dives (1973-1982) and transect swims (1975-1982),

collected in standard series field samples

(1973-1982), and impinged (1975-1982) at the

D. C« Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling 109

17 Comparison of relative ranked abundance of
johnny darters observed by divers during all
dives (1973-1982) and transect swims (1975-1982),
collected in standard series field samples
(1973-1982), and impinged (1975-1982) at the

D. C. Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling Ill

18 Comparison of relative ranked abundance of
white suckers observed by divers during all
dives (1973-1982) and transect swims (1975-1982),
collected in standard series field samples
(1973-1982), and impinged (1975-1982) at the

D. C. Cook Nuclear Plant, southeastern

Lake Michigan. Ordinate scale is inverted

and extends from lowest to highest rank of

relative abundance. Blanks indicate zero

observations or catch. ND = no diving or

sampling 114

vxi

LIST OF TABLES

Table Number Page

1 Summary of day and night dives performed

during 1973-1982 in southeastern Lake Michigan

in the vicinity of the D. C. Cook Nuclear Plant

near Br idgman , Michigan 6

Direction of generation (quadrant), height

(trough-to-crest), and width (crest-to-crest)

of ripple marks observed by divers in reference

areas north and south of the D. C, Cook Nuclear

Plant, during some months from 1973 to 1982 •

Quadrant: I = north to east (0-90**); II = east to south

(90-180**); III = south to west (180-270^); IV = west to

north (270-360**); Asym = asymmetric (no clear

direction of generation). Dimensions are in cm.

Blanks indica te no da ta 23

Depth (mm) of flocculent surficial sediment

measured on riprap surrounding the D. G. Cook

Nuclear Plant intake structures and at

reference stations north and south of the plant,

1973-1982. T (trace) = detectable, but

unmeasurable. Blanks indicate no measurements

were made 27

Horizontal visibility (m) as measured by

divers on the bottom near Cook Plant intake

structures (9 m) and in reference areas (6 m)

north and south of the plant, 1973-1982.

Asterisk (*) shows months when measurements

were not made on the same day at intake and

reference stations. Measurements at reference

stations were always made on the same day for

any given month. Omitted months and blanks

indicate no measurements made 32

Frequency of observation (%) of organic detritus

on the bottom of southeastern Lake Michigan during

standard series dives in the vicinity of the D. C.

Cook Nuclear Plant, 1973-1982. Observations

of fish (F) are expressed in absolute numbers of

fish counted during dives 39

Record of dead fish observed during all dives

in the vicinity of the D. C. Cook Nuclear

Plant, southeastern Lake Michigan, 1973-1982.

Blanks indica te no da ta 43

vixi

LIST OF TABLES
(Continued)

7 Total number and number of previously unrecorded
taxa of periphyton identified in diver-collected
samples scraped from the top of the south intake
structure of the D. C. Cook Nuclear Plant,
1974-1981. One sample per month, April-October,
was collected each year with the exception of
1974 (all months but June omitted), 1975 (April and
September omitted), 1976 (October omitted), and
1977 (April, May, and October omitted). Fraction (%)
of total periphyton taxa that were identified
in samples of entrained phytoplankton collected
from the plant forebay is also listed.
Blanks indicate no samples collected. 52

8. Composition by number (and percent) of the number
of taxa found in diver-collected periphyton samples
scraped from the top of the D. C. Cook Nuclear
Plant south intake structure during 1974-1981.
One sample per month, April-October, was collected
each year with the exception of 1974 (all months but
June omitted), 1975 (April and September omitted),
1976 (October omitted), and 1977 (April, May, and
October omitted). Algae were categorized as follows:
diatoms = Bacillariophy ta, green algae = Chlorophyta,
blue-green algae = Cyanophyta, golden- brown algae =
Chrysophyta, red algae = Rhodophyta, and other
algae = Euglenophyta and Pyrrophyta 52

9 Annual relative ranked abundance of fish observed
during all diving in sou theias tern Lake Michigan near
the D. C. Cook Nuclear Plant, 1973-1982.
Fish were grouped according to frequency of obser-
vation. Blanks indicate no observation. Common
names of fish assigned according to Robins et al.
(1980) 81

10 Annual relative ranked abundance of fish observed
during duplicate observations made during transect
swims in southeastern Lake Michigan, 1975-1982.
Observations were made by itiwo divers swimming side-
by-side for 10 m along the base of the south intake
structure of the D. C. Cook Nuclear Plant.
Each diver examined an area 1 m wide; observations
were summed and then ranked for the total area (20 m^)
examined. Fish were grouped according to frequency
of observation. Blanks indicate no observation.
Common names of fish assigned according to Robins
et al. (1980) 84

LIST OF APPENDICES

Appendix Number Page

1 Summary of observations made during dives
on riprap substrate surrounding the

D. C. Cook Nuclear Plant intake and

discharge structures in southeastern Lake

Michigan, 1973-1982 145

2 Duplicate observations made during transect
swims in southeastern Lake Michigan, April
through October, 1975-1982. Observations were
made by two divers swimming side-by-side for

10 m along the base of the south intake structure

of the D. C. Cook Nuclear Plant. Each diver

examined an area 1 m wide. Total area of each

transect was 10 m^. Omitted swims are

indicated by an asterisk (*) .....o... 156

3 Scientific name, common name, and abbreviations
for species of fish observed by divers in
southeastern Lake Michigan near the D. C. Cook
Nuclear Plant, 1973-1982. Names were assigned
according to Robins et al. (1980) 160

ACKNOWLEDGMENTS

We would like to thank the present project director Ronald Rossmann and
past directors John Ayers and Erwin Seibel for their support, guidance, and
editorial acumen. Valuable on-site assistance and practical expertise were
provided to us by past and present Indiana & Michigan Power Company staff
members Jon Barnes, Tom Kriesel, and Eric Mallen. We would like to recognize
our colleagues Jim Barres and Laurie Feldt for their efforts to identify the
periphyton collected during the study. Thanks are extended to Sam Ritter
who drafted the figures found in this report, and to Beverly McClellan and
Marion Luckhardt who assisted in the technical preparation of the report.
Many useful suggestions for improvement of the text v/ere provided by Jim
Bowers.

We would like to recognize and gratefully acknowledge the extensive time,
effort, and dedication of Lee Somers who has supervised and guided the devel-
opment of diving activities at The University of Michigan and without whose
support and assistance this study could not have been conducted. Finally, our
deepest appreciation is extended to the many divers whose efforts, dedication,
and sacrifices contributed during the many hours of physically and mentally
demanding work made this study possible.

This project was funded by a grant from the Indiana & Michigan Power
Company, a subsidiary of the American Electric Power Service Corporation.
We thank Alan Gaulke for his liaison work throughout the study.

INTRODUCTION

This report is a summary and analysis of observations made by divers in
southeastern Lake Michigan near the D. C. Cook Nuclear Plant, 1973-1982.
This investigation was one component of a multi-disciplinary environmental
impact study conducted by the Great Lakes Research Division, University of
Michigan, for the Donald C. Cook Nuclear Plant from 1970 through 1982.
Overall scope of work included: physical studies - hydrology, sediments,
shore erosion, ice effects; chemical studies - standard water chemistry,
nutrients, trace metals; and biological studies - psammo- littoral organisms,
periphyton, algae, zooplankton, benthos, and fish. In addition, studies by
other agencies included radiological work, weather and currents, thermal plume
mapping, terrestrial flora and fauna, and other environmental, sociological,
and economic assessments associated with plant site selection and pre-
construction activities. In 1986, the various s'tudies conducted by Great
Lakes Research Division were integrated into an overview of the aquatic
environment in the study area.

The purpose of the underwater assessment program was to gather data via
direct observation or analysis of hand-collected samples. Information amassed
through these efforts was used to collaborate or augment other studies at the
Cook Plant and to provide a unique assessment of the aquatic environment, its
ecology, and plant- induced effects.

The D. C. Cook Nuclear Plant is located in Berrien County on the shore of
southeastern Lake Michigan near Bridgman, Michigan. The plant site was
purchased in 1959 and pre-cons truction activities began in the 1960s.
Construction of the two-unit, 2,200 megawatt plant began in the late 1960s.
Placement of in- lake structures (intake and discharge pipes and structures.

and riprap field) was completed in late 1972 • Unit 1 achieved "on-line"
status during 1975, following a prior startup period in 1974, Unit 2 went on-
line during 1977. Great Lakes Research Division studies began at the Cook
Plant in 1970 and were divided into two general phases: preoperational and
operational. Underwater studies were conducted during 1973-1982 and included
10 annual periods of observation from April through October during most years.
In accordance with the plant construction schedule, tlie preoperational study
period began in 1970 and extended through 1974 when Unit 1 went on-line.
Therefore, the preoperational database for diving observations encompassed the
2-yr period from 1973 to 1974. Operational studies were conducted from 1975
through 1982, although full operational status was not attained until late in
the study.

An important feature of Cook Plant structure and operation regarding its
potential effects on the lake was the presence of in- lake structures and once-
through circulation of water to cool the plant reactors. At peak operation,
6.1 million liters per minute (1.6 million gpm) of water are drawn through a
system of three water intakes located 223 m (2,250 ft) offshore in 9 m of
water, circulated once through the plant, and returned to the lake via two
discharge structures located 109 m (1,100 ft) offshore in 6 ra of water.
Aquatic biota entrained in the cooling water are exposed to physical and
thermal effects, as is the environment immediately surrounding the discharge
area. Also, the presence of in- lake plant structures (intakes and riprap)
creates a physical environment that is atypical of the surrounding area.

Nearshore surficial sediments in the study area are typically composed of
coarse- to medium-sized grained sand (1.0-0.25-mm diameter) with fine- to very
fine-sized sand (0.25-0.06-mm diameter) becoming predominant offshore (Davis

and McGeary 1965, Hawley and Judge 1969), A distinct change in sediment
composition that occurs offshore at about 24 m is a function of depth and
severity of nearshore physical processes (Seibel et al. 1974, Rossmann and
Seibel 1977). An accumulation of 1-10 mm of fine particulate material
consisting of sediment, periphyton, organic detritus, and diatom tests often
covers the bottom (Dorr and Jude 1980a, b). Inshore surficial sediments are
unstable, and topography can be attributed to nearshore physical processes
including waves and currents. Typical manifestations in the study area are an
inner and outer bar and a gentle slope of 1:100 or less beyond a depth of 4 m
(Davis and McGeary 1965). Thus most areas of the bottom exhibit only little
relief and provide minimal to no surficial shelter or protection for
macroscopic biota, e.g., fish, crustaceans, and molluscs. In contrast,
substrate surrounding the intake and discharge structures and sub-surface
water circulation pipes consists of crushed limestone riprap (0.1-1.0 m in
diameter). It was installed during plant construction to reduce scour by
plant discharge water on in- lake, cooling-water structures. In its central
area, the riprap bed is mounded 1-2 m above bottom, and the structures rise an
additional 3 m above the riprap. Consequently, the surface profile in the
water intake and discharge areas is considerably more rugose than the
surrounding natural environment.

The focus of our underwater studies was to examine selected features of
this man-made environment and to compare and contrast them with those of the
surrounding area. Through these observations, a better understanding of the
aquatic environment in the vicinity of the plant was achieved, as well as of
the plant impact on that environment. Patterns of colonization of aquatic
biota were also delineated.

V/ithin the report, Cook plant data and findings are Integrated with other
underwater studies conducted In Lake Michigan. Changes In the ecology of the
Cook Plant area related to the Impact of the plant are also discussed.

The knowledge gained through the underwater assessment study has provided
unique Insight Into the Inshore southeastern Lake Michigan environment. This
Insight augments that obtained from other components of the Cook Plant
environmental study. Our results should help guide future similar studies, as
well as add to the understanding of physical and biological processes In the
Great Lakes and elsewhere.

METHODS

The underwater assessment study at the Cook Plant is unique to the Great
Lakes in two respects: its duration, which encompassed 10 separate field
seasons, and its design. Diving began in 1973 and continued through 1982.
During this period, 281 (221 day, 60 night) dives were performed in the study
area (Table 1), and more than 161 h of underwater time were amassed. The area
was examined by divers each month, April-October, for 8-10 seasons.

The second unique aspect of this study was the extent to which
observational techniques, effort, and sampling were standardized. During
1973-1974, diving and underwater assessment techniques were developed for the
study area and were incorporated into the Cook Plant environmental monitoring
scheme for plant operation as required by the Nuclear Regulatory Commission
and the Michigan Department of Natural Resources. These environmental
technical specifications (U.S. Atomic Energy Commission 1975) were in effect
from 1975 through completion of our field studies in 1982, and stringently
defined baseline study objectives and sampling regimes for all sections of the
Cook Plant environmental survey including underwater studies. Strict
adherence to these specifications resulted in a sampling program that was both
rigorous and relatively inflexible with regard to modifications. However, it
had the advantage of generating a continuum of data that permitted
identification and analysis of ecological patterns, changes, and plant impacts
on the environment over a period of years.,

Environmental technical specifications stipulated that visual

observations would be conducted at least once per month, April through

October, at five specified locations, including two dives (one day, one night)

in the area of the intake structures, one day dive in the area of the

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discharge structures, and two day dives in reference areas (one north and one
south of the plant) (Fig. 1). Station names were abbreviated as follows:
south intake station - SI, middle intake station - MI, north intake station -
NI, south discharge station - SD, north discharge station - ND, south
reference station III - SR-III, south reference station II - SR II, south
reference station I - SR-I, north reference station III - NR-III, north
reference station II - NR-II, and north reference station I - NR-I.

Dives were separated into two categories: standard series dives (those
which were performed to satisfy technical specifications) and supplemental
dives. Standard series dives were conducted according to fixed procedures
which described the area examined by divers, observational and sampling
techniques, and recording of data. The formats for supplemental dives were
flexible in response to the objectives of the dive.

During standard series dives, two divers equipped with scuba swam side-
by-side and either 1 or 2 m apart. Divers made observations and collected
samples at the intake structure stations by swimming around the top (61 m in
circumference) and base (78 m in circumference) of the structure. While
swimming, each diver examined a plot of 2 m in width; the areas examined on
top and around the base of the structures were approximately 244 m^ and 312
m^, respectively. In addition, divers swam a 10-m transect along the north
side of the south intake structure base following an anchored line placed
there for the duration of the study. While swimming a transect along this
line, each diver examined adjacent plots 1 m in width, resulting in
observations collected from 1 x 10 m (10 m^) plots. These observational
efforts in measured areas provided a quantified data base. Swims and
observations at the discharge stations were conducted in exactly the same

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manner as described for the intake structure stations. Areas examined on top
(213 m^) and around the base (256 ra^) of the discharge structures differed
slightly in size from areas examined at the intake structures; however,
transect swims along anchored lines at the two locations were conducted
identically. Often, but not always, areas in addition to those described were
examined during a dive. This was done to increase the total area examined in
the vicinity of the plant structures.

At reference stations north and south of the Cook Plant (outside riprap
zone in Fig. 1), two 1 x 10 m (10 m^), side- by-side transects were swum
parallel to shore in line with the discharge structures. At each reference
station, a 10-m line was temporarily anchored for the duration of the transect
swim and divers swam out to the full extent of the anchored line. In addition
to the two 10-m2 plots examined at a reference station, a 5- to 10-min swim
was conducted parallel to shore and toward the discharge structures, following
completion of each 10-m transect swim. The 10-m transect swims at the
reference stations provided quantified data to compare with those obtained
within the plant- structure area (stippled zone in Fig. 1). The 5-10-m swims
increased the area examined at the reference stations.

The previously described stations and observational methods comprised our
monthly standard series sampling effort. Whenever possible, this complete
standard series effort was conducted April through October, 1975-1982.

Occasionally, bad weather or other unsafe diving conditions forced a
reduction in this standard series sampling effort, particularly at the
beginning and end of the field season. Also, over the duration of the study
several basic alterations occurred in the standard series diving effort. As
noted earlier, 1973-1974 diving preceded the environmental monitoring

specifications and slight differences occurred in diving efforts and
techniques. During mid-1977, two-unit operation was achieved and water was
discharged from both structures. Consequently, this area became unsafe for
divers to enter and the standard series dive at this location was eliminated.
Occasionally after this, when water was not being discharged from one of the
structures, supplementary dives were made in this area. Finally, in June
1982, the technical specifications for environmental monitoring were altered
and the monthly standard series diving was reduced to one day and one night
dive in the vicinity of the south intake structure.

Observations were made following a prescribed format (Fig. 2) and were
recorded underwater on water-resistant paper. Occasionally, observations were
committed to memory and transcribed at the surface or dictated in a tape
recorder for later reference. Observations made by both divers during non-
transect swims (e.g., swims around the top and base of the structures, 5- 10-
min swims at reference stations, and during supplementary dives) were pooled
and discussed as total observations, observations per unit area (m^), or as
subjective descriptions of abundance. Transect observations were pooled and a
mean and standard error (SE) calculated. For most data, numbers were
expressed as numbers per 10 m, 100 m, or 1,000 m to avoid fractional units.

Although data were collected in both a qualitative (descriptions or
numerical estimations) and quantitative (counts) manner, suspected violations
of assumptions associated with normal- based statistical analyses precluded
reliable parametric analysis (see Dorr and Jude 1980a for a discussion of
these violations as they pertain to underwater observations and studies).
Therefore, analytical procedures were limited to subjective interpretation of
data, and development and interpretation of ranked orders of abundance.

Observer

Temp: Sur

location

Bot

Swell action (bottom): NO YES Visib

Turbid: V LOW LOW MED HI V HI Current: NO YES; From

Bottom Comp (%): Silt Sand Gravel

Organic debries (Num/Den): ALGAE /

TERR. PLANTS / . BARK /-

BRANCHES / TRUNKS /

Rock.

DUNE GRASS .
LEAVES.

UNID PLANT

./-

Inorganic debris (Item, Num, Den): .

STUMPS .

Depth (ft) .

Speed

UNID ANIAAL

Floe (mm)

CHIPS /-_

TWIGS 1^

CLAM SHELLS /__

OTHER /-

Ripple marks: From: —

Loose algae: NO YES; Color
Descr:

Ht.

Wdth .

Len.

Scour: NO YES.

Size

Num/Den

Periphyton: NO YES; Color

Descr: SPARSE MED LUXURIANT

Gastropods: Num Shells

Descr (location, behov)

Clams: Num Shells

Live: NO YES; Num/Den

Crayfish: Dead; NO YES (Num)
Descr (size, location, behov)

Len.

Live: NO YES; Num/Den

Trails: NO YES (Descr) .,
Descr

. % Coverage .

Live: NO YES Num/Den.

Rel. size .

Fish eggs: NO YES; Location

Num/Den

% Clear % Opaque .

Misc invert, (sponge, hydro, bryozoo, insects, crustaceans)

, Substrate .

Color ,

% Fungus .

Other .

Fish

Number

Density

Size

lorv. YOY
juv. adt.

Location

Behovior

Numerico! estimating code: Actual count or; Few (F) = 1-10 Many (M) = 1 1-50 Numerous (N) ~ 50-100 Abundant (A) = 100+

V«ry abundant (T) - 1000+
Comments:

Fig. 2. Prescribed format in which observations and measurements were re-
corded underwater on water-resistant paper during dives in southeastern Lake
Michigan near the D. C. Cook Nuclear Plant, 1973-1982.

Observations and findings presented are based on objective and subjective
analysis of quantified data, tempered by our qualitative data, general
knowledge of the study area, and interpretation of the literature.

Dorr and Jude (1980a) discussed limitations associated with underwater
visual assessments which include equipment and personnel training limitations
and physical and psychological stress, all of which serve to reduce the
accuracy and precision of observational data. Under conditions of limited
visibility (often less than 3 m in the study area), abundance of pelagic
organisms is usually underestimated by divers, particularly for highly mobile
animals such as large fish. Where substrate is uneven, abundance of demersal
or cryptozoic organisms may also be underestimated. Through standardization
of our observational techniques, we attempted to obtain at least consistently
biased (underestimated) parameter estimates where the error was proportional
to the true population size.

Finally, Miller (1956) described the plateau effect which is related to
perceptual handling of simultaneously presented stimuli. Shaw (1975)
discussed implications of this plateau effect related to fish schooling and
"flash expansion" of schools to present multiple moving targets and promote
predator avoidance. In a sense, a diver is also a predator subjected to the
confusing effect of these avoidance responses. Experience has shown that the
visual plateau for divers ranges from 8 to 15 targets when present
simultaneously, depending on visibility and duration of the observation
period. As a consequence, we developed a standardized code for estimating
numbers of objects in a consistent manner. They included: few = 1-10, many =
10-50; numerous = 50-100, abundant = MOO. When pooling data (counts) such as
these, estimates could be averaged (e.g., few + many = 1-10 + 10-50, or 5 + 30

= 35) or lower (1 + 10 = 11) and upper (10 + 50 = 60) limits placed on
parameter estimates. Small aggregations of animals or objects were estimated
or counted in total, large aggregations were visually partitioned and the
number of items in a single partition counted or estimated and multiplied by
the number of partitions to obtain an estimate of total number. These
estimates were used during subjective evaluation of fish abundance based upon
combined counted and estimated numbers.

The preceding discussion underscores our efforts to develop a continuous
and consistent data base. Sampling locations were examined in a spatially and
temporally consistent manner. Observational targets (Fig. 2) and efforts were
standardized. Subjective descriptions (Fig. 2) and numerical estimation
techniques were also standardized and learned by divers. Finally, to reduce
variation associated with differences in personal diving techniques and
capabilities, tlie senior author performed all but two months of diving during
the entire study. Therefore, about one half of the observational data base
included no diver-to-diver variation.

The operational and observational diving techniques used during this
study were developed over a 10-yr period 1973-1982. Many of these techniques
are described in other underwater studies that we have conducted in the Great
Lakes, the results of which are often related to tliis study. They include:
Dorr (1982), Dorr and Jude (1980a, 1980b), Dorr et al. (1981a, 1981b), Jude et
al. (1981a, 1982), Rutecki et al. (1983, 1985), Schneeberger (1982), and
Schneeberger et al. (1982).

During June 1974, and April-October 1975-1981, divers collected samples
of periphyton from the top of the south intake structure and riprap
surrounding the base of the structure. Periphyton was scraped from the

structure with a putty knife into a plastic mason jar. Efforts were directed
toward collection of an adequate-sized sample; no attempt was made to sample a
quantified or consistently-sized area. A small piece of riprap about 4 cm in
diameter which supported a noticeable amount of periphyton was selected and
placed in a second jar. These samples were preserved in 10% formaldehyde for
laboratory analysis, but because of time constraints, only the samples
collected from the intake structure were examined. In the laboratory, the
sample of scrapings was stirred thoroughly, and a subsample was removed for
wet-mounting in water. Algal identifications were made at 400-600X using a
Leitz-Wetzlar Ortholux microscope. Taxa identified in these wet-mounts became
the yearly lists of periphyton collected from the Cook Plant area.

Data used for comparison with diving observations were derived from
companion studies on impinged fish (Thurber and Jude 1984, 1985) and field-
collected fish (Tesar et al. 1985, Tesar and Jude 1985). Impinged fish were
collected and processed every day during 1975 and every fourth day during
1976-1982. Fish were sampled in Lake Michigan using seines, trawls, and gill
nets at a variety of stations from April-November, 1973-1982.

RESULTS AND DISCUSSION
PHYSICAL FEATURES
Waves and Currents
Surface Waves

The fetch of Lake Michigan ranges from about 100 km west to about 350 km
north. For large lakes such as this, the maximum wave height (h) is related
to the fetch (x) of the lake as follows^ h = 0.105x (Mortimer 1975, Wetzel
1975). Based on this, maximum wave heights at the study site would range from
3; 3 m from the west to 6.2 m from the north.

We observed storm waves with a cycloid diameter or height (trough-to-
crest distance) in excess of 4 m, while vrave heights of 1-2 m were common
during periods of onshore winds. However, it was unsafe for us to dive when
wave heights exceeded 1.5 m; therefore, our observations were biased toward
conditions extant during quiescent periods in the lake.

Wetzel (1975, p. 94) stated that for travelling surface waves with a
cycloid cross-sectional path, "the decrease of vertical movement (of the
water) with increasing depth can be approximately described as a halfing of
the cycloid diameter for every depth increase of A/9", where A is the
wavelength measured as crest-to-crest distance. Wetzel further stated that
the ratio of amplitude to wavelength is highly variable from 1:100 to 1:10,
but that except at shallow beach areas, wave lengths of short surface waves
are less than the depth. Given this, the wavelength of a wave 1.5 m high
should not exceed 10 m when water depth is less than 10 m. For a wave with a
height of 1.5 m and and a wavelength of 9 m (as might have occurred during our
dives at the 9-m stations), the vertical displacement of water on the bottom
should be about 3 mm. On top of the Cook Plant intake structures, which are

about 4 m below the surface, the vertical displacement of water should be
about 90 mm. These calculations are in agreement with conditions that we
observed during dives in the study area. If surface waves exceeded 1 m in
height, some water displacement was noticeable on the bottom at all 6- and 9-m
stations. Water displacement was usually evidenced by a swaying of the
periphyton or sloshing movements of surficial floe. On top of the intake or
discharge structures this movement was greatly accentuated relative to
conditions on the bottom. Because the riprap was mounded from lake bottom
level at its periphery to several meters off bottom at the base of the intake
and discharge structures, the movement of water caused by surface wave action
attenuated as divers swam from the structures across the riprap and down to
level bottom. Movement of water on the bottom at <9 m occurred when surface
waves were less than 1 m high, but the effects were unnoticeable to divers.

These observations suggest that circulation of water and resuspension of
surficial sediment and flocculent organic material occurs through surface wave
action. The threshold for these effects probably occurs when wave heights are
between 0.5-1.0 m; effects increase rapidly with increasing wave height.
Evidence that the riprap traps sediment will be presented later. This factor
in combination with surface wave action probably contributed to the increased
levels of suspended materials observed by divers near bottom in riprapped
areas relative to the surrounding sand areas, when lake surface conditions
were rough. Barres et al. (1984) noted elevated levels of particulates in
phy toplankton samples collected from the Cook Plant forebay during periods of
stormy weather and nearshore turbulence. As discussed later, plant intake
water was often noted by divers to be drawn from the bottom of the water
column at the base of the intake structures. The resuspension of surficial

material noted by divers during and immediately after periods of rough lake
conditions may account for the elevated levels of particulates noted in these
samples. Rossmann et al. (1982) suggested that elevated concentrations of
orthophosphate and dissolved silica in water samples collected in the study
area may also have resulted from storm-induced turbulence.

These observations indicate that surface wave action increased the amount
of suspended material in the riprap areas, relative to surrounding areas.
Attached algae and invertebrates (sponge,, bryozoans, Hydra ); benthic
invertebrates, such as worms, insect larvae, snails, and crayfish; and fish
with demersal life stages concentrated in the riprap areas were exposed to
effects of this increased suspension. Such effects may have included
increased siltation and impairment of filter feeding. Surface wave action
undoubtedly promoted circulation of water in and around the riprap. The rise
of the riprap off bottom in combination with its many interstices permitted
surface wave action to more effectively perfuse this substrate. This in turn
would improve the availability of oxygen and exchange of gases, while serving
to continually remove floe from the surface of the substrate.

Currents

Wind friction and atmospheric pressure changes result in seiches, differ-
ential heating of the lake, diffusion of dissolved materials from the sedi-
ments, influx and outflow of water, and geostrophic (e.g., Coriolis) effects
(Mortimer 1975). In Lake Michigan, surface currents often circulate in large
swirls or gyres (Ayers et al. 1958) which in turn are subject to modifications
by standing wave motions. Lake basin morphometry also influences direction
and speed of surface water currents. Although general current patterns may be

established in large bodies of water such as the southern basin of Lake
Michigan, current velocity at any given point may vary with local conditions.
This is particularly true for the inshore region where local effects such as
presence of offshore winds or sand bars may influence current flow.

Studies on currents were conducted in 1975 and 1978 (Indiana & Michigan
Power Company 1975, 1976; ETA 1980) at locations about 600 m north and south
of the Cook Plant at the 3- and 6-m depth contours. Generally, current speeds
measured during 1975 ranged from 6 to 12 cm/s (0.2-0.4 fps) with a maximum
speed approaching 60 cm/s (2 fps). Currents tended to flow to the north,
although considerable day-to-day variation occurred. These data suggest that
considerable variability existed in both current speed and direction in space
and time. Mortimer (1975) has found that current vectors nearshore are
predominantly shore-parallel, while offshore, the clockwise rotating current
vectors of Poincdre waves dominate the lake.

Efforts by divers to establish general current direction and speed at a
given location were unsuccessful. Considerable variability was measured among
locations separated by only 200 m as well as differences at various depths in
the water column. Consequently, no attempt was made by divers to assess
current velocities, although effects of currents were recorded when observed.

Absence or presence of currents was best observed by the horizontal
transport of suspended material past a stationary diver. When surface waves
exceeded 0.5 m in height, vertical displacement of the water obscured the
horizontal movement of suspended material at depths less than 3 m. When
currents were present, horizontal movement of suspended material could be
discerned within 1 m of the bottom at 6 m and 9 m, regardless of wave heights
at the surface. This was the result of the rapid attenuation of vertical

displacement of water with increasing depth. In areas where sediment
accumulated, such as localized depressions in the sand observed at the
reference station or at the periphery of the riprap field, both current and
surface waves acted to resuspend sediment.

In general, current flow and direction appeared to be influenced by
proximity to the intake and discharge structures at the surface and on the
bottom. Strong currents were encountered throughout the water column at
stations 100 m north and south of the respective discharge structure during
discharge of water. As best as could be determined, the direction of flow was
always away from the structure. Strong eddy currents were encountered during
dives at a station located in line with, and mid-way between, the two
discharge structures. But at the reference stations located 900 m north and
1200 m south of the Cook Plant, no effect of plant water discharge on local
water current was discerned.

Within the riprap area, pronounced currents associated with plant water
circulation obscured any general current patterns noticeable to divers. Large
differences in the force of the intake current could be felt at different
points around the base of each structure. These differences ranged from
currents that were almost undetectable to those that were difficult to swim
against. The direction and speed of the natural lake current and the
recirculation patterns established between the intake and discharge structures
influenced the direction and strength of the intake current and the withdrawal
of water from various levels of the water column.

In both riprap areas and on open lake bottom increased rugosity of the
bottom profile acted to reduce current speed within a few centimeters of the
bottom. This observation is in keeping with the existence of a boundary layer

of slack water known to exist as a function of vertical relief dimensions and
variability and of current force and direction. Both riprap and large ripple
marks would contribute to variability in vertical relief and current flow at
the water- sediment interface.

Thermal Effects

Water temperature regimes encountered during our underwater studies
paralleled those characteristic of southern Lake Michigan. Water temperatures
were 4-8 °G during April and increased rapidly during May-June. Temperatures
less than lO'^C were rarely encountered during June-September. During fall,
temperatures declined and reached 10 ''C during late October-early November as
determined from other dive studies in the region (Dorr and Jude 1980a, Dorr et
al. 1981b).

Divers experienced three major thermal effects. The first was vertical
thermal stratification during June-August. It was common to encounter a 1-m
thick layer of very warm water at the surface, particularly when the lake
surface was calm. An abrupt drop in water temperature could be felt on
exposed skin as divers descended through this layer. Temperatures in the
adjoining layer remained nearly constant until 1-2 m off bottom. At this
point, a second abrupt thermal decline was noticed. This layer of cold water
on the bottom was often more turbid than overlying water, and contained
higher amounts of suspended particles. It was believed that these were
relatively distinct thermal layers and that mixing of water among layers was
reduced relative to horaothermal conditions. Observation of the distinct cold
nepheloid layer on bottom supports this contention.

The second effect experienced by divers was that of horizontal thermal
stratification. This condition was again encountered during the warm-water
months and was particularly noticeable during the 5-min swims at reference
stations. Divers often swam through water masses of different temperatures;
thermal interfaces were usually distinct and only a few meters thick. Because
all swims were conducted on the bottom at 6 m little is known of conditions in
mid-water. It is possible that isolated masses of cooler water were present
on the bottom and surrounded by warmer water, perhaps as a result of uneven
development or breakdown of vertical stratification following a change in lake
conditions (e.g., surface waves, currents, upwelling) .

The final thermal effect encountered by divers was summer upwelling of
cold water inshore following periods of strong offshore winds. Unusually cold
water was occasionally encountered during typically warm-water periods, i.e.,
July or August. On some occasions, water temperatures declined considerably
during diving which occurred over a 2-day period. Again, cold-water
upwellings were often accompanied by increased turbidity and pronounced
decreases in underwater visibility.

Because of lake size and its gentle sloping bottom, the major thermocline
between the epilimnion and the hypolimnion lay well offshore of the study area
during the period of maximum vertical thermal stratification. During
occasional dives in deep water (>12 m) , a distinct thermocline was encountered
along with a large difference in temperature between the epilimnion and
hypolimnion.

Surficlal Features

Presence of riprap and in- lake plant structures created artificial
features and atypical habitat. Most of the lake bottom in inshore south-
eastern Lake Michigan is composed of coarse- to fine-grained sand with
occasional areas of pebbles, and presents a flat, unbroken profile. Only iso-
lated rocks and an occasional log or branch were encountered during our
studies. Dorr (1982), Dorr and Jude (1980b), and Jude et al. (1978) conducted
extensive diver surveys of areas containing rough substrate of natural
(moraines, clay banks) and artificial (reefs, utility structures, harbor
breakwalls) origin from Muskegon, Michigan, south to Michigan City, Indiana.
Areas of rough substrate were isolated within the total inshore system and
represented only a small portion (<1%) of the total inshore area.

Ripple marks and occasional large depressions were observed at the
reference stations and during swims along the 6-m contour. The dimensions and
direction of ripple marks observed 1000 m north (Station III) and 1200 m south
(station III) of the plant were measured and recorded during 1973-1982
(Table 2). Most often, ripple marks were generated from a westerly- to-
northerly direction (quadrant IV - 270-360^). This was the situation during
84% of the dives at the north station, and 74% of the dives at the south
station. The slight reduction (10%) in frequency of generation from the
fourth quadrant observed at the south station was probably created by the
riprap north of the south station. This hypothesis is supported by our
observations that ripple marks were consistently smallest at the south
reference station (station I) closest to the riprap. Discharge of water in a
north and westerly direction combined with the "reef-like" barrier that the
riprap and discharge structures presented, undoubtedly acted to diminish the

Table 2. Direction of generation (quadrant), height (trough-
to-crest), and width (crest-to-crest) of ripple marks observed
by divers in reference areas north and south of the D. C* Cook
Nuclear Plant, during some months from 1973 to 1982.
Quadrant: I.= north to east (0-90*^); II = east to south
(90-180''); III = south to west (180-270''); IV = west to north
(270-360*^); Asym = asymmetric (no clear direction of genera-
tion). Dimensions are in cm. Blanks indicate no data.

North Reference

Areas

South Reference

Areas

Month

Quadrant

Height

Width

Quadrant

Height

Width

1973

Sep

1974

Apr

Jun

Jul

1975

May

Jun

Jul

III

Aug

III

Sep

Oct

1976

Apr

III

May

III

Jun

Jul

Aug

Sep

1977

Apr

100

May

Jun

Asym

Jul

Aug

(Continued) .

Table 2, Continued.

North Reference

Areas

South Reference

Areas

Month

Quadrant

Height

Width

Quadrant

Height

Width

1978

Apr

III

May

III

Asym

Jun

III

Jul

Aug

Sep

Oct

1979

May

Jun

Jul

150

Aug

Oct

1980

Apr

May

Asym

Jun

Jul

Aug

Sep

Oct

1981

Apr

100

May

Jun

Jul

Aug

Sep

Oct

1982

Apr

May

Asym

strength of waves and currents approaching from that direction, which is the
prevailing direction of approach at this location on the lake. In general,
ripple marks were smallest and most asymmetrically developed at reference
stations (stations I and II) closest to the riprap and discharge area.

Very large ripple marks with amplitudes (heights) exceeding 10 cm were
occasionally observed at the two most northerly reference stations. These
marks often had wavelengths of 50-100 cm, and extended for 10 m or more along
the bottom. They were always generated from the 270-360° quadrant (quadrant
IV - west-north), and were never observed at south reference stations. These
large marks usually occurred in isolated patches along the 6-m contour and
were separated by extensive areas containing much smaller ripple marks. Often
these smaller marks were generated from a different direction and cross-
hatched the large marks. Most likely, these large ripple marks were the
remnants of marks generated during conditions of high winds and large surface
waves coming from a westerly to northerly direction. Large marks were never
observed at the north reference station (station I) closest to the discharge
area, again probably a result of the disruptive effect of the north-westerly
directed discharge current on incoming waves. In fact, the disruption of
surface waves by the plant's water discharge is observable from shore.

The other surficial feature of the bottom observed in the vicinity of the
reference stations was the presence of localized depressions in the lake
bottom. These depressions were only observed during swims parallel to shore
between north reference station II and sitation III. During the 5-10-min
swims, divers occasionally encountered depressions about 1 m deep and 5-10 m
across; because the third dimension was not measured, the actual shape of
these depressions is not known. We suspect that they may have been roughly

oval in shape with the long axis oriented more closely perpendicular to shore
than the short axis. These depressions were surficial features of the bottom
that were distinctly different from the major troughs that were located
between the major sand bars. One possibility is that these depressions were
trenches or cuts across these major bars and that the depressions connected
adjoining troughs. Another possibility is that the depressions were remnants
of old troughs that had been mostly filled in during the relocation of a bar.
These features are not unique to the Cook Plant area, since we observed them
during other underwater studies in inshore southeastern Lake Michigan.

Sediment

Qualitative microscopic analysis of the flocculent ("floe") layer of
material overlying the riprap and sand revealed it to be composed primarily
of sediment, diatom tests, and some organic detritus (primarily algae).
The thickness of this layer ranged from complete absence to about 10 mm;
a layer 2-3 mm thick was typical of the area (Table 3).

When present, similar amounts of floe were observed in both reference
areas and on the riprap. However, only once, in April 1982, was floe totally
absent from the riprap surrounding the intake structures, whereas, complete
absence of floe in reference areas was more common (8 occurrences at north
reference station III, 11 occurrences at south reference station III).
Observations of floe deeper than 10 mm were made on two occasions north of the
plant and once south of it. The floe layer on the riprap was never thicker
than 6 mm between 1975 and 1982.

We attribute the more continuous presence of floe on riprap compared with
sand to be the result of the better trapping action of the riprap surface.

Table 3. Depth (mm) of flocculent surficial sediment measured
on riprap surrounding the D. C. Cook Nuclear Plant intake
structures and at reference stations north and south of the
plant, 1973-1982. T (trace) = detectable, but unmeasurable.
Blanks indicate no measurements made.

A rea

Month Intake N. Reference S. Reference

1973

Jun

Aug

Sep

1974

Apr

>10

May

5-10

Jun

Oct

1975

May

Jun

Jul

Aug

Sep

Oct

1976

Apr

May

Jun

Jul

Aug

Sep

Oct

1977

(Continued)

5-10

<5
T
2

2 2

20 3

1 1

2 2
5
2

Apr 3 15

May 3 2

Jun 2

Jul 3

Aug 4X0

Sep 2

Table 3.

Continued,

Area

Month

Intake

N. Reference

S. Reference

1978

Apr

May

Jun

Jul

Aug

Sep

Oct

1979

Apr

May

Jun

Jul

Aug

Sep

Oct

1980

Apr

May

Jun

Jul

Aug

Sep

Oct

1981

Apr

May

Jun

Jul

Aug

Sep

Oct

1982

Apr

May

Aug

Oct

3 3

3 2

8 4

2 2

3 5

8 3

1 3

2 2

3 4
2 2

2 3
20
2 2

2 4

5 4

2 5

2 2

3 4

8 6

2 3

The uneven surface of individual clasts and the presence of periphyton caused
floe to be retained more effectively than on the smooth surface of the sand
bottom. Two general observations support this contention: (1) floe accumu-
lated in the troughs of the ripple marks, and not on the sides or crests, and
(2) surface wave action often caused movement of floe on the sand bottom but
not on the riprap. Rarely did floe accumulate on the sides or crests of
ripple marks. Most often, it was carried into the troughs by water movement.
It was noted earlier that surface wave action could be felt on the bottom at
6 m when waves exceeded 1 m in height. Also, the threshold for noticeable
water movement occurred when waves were 0.5-1.0 m in height. When surface
waves were 1 m, a slight oscillation or movement of the floe in the troughs of
ripple marks was apparent. Under these same conditions, the periphyton on
riprap was observed to sway, but no movement of the floe could be seen.

Additional evidence that uneven surfaces trapped sediment more
effectively that smooth surfaces was provided by the occasional deep
accumulations of floe in depressions observed in the sand bottom in the north
reference area (see previous section - Surficial Features). Floe 10-20 cm
deep was measured in some of these depressions (Table 3). Suspended material,
transported along the bottom, probably encountered these depressions where
water velocities were reduced resulting in this material being deposited in
thick layers. In a sense, these large depressions were analogous to small
pockets or interstices in the surface of the riprap. A small trough (1-2 m
wide and less than 1 m deep) in the sand bottom adjacent to the riprap often
formed along the perimeter of the riprap,. Quite often, floe accumulated in
this restricted area to depths of 10-20 mm. Most likely, this was the result
of a small area of stagnant water created by the barrier which the riprap

imposed as it rose off the bottom at this point. Observations made during
studies of other areas of naturally formed sand (Jude et al. 1978, Dorr and
Jude 1980b), rock or clay bottom (Dorr 1982), and artificial substrates (Dorr
et al. 1981b, Dorr 1982) confirm that rugose surfaces trap sediment more
effectively than smooth surfaces.

There appeared to be a direct relationship between absence or presence of
floe and water depth. In this study and others (Dorr 1974, Dorr and Miller
1975, Dorr 1982), floe was rarely observed at depths less than 6 m. However,
it was always present at 12 m or more. Seibel et al. (1974) and Rossmann and
Seibel (1977) noted a distinct demarcation at 24 m where finer-grained
sediment predominated. Its occurrence was a function of depth and severity of
nearshore physical processes, including wave action and currents. Our
observations, combined with the calculated attenuation of even the largest
surface waves observed during any period of several years, suggest that at
depths greater tlian 12 m, the movement of water is not sufficient to sweep
even smooth bottom clear of flocculent material, much less rugose surfaces.
This observation has significant implications regarding the depth location of
structures such as artificial reefs or natural lake trout spawning reefs,
where the removal or absence of floe from the surfaces or interstices of the
substrate by natural movements of the water is desired.

In a 1977 experiment, we positioned several vertical sediment-collecting
tubes 1 m off bottom over Cook Plant intake riprap. Following a 21-day period
(25 May-16 June), 74 mm of material was collected in the 3.8-cm diameter
tubes. The tubes were constructed to permit diffusion of formaldehyde from an
attached reservoir into the collection chamber, thereby preserving the mater-
ial from decomposition. About 90% of the floe collected was sediment;

the remaining portion was composed of diatom tests and organic detritus.
This experiment confirmed the potential for rapid deposition and accumulation
of sediment in inshore depressions.

Flocculent material may change the circulation of water, dissolved gas
exchange, and sediment oxygen demand (SOD) in microhabitats such as surfaces
and interstices of substrates, which might adversely impact biological
entities such as incubating lake trout eggs.

Transparency

Water transparency, the maximum distance between two divers at which they
remained visible, was measured on the bottom with a line marked at 0.5-m
intervals; values were relatively comparable among riprap and reference
stations (Table 4). Highest visibility recorded was 6.8 m at the 9-m intake
station, while the lowest was 0.6 m at a north reference station. Typical
values were 2-3 m at all stations.

Visibility tended to be highest during summer months (June-August). This
was probably the result of summer thermal stratification, followed by
depletion of nutrients, and reduced plankton productivity. Also, fewer severe
storms and reduced turbulence during summer permitted suspended material to
settle. Highest visibilities occurred following a period of one to two weeks
of calm lake conditions.

Several patterns were noted in the visibility among stations.
Visibilities were usually lower at the two stations closest to the discharge
structures (NR-1, SR-1 ) than at other reference or riprap stations. Also,
there was a noticeable decrease in visibility from surface to bottom (6 m) at
these two stations. The reduction in visibility at these locations was the

Table 4, Horizontal visibility (m) as measured by divers on
the bottom near Cook Plant intake structures (9 m) and in
reference areas (6 m) north and south of the plant, 1973-1982.
Asterisk (*) shows months when measurements were not made on
the same day at intake and reference stations. Measurements
at reference stations were always made on the same day for any
given month. Omitted months and blanks indicate no measure-
ments made.

Month

Intake

Area

N. Reference

S. Reference

1973

Jun*

2.0

Aug

4.5

Sep

1.2

1974

Apr*

1.0

May

3.8

Jun

3.3

Jul

Oct

1.2

1975

May*

2.1

Jun

7.6

Jul

4.5

Aug*

3.0

Sep

2.7

Oct

2.7

1976

Apr*

2.5

May*

2.0

Jun

4.0

Jul

1.5

Aug*

3.0

Sep

2.0

Oct

3.0

1977

May

Jun

Jul*

Aug

Sep

(Continued)

3.0
6.8
5.0
6.0
2.5

0.6

2.0
6.1
4.0
3.0
2.7
2.0

1.8
1.8
4.5
1.5
3.0
1.5

6.1
3.0
4.0
2.0

2.0
1.8

3.3
1.7

4.5
1.5

2.5

1.0
1.2
3.0
2.0
3.0

6.0
4.5
4.0
2.0

Table 4. Continued,

Area

Month

Intake

N. Reference

S. Reference

1978

Apr

1.0

May

1.0

2.0

Jun

3.0

Jul*

2.0

3.0

Aug

2.5

3.0

Sep

2.0

Oct

1.0

3.0

1979

Apr

2.0

May

2.0

2.5

2.0

Jun

2.0

Jul

4.5

4.0

Aug

3.0

Sep

3.0

Oct*

1.3

2.0

1980

Apr

2.0

3.0

2.0

May

3.0

2.5

Jun

3.0

Jul

1.0

2.5

1.5

Aug*

2.0

Sep*

2.0

2.5

Oct*

2.5

2.0

2.5

1981

Apr

1.5

2.0

May

2.0

Jun

3.0

Jul

2.0

3.0

1.0

Aug

3.0

4.0

3.0

Sep

3.0

2.5

2.0

Oct

1.5

1.0

2.0

1982

Apr

1.5

1.0

May*

3.0

Jun

4.0

Jul

4.0

Aug

4.0

Sep

3.0

Oct

3.0

result of increased turbulence and suspension of sediment near the point of
water discharge. No effect of plant- induced turbulence and reduced visibility
was noted at reference stations farthest from the discharge structures.

On several occasions (Table 4), visibility at intake structures was
greater than at reference stations. This situation occurred during summer
months when a slight thermal stratification developed inshore (see previous
section - Thermal Effects). A warm, clear layer of water occasionally
overlaid a narrow band (1-2 m thick) of colder, more turbid water adjacent to
the bottom. At reference stations where these layers were undisturbed,
visibility was markedly reduced by one- half or more compared to the intake
area. The overlying water layer was often drawn down into the lower layer at
the intake structures, thus displacing the cooler, more turbid water and
accounting for lower visibilities at reference stations. While diving on the
bottom around the base of the intake structures, divers often swam in and out
of these two water masses. This probably occurred because the water was not
drawn evenly from both layers at all points around the structures.

Our studies in other inshore areas of southeastern Lake Michigan revealed
that water transparency, measured as underwater visibility, did not vary
consistently among locations. Underwater visibilities recorded at the Cook
Plant were typical of the area. But, in another study (Dorr 1982) south of
the plant near New Buffalo, Michigan, we found visibility on the bottom (6-12
m) in an isolated area of clay substrate and extensive submarine trenches to
be consistently lower than the surrounding area, including that of the Cook
Plant. This was the result of erosion of the clay substrate combined with
relatively stagnant water contained in trenches. The water was usually much
more transparent several meters above bottom.

Observations at the Cook Plant and elsewhere in the area suggest that
inshore visibility (transparency) is largely a function of water movements or
currents that suspend sediment off bottom. During quiescent periods, this
material settles and transparency increases significantly. Presence of
accumulations of sediment or erodable material such as clay may reduce
visibility locally.

Inorganic Debris

We distinguished between inorganic debris observed in the study area and
organic material which was termed detritus. Two general types of debris were
noted: that which was deposited during initial construction and subsequent
repair of in- lake plant structures, and debris which accumulated as a result
of activities unrelated to plant construction and maintenance operations.

A variety of materials was deposited on the riprap during construction
including: steel girders and plates, metal pipe, plastic, steel cable, and
tires. For the most part, heavy objects remained in place for the duration of
the study. Subsequent repair work on these structures (e.g., replacement of
broken ice guards on the structures, addition of riprap or cement scour pads,
etc.) resulted in accumulation of debris which remained in the area. However,
some transport of lighter materials (plastic, tires, containers, etc.) from
the area occurred during major storms.

In contrast with the riprap area, debris from plant construction was
never observed on the surrounding sand bottom. If such debris were deposited
in this area, lighter materials were probably rapidly transported from the
area, while heavy objects sank into the bottom and were covered over by sand.
The end result was that plant construction debris did not remain exposed in

sand bottom areas for an extended time. In contrast, inorganic debris and
organic detritus deposited on the riprap could not sink into the substrate,
but snagged on the projections and in the crevices of the rugose substrate and
was held in place. This debris served to expand the variety of substrates and
habitats available to local biota.

The other general type of debris that was noted in the area was that
which resulted from the dumping of trash into the lake. Some of this material
(beverage containers, clothing, fishing tackle, household items, etc.) was
dumped directly into the area by people fishing from small boats. It was not
uncommon to count 20 or more small boats over the riprap area on a summer day.
The other source of this trash came from refuse dumped in surrounding areas of
the lake or eroded from the beach.

In general, the bulk of this trash was composed of lighter items which
were eventually transported from the area. Trash was less abundant in the
early spring following the prolonged absence of fishermen from the area
coupled with the intense fall and spring storms which swept trash from the
area. Evidence of such transport was provided by the occasional observation
of such trash at all reference stations. Our observations during this and
other studies reveal that while most trash is washed onshore or buried and
eventually degraded in the substrate, considerable amounts of litter must be
exposed and washed along the bottom of the lake at any given time. We base
this observation on consideration of the relatively small areas of the lake
bottom observed by divers, and the fairly high frequency at which trash was
observed. With the exception of the riprap area itself, accumulations and
observations of trash near the Cook Plant were similar to those noted
elsewhere in the lake.

While plant construction materials that remained in place on the riprap
provided expanded substrate and habitat, the trash did not. Trash was an
inevitable result of the intensive use of a small area of the lake by the
fishing populace.

BIOLOGICAL FEATURES
Organic Detritus

Organic detritus observed in the study area by divers was classified into
two groups: microscopic and macroscopic. Microscopic organic detritus was
defined as organic material whose original form could not be discerned by the
unaided eye. These materials included remains of planktonic organisms or
parts of larger organisms that were finely divided, such as shredded plants or
decomposed animal tissue. Macroscopic organic detritus included dead algae,
parts of plants (e.g., grasses, bark, twigs, limbs, trunks), and dead animals
(e.g., crayfish and fish).

Accumulations of sediment greater than 10 mm thick were uncommon but
amounts less than 5 mm thick were frequently observed in the study area. No
diver-collected samples were analyzed for loss of organic material upon igni-
tion, at which time organic material would be oxidized to carbon dioxide and
water. However, in a separate study, analysis of 34 samples collected at
depths less than 18 m in the vicinity of the study area showed a mean loss in
sample weight upon ignition of 4.3% with a standard deviation of 4.1%
(Rossmann and Seibel 1977). Combined with diving observations, these results
suggest that both the total accumulation of surficial sediment and its organic
component are variable in inshore southeastern Lake Michigan. Typical values
for thickness and organic content of inshore surficial sediment are 3-5 mm and

4,3% total weight, respectively. These observations also suggest that small
amounts of microscopic organic material are consistently available to benthic
detritivores including epibenthic zooplankton, sponges, bryozoans, Hydra ,
snails, clams, crayfish, insect larvae, and fish. Not surprisingly, all of
these organisms were found in the study area, although they were unevenly
distributed.

Presence of macroscopic organic detritus was recorded in one of several
categories contained in the prescribed record format (Figure 2). Some of
these groups were later combined and summarized in six general categories of
macroscopic material: algae (A), dune grass (B), shreds or chips of wood (C),
twigs and branches (D), tree trunks and stumps (E), and fish (F) (Table 5).
Other materials such as mollusc shells, insect larvae exuviae, crayfish, and
fish feces were seen on occasion, but not often enough to warrant inclusion in
the general summarization of observations. It was not possible to discern or
count individual detrital objects. Therefore, only presence (or absence) of
detritus within the various categories was noted and summarized as frequency
of occurrence (%) among stations and years (Table 5).

Most types of organic detritus were observed at one time or another at
all stations. Twigs and branches were most common and were seen at all
stations at least once in all years. Clumps of loose algae were seen during
22% and 26% of all dives at the north- and south- reference stations,
respectively. Dune grass was noted more often at the reference stations than
at the intake or discharge stations. Shreds and chips of wood were
consistently seen at all stations, but were observed more frequently in
reference areas. The smooth, flat bottom at the reference stations
facilitated diver observation of small detrital objects such as algae, dune

Table 5. Frequency of observation (%) of organic detritus on
the bottom of southeastern Lake Michigan during standard series
dives in the vicinity of the D.C. Cook Nuclear Plant, 1973-1982. ^
Observations of fish (F) are expressed in absolute numbers of
fish counted during dives.

and

No. of '

Category

Year

s ta tion^

dives

1973

100

10 AL

1 YP

1974

100

5 AL

1 SS,

1 YP, 1 XX

1975

1 AL

4 AL,

1 YP

1 AL

100

1976

1 AL

100

7 AL

1977

4 AL,

1 SP

2 AL,

1 SM

9 AL,

1 CP, 1 SS

1978

2 AL

1 CC,

1 XX

1979

2 AL

(Continued)

•

Table 5. Continued,

No. of

tegory^

Year and

station^

dives

1980

AL,

1 YP

1981

AL,

2 YP

1982

100

All years

14
1

AL,
SP

3 JD,

57
1

AL,
CC,

3 YP,
1 XX

116

AL,

1 YP

16
2

AL,
SS,
XX

2 YP,
1 CP,

Total

257

100
3
1
1
2

AL,
JD,
CC,
SM,
XX

6 YP,
2 SS,
1 CP,
1 SP,

Frequency of observation (%) = ~ x 100

where i

No = no. dives at station when observed,

Nt = total no. of yearly dives at station.

NR = north reference stations, SR = south reference
stations, I = intake station, D = discharge station.

A = loose algae, B = dune grass, G = shreds or chips of wood,
D = twigs and branches, E = trunks and stumps, F = fish
(AL = alewife, CC = channel catfish, CP = common carp,
JD = johnny darter, SM = rainbow smelt, SP = spottail shiner,
SS = sculpin, YP = yellow perch, XX = unidentified fish).

grass, and shreds or chips of wood. At the intake and discharge stations, the
uneven surface of riprap and abundance of interstices made observation of
these small objects more difficult than at reference stations.

Tree stumps and trunks were observed infrequently (5% of total dives)
and only once at a reference station. Stumps and trunks were most often
observed at the discharge station. Their projections snagged on the uneven
substrate. The solid foundation formed by the riprap also prevented the heavy
stumps and trunks from sinking into the substrate. Water discharge currents
from the Cook Plant kept these objects washed free of sediment that might
otherwise have eventually covered them. On several occasions (1974-1976),
divers observed tree trunks which were adjacent to ithe discharge structures
and remained in place for several months, including winter.

In areas of sand substrate, moderately heavy objects resting on the
bottom sank into the substrate and were rapidly covered by sediment.
We observed many large chunks of wood, logs, and stumps during excavation of
the lake bottom for placement of plant intake and discharge pipes. A portion
of an excavated stump was examined and thought to hsive been buried along the
shoreline during a previous low- level stage of the lake; possibly during the
Chippewa (5,000-6,000 years ago) or Nipissing (4,000-5,000 years ago) stages
(Hough 1958; personal communication, C. I. Smith, Department of Geology,
University of Michigan)..

Shells of snails and sphaeriid clams were observed occasionally, most
often in troughs of large ripple marks or in shallow, flat- bottomed
depressions in the riprap. These shells were often fragmented and many were
severely eroded. This suggests that the shells were transported by waves and
currents and accumulated in these areas of slack water. Divers often

encountered shells or fragments when sifting through coarse sand, but rarely
when examining fine sand. Again, this was probably the result of the sorting
of sediments by water movement; shell fragments contained in the fine sand
were too small to be observed by the unaided eye.

Fish feces were commonly observed at reference stations. Alewife feces
were most abundant during May-June when these fish concentrated in the area.
Following commencement of heated water discharge from the plant during 1975,
common carp began to be attracted to the area and feces of this fish were
often found in abundance at reference stations closest to the discharge
structures. The feces of these alewives and common carp undoubtedly increased
the supply of organic material to detritivores and recycled nutrients to algae
in the local area, but the significance of tliis contribution is unknown.

On a few occasions, dead crayfish were observed in the riprap zone but no
pattern was detected in their occurrence. However, crayfish are often used by
fishermen as bait for yellow perch that congregate over the riprap. Some of
the dead crayfish seen by divers may have been discarded by these local
fishermen.

Dead insect larvae and shells were observed occasionally but never in
large numbers. Larvae of mayflies, water bugs, caddisflies, and water beetles
were seen at both sand and riprap stations.

The preceding observations indicate that a spectrum of plant and animal
material is available to detritivores inhabiting the inshore region of
southeastern Lake Michigan. The role that detrital- feeding organisms play in
lake ecology is discussed in more detail later in this report (see ECOLOGY).

Large accumulations of dead fish were never observed during dives in the
vicinity of the Cook Plant (Table 6). The largest number of dead fish

Table 6. Record of dead fish observed during all dives in
the vicinity of the D. C. Cook Nuclear Plant, southeastern
Lake Michigan, 1973-1982. Blanks indicate no data.

Water

temp.(°C)

Fish

obsei

rved

Date

Time

Surface

Bottom

Species-'-

Dead

Live2

Nor

th reference stations

25 Jun

1945

19.0

13 May

1333

13.0

12.0

9 Jun

1730

21.7

16.2

75-100

19 May

1530

19.0

16.0

13 Jul

1745

23.7

21.6

28 Jun

1515

20.5

16.5

25 Jun

1605

13.5

9.5

24 Jun

1605

19.0

17.4

26 May

1615

14.8

12.3

Sou

th reference stations

18 Jun

1717

22.0

18.0

22 Jul

1945

15.6

10.0

23 Jul

1445

15.6

7.8

17 Jul

1450

25.0

22.8

AL
YP

15 Jul

1910

23.5

22.7

>1,000

19 May

1630

19.5

16.5

AL
SM

25-30

28 Jun

1620

20.5

19.5

18 Jul

1556

18.0

15.0

28 May

1804

13.6

11.9

26 May

1635

14.5

12.5

23 Jun

1835

17.4

16.0

AL
YP

1 Jul

1630

AL
YP

19 May

1722

19.0

17.0
Intake s

AL
tation

>100

16 Jul

1425

22.2

8 Jun

2145

19.0

16.2

>1,000

15 Jul

1705

23.5

22.6

28 May

1559

13.0

10.5

28 Jul

0400

18.0

12.5

26 May

1720

15.5

12.0

23 Jun

1900 .

18.0

16.5

1 Jul

1730

18.0

13.0

(Continued) .

Table 6. Continued.

Date

Time

Water
Surface

temp. CO
Bottom

Fish

observed

Species ■'■

Dead

Live2

Discharge

station

Aug 73

1103

21.1

17.8

May 74

1150

12.0

11.0

SS
YP
XX

May 76

1540

14.4

11.8

May 77

1330

19.6

15.4

AL
SS
CP

1
18

Jun 77

1920

19.0

16.2

>100

AL = alewife, YP

= yellow perch, SS = sculpin (C.

cognatus

or C. bairdl),

JD = johnny darter,

CC = channel catfish,

CP = common carp, SM = rainbow smelt, SP - spottail shiner,
XX = unidentified fish. See Appendix 3 for scientific
names,

2 Number of live fish of same species observed during same
dive.

observed during a single dive was 30 alewives, which were seen during a dive
in June 1981 at a south reference station. Observation of more than 5 dead
fish during a dive was rare, and of the 281 dives made in the vicinity of the
Cook Plant during 1973-1982 (Table 1), dead fish were observed on only 35
occasions (12% of the dives).

During the 281 dives made near the Cook Plant, 125 dead fish were count-
ed. Of this total, 107 or 86% of the fish were alewives (see Appendix 3 for
scientific names); the remainder was comprised of yellow perch (5), slimy
sculpin and johnny darter (3 each), common carp (2)<, spottail shiner (1),
channel catfish (1), rainbow smelt (1), and 2 unidentified fish. All of these
fish species were abundant in the study area (Tesar and Jude 1985) and were
commonly observed by divers, with the exception of channel catfish.

No particular pattern or trend was detected in numbers of dead fish
observed among stations or years. However, 71% of the dives during which dead
fish were seen were conducted during May-June. This observation was not
surprising because of the high percentage (86%) of dead fish that were
alewives. Annual dieoffs of alewives have typically occurred during May-June
in southeastern Lake Michigan since the late 1960s (Brown 1968, Jude et al.
1979). In fact, considering the thousands of dead fish occasionally seen
floating on the surface of the lake above the divers and washed up directly
onshore, the small number of carcasses seen on bottom was unexpected. An
unquantified but probably small proportion of the alewife carcasses that sank
to the bottom may have been eaten or decayed, but severely eroded or decayed
fish were seldom seen. Most dead alewives seen inshore of the 10-m depth
contour of the lake probably floated on the surface or bottom until they
eventually washed up onshore. The continuous exposure of this inshore region

of the lake to waves and currents undoubtedly quickened the transport of dead
fish to the beach.

Dead fish were never observed during April, September, and October.
Inshore water temperatures were lower during these months than in May-August,
and adult alewife and yellow perch remained farther offshore. The few dead
yellow perch (5) observed during the underwater study were probably caught and
discarded by local fishermen fishing from boats above the riprap and in-lake
plant structures. Observations of all other species of dead fish were
incidental and showed no pattern or particular significance.

Periphyton

Installation of the Cook Plant intake structures and associated riprap
field was completed in late 1972. The surfaces of these objects then
underwent a rapid sequence of initial rusting (of metallic surfaces),
accumulation of sediment and organic detritus, and formation of bacterial
slime. Much of this occurred in 1972-1973.

As the inshore water warmed during spring 1973, the surfaces of the
structures and riprap began to be colonized by periphyton (attached algae),
associated zooplankton, and other microscopic invertebrates. Macroscopic at-
tached invertebrates such as sponges, bryozoans, and Hydra also appeared in
small numbers on these surfaces.

The structures and riprap field were first examined by divers in June
1973. From 1973-1982, the length of periphyton on the top of the south intake
structure and on riprap surrounding its base was measured by divers during
most monthly dives (Appendix 1). Extensive colonization and growth of
periphyton on the top of the intake structure occurred during its first year

in the lake because the periphyton was already 3,7 cm long when first examined
in June 1973. Periphyton 0.5 cm in length also appeared on the upper surfaces
of riprap surrounding the structure at this time. Periphyton grew rapidly on
top of the structure during late spring and attained peak lengths during mid-
summer. This was followed by sloughing of the algae during late summer and
over-wintering at minimal lengths (Fig. 3). Although the pattern of growth
for periphyton on top of the structure was similar for all years, peak length
attained each year varied. This was primarily the result of mechanical
abrasion by ropes tied to buoys surrounding the structure and diver-
construction activities during some years. Periphyton attained greatest
lengths on protected portions of the structure (e.g.,, crevices, flanges, etc.)
and along the top edges of the structure.

Periphyton growth on riprap surrounding the base of the south intake
structure followed an annual pattern that paralleled that on top of the
structure. Peak lengths were usually less than those attained on top of the
structure, except during years of abrasion to the top of the structure. The
primary reason for reduced growth of periphyton on the riprap was the
increased depth (an additional 3 m) and commensurate reduction in light.

Some basic patterns in periphyton growth on the structure or surrounding
riprap were detected during the 10 seasons that the area was examined
(Fig. 1). Periphyton growth was most luxuriant at the edges of the structure
top and within 5 m of the base of the structure, probably the result of
maximal water currents which occurred at these locations. The movement of
water kept the periphyton free of sediment and increased exchange of gases and
nutrients. Periphyton growth was limited on vertical surfaces and non-

CM .

o.
01 •

) \

i 1

6*.
o

O CM-

I ! ! MM I I I

f MAMj JA SOM

l»73 1974

Tif M l

I If M M M ll| Ti n l i M m

r MAM J JA S0*« Dl ^ MAM Jj A S OM0( ^

1975 »76

n y^ M n ! I f I

! r I ! f I I H I » ■ ]

IJjASOMOirMAMJjASONDl

r9r7 1978

X * TOP OF STRUCTURE (3.5- m stratuml

• • RIPRAP SURROUNDING BASE OF

STRUCTURE (7.3-m depth)

1 1 1 il'i I 11 i I 1 1

I FMAM J JA SONO

lf »»H

r979

f MAM JJ ASONO

1960 1961

DATE

I f < > i M M M I

F MAM JJ ASOMOl

Hh-rrr

M I I
FHAajJ* SONC I
C 1983

Fig. 3- Length of periphyton (mm) on top of the south intake structure
(at the 3-m depth stratum) and on the upper surfaces of riprap (at the 7.4-m
depth stratum) adjacent to the base of the structure. Measurements were made
during dives in southeastern Lake Michigan near the D. C. Cook Nuclear Plant,
1973-1982.

existent on the undersides of the structure, riprap, and other unlighted
surfaces at all depths.

The rapid attenuation of light with increasing depth also limited growth
of periphytic algae. Periphyton growth at depths exceeding 10 m was minimal
in comparison with that which occurred at lesser depths. A similar ob-
servation was made during our underwater examinations in 1978-1981 of fine-
mesh screens, intake structures, and riprap at the J. H. Campbell Power Plant
at Port Sheldon, Michigan, located 100 km north of the Cook Plant (Jude et al.
1982). Periphyton growth on all objects was depauperate in comparison with
that observed on the upper surfaces of the Cook Plant structures and riprap.
However, depths at the Cook Plant ranged from 4 to 9 m, while those at the
Campbell Plant exceeded 10 m. At Hamilton Reef, located near Muskegon,
Michigan, about 140 km north of the Cook Plant, periphyton was very sparse and
Cladophora was absent (Cornelius 1984). The minimum depth of this reef is
8.3 m. Observations on the Campbell and Hamilton reefs suggest that periphy-
ton growth is limited at depths greater than 7-8 m in eastern Lake Michigan.

These observations also suggest that, given the general light, tem-
perature, and water transparency regime in southeastern Lake Michigan,
clogging of water intake structures by periphytic algae should be limited to
horizontal surfaces exposed to direct sunlight at depths less than 8 m.
However, clogging of structures by attached invertebrates such as sponges,
bryozoans, and Hydra would not necessarily be eliminated by increasing depth,
and in fact these organisms became very dense on the Campbell Plant intake
screens (Rutecki et al. 1985, Jude et al. 1982).

For several years prior to 1975, periphyton samples were collected from
artificial substrates placed in the lake. Analysis of these samples provided

baseline information on the taxonoraic composition of periphyton in the study
area. Preliminary studies in 1974 and full sampling efforts occurred from
1975 through 1981. During this time, the sampling program was altered so that
samples of periphyton were collected from the top of the south intake
structure and surrounding riprap by divers. Comparison of the 1974-1981
diver-collected samples with those collected earlier from the artificial
substrates revealed that direct sampling of periphyton from the structures and
riprap to qualitatively assess colonization and growth of periphytic algae on
these objects was preferable to use of hand-placed artificial substrates.

A distinct trend occurred toward increasing numbers of taxa, or taxonomic
diversity, with time (Fig. 4; Table 7). Total numbers of taxa increased from
97 in 1975 to 189 in 1981. Numbers of previously unrecorded taxa followed a
trend similar to that observed for total taxa but was less pronounced. This
trend was mostly the result of an increasingly diverse diatom flora. The
fraction diatom (Bacillariophy ta) taxa made of total taxa increased every year
(except 1980) from 58% in 1975 to 75% in 1981 (Table 8); data from 1974 were
considered inconclusive because they were based on analysis of only one sample
from June. The percentage of the total that green algae (Chlorophy ta)
comprised decreased by 14% during the same period. Percent composition of
blue-green algae (Cyanophyta) remained relatively stable and varied from 4%
in 1976 to 9% in 1978 (range =5%). Other algae (Chrysophy ta, Euglenophy ta,
Pyrrophyta, and Rhodophyta) comprised from 1% (1979) to 8% (1975) by number of
the total taxa recorded for each year.

The increase in algal taxonomic diversity was accompanied by a decrease
in numbers of dominant forms. In 1977, 8 of 97 taxa occurred in all samples;
in 1978, 3 of 117 taxa were present in all samples; in 1979, no taxon was

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Table 7. Total number and number of previously unrecorded taxa
of periphyton identified in diver-collected samples scraped
from the top of the south intake structure of the D. G. Cook
Nuclear Plant, 1974-1981. One sample per month, April-
October, was collected each year with the exception of 1974
(all months but June omitted), 1975 (April and September omit-
ted), 1976 (October omitted), and 1977 (April, May, and Octo-
ber omitted). Fraction (%) of total periphyton taxa that were
also identified in samples of entrained phy toplankton collected
from the plant forebay is also listed. Blanks indicate no
samples collec ted .

Total

No. (%) taxa

Percentage

. of

no. of

previously

of taxa

Year

samples

taxa

unrecorded

entrained

1974

21 (100)

1975

66 (68)

1976

1 (1)

1977

34 (35)

1978

117

43 (37)

1979

131

45 (34)

1980

141

38 (27)

1981

189

54 (29)

Table 8. Composition by number (and percent) of the number of
taxa found in diver-collected periphyton samples scraped from
the top of the D. C. Cook Nuclear Plant south intake structure
during 1974-1981. One sample per month, April-October, was
collected each year with the exception of 1974 (all months
but June omitted), 1975 (April and September omitted), 1976
(October omitted), and 1977 (April, May, and October omitted).
Algae were categorized as follows: diatoms = Bacillariophy ta,
green algae = Chlorophyta, blue-green algae = Cyanophyta,
golden-brown algae = Chrysophyta, red algae = Rhodophyta, and
other algae = Euglenophyta and Pyrrophyta.

Blue-

Golden-

Green

green

brown

Red

Other

Year

Diatoms

algae

1974

15 (71)

5 (24)

1 (5)

1975

56 (58)

28 (29)

5 (5)

1 (1)

2 (2)

1976

44 (63)

19 (27)

3 (4)

1 (2)

1977

61 (63)

25 (26)

5 (5)

2 (2)

1 (1)

3 (3)

1978

75 (63)

29 (25)

10 (9)

1 (1)

2 (2)

1979

101 (70)

31 (21)

11 (8)

1 (1)

1980

91 (64)

37 (26)

11 (7)

1 (1)

1981

142 (75)

29 (15)

9 (5)

4 (2)

5 (3)

present in all samples; in 1980 and 1981, one taxon was present in all
samples. During the period 1975-1980, the dominant green algae on the
structure were species of Gladophora . During 1979-1981, length and density of
Gladophora filaments growing on the structure were reduced relative to earlier
years. Oscillatoria spp. were the dominant blue-green algae during all years
expect 1981 when Anacystis incerta was most abundant. Diatoms of the genera
Asterionella , Cymbella , Fragilaria , Melosira , Navicula , Nitzschia ,
Stephanodiscus , and Tabellaria were common in nearly all years. The golden-
brown algae Dinobryon sp. was commonly recorded in samples, while red algae,
flagellates, and euglenoids were occasionally noted.

Successive comparison of total numbers of taxa identified annually in the
periphyton samples revealed: 54 taxa were present in 1981 only; 48 taxa were
present in 2 of the 7 years; 23 taxa were present in 3 of the 7 years; 17 taxa
were present in 4 of the 7 years; 10 taxa were present in 5 of the 7 years; 17
taxa were present in 6 of the 7 years; and 37 taxa were present in all years.

The fraction of periphyton taxa observed in samples of entrained
phy toplankton collected from the Cook Plant forebay was consistently high,
varying from 74% to 81% during 1977-1981 (Table 7). This observation suggests
that considerable sloughing of periphyton occurs each year. Most likely,
sloughing rates are highest during late summer and early fall as decreasing
light levels and water temperatures result in die-off of much of the
periphyton. Comparison between taxonomic lists of algae collected by divers
and those collected in entrainment samples pumped from the plant forebay,
suggests that entrainment sampling is an effective method for qualitatively
assessing the diversity of periphyton attached to in- lake power plant
structures during months when diving is not possible.

5 3

Several conclusions may be drawn from the observations presented in this
section. Almost immediately upon their placement in the lake, underwater
structures were colonized by periphyton, and considerable taxonomic diversity
was achieved during the first year. However, there was a steady increase in
the total number of taxa recorded each year, which was accompanied by a
decline in number of dominant forms noted. A substantial number of rare taxa
was recorded each year, and long-term dominant taxa were few in number. The
largest number of previously unrecorded taxa was identified in 1981 samples,
during the fifth and final year of the periphyton study. This suggests that
ecological succession continued to occur 7 years after the structures and
riprap had been placed in the lake, and that the taxonomic composition and
relative abundance of periphyton had not yet stabilized at the end of this
period. Evidence (Fig. 4) also indicated that periphytic succession would
continue and that taxonomic stabilization was not imminent.

The decline in abundance of Cladophora during 1979-1981 was significant
because, prior to that, these algae comprised most of the mass of periphyton
seen and sampled from the area. Reasons for this decline are not known, but
reduced abundance of Cladophora is related to declining phosphorus levels in
Lake Michigan due to the phosphate ban in 1977 and reduced discharges at
Chicago and Waukegan, Illinois. Presence (or absence) of Cladophora on
substrates was shown to affect the distribution of some invertebrates
(Lauritsen and White 1981).

Attached Macroinvertebrates

Several taxa of invertebrates having one or more sessile stages during
which they must attach to a substrate were observed by divers and included:

freshwater sponge, bryozoans, and Hydra spp. Observations of these animals
were generally incidental relative to those of other invertebrates (snails and
crayfish), but a few patterns emerged from the limited data (Appendix 1),
Attached invertebrates were only observed on substrates in the riprap zone.
Attached invertebrates were not observed in reference areas because of the
absence of stable substrate.

Branched or multi-filamentous Hydra were first observed during September
1973 and were attached to riprap surrounding the intake structures. They were
not observed again until 1978 when they were seen during standard series
diving in October. Hydra were subsequently observed twice in 1979, and once
in 1980 and 1982. These data are somewhat misleading in that they suggest the
abundance of Hydra was low in the study area. When observed, Hydra occurred
in tremendous numbers and often completely covered the upper surfaces of the
riprap. During February 1977, a supplemental dive was made in the Cook Plant
forebay where mats of Hydra 1-2 cm thick and more than 10 m in diameter were
seen attached to the forebay walls. Commercial divers noted similar
occurrences of Hydra during inspection of the interior walls of the plant
intake and discharge pipes (personal communication, A. Sebrechts, Bridgman,
Mich.). The abundance of Hydra on the intake structures and pipe explains its
consistent occurrence in large numbers in entrainment samples.

In the open lake. Hydra were seen only during May and August-October,
suggesting that conditions (e.g., water temperature, availability of specific
planktonic prey) during June-July were not conducive to Hydra growth. Another
possibility is that Hydra competed for substrate with algal periphyton which
attained maximum growth during June-July. This hypothesis is consistent with

diver observations that Hydra were concentrated on the lateral and undersides
of the riprap and plant structures where periphyton was absent.

The long-term distribution of Hydra showed a distinct pattern of initial
colonization within one year of placement of substrates in the lake, followed
by an extended period (1974-1977) of gradual expansion in distribution and
density on these substrates. Peak abundance was achieved during 1978-1980,
although Hydra continued to be observed throughout the duration of the study.

Bryozoans were observed during monthly dives once in 1974, three times in
1976, once in 1977, 1978, and 1980, and twice in 1981. Colonies were isolated
and generally small, never exceeding a centimeter in diameter. No seasonal or
temporal pattern in the abundance or distribution of this organism was
detected during this study. Colonization of the structure and riprap by
bryozoans occurred during the first two years that these substrates were in
the lake.

Freshwater sponges were not observed in the study area until 1975, when
they were seen during two monthly dives. Subsequently, they were seen during
3 mo in 1976, all months in 1977, 4 mo in 1978, 3 mo in 1979, 1 mo in 1980, 4
mo in 1981, and 1 mo in 1982. Both its seasonal and temporal distributions
were more continuous than that of Hydra or bryozoans.

About two years were required for sponges to colonize the plant
structures and riprap in sufficient numbers to be noticed by divers. It is
possible that colonization of these substrates may have occurred more slowly
than for Hydra or bryozoans, although this cannot be substantiated by our
data. Numbers of sponge colonies appeared to stabilize during 1976-1978 and
remained at similar levels of abundance through the remainder of the study.

Both the structures and riprap served as substrates for attachment of sponges,
although they were observed most frequently on the riprap.

Sponges were not observed during dives in early spring (April-May) except
in 1977. Generally, colonies were first observed during June, continued to
increase in numbers throughout the summer, and remained abundant during the
fall (September-October). In late summer, sponges were often bright green in
color, a result of the inclusion of algal cells in the sponge matrix.
Colonies usually appeared as flattened disks up to 1 cm in thickness and 10 cm
in diameter, but occasionally formed finger-like outgrowths 2-3 cm in length.
During late fall, sponge colonies became flattened and tan or white in color
as the algal cells died, and a reduction or die-off of sponge was suspected to
occur during the winter. Winter die-off and dormancy of most living cells
contained in upper strata of the underlying skeletal matrix is typical of
temperate freshwater sponges (Pennak 1953).

The general pattern of colonization of Cook Plant substrates by attached
invertebrates was one of early appearance followed by slow expansion to avail-
able substrates. Riprap appeared to provide a more suitable substrate than
did the metal structures, perhaps because rusting and sloughing of the metal
surface occurred throughout the study, although the rate at which this process
occurred declined in later years of the study. Peak abundance of attached
macroinvertebrates occurred four to six years after placement of substrates in
the lake. During the last several years of the study, the abundance of Hydra
and bryozoans declined, while numbers of sponge colonies continued to fluc-
tuate and showed no particular pattern or trend. Availability of substrate
combined with moving (plant-circulated) water and presence of surficial sedi-
ment, organic detritus, and periphyton combined to provide a hospitable but

isolated micro-environment that was atypical of the surrounding inshore en-
vironment.

Underwater observations at both the Campbell Plant reef near Port Shel-
don, Michigan (Jude et al. 1982) and Hamilton Reef near Muskegon, Michigan
(Cornelius 1984) documented the colonization of riprap by sponges within one
to two years of substrate placement in Lake Michigan. At the Campbell Plant,
sponge colonies attached to wedge-wire intake screens in addition to the
riprap, eventually necessitated cleaning of these screens. Farther north of
the Campbell Plant at Hamilton Reef, sponges and unidentified fungi were
common in diver-collected samples of invertebrates attached to the riprap
(Cornelius 1984).

Free-living Macroinvertebrates

Diver observation of unattached or free-living macroinvertebrates in the
study area included aquatic stages of insect larvae, molluscs (clams and
snails), and crustaceans (crayfish). These observations are summarized in
Appendices 1-2.

Within and outside the riprap zone, divers observed larvae of Diptera
(Chironomidae - true midges), Ephemeroptera (mayflies), and Trichoptera
(caddisflies) . Observations of these larvae were infrequent with no clear
pattern. However, insect larvae were observed only during mid-spring (April-
May) in the study area.

Other invertebrates observed in the area included the crustaceans Mysis
(opossum shrimp) and Pontoporeia (scuds), and an adult of the insect family
Notonectidae (back swimmers). Pontoporeia were observed only during late
summer and fall (August-October) and never during spring or early summer.

Sightings of the above invertebrates were generally limited to the riprap
zone. Often, these organisms were seen clinging to the sides or undersurfaces
of stones. These animals were rarely seen in areas north or south of the
plant. Most likely, invertebrates living in such areas of shifting sand
substrate either buried themselves in the upper layers of the sediment and
were not visible to the divers or were quickly eaten by fish.

Molluscs observed during the study included Sphaeriidae (fingernail clam)
and Gastropoda (snails). Live sphaeriids were not observed because they were
buried in the sediment. However, large numbers of empty shells were commonly
seen at all stations. Sphaeriid shells accumulated in the troughs of ripple
marks and in open depressions among the riprap. These accumulations were
often several centimeters thick and several meters in length or diameter and
attested to the abundance of these organisms in the study area. On one
occasion one valve of a large pocketbook clam ( Lampsilis ventricosa ) was found
at 6 m at the most northerly reference station (Fig. 2). Whether the specimen
came from Lake Michigan or was transported from a connected inland lake was
not known. However, we found lampsilid clams in abundance in the Grand Mere
Lakes, a chain of shallow bar lakes located about 3 km north of the Cook Plant
and which connect to Lake Michigan via an intermittent outlet.

Gastropods (snails) observed in the area during 1973-1982 included Physa ,
Goniobasis , and Lymnaea . Lymnaea were easily recognized by the high, sharp
spire of their shell. Only shells of this snail were seen on a few occasions,
and live specimens were never observed. Physa and Goniobasis were
distinguished underwater by differences in the coil of their shell (sinistral
and dextral, respectively). Laboratory identification of snails collected
over a period of several years revealed that most specimens were Physa Integra

and documented this snail to be the predominant gastropod inhabiting the Cook
Plant riprap.

Gastropod speciation at the J. H. Campbell Plant differed considerably from
that observed for the Cook Plant. The Campbell Plant riprap was initially
colonized by Valvata which were later displaced by Lymnaea , and Physa were
never observed at the Campbell Plant (Rutecki et al. 1985). Interestingly,
Valvata were seen in great abundance during a pre-construction underwater
survey of the site in 1977 (Jude et al. 1978) and were the most abundant
gastropod in Ponar grab samples of sediment collected during 1977-1979 from
areas north and south of the plant (Winnell and Jude 1981).

The diffierence in species distribution of gastropods between the Cook and
Campbell reefs was probably related to differences in physical and biological
conditions at the two reefs. The increased size of the riprap and
interstitial spaces, combined with greater depth and subsequently reduced
storm-generated water turbulence, less periphyton, and absence of Cladophora
on the Campbell Plant reef, may have favored or excluded certain species of
snails. Pennak (1953) noted that Physa occurs in greatest abundance where
there is a moderate amount of aquatic vegetation but is rare in areas where
there are dense mats of vegetation. This may, in part, explain why Physa
initially colonized the Cook riprap but disappeared in later years as
periphyton became more abundant on the reef. Absence of periphyton or other
vegetation on the Campbell riprap may have discouraged colonization of this
reef by Physa . On the other hand, Lymnaea is found in a wide variety of
habitats (Pennak 1953). This snail was abundant on the Campbell reef and its
shells were occasionally collected at the Cook reef. No exact explanation
could be made for the presence of Valvata on the Campbell reef and its absence

on the Cook reef. However, there is a major anatomical and physiological
difference in the respiratory mechanism of the Valvatidae when compared with
the Physidae and Lymnaeidae. The Valvatidae have external plumose gills;
whereas, the Physidae and Lymnaeidae have a "lung" or pulmonary cavity.
Also, most pulmonate snails come to the surface to breathe (although a large
number do not) and therefore generally tend to inhabitat shallow water.
The increased depth of the Campbell reef along with absence of periphyton that
might interfere with external gills may have favored the valvatid snails.

Numbers of snails (primarily Physa ) at the Cook Plant did not show any
strong pattern of seasonal abundance during April-October, except that they
tended to be most abundant during April-June and August-October and were never
abundant during July (Fig. 5). However, a clear pattern of temporal abundance
emerged during the study. Snails were observed in large numbers during 1973-
1975 and peaked in abundance during May 1975 when 30-100 snails/m^ were
counted during dives at the south intake station. These numbers include only
snails immediately visible to divers without disturbing the riprap. In
actuality, the density of snails was probably several times greater than 30-
lOO/m^, because they were abundant on the sides and undersurfaces of the
riprap as well as on stones beneath the surficial layer of riprap. Following
1975, a precipitous decline in snail abundance occurred during 1976-1978. No
snails were observed in the study area from 1979 through 1982.

The riprap was colonized by snails during its first year in the lake and
supported large populations of Physa for about three years. At that point,
habitat conditions or some other ecological effect occurred that rendered the
riprap unsuitable for Physa . As previously noted, it is possible that after
several years, the accumulation of sediment and periphyton on the surface of

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t±e riprap reached a point at which it interfered with the respiration or
movement of the snails. Another possibility is that composition of
microscopic flora and fauna that snails fed upon was altered through the
accumulation of sediment and periphyton, and eventually the riprap surfaces no
longer provided suitable food for the snails. Yet another possibility is
based on the observation that snail egg cases were commonly observed during
the first few years of diving but not in later years. Perhaps as the surface
of the riprap aged and accumulated material, it was no longer sufficiently
clean to serve as substrate for the attachment and incubation of these eggs.

On a few occasions, live snails were seen on the metal surfaces of the
intake and discharge structures. However, only isolated animals were observed
and densities never exceeded one snail per several square meters. The surface
of the structures was always covered with either periphyton and sediment, or,
when periphyton was absent, rust. The snails may have avoided all such
surfaces. Also, snails were quite obvious on the flat surface of the
structure and may have been more susceptible to predation by fish.

In contrast to sightings of Valvata in areas surrounding the Campbell
reef, live snails were never observed by divers in sand-substrate areas
surrounding the Cook Plant riprap zone. No explanation can be offered for
this difference. However, snails were observed in areas of natural (clay,
cobble) rough substrate north and south of the Cook Plant (Dorr 1982). These
isolated areas of naturally occurring, stable substrate probably served as
preserves on the lake bottom where snails, along with crayfish and attached
invertebrates could survive and emigrate to areas of newly placed artificial
substrate.

Information on the abundance and distribution of decapods (crayfish) in
the study area originated from two sources: diving observations made during
1973-1982 and records of their impingement from 1975 through 1981 on Cook
Plant traveling screens (Fig. 6). Three species of crayfish were present in
impingement samples; Qrconectes propinquus , £. virilis , and Cambarus diogenes
diogenes . Only isolated specimens of the latter two species were collected,
representing only a fraction of a percent (0.08%) of all crayfish collected
(Winnell 1984). Crayfish were observed during all years of the underwater
study, although their abundance fluctuated during this period. It was assumed
that most crayfish observed by divers were jO. propinquus , based on the
predominance of that species in impingement samples.

Crayfish were observed more frequently at night than during the day
(Fig. 7). This was in accordance with the generally nocturnal habits of this
animal which remains hidden in burrows or under substrate during the daytime
(Pennak 1953). At the Cook Plant, crayfish could be found during daytime by
excavating some of the riprap. At night, crayfish emerged and rested on top
of the stones or among the interstices.

Comparison of total numbers of crayfish observed by divers each month
with numbers of crayfish impinged documented a general pattern of initial low
abundance, followed by rapid population growth, and then by a decline to about
one- tenth of peak abundance. Crayfish were observed in 1973 and had therefore
colonized the reef within one year of its placement in the lake. During 1979-
1982, numbers of crayfish observed and impinged fluctuated but remained within
the same general upper and lower limits during the period.

During April-October, 1975-1982, day and night observations were made at
two side-by-side, 1 x 10 m transects adjacent to the base of the south intake

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Fig. 6. Numbers of crayfish observed by divers (1973-1982) and impinged on
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structure. These observations were pooled to yield numbers of crayfish (and
other organisms) observed per 20 m^. These quantified observations were based
on standardized methodology and constituted the most reliable database from
which conclusions could be drawn based on underwater observations. Comparison
of transect observations of crayfish (Fig. 7) with total numbers of crayfish
observed and impinged in the study area (Fig. 6) revealed a corroborating
pattern of temporal abundance. As with total numbers of crayfish observed and
impinged, peak abundance of crayfish recorded during transect observations
(72/20 m^) also occurred during 1976, although more were seen during September
than August. Transect observations also support the conclusion that crayfish
were most abundant on the Cook Plant riprap during 1975-1977 and that their
abundance declined precipitously during 1978. They continued to be observed
in small numbers through 1981 but none was seen in 1982.

The reason for the abrupt decline in abundance of crayfish in 1978 is
unknown. Peak numbers of crayfish impinged during 1978 approached 1977 levels
but sustained impingement during 1978 was clearly less than that of 1977.
Total and transect observations of crayfish declined by a factor of 10 during
the period 1977-1978. It appears that some environmental factor or ecological
relationship changed during the period fall 1977-spring 1978 and caused a
rapid decline in abundance of crayfish on the Cook Plant riprap. A similar
decline in abundance of snails was discussed earlier, although it occurred
during 1976, about two years in advance of the crayfish population decline.

Peak abundance of crayfish recorded during transect observations
(September 1976 - Fig. 7) was 72/20 m^ or about A/m^. However, this number
included only those animals visible to the divers who did not displace the
riprap during transect swims. Based on non- transect observations during which

the riprip was overturned, it is possible that actual abundance of crayfish
may have peaked at S-lO/m^. Based on numbers and weights of crayfish impinged
during the same month, the average weight of these crayfish was 5.1 g.
This extrapolates to an observed abundance of 20.4 g/m^ (162 lbs/acre) and an
estimated abundance of 41-51 g/m^ (364-445 lbs/acre). Pennak (1953) noted
that pond populations of crayfish generally do not exceed 100 lbs/acre but in
exceptional cases may attain 500-1,500 lbs/acre. These data suggest that at
peak abundance, the riprap supported a relatively dense population of cray-
fish. It is possible that within two to three years the carrying capacity of
the habitat may have been exceeded which resulted in the subsequent decline in
crayfish abundance observed during later years of the study.

Unlike the Cook Plant reef, no crayfish were observed during four years
of diving (1978-1981) on the Campbell Plant reef. Rutecki et al. (1985)
attributed this disparity to differences in reef composition and
configuration. Surficial riprap surrounding the Cook Plant intakes was
composed of stone ranging from about 0.1-0.6 m in diameter and weighing about
1-50 kg. Campbell Plant riprap was considerably larger than Cook Plant
riprap, usually exceeding 1 m in diameter and weighing 225-900 kg. The
interstices among the Campbell riprap were much larger than those of the Cook
Plant and may have provided crayfish with less protection from fish predation
(e.g., slimy sculpin, yellow perch), especially during the egg and juvenile
stages.

Another possible explanation for the absence of crayfish on the Campbell
reef is that, in contrast to the Cook riprap, periphyton was extremely
depauperate on the Campbell riprap and Cladophora was absent. Prince et al.
(1975) found that in Smith Mountain Lake, crayfish were abundant in areas

supporting luxuriant Gladophora and absent from areas with little or no growth
of this alga. Crayfish are omnivorous and are known to eat aquatic vegetation
(Pennak 1953). It is possible that Gladophora constituted an important
component of the diet of crayfish at the Cook Plant and that absence of this
or other aquatic vegetation on the Campbell riprap resulted in an inadequate
supply of food. Lauritsen and White (1981) found that the seasonal abundance
of some predacious and filter- feeding zoobenthos was correlated with the the
luxuriance of Gladophora on the Cook Plant riprap. These zoobenthos may have
served as prey for crayfish, thus providing a trophic link through which the
abundance of Gladophora . could affect the abundance of crayfish on the reef.

These observations correspond with those of Cornelius (1984) for Hamilton
Reef near Muskegon, Michigan. This artificial reef is similar in composition
and location to the Campbell reef, although its configuration is somewhat
different in that the riprap is separated into numerous piles several meters
apart which are interspersed by areas of sand. Like the Campbell reef,
periphyton was scarce on the Muskegon reef, Gladophora was absent, and
crayfish were not observed during three field seasons of diving. Elsewhere in
the area. Dorr (1982) documented the presence of crayfish in areas of
naturally occurring cobble substrate located near Saugatuck and South Haven,
Mich., between the Campbell and Cook Plants. These substrates also supported
periphyton, although growths were never as luxuriant as those seen at the Cook
Plant. However, abundance of crayfish was also lower at these locations than
at the Cook Plant. The above observations argue for the existence of a
relationship between abundance of periphyton, Gladophora in particular, and
that of crayfish on inshore reefs in eastern Lake Michigan.

During 10 years of diving at the Cook Plant, only one crayfish was seen
in an area of sand substrate outside the riprap zone. This attests to the
critical role that substrate plays as a limiting factor in the life history
and distribution of crayfish, particularly in such a harsh environment as
occurs inshore in eastern Lake Michigan.

Fish Spawning

Spawning by numerous species of fish has been inferred from catches of
male and female fish with ripe- running gonads in the inshore region of Lake
Michigan near the Cook Plant (Jude et al. 1979, Tesar et al. 1985).
Occurrence of newly hatched yolk-sac larvae in plankton net hauls in the lake
and entrainment samples collected from the plant forebay (Bimber et al. 1984,
Noguchi et al. 1985) supports this inference. More direct evidence of fish
spawning in the immediate vicinity of the Cook Plant was provided by in situ
observation of eggs of five fish species: alewife, spottail shiner, yellow
perch, johnny darter, and slimy sculpin.

Fish eggs were observed during all years of the study except 1982
(Appendix 1). Eggs were observed exclusively during May-August (Fig. 8).
Duration of occurrence for a given species ranged from about 3 weeks for
yellow perch and sculpin to about 10 weeks for alewife.

The line graphs in Figure 8 must be interpreted with care because they
present information on different components of the reproductive cycle.
The basic progression of events during reproduction should be the appearance
of ripe- running fish in the area followed (or paralleled) by spawning and
deposition of eggs. Next would come a period of egg incubation during which

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eggs might be observed ^ situ followed by hatching and appearance of yolk-sac
larvae in the area.

Most data presented in Figure 8 were compiled exclusively from diving
observations and concurrent studies of adult and larval fish at the Cook
Plant, with the exception of the literature survey. Therefore, some disparity
between reported spawning periods and the timing of other events in the
reproductive cycle shown in Fig 8. was expected. This occurred because the
literature survey included habitats other than the Cook Plant where environ-
mental conditions might elicit spawning at other times of the year. For ex-
ample, temperature-dependent spawning of fish may occur earlier in the year in
a shallow inland lake where the water warms more rapidly in spring than in
Lake Michigan.

Another cause for the disparity among events depicted in Figure 8 may
be that these data summarize the findings from several years of study.
Some variability occurred among years in the timing of reproductive events
(e.g., maturation of gonads, deposition of eggs, and hatching of larvae).
Therefore, for any given year, the duration of reproductive events was
probably shorter than the periods shown.

Alewife showed the most protracted period of reproductive activity among
the five species. Over a 4-6-yr period, yolk-sac larvae were taken in field
samples as early as April and appeared in both field and entrainment samples
until the beginning of October. Occurrence of ripe adults (early May-mid-
July) and observation of eggs (June-mid-August) were in close agreement in
terms of the sequence of these reproductive events. The spawning period
reported in the literature for alewife was longer than that suggested by adult
fish studies and diving observations but agreed with the occurrence of yolk-

sac larvae late in the summer. The appearance of yolk-sac larvae in field and
entrainment samples during April was difficult to explain in terras of the data
presented in Figure 8 but may have resulted from exceptionally early spawning
by a few fish. Yolk-sac larvae were never captured in large numbers during
April or early May. The period from mid-May through July appeared to
encompass the bulk of alewife spawning and egg incubation in the study area.
Most eggs observed during late July and August were either opaque or fungused,
indicating that they were no longer viable.

Of these five fish, alewife, spottail shiner, yellow perch, johnny
darter, and slimy sculpin, only alewife has pelagic eggs that are randomly
broadcast during spawning; the other four species have demersal eggs that
adhere to the substrate. Also, only alewife eggs were observed in areas
outside the riprap zone. The eggs often accumulated and formed a thin layer
in the troughs of the ripple marks at the sand- substrate reference stations
north and south of the plant. Alewife eggs were commonly observed on top of
the riprap and plant structures, trapped among the filaments of periphyton.
Eggs were seen in about equal abundance in the riprap zone and at reference
stations. No indication of area- or substrate-selective spawning was noted.

During 1973-1982 adult fish studies near the D. C. Cook Nuclear Plant,
several thousand yellow perch stomachs were examined. Many were found to
contain alewife eggs, thereby documenting predation by yellow perch on these
eggs (unpublished data, Great Lakes Res. Div., Univ. Mich., Ann Arbor, Mich.).
These studies and those of Dorr (1982) showed extensive yellow perch predation
on young-of-the-year and adult alewife as well. Yellow perch predation on
large larval alewives was suspected, but larvae were not found in the stomachs
of yellow perch, probably because of the rapid rate at which this material was

7 3

digested beyond recognition. The Cook Plant adult fish studies also
documented a dramatic increase in abundance of yellow perch in the area and a
concurrent decline in abundance of alewife (Tesar and Jude 1985, Jude and
Tesar 1985 )• The recent decline in abundance of alewife in Lake Michigan
probably resulted from salmonine predation. Increased abundance and predation
of yellow perch on eggs, larvae, juveniles, and adult alewife combined with
tliat from stocked salmonids may cause a possible future collapse of alewife
stocks in Lake Michigan.

Spot tail shiners were observed spawning on top of the south intake
structure during a night dive in 1973. As the eggs were broadcast over the
mat of periphyton that covered the surface of the structure, they settled into
the periphyton and adhered to the algal filaments. Spawning was not observed
on the riprap. On several occasions during later years, a few eggs were
collected from the top of the structure and incubated in the laboratory, and
the newly hatched larvae were identified as spot tail shiners.

The chronology of reproductive events observed for spot tail shiners in
the study area (Fig. 8) closely paralleled the expected timing of events.
Ripe fish were caught during mid-April-mid-July. Spawning and eggs were
observed during June. Yolk-sac larvae appeared in field samples from June
through mid-August and in entrainment samples from June through mid-October.
The bulk of spot tail shiner spawning, egg incubation, and hatching occurred
during June-mid-July in the study area. The only unexplained component of the
data (Fig. 8) was the observation of yolk-sac larvae in entrainment samples
during September and October, one to two months after ripe fish ceased to be
collected in the area. The spawning period reported in the literature for

spottail shiners was in close agreement with that which would have been
predicted from field study data.

Spottail shiner eggs were occasionally seen on the riprap but never at
reference stations. This is probably due to the more nearshore distribution
(O m) of their eggs.

Maturation, spawning, egg incubation, and hatching of yellow perch in the
study area was examined in detail by Dorr (1982). He documented that spawning
and incubation of yellow perch eggs was limited to areas of rough (natural or
artificial) substrate. Yellow perch egg masses were never observed on sand
substrate during nearly 500 dives in the study area which encompassed 10
spawning seasons (Dorr and Jude 1980a, b; Dorr 1982). These findings concur
with those reported in the literature and clearly establish that in
southeastern Lake Michigan yellow perch spawned selectively on stable, rugose
substrate. These substrates probably serve to anchor the eggs and suspend
them slightly above bottom, thereby reducing settling of eggs into the
substrate or transport to areas with conditions less favorable to survival,
e.g., the turbulent beach zone.

In addition to the Cook Plant reef, evidence of yellow perch spawning on
two other artificial reefs in eastern Lake Michigan lias been compiled. Al-
though yellow perch egg masses were never observed on the Campbell Plant reef
(Rutecki et al. 1985), the high abundance of ripe fish and yolk-sac larvae in
field samples and predominance of yellow perch larvae in entrainment samples
(Jude et al. 1982) suggest that perch spawned on this reef. Yellow perch eggs
were usually observed in situ for no more than 2 weeks (Dorr 1982); most like-
ly, the timing and intensity of diving on the Campbell reef was inadequate to

permit observation of eggs. Biener (1982) reported aggregation and spawning
of yellow perch on Hamilton Reef near Muskegon, Michigan, in 1981.

Yellow perch egg masses were also observed in areas of natural rough
substrate by Dorr (1982). Masses were seen at 6-9 m on cobble substrate near
Saugatuck and South Haven, Michigan, and on rugose clay substrate 3 km north
of the Cook Plant and on New Buffalo shoals south of the plant. Egg masses
have also been seen on clay substrate near Michigan City, Indiana (personal
communication, G. McDonald, Ball State Univ., Muncie, Indiana).

Capture of ripe yellow perch during early April-early June and observa-
tion of eggs during mid-May-early June corresponded with the expected timing
of these events. Occurrence of yolk-sac larvae in field and entrainment sam-
ples during mid-May-July corresponded with maturation and spawning. The oc-
currence of yolk-sac larvae in the study area during April and early May has
been attributed to riverine input of larvae spawned in inland waters that warm
to spawning temperatures earlier in the spring than inshore Lake Michigan
waters (Wells 1973; Jude et al. 1979, 1981a; Dorr 1982; Perrone et al. 1983).
Appearance of yolk-sac larvae in August entrainment samples may have been the
result of some isolated late spawning or unusually slow maturation of larvae.

The spawning period (mid-May to mid-June) reported for yellow perch in
southern Lake Michigan corresponded closely with that predicted from Cook
Plant fish and underwater studies. Lake Michigan yellow perch have a short
reproductive season relative to other fish species, and the bulk of spawning,
incubation, and hatching occurs during a 3-4-week period from mid-May through
early June in this area of the lake.

Johnny darter eggs were found on two occasions in 1977, during May and
June. In May, one cluster of eggs was found attached to the underside of a

fiberglass washtub and another was attached to the underside of a swim fin.
Both of these objects had been lost from the dive boat during the previous
month. In June, two more clusters of eggs were found attached to the
underside of a flat slab of wood. The female darter often lays her eggs in
several clusters each containing 20-200 eggs (Scott and Grossman 1973); the
two clusters of eggs found on the wood slab may have been spawned by a single
fish. The clusters were 2-3 cm in diameter and were composed of several
hundred eggs packed closely together in a single layer. The eggs were
collected, hatched in the laboratory, and larvae verified as johnny darters.

The concurrent appearance of ripe fish in field samples and observation
of eggs during mid-May to mid-June (Fig. 8) defined a short spawning period
for johnny darters in the study area. The occurrence of yolk-sac larvae in
field and en trainmen t samples during mid-May-July was in general accord with
the timing of spawning and incubation of eggs, as was the spawning period
reported in the literature. But, like the other species, both early and late
occurrences of yolk-sac larvae were noted. These data suggest that the bulk
of johnny darter spawning, incubation, and hatching occurs from mid-May
through late June in the study area.

Sculpin eggs were found on two occasions, in May of 1974 and 1978.
In both instances, the eggs occurred as a flattened mass attached on the
underside of a piece of riprap. These masses were similar in appearance to
the johnny darter egg clusters except that both the individual sculpin eggs
and size of the egg mass were larger than those of the darter. On both
occasions, the collected eggs were incubated in the laboratory until the
larvae hatched and were identified as slimy sculpin (Cottus cognatus) .

The chronology of reproductive events documented for slimy sculpin by
Cook Plant fish and diving studies was nearly perfect, in biological terms.
Ripe adults were caught during April- mid-May, and eggs were observed during
the first three weeks of May. Yolk-sac larvae appeared in entrainment samples
from mid-May through June and in field samples during June. Larvae appeared
in entrainment samples about two weeks earlier than in field samples, because
sculpin spawning was concentrated in the riprap zone where field net tows were
not conducted. Netting was conducted north and south of the riprap, and some
time probably elapsed before the newly hatched larvae migrated from their
nests in the riprap zone to surrounding areas of the lake where they were
subsequently netted. The spawning period reported in the literature generally
agreed with that predicted from Cook Plant data. Again, spawning reported
during March-early April probably occurred in inland waters that warm to
spawning temperatures more rapidly than inshore Lake Michigan. These data
(Fig. 8) indicate spawning, egg incubation, and hatching of sculpins occurs
during a relative brief period, with the bulk of this activity taking place
during late April- late May.

Several conclusions may be drawn from the preceding discussion on
reproductive activity of fish in the study area. Two general modes of
spawning were noted: fish that broadcast their eggs at random without regard
to substrate type and fish with substrate-specific spawning requirements.
Alewife was a primary example of the first category of spawner. Its eggs were
pelagic and ubiquitously distributed. Examples of the other spawning mode
included spottail shiner, yellow perch, johnny darter, and slimy sculpin.
Spottail shiner eggs were demersal and adhesive and were found attached to a
variety of stable substrates. It appeared that while this species selects

stable substrates for spawning, the composition and configuration of that
substrate is not a critical factor in the selection process. Johnny darter
and slimy sculpin were more selective in that eggs were laid on the flat,
clean undersides of riprap and inorganic or organic debris. As in other
studies in the area (Biener 1982, Dorr 1982, Rutecki et al. 1985), yellow
perch were found to have rather specific substrate requirements that focused
on substrate configuration and rugosity. Finally, related studies (Dorr and
Jude 1981a, Dorr et al. 1981b, Jude et al. 1981b) in the area have compiled
evidence that some species such as lake trout have extremely specific
spawning-substrate requirements that include characteristics such as
composition, configuration, rugosity, and interstitial dimensions.

With the exception of alewife and spot tail shiner, spawning was
concentrated in the riprap zone, and much of the reproduction of the species
discussed occurred during May-June. During this period, survival and growth
of these fish populations could be affected by perturbations of specific
events (spawning, incubation, hatching and early survival) in their
reproductive cycle. Populations of pelagic spawners such as alewife that
broadcast their eggs randomly over a wide area are less likely to be affected
by a point ecological impact than populations of fish which concentrate their
spawning in the area of the impact. With regard to johnny darters, slimy
sculpins, and to a small degree spottail shiners, an ecological trade-off
exists between reproduction and plant operation. These species concentrate
around and spawn on in- lake plant structures, thus increasing their
vulnerability to impingement, entrainment, and physical (heat) and chemical
(chlorine) discharges. But at the same time, populations of these fish have

been enhanced by the creation of this artificial substrate and would not exist
in such abundance if the plant structure were not present.

Juvenile and Adult Fish

Twenty- two taxa encompassing 24 species of fish were observed by divers
during the study and were grouped according to frequency of observation
(Table 9) from data presented in Appendix 1. Frequently observed species
included alewife, yellow perch, sculpins (slimy sculpin and mottled sculpin),
johnny darter, and spot tail shiner. All of these fish were seen at least once
during each year of the study. Commonly observed species included trout-
perch, common carp, rainbow smelt, burbot, and white sucker. These fish were
seen during seven to nine years of the study. Uncommonly observed species
included largemouth bass, lake trout, channel catfish, black bullhead,
smallmouth bass, and longnose sucker. These fish were seen in more than one
year but less than half of all study years. Species that were rarely observed
and were seen during only one year included emerald shiner, brown trout,
quillback, walleye, coregonids (bloater and lake herring), and shorthead
redhorse. The 10 taxa that were frequently or commonly observed composed the
bulk of the observations of fish. The remaining 12 taxa were seen too
infrequently to make detailed inferences based on underwater observations.

A total of 72 species of fish were identified among the 1.1 million fish
collected during 1973-1982 field studies near the Cook Plant (Tesar and Jude
1985) and 5.8 million fish impinged on its traveling screens during 1975-1982
(Thurber and Jude 1985). Therefore, about one third (31%) of the species
documented in the study area by Cook Plant studies were observed by divers.
These observations suggest that a large number of the species that occurred in

Table 9. Annual relative ranked abundance of fish observed during all
diving in sout±ieastern Lake Michigan near the D. C. Cook Nuclear Plant,
1973-1982. Fish were grouped according to frequency of observation.
Blanks indicate no observation. Common names of fish assigned accord-
ing to Robins et al. (1980).

Species

Year

No. yrs

observed 73 74 75 75 77 73 79 80 81 82

Frequent

Alewife
Yellow perch
Cottus spp.^
Johnny darter
Spottail shiner

Common

Largemouth bass
Lake trout
Channel catfish
Black bullhead
Smallmouth bass
Longnose sucker

10
10
10
10
10

2
3
5
6
1

6
4
1
3
2

1
3
2
4
5

1
4
2

1
3
5
2
7

1
3
4
4
7

1 1

2 4
5 5
4 6

3 3

9 10

1 1

2 2
5 4
4 6
7 5

Trout- perch

4 5

Common carp

^ 9

Rainbow smelt

Burbot

White sucker

Uncommon

10 9

9 10

Rare

Emerald shiner
Brown trout
Quillback
Walleye

Goregonus spp.^
Shorthead redhorse

Total taxa

6 12 12 11 10 11 11 13 10 14

i Includes both C^. cognatus (slimy sculpin) and C. bairdi
(mottled sculpin) .

^ Includes both C^. artedii (cisco or lake herring) and C^. hoyi
(bloater).

the area were rare and that diver observations of fish were limited to the
more abundant species. The 5 fish taxa most frequently observed by divers
were also among the 10 fish taxa most frequently collected in field and
impingement samples.

Total number of fish taxa observed each year varied from 6 to 14 (Table
9). If 1973 data are ignored (both the diving methodology and schedule were
incomplete that year), numbers of fish taxa observed ranged from 10 to 14,
annually. Considering that 11 taxa were seen at least 7 out of 10 years, and
5 taxa were seen every year, the diversity of species regularly observed by
divers was low in comparison with total number of species occurring in the
area. However, tlie most abundant species in field and impingement samples
were nearly always observed by the divers. These observations suggest that
diving is effective for documenting the presence of abundant species but
ineffective for studying rare species.

Fish species observed by divers could be divided into two categories
based on their behavior and response to the presence of the Cook Plant.
The first category described orientation of fish in the water column - pelagic
or demersal. The second category was related to the response of fish to the
physical presence or aspects of plant operation - attracted or indifferent
(species repelled by the plant were not discerned by this study) (see Tesar
and Jude 1985). Four combinations of these behavior-response categories were
represented in the observational data base: pelagic fish that were attracted
to the plant (pelagic-attracted), pelagic fish that were indifferent to the
plant ( pelagic- indif f eren t) , demersal fish that were attracted to the plant
(demersal-attracted), and demersal fish that were indifferent to the plant
(demersal-indifferent) .

Pelagic fish that appeared to be attracted to the in- lake structures or
operation of the plant included yellow perch and common carp and possibly
largemouth bass, smallmouth bass, and walleye. Pelagic species that appeared
generally indifferent to the in-lake presence or operation of the plant
included alewife, spottail shiner, trout-perch, rainbow smelt, lake trout,
emerald shiner, brown trout, and coregonids. Demersal fish that appeared to
be attracted to the in- lake presence or operation of the plant included
sculpins, burbot, channel catfish, and black bullhead. Demersal fish that
appeared indifferent to the in- lake presence or operation of the plant
included johnny darter, white sucker, longnose sucker, quillback, and
shorthead redhorse.

Inspection of relative ranked abundance of fish within and among years
revealed that in most years alewife was most abundant. Yellow perch always
attained one of the next three ranks (second-fourth). Alewife, yellow perch,
johnny darter, spottail shiner, and sculpins always comprised at least four of
the top five ranks each year.

Relative ranked abundance of fish species observed during transect swims
along the base of the south intake structure (Table 10) generally paralleled
that established for total dives (Table 9). Total number of fish species
observed each year ranged from five to nine. Number of species observed
during transect dives was always less than the total number observed for any
given year, primarily because the observational effort for transect swims was
much less than for total dives. However, during transect swims, observations
were focused on the bottom and did not extend above bottom beyond the range of
visibility, which was usually between 2 and 3 m (Table 4). Consequently, a
slightly higher percentage (44%) of those species classified as demersal was

Table 10. Annual relative ranked abundance of fish observed during
duplicate observations made during transect swims in southeastern Lake
Michigan, 1975-1982. Observations were made by two divers swimming
side-by-side for 10 m along the base of the south intake structure of
the D. C. Cook Nuclear Plant. Each diver examined an area 1 m wide;
observations were summed and then ranked for the total area (20 m^)
examined. Fish were grouped according to frequency of observation.
Blanks indicate no observation. Common names of fish assigned according
to Robins et al. (1980).

Species

No. yrs

Year

observed

Frequent

Alewife
Yellow perch
Cottus spp.-^

Common

Johnny darter
Spottail shiner
Rainbow smelt
Trout-perch

Uncommon
Burbot

Rare

Black bullhead

Total taxa

^ Includes both £. cognatus (slimy sculpin) and C^. bairdi (mottled
sculpin) .

seen than of t±ose classified as pelagic (38%). Of those species frequently or
commonly observed during the total diving effort, only burbot and white sucker
did not appear in these same observational frequency categories during
transect dives. These two species were not abundant and never attained a rank
higher than ninth in total dives conducted after 1974.

As with total dives, alewife was the most frequently observed fish
species during transect dives. Sculpins displaced yellow perch as the second-
most abundant fish species observed during transect swims. This was not
unexpected considering the generally high abundance and demersal behavior of
sculpin. Yellow perch was generally the third-most abundant species seen
during transect swims. Johnny darter and spottail shiner occupied a lower
frequency category for transect dives than for total dives. However, the
significance of this shift was relatively inconsequential considering the
overall abundance of these two species in the study area. No pelagic species
classified as uncommon or rare among total diving observations (Table 9) were
observed during transect swims (Table 10).

In addition to total diving observations (summarized from Appendix 1)
and transect observations (summarized from Appendix 2), summary data are
presented from standard series field sampling (Tesar and Jude 1985) and
studies on impingement of fish on the Cook Plant traveling screens (Thurber
and Jude 1984, 1985) for 10 species of fish: yellow perch, common carp,
alewife, spottail shiner, trout-perch, rainbow smelt, sculpins, burbot, johnny
darter, and white sucker. The remaining 12 species of fish observed during
underwater studies at the Cook plant were seen too infrequently to permit
meaningful analyses based on observational data. Species discussions are

grouped according to the four behavioral categories noted earlier: pelagic-
attracted, pelagic-indifferent, demersal-attracted, and demersal-indifferent.

Pelagic-Attracted —

The species complex of diver-observed pelagic fish that appeared to be
attracted to the in- lake structures or plant operation included yellow perch,
common carp, and possibly largemouth bass, smallmouth bass, and walleye.
Sufficient evidence (Tables 9, 10) was compiled during the study to infer the
attraction of yellow perch and common carp to the plant. The attraction of
the other three species to the plant was hypothesized more from general
knowledge of the species and their habits than from empirical data.

Yellow perch was usually the second- or third-most abundant species
observed during all dives and transect swims and was never lower than fourth
(Fig. 9). It was also among the five most abundant species in field and
impingement samples. During 1973-1977, the relative ranked abundance of
yellow perch fluctuated among the four sampling categories. A distinct
decline in abundance occurred in field and impingement samples between 1977
and 1978 and was followed by a steady increase in relative abundance.
Although this pattern was not reflected in diving observations, yellow perch
were frequently observed during 1978-1982 underwater studies.

The disparity in trends of relative ranked abundance between field and
impingement sampling and all dives and transect swims may be explained by the
documented affinity that yellow perch have for rough substrate in the
generally smooth, sandy-bottom areas of inshore eastern Lake Michigan (Dorr
1982, Rutecki et al. 1985). The attraction of yellow perch to the riprap
zone, established through underwater observations, elevated their local

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00'

III o>-\ ^m ^m ^m ^m ^m ^m ^m ^m imrngement

9- | ND ND ^^^^^^^^ SAMPLES

— — 1 r~

iulUlUll

luUliI

^ 00-

I— <^-| ^H ^M ^M ^M ^M ^M ^M ^M transect

^ 2-1 NO NO ^m ^H ^1 ^m ^m H ^H ^1 ^^'^^
«l ^ J~T r-^^^^^^ ^ ^ ^ ^ ^

UJc

OCtn

luuuiilL

1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

YEAR

Fig. 9. Comparison of relative ranked abundance of yellow perch observed by
divers during all dives (1973-1982) and transect swims (1975-1982), collected
in standard series field samples (1973-1982), and impinged (1975-1982) at the
D. C, Cook Nuclear Plant, southeastern Lake Michigan. Ordinate scale is
inverted and extends from lowest to highest rank of relative abundance.
Blanks indicate zero observations or catch; ND = no diving or sampling.

abundance in comparison with field sampling, which was conducted only in areas
of sand substrate (Fig. 9). The parallel in ranked abundance of yellow perch
in impingement samples with that of field samples suggests that rate of
impingement was related more closely to their general field abundance than
their attraction to the riprap zone.

Most yellow perch observed by divers were adults; juveniles were seldom
seen, although they were abundant in field and impingement samples. A dis-
tinct pattern in the temporal distribution of yellow perch was noted. Adult
fish moved inshore into the study area during April. This movement appeared
to be more closely related to inshore spawning than initial feeding, because
most fish did not eat until spawning was completed (Dorr 1982). Spawning oc-
curred in the study area during late May, and yellow perch remained concen-
trated in the riprap zone throughout the summer. Feeding commenced shortly
after spawning was completed. During fall, yellow perch moved offshore and
were seldom seen by divers during October dives. Largest numbers of adult
fish were collected in field samples during May-August. Young-of- the-year
were collected in trawl and seine hauls during late summer and fall and in
impingement samples during fall and winter.

At least two patterns in the spatial distribution of yellow perch were
discerned by this and related studies. The first pattern was the seasonal
inshore migration of adults in spring and an offshore migration during fall.
These movements were documented by underwater observations, field studies
(Tesar and Jude 1985), and impingement studies at the Cook Plant (Thurber and
Jude 1984, 1985). Juvenile yellow perch inhabited the inshore area throughout
fall and winter, as evidenced by their impingement at the Cook Plant during
these months. The second pattern in spatial distribution was the

concentration of adult fish in areas of rough substrate. As water
temperatures increased in spring, adult fish moved inshore and onto natural
and artificial reefs present in the area. Although Dorr (1982) compiled some
evidence that limited movement off the reefs occurred after spawning, the bulk
of the fish appeared to remain close to areas of rough substrate. Yellow
perch were never observed at smooth-bottomed reference stations; however, they
were commonly collected there during summer months in trawls and gill nets
(Tesar and Jude 1985).

Adult yellow perch were distinctly day-active and at night rested on the
bottom, often in crevices formed by the riprap. As further evidence of yellow
perch nocturnal inactivity, divers were able to grasp fish at night. During
the day, fish on several occasions were fed crayfish by divers. Fish formed
loose schools composed of various sizes of fish with a length range often
exceeding 100 mm. Random swimming or "milling" was typical; closely
coordinated group movements were not observed. Both solitary fish and schools
remained within 1-3 m of the bottom or the plant structures.

Common carp was the sixth or seventh most commonly observed fish in the
study area; they were seen during all years except 1973. Field sampling and
impingement of common carp at the plant suggested that the overall abundance
of this species in the study area was relatively constant during the study
period (Fig. 10). However, several patterns and changes in the temporal and
spatial distribution of common carp were evidenced by underwater observations
and other studies of adult and larval fish.

Diving observations documented a distinct increase in abundance of these
fish near the plant following the start-up of warm-water discharge. This
local increase was paralleled in field study catches (Tesar and Jude 1985). Of

8 9

CVJ-

to-

00.

Ill <^

IMPINGEMENT
SAMPLES

CVJ

mmJ lO
CD <0

Li.o>
Oo

^ ro-

FIELD

SAMPLES

>«

to-

(£>•

00-
O)-

NO ND

1 r-

TRANSECT
SWIMS

JllUUll

1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

ALL DIVES

YEAR

Fig. 10. Comparison of relative ranked abundance of common carp observed by
divers during all dives (1973-1982) and transect swims (1975-1982), collected
in standard series field samples (1973-1982), and impinged (1975-1982) at the
D. C. Cook Nuclear Plant, southeastern Lake Michigan. Ordinate scale is
inverted and extends from lowest to highest rank of relative abundance.
Blanks indicate zero observations or catch; ND = no diving or sampling.

the more than 460 common carp observed during the study, none was seen in
1973, and only two were seen in 1974, preoperational years. Nine fish were
seen in 1975. From 1976 to 1982, numbers of fish observed annually varied
from 14 to more than 200 (Appendix 1) and averaged about 40. Larval common
carp were never collected in preoperational years 1973-1974 at the Cook Plant
but were collected and entrained at the plant during its first operational
year (1975) and in most later years of the study (Noguchi et al. 1985).
Larval common carp were not collected during 1973-1979 at reference stations
located 7 km south of the Cook Plant near Warren Dunes State Park, but a few
larvae were taken at these reference stations during the last years of the
study. Bimber et al. (1984) attributed this uneven distribution of larval
common carp to spawning in the warm-water plume of the plant. Although common
carp were attracted to the plant, annual impingement was low and ranged from
zero to 34 fish between 1975 and 1982 (Thurber and Jude 1985). This suggests
that the fish were not particularly susceptible to entrapment at the intake
structures, probably because they concentrated near the discharge area.

Further evidence of attraction of common carp to the warm-water plume was
that of the more than 460 fish observed by divers, only 12 were seen at the
intakes and none was seen at reference stations. All other observations were
made in the vicinity of the discharge stations. On several occasions during
late spring and summer, divers in boats and on shore observed schools of
common carp swimming in the vicinity of the discharge structures; none was
seen in the vicinity of the intake structures.

Divers observed common carp in greatest abundance during the period May-
August. Most fish taken in field samples were collected during the same
period. However, the impingement of common carp did not show any temporal

pattern, probably because their susceptibility was low even when they were
abundant in the vicinity of the discharge.

Common carp were day-active and seldom seen at night. The few fish that
were observed during night dives were on the bottom, solitary, and inactive.
Most often, common carp were seen in groups rather than individually. Most
diver-observed fish were swimming randomly in the vicinity of the discharge
structures. They often approached the divers closely and on several occasions
swam into the divers. As noted earlier, their feces were often abundant at
the closest reference station north of the discharges (north reference station
I - Fig. 1) but were rarely seen at other diving stations.

Largemouth bass, sraallmouth bass, and walleye were seen three times,
twice, and once, respectively, during the study (Table 9) and never during
transect swims (Table 10) or at reference stations. In all instances, the
fish were seen in close proximity to the intake or discharge structures.
It is believed that these fish were attracted to the structures and not just
the surrounding rough substrate, perhaps because of the elevated profile that
the structures presented. All fish were seen during the warm-water months
(May-September) and during the day. Only solitary fish were observed.

Pelagic-Indifferent —

The species complex of diver-observed pelagic fish indifferent to the
in-lake structures or plant operation included alewife, spottail shiner,
trout-perch, rainbow smelt, lake trout, emerald shiner, brown trout, and
unidentified coregonids (bloater or lake herring). Sufficient observational
data were compiled on the first four species to permit meaningful discussion

and inferences. The remaining fish species were seen infrequently and little
can be concluded based on these sightings.

Alewife was generally the most abundant species observed and collected in
the study area. Comparison of summary data (Fig. 11) revealed few
fluctuations in annual relative ranked abundance within each of the four data
categories. Field sampling data and other evidence indicated that the
abundance of alewife in the study area declined during 1980-1982 relative to
previous years (Jude and Tesar 1985). This decline was paralleled by transect
swim data where annual observational effort was standardized. The decline was
not reflected in data compiled from all dives. It is possible that the small
annual variation in total diving effort that occurred during 1975-82 may have
obscured this decline, although more dives were conducted annually during
1975-1979 (17-19 dives yearly) than during 1980-1982 (15-17 dives yearly).
Another explanation may be that large schools of alewives were rarely
encountered during transect swims; whereas, they were frequently encountered
during non- transect diving. Also, estimation of these large schools of fish
(often containing more than 1,000 individuals) may have smoothed and obscured
yearly variations in abundance. Nonetheless, alewife were the most abundant
and ubiquitously distributed fish in the study area.

No patterns or trends were observed in the spatial distribution of
alewife during the underwater study. Individual and schooling fish were
observed at both riprap and reference stations.

A distinct temporal pattern was noted in the abundance of alewife.
Alewife were rarely observed during April but were usually seen in great
abundance during May-June, and the impingement of alewives usually peaked
during the same period. Adult fish were collected in field samples in

IMPINGEMENT
SAMPLES

FIELD

SAMPLES

TRANSECT
SWIMS

ALL DIVES

1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

YEAR

Fig. 11. Comparison of relative ranked abundance of alewives observed by
divers during all dives (1973-1982) and transect swims (1975-1982), collected
in standard series field samples (1973-1982), and impinged (1975-1982) at the
D. C. Cook Nuclear Plant, southeastern Lake Michigan. Ordinate scale is
inverted and extends from lowest to highest rank of relative abundance.
Blanks indicate zero observations or catch; ND = no diving or sampling.

greatest abundance during the same period a The abundance of alewife in the
study area during this period corresponded with their spring migration from
offshore areas of the lake to the more rapidly warming inshore waters where
they subsequently spawned during late May-August, Adult fish continued to be
observed throughout the summer, although numbers of fish observed were reduced
from peak levels that occurred during May-June. Numbers of adult fish seen
during October were always low and corresponded with the fall migration of
fish to offshore areas.

Young-of-the-year (YOY) alewives' were usually first observed by divers
during August or September and large schools were often seen during September-
October. This fall pattern was paralleled by an increase in impingement of
YOY alewives, which by this time were large enough (>50 mm) to be retained by
the traveling screens (Thurber and Jude 1984, 1985). Young-of-the-year fish
were often seined in great abundance during August-September.

When observed, schools of both adult and YOY alewives were distributed
throughout the water column. Schooling of adult fish was observed only during
the day. Movements of individual fish were rarely coordinated into
simultaneous group movements and considerable "milling" of fish occurred.
Solitary fish were commonly seen. At night, fish often occurred in groups or
clustered at various locations around the intake structure. Although the fish
were active at night, swimming appeared undirected, and fish could often be
approached closely or touched by divers. Schools of YOY alewife were only
observed at night and were closer to the surface than the bottom. On several
occasions, adult fish were observed to group near the intake structure and
face into the oncoming current. Some individuals made snapping or sucking

(not coughing) movements with their mouth and may have been ingesting
zooplankton in the water.

Spot tail shiner was included among the group of frequently observed
species; they were seen during all years of the study. It was also included
among the five most-abundant species in field and impingement samples.
The relative ranked abundance of spottail shiners in impingement catches
fluctuated somewhat among years but remained nearly constant for field samples
(Fig. 12). A nearly constant level of relative abundance was also reflected
in transect- swim data. Pooled observations from all dives suggested that the
relative abundance of spottail shiners declined during the late 1970s, but
this decline was not reflected among the other three data bases. Therefore,
it was concluded that the relative ranked abundance of spottail shiners
remained relatively unchanged during the study.

Spottail shiners were not observed at reference stations, but field and
impingement studies did not indicate any notable differences in spatial
distribution. However, diving was more extensive in the riprap area and the
small size of the fish made them difficult to see off bottom, particularly
when visibility was low. No other evidence of substrate-selective behavior or
attraction to plant structures or operation was compiled during the underwater
studies.

A distinct temporal pattern was noted in the seasonal distribution of
spottail shiners as observed by divers. Fish were rarely seen in the study
area in April and October and were most often observed during June-August.
A similar pattern of seasonal abundance was reflected in field catches of
spottail shiner (Tesar and Jude 1985). This temporal pattern of abundance
resulted from movement of fish into the inshore area of the lake during June-

CVJ-

ibllui

^ ' ■ IMPINGEMENT

ND ND ^'^^^

T r

Hui

^Z) to

CD (0

oi:ni ■■■■■■■■ '"sVes

llJ

...^ 00

h- ^~l ^1 ^1 ^1 ^1 ^1 ^1 ^M TRANSECT

_i_ "-T — I ^^ I ^n r"r"r

iIIIipJmI...

1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

YEAR

Fig. 12. Comparison of relative ranked abundance of spottail shiners observed
by divers during all dives (1973-1982) and transect swims (1975-1982), col-
lected in standard series field samples (1973-1982), and impinged (1975-1982)
at the D. C. Cook Nuclear Plant, southeastern Lake Michigan. Ordinate scale
is inverted and extends from lowest to highest rank of relative abundance.
Blanks indicate zero observations or catch; ND = no diving or sampling.

August when spawning and feeding occurred. During fall, fish moved offshore.
Although peak impingement of spot tail shiners usually occurred during May-
August, fish were often impinged in large numbers throughout the year. The
relatively high impingement of fish during periods of low field abundance may
have resulted from their seeking shelter near the structures during fall and
winter storms or from their general disorientation and increased
susceptibility to entrapment during these periods of severe inshore
turbulence.

Spot tail shiners were more commonly observed at night than during the
day, but this was believed to be more the result of increased vulnerability to
approach and observation at night because of reduced light than to actual
increases in nocturnal activity. This belief was based on the observed
similarity between daytime and nighttime behavior, including levels of
activity and alertness.

Most spottail shiners seen by divers were adults; juveniles and YOY fish
were rarely observed. Although schooling probably occurs for this species
(Nursall 1973), it was not observed by divers. No differences in diel
activity were noted. Fish were seen throughout the water column and did not
appear attracted to the structures or riprap.

During a 1973 night dive on the south intake structure, several thousand
spottail shiners were observed, some of which were seen to broadcast their
eggs over the periphyton growing on top of the structure. Spawning was not
observed in subsequent years, but spottail shiners were usually seen in
considerable abundance during June night dives in the vicinity of the
structures. The fish are abundant and widely distributed in Lake Michigan,
and no evidence supporting substrate-selective spawning was compiled during

this study. Spottail shiner eggs are demersal, adhesive, and probably
randomly broadcast without regard to substrate configuration or composition.
Most spawning occurs in the <3 m depth zone (Tesar and Jude 1985, Noguchi et
al. 1985).

Trout- perch were seen during 9 of the 10 study years (Table 9) but
usually not in great abundance, i.e., more than 60 fish during any set of
monthly dives (Appendix 1). Trout-perch were never seen in abundance during
transect swims along the base of the south intake structure (Table 10).
This was attributed to their tendency to remain off-bottom during the day,
which encompassed half of the transect diving effort. The relative ranked
abundance of trout-perch remained similar among years for impingement and
field samples and transect swims (Fig. 13). A decline in relative ranked
abundance occurred in data summarized from all dives, but this decline was not
reflected in the other three data sets.

Although trout-perch were never seen at reference stations, no evidence
was compiled during field sampling and impingement studies to suggest that
they were attracted to tlie plant structures or riprap or by plant operation.
A seasonal pattern was evident in the temporal distribution of the fish.
Generally, trout-perch were seen most frequently during May-August; sightings
during other months were rare. Both field and impingement catches of trout-
perch were largest during May-September and small during the winter. No pat-
tern was noted in the diel distribution of fish as observed by divers.

All fish observed were solitary. During the day, trout-perch were alert
and active and were difficult to approach. At night, most fish were seen
within 1-2 m of the bottom, and although they were active, swimming was

JlUuii

I I r 111 I I I I

CVJ-
^-

00-

^m ^m ^m ^m ^m h ^m ^m imrngement

ND ■■■■■■■■

gcvi-

2^ CSJ-

lUlUlUI

CD (0-

® ^_ ^_ ^_ ^_ ^_ ^_ ^_ ^ ^_ ^_

Li- o> H ^ ^1 ^1 ^1 ^1 ^1 ^1 ^M ^M ^M F'lELD

O 2- H H ^H ^1 ^H ^^1 ^M H ^1 ^1 SAMPLES

1 r

*JU

00-

oi-\ ^m ^m H ^m transect

NO ND ■ ■ ■ ■ SWIMS

CVJ-

hBBBB ■BBBB all dives

I I I I I I I I ^"^T^

1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

YEAR

Fig. 13. Comparison of relative ranked abundance of trout-perch observed by
divers during all dives (1973-1982) and transect swims (1975-1982), collected
in standard series field samples (1973-1982), and impinged (1975-1982) at the
D. C. Cook Nuclear Plant, southeastern Lake Michigan. Ordinate scale is
inverted and extends from lowest to highest rank of relative abundance.
Blanks indicate zero observations or catch; ND = no diving or sampling.

100

undirected and sporadic, and the fish appeared disoriented and often darted
against the bottom when approached.

Rainbow smelt were seen during 8 of the 10 study years. Adult fish were
never seen in abundance although schools of YOY fish were occasionally ob-
served during September and October. The relative ranked abundance of rainbow
smelt remained similar among years for field samples but varied among impinge-
ment samples, transect swims, and overall diving observations (Fig. 14).

A pronounced seasonal pattern was noted in the temporal distribution of
rainbow smelt. Fish were most commonly collected in field and impingement
samples during the early spring when the fish moved inshore to spawn and
during fall after the lake water cooled. Exceptions to this pattern occurred
during summer when upwellings brought fish associated with offshore cold-water
masses into the study area. Much of the variability among years for diving
observations was attributed to the sporadic occurrence of upwellings inshore
during summer months and the association of rainbow smelt with these masses of
cold water. Rainbow smelt were not observed at reference stations, but no
pattern or differences in spatial abundance of fish were established during
the underwater studies. Quite likely, fish avoided the warm-water discharge
area and plume, but this was undoubtedly a local effect and had negligible
impact on the overall inshore distribution or abundance of rainbow smelt.

Adult fish were seen more often at night than during the day. Fish were
solitary, active, and alert. They were usually seen off-bottom and did not
exhibit any affinity for the structures or riprap. Schooling was not observed
for adult fish, but small schools of YOY fish were seen during some night
dives in September and October.

101

IMRN6EMENT
SAMPLES

FIELD

SAMPLES

rz a>- ^m ^M ^M ^M ^M transect

S 2~ NO NO ^H ^H ^H ^1^1 SHMS
_j ^ '— T 1 1 ^ 1 ^ 1

% ll I

— I — I III I I I I I

1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

YEAR

Fig. 14. Comparison of relative ranked abundance of rainbow smelt observed by
divers during all dives (1973-1982) and transect swims (1975-1982), collected
in standard series field samples (1973-1982), and impinged (1975-1982) at the
D. C. Cook Nuclear Plant, southeastern Lake Michigan. Ordinate scale is
inverted and extends from lowest to highest rank of relative abundance.
Blanks indicate zero observations or catch; ND = no diving or sampling.

102

Lake trout were seen during three of the study years, and emerald shiner,
brown trout, and unidentified coregonids (bloaters or lake herring) were seen
during one year. Brown trout, emerald shiner, and unidentified coregonids
were seen too infrequently to permit meaningful inferences regarding these
fish. However, no evidence was compiled during the underwater studies which
indicated that any of these four species of fish were attracted or repelled by
presence of in- lake structures or riprap or by operation of the plant.

In a separate study, lake trout were seen in abundance in the Cook Plant
intake area and at 6 m in an area of rough clay substrate 5 km north of the
Cook Plant off the Grand Mere Lakes during night dives conducted on 14 Novem-
ber 1977. The fish were active, alert, and occurred in groups, but spawning
was not observed. The substrate was examined closely, but no eggs were found
(unpublished data, Great Lakes Research Division, University of Michigan,
Ann Arbor, Michigan). The only other observations of lake trout were inci-
dental sightings of solitary fish made primarily at night. During 9-10 Novem-
ber 1975, an intense storm passed through the Great Lakes region, and thou-
sands of windrowed lake trout eggs were observed along the beach at the Cook
Plant (personal communication, J. Barnes, Indiana & Michigan Power Company,
Bridgman, Mich.) as well as near Charlevoix, Michigan (personal communication,
T. Stauffer, Marquette Fisheries Research Station, Marquette, Michigan).
However, lake trout eggs were never observed by divers or taken in entrainment
samples pumped from the plant forebay. On a few occasions, salmonid eggs were
found in the stomachs of slimy sculpins impinged at the Cook Plant, but the
species and location where the eggs were spawned and eaten were not estab-
lished. During 10 years of study, no evidence was compiled to suggest that
lake trout spawned on the Cook Plant riprap.

103

Demersal-Attracted —

The species complex of diver-observed demersal fish tliat appeared to be
attracted to the in- lake structures or plant operation included sculpin
( Cottus cognatus or jC. bairdi ) , burbot, channel catfish, and black bullhead.
We believe sculpins and burbot were attracted to the plant area. The at-
traction of channel catfish and black bullhead to the plant area was hypothe-
sized more from general knowledge of the species and their habits than from
empirical data.

Three species of sculpin were found in field and impingement samples col-
lected in the study area: Cottus cognatus or slimy sculpin, C^. bairdi or mot-
tled sculpin, and Myoxocephalus thompsoni or deepwater sculpin. Deepwater
sculpins were rarely collected and are excluded from this discussion.
Both slimy sculpins and mottled sculpins were identified in field and impinge-
ment catches made in the study area (Tesar and Jude 1985; Thurber and Jude
1984, 1985). There was some evidence that mottled sculpin were more abundant
inshore during summer than slimy sculpin. However, it was not possible for
divers to distinguish between the two species; therefore, they are treated as
a single group and referred to collectively as sculpins.

Sculpins were seen during every year of the study for both total standard
series dives (Table 9) and transect swims along the base of the south intake
structure (Table 10). Overall, it ranked as the fourth- or fifth-most
abundant fish species seen by divers during the study. Comparison of the
relative ranked abundance of sculpins observed during all dives and transect
swims with their ranked abundance in impingement and field samples indicated
the attraction of this fish to the plant area (Fig. 15). Sculpins ranked as
only the sixth- to ninth-most abundant fish in field samples that were

104

IMRNGEMENT
SAMPLES

FIELD

SAMPLES

< Sj ND ND

TRANSECT
SWIMS

lilUuiuu

1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

YEAR

Fig. 15. Comparison of relative ranked abundance of slimy sculpins (Cottus
cognatus or C. bairdi) observed by divers during all dives (1973-1982) and
transect swims (1975-1982), collected in standard series field samples (1973-
1982), and impinged (1975-1982) at the D. C. Cook Nuclear Plant, southeastern
Lake Michigan. Ordinate scale is inverted and extends from lowest to highest
rank of relative abundance. Blanks indicate zero observations or catch; ND =
no diving or sampling.

105

collected exclusively in sand-bottom areas. But in impingement samples, they
ranked as the fifth to sixth most abundant species and were always among the
first five most abundant species in transect and total diving observations.
Sculpins are cryptozoic in their behavior which is reflected in their
preference for rugose substrate (Scott and Grossman 1973). The interstices
among the riprap provided ideal shelter and habitat for these fish. Sculpins
were probably attracted to the riprap and the protection it afforded rather
than to any specific factor associated with plant operation (e.g.,
circulation, heated-water discharge, turbulence, suspension of sediments and
locally elevated turbidity, etc.)

Evaluation of the temporal abundance of sculpins as reflected in their
relative abundance among years showed that a decline occurred during 1976-
1977, which was followed by a gradual recovery during 1978-1982 (Fig. 15).
This decline and recovery was noted in both field and impingement collections
as well as in diver observations of sculpins. No explanation can be offered
for these changes in annual abundance. Of all fish observed by divers,
sculpins were the most evenly distributed throughout the observational period
(April-October). Unlike most other fish, sculpins were frequently observed in
the study area during April-May and September-October. Although sculpins were
impinged during most months, numbers of fish taken during April-May usually
peaked at levels 10-fold higher than during other months (Thurber and Jude
1984, 1985). This was probably related to higher levels of activity and
movement associated with spawning in riprap areas surrounding the intakes and
subsequently, increased vulnerability to impingement. Elsewhere in the area,
sculpins were found to move shoreward in early spring to spawn but generally
avoided the warm inshore waters during summer (Tesar and Jude 1985).

106

Comparison of diving observations and impingement catches with the field
distribution of sculpins underlines the attraction and concentration of fish
in the riprap zone during periods (summer) when the overall abundance in the
inshore area was low.

The uneven spatial distribution of sculpins reflects their preference for
rough substrate and their attraction to the riprap. Sculpins were rarely
observed in sand-bottom areas surrounding the riprap, although small numbers
of fish were trawled and seined from these areas (Tesar and Jude 1985).
Sculpins were also observed during other underwater studies in areas of
natural rough substrate north and south of the Cook Plant (unpublished data,
Great Lakes Research Division, Univ. Mich,,, Ann Arbor, Mich.).

All sculpins observed by divers were solitary. Most fish were adults,
but juveniles were occasionally seen during late summer. Sculpins showed a
distinctly nocturnal activity pattern which was reflected in the large number
of fish observed during night transect swims (Appendix 2). During the day,
fish remained hidden below the top layer of riprap and were less frequently
observed. At night, they moved onto the upper surfaces of the stones where
they remained active and alert. None was ever seen swimming off bottom,
and only an occasional fish was sighted at night on top of the intake
structures.

Burbot were commonly observed in the riprap area and were seen during 7
of the 10 study years. They were consistently the ninth-most abundant fish
observed during all dives (Table 10) but were among the least frequently
observed fish species seen during transect swims (Table 10). Similar to
sculpins, burbot were relatively less abundant in field samples collected
outside the riprap area than in impingement catches and diver observations

107

which sampled the population on the riprap (Fig. 16). These data suggest that
burbot concentrated in the riprap area. The attraction was probably related
to the increased protection that the more rugose substrate provided and not to
some aspect of plant operation.

Diving observations revealed no temporal patteL-n in the seasonal inshore
abundance or distribution of burbot, although field sampling and impingement
catches indicated that the fish left the inshore area during summer months.
Underwater observations of burbot revealed a clear pattern in their diel
distribution. Nearly all fish were seen at night, and they remained out of
sight during tlie day. As with sculpins, all burbot observed were solitary,
alert, and active, although they could usually be approached and grasped by
divers. They were always seen on the bottom and were usually entwined among
the riprap.

Despite the relatively low abundance of burbot in the area, on one
occasion a specimen was found lodged headdown inside a 7-cm diameter tube that
had been suspended perpendicular to and 1 m off the bottom for three weeks to
collect suspended sediment. This attested to the active exploration of the
area by this particular species.

Burbot were never observed at reference stations, and their spatial
distribution reflected tiieir attraction and concentration in the riprap area.
The relatively frequent impingement of burbot in relation to their low field
abundance also reflected their concentration in the area. Construction divers
working inside the intake and discharge pipes and plant forebay reported
seeing burbot in high abundance relative to the riprap area (personal communi-
cation, A. Sebrechts, Sebrechts Inc., Bridgman, Michigan). Quite possibly.

108

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00-

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ND
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IMRNGEMENT
SAMPLES

«J in-

CD (0

FIELD

SAMPLES

-J

cr.

CJ-

CVI-

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If).

00-

JSJD ND

T 1 1 \ r

TRANSECT
SWIMS

1 r

*iU»

ALL DIVES

1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

YEAR

Fig. 16. Comparison of relative ranked abundance of burbot observed by divers
during all dives (1973-1982) and transect swims (1975-1982), collected in
standard series field samples (1973-1982), and impinged (1975-1982) at the
D. C. Cook Nuclear Plant, southeastern Lake Michigan. Ordinate scale is
inverted and extends from lowest to highest rank of relative abundance.
Blanks indicate zero observations or catch; ND = no diving or sampling.

109

t±e fish were attracted to the dark Interior of these structures, and ended up
being impinged as a result.

Channel catfish and black bullheads were seen during two years of the
study (Table 9), and a black bullhead was seen once during a night transect
swim along the base of the south intake structure (Table 10) • These fish were
never observed at reference stations and were not seen in abundance on the
reef. Most sightings occurred at night; fish were solitary and alert.
No fish were seen swimming off bottom, and they were usually found in the
interstices among the riprap rather than on top of it.

Demersal-Indifferent —

The species complex of diver-observed demersal fish that appeared to be
indifferent to the in- lake structures or plant operation included johnny
darter, white sucker, longnose sucker, quillback, and shorthead redhorse.
The composite of diving observations, field studies, and impingement sampling
indicated that these fish were distributed throughout the study area and did
not appear to congregate in the riprap area.

Johnny darters were observed during all study years (Table 9) and during
transect dives in all but the last year of diving (Table 10). They were
typically about the fourth-most frequently observed species of fish. Although
johnny darters were observed in abundance in the riprap area, they were also
frequently seined in the beach zone and trawled at 6- and 9-m stations during
field studies of fish (Tesar et al. 1985, Tesar and Jude 1985). Comparison of
the relative ranked abundance of johnny darters showed that they were the
sixth- to eighth-most frequently collected species in field sampling and the
seventh- to ninth-most frequently impinged species (Fig. 17). The difference

110

CM-

111 ^4 ^M ^M ^M ^m ^m ^m ^M ^m IMPINGEMENT

O Qj ND ND ■■■■■■■■ SAMPLES

man

^ rOH ^^ ^H flHHi

csi

CD <o

oi«| ■■■■■■■■ ^'^^sSmp^s

■Ih

IT)

< Q-INDND

UlliiL.

UJcsi

OC to

illlllllll ..

0>- ^^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^

ALL DIVES
1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

YEAR

Fig, 17. Comparison of relative ranked abundance of johnny darters observed
by divers during all dives (1973-1982) and transect swims (1975-1982), col-
lected in standard series field samples (1973-1982), and impinged (1975-1982)
at the D. C. Cook Nuclear Plant, southeastern Lake Michigan. Ordinate scale
is inverted and extends from lowest to highest rank of relative abundance.
Blanks indicate zero observations or catch; ND = no diving or sampling.

Ill

in absolute value of annual rank between these data sets never exceeded three
and was often only one. These differences were probably not significant and
did not suggest an unusually high rate of impingement of fish in relation to
their general field abundance. Johnny darters were occasionally observed at
dive study reference stations, although they were seen in far greater
abundance on the riprap. The relative ranked abundance of johnny darters
observed during transect swims and for all dives differed slightly in absolute
value but followed nearly identical patterns in terms of annual variation.
The close similarity in these patterns of abundance was attributed to the
abundance, demersal behavior, and rather even distribution of johnny darters
on the riprap. As a result, the small areas of riprap examined during
transect swims served well as representative samples of the abundance of
johnny darters.

Several patterns appeared in the temporal abundance and distribution of
johnny darters. Diver observations and field and impingement catches
suggested that the abundance of johnny darters relative to other species
declined after 1977 and then fluctuated at lower levels during remaining years
of study. The rebound in relative abundance was more apparent in field
samples than in impingement samples or diver observations. This suggests that
the decline was more pronounced in the riprap area relative to the surrounding
area and that recovery to former levels of relative abundance was slower.
Quantitative substantiation and explanation for a differential decline and
recovery in abundance of johnny darter between the riprap and surrounding sand
area are lacking.

Secondly, johnny darters were absent from the area during April and
October, in contrast with their high abundance and widespread distribution

112

during warm-water months (May-September), Monthly peaks in numbers of fish
observed, impinged, and collected in field samples often occurred in May and
coincided with the spawning period for these fish (Fig, 8),

A final temporal pattern occurred in diel abundance. Although johnny
darters were commonly seen during the day, numbers observed during transect
swims were consistently higher at night (Appendix 2).

As noted earlier, although johnny darters were seen in much greater
abundance at riprap stations than at reference stations, no overall patterns
or differences in the spatial distribution of this species were supported
among the three general studies (diving, field, impingement). While johnny
darters may prefer rough substrate, particularly for spawning, they appear to
be widely distributed inshore during spring, summer, and fall. The decline in
rate of impingement of johnny darters during winter suggested that either the
fish moved offshore, or their activity and susceptibility to impingement were
lower during this period.

Nearly all johnny darters seen were adult fish, which were solitary,
alert, and active during day and night. All fish were seen on the bottom and
often rested on the upper surfaces of the riprap. Occasionally, a fish was
observed on top of the intake structure.

White suckers were seen during 7 of the 10 study years and ranked as the
ninth- or tenth-most frequently observed species of fish (Table 9). White
suckers were never observed during transect swims, primarily because of their
low abundance in the area. The relative ranked abundance of white suckers in
field samples remained the same (seventh) for all but two years, when it
declined by one rank (Fig. 18). Relative ranked abundance of white suckers in
impingement samples fluctuated slightly but showed no strong patterns or

113

ys:

CM-
^'
iO-
00'

ND ND
"T r-

IMRN6EMENT
SAMPLES

GD <o
Oo

FIELD

SAMPLES

uj:.

!5
-J

CM'
fO-
^•
If).

00-

ND ND
n T"

TRANSECT
SWIMS

ALL DIVES

1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

YEAR

Fig. 18. Comparison of relative ranked abundance of white suckers observed by
divers during all dives (1973-1982) and transect swims (1975-1982), collected
in standard series field samples (1973-1982), and impinged (1975-1982) at the
D. C. Cook Nuclear Plant, southeastern Lake Michigan. Ordinate scale is
inverted and extends from lowest to highest rank of relative abundance.
Blanks indicate zero observations or catch; ND = no diving or sampling.

114

trends. White suckers were observed consistently but in low numbers during
most years of the underwater study.

A seasonal pattern in the temporal abundance of white suckers appeared in
both underwater observations and field catch of this species. Fish were
observed exclusively during May-August except on one occasion in September;
most collected in field samples were also taken during May-August.
Impingement of these fish tended to be greater in summer, but white suckers
were impinged during most months and occasionally in relatively high numbers
during winter. These data suggest that white suckers are generally more
abundant inshore during warm-water months. It is possible that they move
offshore during winter or some fish may have sought shelter from storms and
ice inside the intake structures and pipes, thus accounting for the relatively
high impingement during winter when field abundance was relatively low.
White suckers were most often seen at night when they were solitary, alert,
and active. Tesar and Jude (1985) found that this species moved shoreward at
night in the study area.

Although white suckers were not observed at reference stations, there was
no evidence that they were attracted to the plant structures or riprap or that
operational factors affected their distribution. In fact, analysis of gill
net data revealed that white suckers were significantly less abundant near the
Cook Plant than at a reference station located 11 km south off Warren Dunes
State Park, Michigan (Tesar and Jude 1985). These data indicate that white
suckers may actually have avoided the Cook Plant area, perhaps in response to
some operational factor such as discharge of heated water. A similar pattern
of avoidance was noted at the J. H. Campbell Plant located north of the Cook
Plant (Jude et al. 1982).

115

Longnose suckers were seen on several occasions during the study.
Quillback and shorthead redhorse were each observed on one occasion. All of
these fish were observed in the riprap zone, but attraction of these species
to the area was not established.

The overall abundance and distribution of most fish observed by divers
were influenced by several factors. One factor was the annual water
temperature regime. Fish abundance, diversity, and levels of activity as
observed by divers were generally highest during the warm-water months (May-
September), with lowest levels of abundance, diversity, and activity occurring
during April.

Abundance and diversity of fish observed by divers was generally higher
at night than during the day. Part of this was because many fish were less
wary at night and did not flee the area as divers approached. Also, many
species of fish seen were nocturnal or showed no clear pattern of diel
activity. Those species that were day-active often remained on bottom at
night where they were readily visible to the divers.

Inshore turbulence associated with storms and surface waves appeared to
cause many fish to retreat from the area. Offshore movements were most
likely, but some fish (alewife and yellow perch) in the immediate vicinity of
the Cook Plant appeared to seek shelter in the lee of the intake structures
and were consequently more vulnerable to impingement during these periods.
This response to storms was also documented by Lifton and Storr (1977).

Finally, for many of the species of fish observed during this underwater
study, their onshore movements and peak abundance in the study area were often
directly correlated with spawning activities. This was true for species that
were attracted to the plant area for spawning substrate (e.g., yellow perch,

116

sculp ins, johnny darter) or an operational factor (common carp) and for
species that appeared indifferent to the presence or operation of the Cook
plant (e.g., alewife, spottail shiner, rainbow smelt).

The spatial and temporal abundance of Lake Michigan fish found in the
study area appears to be strongly influenced by environmental factors
(substrate conditions, water temperature, storms, turbulence, ice, diel
period) acting in concert with physiological needs of the fish (maturation,
spawning, feeding, survival, growth) and the distribution of other aquatic
biota (predators and prey). Our studies also indicate that the level of
influence that these factors assert on fish abundance, distribution, and
behavior changes as fish pass through various stages in their life history and
physiological needs.

ECOLOGY

Given some annual variation, most of the physical, chemical, and
biological features of the study area remained basically unchanged during
preoperational and operational phases of the Cook Plant (Rossmann 1986). Such
factors included composition and configuration of surficial sediments,
presence of lake currents and occasional occurrence of storms, annual water
temperature regime, nutrient cycling, and the seasonal appearance of various
animal populations in the area. These factors along with many others comprise
the environment and dictate the growth and survival of plants and animals in
the area. In most instances, these environmental interrelations and responses
are complex and difficult to isolate or explain.

However, construction and operation of the Cook Plant resulted in some
gross alterations in local environmental conditions which could be identified

117

and explored. The placement of plant structures and riprap in the lake
created a small, isolated benthic environment that was atypical of the
surrounding area. Subsequent operation of the plant which included withdrawal
of water, circulation and warming of water inside the plant, and discharge of
water back into the lake further affected both the benthic and pelagic
environment in the immediate vicinity. Two basic themes underlie the initial
discussion in this section: the first is an evaluation of the response of
selected biota to the introduction of new habitat or sets of environmental
conditions. The second theme is the response of these biota to habitat aging
and changes in environmental conditions. The discussion is limited to
observations and inferences that are derived from this underwater study.

The inshore physical environment in this region of the lake is variable
in comparison with many other aquatic environments. Waves, currents, shifting
surficial sediments, exposure to ice scour, and widely fluctuating water
temperatures contribute to the set of conditions that stress plants and
animals living in the area. The riprap and in- lake plant structures provided
a stable substrate that afforded increased protection for mobile benthic
organisms and a surface for attachment of sessile biota. This was reflected
in the rapid colonization of this habitat by organisms not found in the
surrounding environment, (e.g., periphyton and attached invertebrates) or
which normally occurred in lesser abundance (e.g., snails, crayfish, and some
fish).

Following placement of the structures and riprap in the lake, aging of
their surfaces commenced and altered the conditions of this micro-environment.
The structure surfaces first rusted and then accumulated bacterial slime, fine
sediment, and particulate organic material. Bacterial slime grew on the

118

surface of the riprap while the holes and crevices, particularly those in its
upper surfaces, trapped sediment and organic matter.

Periphyton rapidly colonized the exposed, upper surfaces of the struc-
tures and riprap, and Cladophora was often abundant. Snails appeared on the
riprap within a year and attached invertebrates ( Hydra , bryozoans, and
sponges) colonized the substrate in the first few years. Crayfish also
appeared on the reef within the first several years. Abundance of snails,
crayfish, and some invertebrates peaked during the first three to five years
and then declined to varying degrees. Snails disappeared completely from the
riprap by the sixth year, and numbers of crayfish observed and impinged
declined dramatically by the seventh year,. The abundance of most attached
invertebrates declined in later years of the study, but these organisms
continued to be observed throughout the 10-yr study period. Interestingly,
fluctuations occurred in the abundance of fish that were attracted to the
area, but clear patterns or trends in their abundance were not evident.
The reason for this may be that those factors which attracted the fish (e.g.,
shelter, circulating water, etc.) were not altered as much during the study as
the micro- environment on the surface of the riprap. This in turn suggests
that attraction of fish to the area may have been more a response to the
physical configuration of the reef than to biological factors such as
availability of prey (e.g., sculpin feeding on snails or perch feeding on
crayfish) .

In a stable environment, associated physical, chemical, and biological
conditions often achieve some balance with each other. Patterns, trends, and
random variations in these conditions are expected to occur during long
periods of observation, but radical changes are either atypical (e.g., damage

119

or destruction of the structures) or at least predictable (upwellings) .
When existing habitat is altered or new habitat is introduced, the extant
environmental conditions change and a new set of physical, chemical, and
biological conditions begin to appear. Usually, some period of time is
required to reform a stable and relatively predictable balance with this new
set of conditions. The response of individual organisms to these environ-
mental changes varies but is eventually reflected in population abundance and
diversity. Populations may increase or decrease in numbers, and the rate at
which this occurs may also vary. However, several basic patterns are known,
and some occurred at the Cook Plant.

One pattern, shown by snails at the Cook Plant, is where population
density follows a J-shaped curve over time. Initially, a positive
acceleration phase occurs, followed by a logarithmic growth phase.
Eventually, population density peaks and is then followed by a logarithmic
decrease in population density and later, a negative acceleration phase
(Knight 1965). Colonization, rapid population increase, peak abundance, and
population decline of snails took place within a 4-yr period; over the next
two years the population trailed off into extinction on the reef. The primary
factor which initially encouraged population growth was most likely the ap-
pearance of clean, stable substrate. The major factor which eventually caused
the extinction of snails on the reef may have been the accumulation of a thick
coating of material (sediment, organic detritus, and algae) on the surface of
the substrate. This material may have interfered with snail movement,
ventilation, or incubation of eggs attached to the substrate. Changes may
also have occurred in the composition of the detrital material upon which
snails fed.

120

A second population density curve which develops in response to changing
environmental conditions is the sigmoid curve. In this instance, the
ascending limb and peak of the curve are followed by a series of oscillations
which may be cyclic or nonper iodic and show trends and patterns or totally
random changes in population abundance over time. Attached invertebrates and
crayfish followed this general form of population density curve. Given time
and eventual stabilization of environmental conditions on the reef, the
population density curves of these organisms might eventually flatten or show
some periodicity or trend. But the duration and intensity of sampling
conducted during this study were insufficient to reveal such features in these
population curves. The seasonal growth of Cladophora followed a variation of
this curve where the length and density of the alga showed cyclic fluctuations
according to season (maximum in summer, minimum in winter). However, no long-
term trend superimposed on these cyclic oscillations was identified during the
study.

Changes in surficial substrate conditions suspected to have affected
snails probably also affected attached invertebrates and crayfish. Evidence
indicated that Cladophora may have had a direct effect on these animals.
In studies of artificial substrates placed on the Cook Plant riprap, Lauritsen
and White (1981) found that Cladophora increased space available for clinging
invertebrates such as Naididae, Oligochaeta, water mites, and amphipods.
With the disappearance of most Cladophora in the fall, the total number of
benthic invertebrates decreased, and filter feeders dominated the fauna.
Prince et al. (1975) found that at Smith Mountain Lake, Virginia, crayfish
were most abundant in areas of luxuriant Cladophora growth and absent from
areas of the reef with little or no Cladophora growth. These observations

121

combined with tJiose of the present study (see Free-living Macroinvertebrates)
suggest that a direct relationship exists between the presence of Cladophora
(or factors which promote growth of the alga) and the abundance of
invertebrates at the Cook Plant. The population growth of snails may have
been repressed by luxuriant Cladophora growth; whereas, the population growth
of crayfish may have been enhanced. Attached invertebrates may have had to
compete with the alga for substrate, and some of the aquatic insect larvae
observed during the study may have fed on organisms living in association with
Cladophora .

Another population density curve is asymptotic in shape. Unlike the J-
shaped curve, no clear peak density is achieved but rather an asymptotic or
flat, linear phase is established. Some possible examples of this curve were
the population densities of yellow perch, sculpin, johnny darter, and burbot
that were attracted to the rough substrate. Unfortunately, diving was not
conducted before and immediately after placement of the substrate in the lake.
Therefore, the initial increase in density which occurred as fish located and
colonized the area was not recorded and the ascending limb of the curve was
not reflected in the data. However, the relative ranked abundance of many of
these fish underwent only minor fluctuations following colonization, and the
actual abundance of these reef fish may have stabilized. As noted earlier,
the attraction of these fish to the reef may have been more a response to its
gross physical configuration and stability which remained nearly unchanged
during the study, than to reef organisms (algae or invertebrates) that served
as prey, or to micro-environmental conditions on the surface of the riprap.
Interestingly, lake trout, which appear to have extremely specific
requirements regarding spawning-substrate conditions, were never found to

122

utilize the Cook Reef for spawning; whereas, other fish (yellow perch, slimy
sculpin, johnny darter, spottail shiner, and alewife) with less stringent
spawning- substrate requirements spawned extensively on the reef. In contrast,
lake trout did spawn on the newly-placed large riprap at the Campbell Plant
(Jude et al. 1981b).

The population density curves of periphytic algae at the Cook Plant reef
followed a pattern typical for colonial algae but unique in comparison with
curves previously discussed. In general, abundance of individual algal forms
peaked soon after colonization and then decreased slowly, thus defining
asymmetric population density curves that were skewed to the right. However,
as individual population densities decreased and more stability was attained,
total diversity of forms increased almost linearly throughout the study.
These opposing processes may have been the result of aging and increased
stability of surficial substrate conditions acting in concert with the large
number of rare forms present in the lake.

Most organisms studied during this investigation exhibited both temporal
and spatial variation in their abundance and distribution. The three most
obvious environmental effects were substrate conditions, water temperature,
and photic conditions. Pronounced effects of substrate were found on the
distribution of periphyton, attached invertebrates, snails, and crayfish and
on the distribution and spawning of some fish. For all animals studied,
presence of stable, rugose substrate attracted and concentrated biota that
were less abundant in the surrounding environment of flat, exposed, shifting-
sand bottom. Most organisms not attracted to the riprap zone (e.g., pelagic
fish) were distributed in the area in a manner similar to that of the
surrounding environment. However, the faunal distributions of some organisms

123

t±iat would undoubtedly have been reduced by the presence of hard substrate,
such as those of burrowing invertebrates, including sphaeriid clams or worms,
were not studied.

Although short- terra fluctuations in water temperature, such as
upwellings, were encountered, their effects on the abundance and distribution
of local biota were difficult to discern through diver observations. However,
seasonal changes in water temperature had obvious effects on both plants and
animals. In general, abundance and diversity of most organisms observed by
divers were far greater during months of warm water than during early spring
(April) or late fall (October). Part of this reduction was likely the result
of reduced metabolic activity and movements as a function of lower water
temperatures. But, frequent storm- genera ted turbulence and scouring of the
bottom by ice made the inshore area considerably more inhospitable during the
cold-weather period of the year.

The diel distribution of some animals was a direct result of phototrophic
responses. Crayfish were distinctly more active at night as were sculpin and
YOY alewives. Yellow perch and common carp were active during the day and
inactive at night. While abundance of adult alewives appeared unaffected by
photoperiod, schooling was a distinctly daytime activity. In general, most
fish were less alert and more approachable by divers at night than during the
day. Also, orientation of fish to the structures and riprap was often clearly
obvious during the day and obscure or absent at night.

Finally, a distinct process of colonization and succession of biota on
the Cook Plant structures and riprap was documented during this study.
Although specific population density curves have been discussed, the overall
pattern was one of initial location of habitat by extant biota, explosive

124

population growth which peaked during the first few years of the reef's
existence, and a decline in population abundance to lower levels of
fluctuating population abundance or extinction. This general pattern was most
strikingly exhibited by sessile biota, perhaps because they were more directly
affected by changes in substrate conditions than were motile organisms such as
fish. These changes probably included shifts in micro- habitat conditions such
as circulation of water and exchange of gases and nutrients at the
substrate/water interface. The physical occlusion of the substrate surface,
pores, cracks, and interstices by an accumulation of algae, sediment, and
organic detritus probably influenced these micro- habitat conditions and
dictated the response of organisms to that habitat.

Generally, artificial reefs are used throughout the world to increase
local biological productivity (Rutecki et al. 1985). Such increases are
achieved by expanding the variety and abundance of habitat available to biota.
These conditions favor the survival and growth of individual organisms and
promote local population increases. The Cook Plant structures and riprap have
provided just such an environment which through its physical presence and
modification of extant environmental conditions acting in combination with
effects of plant operation have had a distinct impact on the local ecology.
From the standpoint of diver-observed effects, this impact appears limited
almost exclusively to the reef itself and has not influenced the ecology of
the surrounding area to any noticeable extent.

PLANT EFFECTS
Physical Presence

The physical presence of Cook Plant in- lake structures and riprap

125

appeared to have several effects on the local environment that were not
related to plant operation (e.g., circulation or discharge of heated water).
These effects were generally related to an expansion of habitat which provided
increased substrate for attachment, shelter, or reproduction of biota.

The structures and riprap provided stable substrate for the attachment
and growth of periphytic algae and attached invertebrates including Hydra ,
bryozoans, and freshwater sponges. These animals were not found on shifting-
sand substrate in the surrounding area.

Snails were attracted to the clean, stable substrate that provided a
surface on which they could move about and lay their eggs. Crayfish may have
fed on Gladophora or other periphyton attached to the riprap but also used the
interstices among the stones for shelter and protection.

Several species of fish were attracted to the structures and riprap.
Yellow perch congregated in the area in the late spring and remained more
concentrated in the riprap zone than the surrounding area throughout the
summer. Although alewives did not show any particular attraction to the area
based on diver observations, impingement records indicated that fish clustered
near the structure during storms and were thereby more vulnerable to
entrapment (Thurber and Jude 1984, 1985). Demersal fish including sculpins,
burbot, johnny darter, black bullheads, and catfish were attracted to the
riprap probably as a result of their cryptozoic behavior. In all cases, the
presence of the structures and riprap increased the amount of protected
habitat available to these fish. Therefore, strictly from the standpoint of
their physical presence, the structures and riprap enhanced and expanded local
populations of some fish species in a manner that would not have occurred in
the absence of this habitat. However, this enhancement must be balanced

126

against the operation of tlie plant which often contributed to mortality of
fish occurring in the area.

The riprap served as spawning substrate for yellow perch, slimy sculpin,
and johnny darter, and through this process may have enhanced the growth of
local populations of these fish. Spot tail shiners were observed to spawn on
periphyton growing on top of the south intake structure, which provided an
additional but probably insignificant amount of spawning habitat.

In overview, the physical presence of in- lake plant structures and riprap
created an atypical, more sheltered, and more diverse habitat as compared to
the surrounding area. These factors served to attract and concentrate biota
which normally would be absent from the area or occur in considerably reduced
numbers. In most instances, the presence of this habitat enhanced local
populations of some plants and animals, while others (e.g., those of burrowing
animals) were likely reduced. But, the attraction and enhancement of these
populations must be balanced against their increased vulnerability to
operational effects of the Cook Plant and plant-induced mortality.

Operational Effects

The entrainment of organisms during intake of plant cooling water and
discharge of heated water and currents associated with the withdrawal and dis-
charge of water were the major effects of plant operation that were noted by
divers. Some of the physical impacts from plant operation have already been
described and are summarized here. A shallow surface layer of warm water was
occasionally encountered by divers at reference stations closest to the dis-
charge structures. Warm water was also encountered when diving in the dis-
charge area during one-unit plant operation. Elevated turbidity was occasion-

127

ally encountered at the north reference station nearest the plant, and on one
dive, debris was flushed from the north discharge during cleaning of the plant
forebay. Intake and discharge of water modified lake currents and waves in
the immediate vicinity of the plant. We observed changes in ripple mark pat-
terns on the bottom, encountered eddy currents at the discharge, and detected
water masses of clearly differing temperature and transparency in the strati-
fied intake water. Although the riprap trapped sediment and organic debris,
some of these materials were re-suspended by plant- genera ted water currents.

Although the pelagic life stages of attached organisms were vulnerable to
entrainment and possible plant- induced mortality, sessile adult organisms were
considerably less susceptible to operational effects of the plant. Diver ob-
servations revealed that portions of the intake structures most directly
exposed to intake water currents often supported the most luxuriant periphyton
growth.

Crayfish were attracted to the riprap. However, intake currents strong
enough to dislodge these animals from the substrate and result in their
subsequent impingement in the plant were never encountered. Crayfish, which
show pronounced negative phototatic behavior (Pennak 1953), most likely were
attracted to the dark interior of tlie intake structures and pipes and
eventually entered or were entrained into the the plant forebay and impinged
on the traveling screens. The same process may have occurred for sculpins
which concentrated in the riprap area; sculpins are also nocturnally active.

Diver-observed effects of plant operation on fish were limited to attrac-
tion of common carp to the heated discharge water and a general responsiveness
of some species to currents at the intake structures. Although common carp
spawned in the warm water as evidenced by the concentration of newly hatched

128

larvae at sampling stations nearest the thermal plume (Bimber et al. 1984),
they may have been attracted to the plume for other reasons. No evidence was
compiled to indicate that common carp would have been attracted to the area
strictly in response to the physical presence of plant structures or riprap.
Several species of fish, including yellovr perch, alewives, and spottail shin-
ers, were observed to exhibit positive rheotaxis and some position-holding in
the area of strong intake currents. On occasion, some of these fish were
observed to selectively congregate at various locations around the intake
where the incoming water was warmer or less turbid than at other points. Cook
Plant impingement records and other studies suggest that both alewives and
yellow perch may have concentrated near the intake structures during storms
and periods of extreme inshore turbulence, perhaps in search of shelter in the
lee of the structures (Lifton and Storr 1977; Thurber and Jude 1984, 1985).
Such concentrations, combined with the increased activity of fish during
storms and possible disorienting effects of extreme turbulence, may have
resulted in increased impingement of fish during and immediately following
severe inshore turbulence.

Pelagic fish, including juvenile and adult alewife, spottail shiner, and
yellow perch, were observed to swim in and out of the intake structures.
This observation suggests that water intcike currents outside the structures
and at many points within the structures were not so strong as to over-power
the fish. Rough measurements of current speed made by divers at the intake
screens of the structures by timing the transport of suspended material along
a measured distance indicated that intake currents at the screens were usually
less than 0.5 m/sec. During seven-pump plant operation, currents at the in-
take screens occasionally approached 1 m/sec at points along the structure

129

which faced directly into the oncoming lake current. Commercial divers re-
pairing the intake structures reported that there were specific locations
within the structures where intake currents would suddenly increase (personal
communication, A. Sebrechts, Sebrechts Inc., Bridgman, Mich.). These loca-
tions varied with the number of pumps operating, direction and speed of lake
currents and surface waves, and eddy currents caused by recirculation of dis-
charge water.

Review of fish swimming performance, summarized by Hocutt and Edinger
(1980), indicates that water velocity at the Cook Plant intake screens is con-
siderably less than the "burst" swimming speeds of most pelagic and juvenile
fish found in the study area and does not exceed the "sustained" swimming
speed for species such as alewife and yellow perch. They also reported that
alewife demonstrate a countercurrent orientation in streams and prefer high
velocity flow; whereas, yellow perch are inconsistent in their orientation to
current.

We theorize that at the Cook Plant most fish voluntarily enter the
structure and then may be unexpectedly subjected to strong currents occurring
at varying locations within the structure. Upon entering the structure and
suddenly encountering these currents, many fish probably retreat to areas of
reduced current within or outside the structure; this scenario may be repeated
many times before the fish eventually leave the area or are entrapped. Intake
currents inside the pipes may approach 1.8 m/sec (6 ft/ sec) during seven-pump
operation, which would be 10 body lengths/sec for a 180 mm fish. Based on
fish swimming performances cited in Hocutt and Edinger (1980), this value (10
lengths/ sec) probably exceeds the "burst" swimming speed for many of the
species of fish commonly impinged at the Cook Plant, particularly small fish.

130

Hocutt and Edinger noted that swimming perfonnance is also related to the rate
of velocity increase. Therefore, if a fish unexpectedly encounters a strong
intake current inside the Cook Plant structure, escape may be difficult,
particularly if the fish has been drawn through the structure and down into
the intake pipe. If fish congregated near the structures for shelter during
storms, the increase in turbulence could well disorient them or mask the
intake current so that the fish might have increased difficulty sensing the
sudden increases in intake current flow inside the structure. The end result
would be that more fish would be entrained and impinged during storms, which
was exactly what was observed at the Cook Plant.

Divers noted plant effects that were the result of the simple physical
presence of the structures and riprap and some that were a function of plant
operation. Most of these effects served to enhance local population densities
of organisms attracted to the area. Negative effects (e.g., primarily
entrainment and impingement) appeared to be limited more to plant operation
than the physical presence of the structures and riprap in the lake and were
inferred from other aspects of the Cook Plant studies. Barring a large change
in the in- lake structure of the Cook Plant or its operation, future diver
observation of additional major or significant ecological changes or plant
impacts are not anticipated.

SUMMARY

The physical, chemical, and biological features of the inshore

environment surrounding the Cook Plant in- lake intake and discharge structures

and riprap defined a harsh regime of environmental conditions relative to many

other aquatic environments. A spectrum of flora and fauna existed in this

131

environment, but the abundance and distribution of most organisms appeared to
be rather strictly dictated by the environmental conditions they encountered.
The inshore Lake Michigan environment evaluated during this underwater study
appeared relatively homogeneous, and considerable opportunity existed for the
mobile life stages of flora and fauna to migrate and colonize new habitat.

Inshore surface waves may attain 4 m in the study area during intense
storms, which contribute to the harsh nature of the environment. Effects of
waves 0.5-1.0 m could be felt on the bottom by divers at depths less than
10 m. Lake currents were occasionally encountered by divers, but their
effects were masked in areas where plant- genera ted currents could be felt.
Both uni-directional and eddy currents were detectable throughout the water
column within 100 m of the discharges; at stations more than 300 m from the
discharges, weak plant-generated currents were noted occasionally, but lake
currents appeared to predominate. Variable current speeds were encountered at
the intake structures, but distinct differences often occurred at various
points around the structures. Currents were strongest during seven-pump
operation, and presence of warm water drawn into the shoreward sides of the
structures suggested some recirculation of discharge water.

Thermal effects encountered during diving included seasonal large-scale
changes in water temperature, short-term processes, including upwellings, and
temperature stratification within the water column. A thin layer of
naturally warmed water was occasionally found at the surface. Plant effects
included presence of warm water near the discharge area and recirculation of
discharge water.

The bottom profile of the inshore Lake Michigan environment was typically
flat and unbroken. Sediments were composed of coarse- and fine-grained

132

shifting sand. Occasional "islands" of rock or clay substrate occurred in the
inshore area of eastern Lake Michigan but were extremely limited in number and
areal extent. These islands included habitat and environmental conditions
more dissimilar to the surrounding area than to the physical conditions
created by the Cook Plant in- lake structures and riprap.

Accumulations of surficial flocculent material typically ranged from 1 to
5 mm thick. Occasionally, large (10-m diameter, 1 m deep) depressions con-
taining 20-40 mm of floe were encountered at reference stations. The riprap
trapped sediment along with other inorganic and organic materials.

Water transparency ranged from less than 1 m to more than 6 m and was
reduced during periods of inshore turbulence. High transparency was usually
associated with extended periods (days to weeks) of stable weather and calm
lake conditions. Transparency was occasionally reduced in the vicinity of the
discharges and at specific points around the intake structure. These reduc-
tions were attributed to discharge turbulence and withdrawal of water from
discrete water masses of differing turbidity.

Inorganic debris and organic detritus were more commonly observed in the
riprap zone than at reference stations. This was believed to be primarily a
function of the increased trapping action of the more rugose surface of the
riprap. Inorganic trash accumulated as a result of plant construction and
items discarded by fishermen angling over the reef. Organic debris was
composed primarily of terrestrial plant material.

Periphyton colonized the structures and riprap within a year of placement
in the lake. Seasonal growth patterns were clearly obvious, with algal
length, density, and taxonomic diversity peaking during summer months. Most
algae sloughed from the substrate during winter. Cladophora was abundant and

133

was suspected to have affected the abundance of other organisms on the reef,
including attached or clinging invertebrates, crayfish, and possibly snails.
No long-term pattern in length or luxuriance of periphyton growing on the
plant structures or riprap was identified. However, taxonoraic diversity and
number of new forms recorded each year increased almost linearly throughout
the study. These observations documented a pattern of colonization and
succession that was typical for periphytic algae and also attested to the
large number of rare forms present in the lake.

Attached invertebrates observed during the study included Hydra ,
bryozoans, and freshwater sponges. Hydra colonized the structure and riprap
during its first year in the lake, as did bryozoans. Freshwater sponges
appeared to require about two years to colonize the substrate. Peak abundance
of these invertebrates on the reef occurred four to six years after placement
in the lake. During the last several years of the study, abundance of Hydra
and bryozoans declined, while numbers of sponge colonies continued to
fluctuate and showed no particular pattern or trend. Riprap appeared to
provide a more suitable substrate than did the metal structure, although large
mats of Hydra were observed on the interior walls of the intake pipes and
plant forebay.

Snails and crayfish colonized the riprap within its first year in the
lake. Abundance of snails ( Physa ) peaked during the third year of the reef
and then declined rapidly. No snails were observed during the last four years
of the study. Extinction was believed to have been caused primarily by
changes in the surface of the substrate as it aged and accumulated sediment,
bacterial slime, periphyton, and organic detritus. Crayfish abundance peaked
one year after that of snails. A rapid decline in abundance then occurred,

134

but unlike snails, crayfish continued to be observed in low numbers throughout
the duration of the study. Decline in crayfish abundance was believed to be
related to changes on the reef substrate surface operating in combination with
initial overpopulation of the habitat. For both snails and crayfish,
predation on eggs, juveniles, and adults by other crayfish and fish may have
contributed to the decline in abundance of these invertebrates.

Several species of fish including yellow perch, slimy sculpin, and johnny
darter spawned on the reef in preference to the surrounding sand- bottom area.
Spot tail shiners were observed to spawn over periphyton growing on top of an
intake structure. Alewife eggs were seen in abundance but were about equally
distributed over riprap and sand substrate, indicating that this spiecies
broadcasts its eggs at random without regard to substrate composition.
Observation of fish eggs was limited to May-August, and spawning activity of
the above species appeared to be concentrated in May-June.

Twenty-two taxa, encompassing 24 species of fish, were observed by divers
during the study and were grouped according to frequency of observation.
Frequently observed species included alewife, yellow perch, sculpins, johnny
darter, and spottail shiner. All of these fish were seen at least once during
every year of the study. Commonly observed species included trout-perch,
common carp, rainbow smelt, burbot, and white sucker. These fish were seen
during seven to nine years of the 10-year study. Uncommonly observed species
included largemouth bass, lake trout, channel catfish, black bullhead,
smallraouth bass, and longnose sucker. These fish were seen in more than one
but less than half of the study years. Species that were rarely observed and
were seen during only one year included emerald shiner, brown trout,
quillback, walleye, unidentified coregonids, and shorthead redhorse.

135

Pelagic fish that appeared to be attracted to the in- lake presence or
operation of the plant included yellow perch and common carp and possibly
largemouth bass, smallmouth bass, and walleye. Pelagic species that appeared
generally indifferent to the in-lake presence or operation of the plant
included alewife, spottail shiner, trout-perch, rainbow smelt, lake trout,
emerald shiner, brown trout, and coregonids. Demersal fish that appeared to
be attracted to the in-lake presence or operation of the plant included slimy
sculpin, burbot, channel catfish, and black bullhead. Demersal fish that
appeared indifferent to the in-lake presence or operation of the plant
included johnny darter, white sucker, longnose sucker, quillback, and
shorthead redhbrse.

Several generalizations related to fish behavior may be made based on
this study. Species diversity and overall abundance of fish were higher
during the warm-water months (June-August) than in the spring or late fall and
higher at night than during the day. Day-active fish included yellow perch,
common carp, and johnny darter. Nocturnally active fish included sculpins and
burbot. Alewife, spottail shiner, trout-perch, and rainbow smelt showed no
obvious pattern in diel activity. Daytime schooling was observed among adult
alewife (500-1 ,000/school) , yellow perch ( 10-50/school) , and common carp (5-
20/school), although aggregations tended to be loose and often included fish
of widely differing sizes. Schooling among YOY fish was observed for alewife,
yellow perch, and rainbow smelt. For all species that were active at night,
swimming was more undirected and slower, and fish were more easily approached
by divers than during the day.

Schools of YOY alewife were observed in September and October during most
years. Schools of YOY yellow perch were occasionally seen in August.

136

Observation of these YOY fish coincided with their appearance inshore at this
time of the year and was further documented in field and impingement catches.

Fish abundance and diversity were greater in the riprap area than in the
surrounding area of sand substrate. Yellow perch, slimy sculpins, johnny
darter, burbot, channel catfish, and black bullheads were probably attracted
to the vertical relief and protection that the rugose substrate offered.
Common carp appeared to be attracted to the warm-water discharge. Largemouth
bass, smallmouth bass, and walleye were seen in close association with the
structures and may have been attracted to the vertical relief that these
objects presented. Alewives were seen in abundance in all of the study area
but may have sought shelter near the structures during periods of inshore
turbulence. Spot tail shiners, rainbow smelt, and trout- perch did not appear
attracted or repelled by the physical presence of the reef or operation of the
plant. Excluding the operational effects of entrainraent and impingement on
fish at various life stages, the physical presence of the structures and
riprap appeared to enhance fish populations by providing additional habitat
for spawning, feeding, and protection from predation and harsh inshore lake
conditions.

The seasonal abundance of fish observed by divers in the study area was
often directly correlated with their spawning activities. This was true for
species that were attracted to the plant area for spawning substrate (e.g.,
yellow perch, sculpins, johnny darter) or by an operational factor (common
carp), as well as for species that appeared indifferent to the presence or
operation of the Cook Plant (e.g., alewife, spottail shiner, rainbow smelt).

The spatial and temporal abundance of Lake Michigan fish found in the
study area appeared to be strongly influenced by environmental factors

137

(substrate conditions, water temperature, storms, turbulence, ice, diel
period) acting in concert with physiological needs of the fish (maturation,
spawning, feeding, survival, growth) and presence of other lake biota
(predators and prey). Our studies also indicated that the level of influence
that these factors assert on fish abundance, distribution, and behavior
changes as fish pass through various stages in their life history and
physiological needs.

The Cook Plant structures and riprap have created habitat atypical of the
surrounding environment. Through its physical presence and modification of
extant environmental conditions acting in combination with effects of plant
operation, it has had a distinct impact on the local ecology. Population
increases for some organisms, including periphytic algae, attached and free-
living invertebrates, and pelagic and demersal fish, have been achieved
through the expansion of substrate to provide increased shelter and a more
diversified habitat relative to the surrounding environment. Environmental
conditions on the reef have favored the survival and growth of individual
organisms and resulted in local population increases. From the standpoint of
diver observations, effects of these changes appeared limited almost exclu-
sively to the reef itself and have not influenced the ecology of the sur-
rounding area to any noticeable extent.

Presence of the riprap served to enhance local population densities of
organisms attracted to the area. The attraction and enhancement of these
populations must be balanced against their increased vulnerability to
operational effects of the Cook Plant and plant-induced mortality. Negative
effects (e.g., primarily entrainment and impingement) appeared to be limited
more to plant operation than the physical presence of the plant structures and

138

riprap in the lake and were inferred more from other components of the Cook
Plant studies than from diver observations. Barring major modifications to
the in- lake structures or operation of the Cook Plant, future diver
observation of additional large or significant ecological changes or plant
impacts are not anticipated*

139

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eastern and east-central Lake Michigan, May-August 1972. Admin. Rep.
Great Lakes Fish. Lab., Ann Arbor, Mich. 24 pp.

Wetzel, R. G. 1975. Limnology. W. B. Saunders Company, Philadelphia, Penn.
743 pp.

Winnell, M. H. 1984. Ecology of the zoobenthos of southeastern Lake Michigan
near the D. C. Cook Nuclear Power Plant. Part 5: Malacostraca (Amphipoda,
Mysidacea, Isopoda, and Decapoda). Spec. Rep. No. 99. Great Lakes Res.
Div., Univ. Mich., Ann Arbor, Mich. 94 pp.

Winnell, M. H. , and D. J. Jude. 1981. Spatial and temporal distribution of

benthic macroinvertebrates and sediments collected in the vicinity of the
J. H. Campbell Plant, eastern Lake Michigan, 1980. Spec. Rep. No. 87.
Great Lakes Res. Div., Univ. Mich., Ann Arbor, Mich. 110 pp.

144

Appendix 1. Summary of observations made during dives on riprap sub-
strate surrounding the D. C. Cook Nuclear Plant intake and discharge
structures in southeastern Lake Michigan, 1973-1982.

Category

Apr May Jun Jul Aug Sep Oct

1973

No. of dives-*-

Periphyton^
S true ture
Riprap

Invertebrates -

Crayfish

Snails

Hydra

Bryozoans

Sponge

Other

Fish^

Fish eggs ^

Riprap

Sand

(Continued) .

3.7
0.5

3.7
2.0

3.2
2.5

1
>100

>1,000

>200

145

Appendix 1.

Continued.

Category

Apr

May

Jun

Jul

Aug

Sep

Oct

No. of dives^

Periphy ton ^

Structure

Riprap

Invertebrates ^

Crayfish

Snails

Hydra

Bryozoans

Sponge

Other

Fish^

Fish eggs ^

Riprap

Sand

1974

3.8 7.5
0.5 1.0

1 5 30
100 >100

>100

3.0
1.3

50 1
75 >100

75 72

(Continued)

146

Appendix 1. Continued,

Category

Apr

May

Jun

Jul

Aug

Sep

Oct

1975

No. of dives^

Periphyton^

Structure

13.

12.5

5.0

1.0

Riprap

0.5

12.

5.0

1.0

Invertebrates-^

Crayfish

103

Snails

>1,000

Hydra

Bryozoans

Sponge

Other

Fish^

>100

>133

>100

>128

>100

>1,000

,000 >1

,000

3+1*

Fish eggs^

Riprap

AL,

SP,YP

Sand

(Continued) .

147

Appendix 1.

Continued.

Category

Apr

May

Jun

Jul

Aug

Sep

Oct

No,

of dives-^

1976

Periphy ton ^

Structure

Riprap

Invertebrates-

Crayfish

Snails

Hydra

Bryozoans

Sponge

Other

Fish^

Fish eggs ^

Riprap

Sand

1.2

2.5 11.5 10.0 6.3
1.5 2.5 1.0 0.5

5.0
0.5

18
2

27 >216 >382 >134
1

X X

107

135

2 >1,000 >100 >243 >1,000
1 1

108

SP,AL
AL

AL
AL

(Continued)

148

Appendix 1. Continued,

Category

No.

of dives^

Apr May Jun Jul Aug Sep Oct

1977

Periphyton^

Structure

Riprap

Invertebrates^

Crayfish

Snails

Hydra

Bryozoans

Sponge

Other

Fish^

LB
BT
LS

QB
SR
XG
WL

Fish eggs ^

Riprap

Sand

0.5 0.5 1.5 1.8 3.0 1.5
0.4 1.0 1.0 1.2 1.5 0.3

>225 122

>125 >298
1

>151

X
X

200

187 13
28 11

39
1

,000

16 >1,000

>102

JD.YP JD,YP,AL AL
AL AL

(Continued)

149

Appendix 1.

Continued.

Category

Apr

May

Jun

Jul

Aug

Sep

Oct

No. of dives-^

Periphyton ^

Structure

Riprap

1978

0.3 0.1 7.5 10.0 3.0 2.0 1.7
1.0 3.5 8.0 7.5 2.5 2.0

Invertebrates -

Crayfish

Snails

Hydra

Bryozoans

Sponge

Other

Fish^

M,C

X
X

10 1
3

>360

,000

>100 >1,000

Fish eggs -

Riprap

Sand

SS AL,SP AL
AL

(Continued) .

150

Appendix 1.

Continued.

Category

Apr

May

Jun

Jul

Aug

Sep

Oct

1979

No. of dives^ 3 3 3

1 2

8 3

8 >1,

,000

327 >1,000

Periphy ton ^

Structure 0.5 1.5 3.0 6.0 1.0 1.0

Riprap 0.5 1.2 3.0 5.5 5.0 3.0 2.5

Invertebrates^

Crayfish 4 8 16 5

Snails

Hydra X X

Bryozoans

Sponge X XX

Other

Fish ^

YP 99 1 170 36 2

CP 8 4 11*

SM 5 3

LS 1

QB 1

Fish eggs ^

Riprap YP AL

Sand AL

( Con tinned) .

151

Appendix 1. Continued,

Category Apr May Jun Jul Aug Sep Oct

1980
No. of divesl 2 2 3 3 2 2 3

Periphyton^

Structure 2.0 1,6 6.5 1.0

Riprap 3.0 1.8 1.5 6.0 1.0 1.3 1.0

Invertebrates ^

Crayfish 4 7 13 10 5 5

Snails

Hydra X

Bryozoans X

Sponge X

Other

Fish^

BT 1

Fish eggs ^

Riprap AL

Sand AL

(Continued) .

114

>106

>103

1
30

210

152

Appendix 1. Continued,

Category Apr May Jun Jul Aug Sep Oct

1981

No. of dives^ 3 2 3

X X

X X
P P

>110

>243

>109

4 1

3 22

>175

3 1

2 >1,000

Periphy ton ^

Structure 1.5 12.5 7.5 1.0 0.8 0.7

Riprap 1.0 2.5 5.0 2.0 1.5 1.8

Invertebrates -^

Crayfish 4 9 3 1

Snails

Hydra

Bryozoans

Sponge

Other

Fish*

CP 18 30

SM 11 15

Fish eggs ^

Riprap YP

Sand

(Continued) ,

153

Appendix 1. Continued.

Category Apr May Jun Jul Aug Sep Oct

1982
No, of divesi 12 2 3 3 2 2

Periphyton ^

Structure

Riprap 0.5 1.0 4.0

Invertebrates ^

Crayfish 3 1

Snails

Hydra X

Bryozoans

Sponge X

Other

Fish^

YP 12 44 >765 >131

JD 5

SS 84 1

TP 1

SP 2

AL 1 >178

CC 1

CP 3* >100 >100+6*

WS - 1

SM 3

LB 1

SR 1

XC 1

5 3

>170

>114 >1,000

Fish eggs ^

Riprap

Sand

154

Total number of standard series dives (usually three) made in the
ripraped area surrounding the plant intake and discharge structures.
From August 1977 to May 1982, diving in the area was reduced to
only those occasions when water was not being discharged from one
of the structures. During June 1982, the technical specifications
for monitoring were reduced to two dives per month in the intake
area only.

Length (cm) of periphyton on top of the structure and on riprap
adjacent to the base of the structure as measured by divers.

Numbers of crayfish and snails were counted by divers. Values
showing the greater than (>) symbol are tot;als which included
open-ended estimates of 100+ or 1,000+ (see Fig. 2 and Methods).
Presence of other invertebrates was noted (X) but animals were not
enumerated. C = Chironomid (midge) larvae, E = Ephemeropterid
(mayfly) larvae, M = Mysis , N = Notonectid (back swimmer),
P = Pontoporeia , T = Trichoptera (caddisfly) larvae.

See Appendix 3 for scientific and common names, and abbreviations
for fish. * = observed at intake stations.

Denotes observation of eggs of the fish species indicated during
standard series dives on riprap substrate or during dives at
reference stations north and south of the plant in areas of sand
substrate.

153

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159

Appendix 3. Scientific name, common name, and abbreviations for species
of fish observed by divers in southeastern Lake Michigan near the D. C.
Cook Nuclear Plant, 1973-1982. Names were assigned according to Robins
et al, (1980).

Scientific name

Alosa pseudoharengus (Wilson)
Carpiodes cyprinus (Lesueur)
Catostomus catostomus (Forster)
Catostomus commersoni (Lacepede)
Coregonus spp.^
Cottus spp.2

Cyprinus carpio Linnaeus
E the o stoma nigrum Raf inesque
Ictalurus melas (Raf inesque)
Ictalurus punctatus (Raf inesque)
Lota lota (Linnaeus)
Micropterus dolomieui Lacepede
Micropterus salmoides (Lacepede)
Moxo stoma macrolepido tum (Lesueur)
Notropis atherinoides Raf inesque
Notropis hudsonius (Clinton)
Qsmerus mordax (Mitchill)
Perca flavescens (Mitchill)
Percopsis omiscomaycus (Walbaum)
Salmo trutta Linnaeus
Salvelinus namaycush (Walbaum)
Stizostedion vitreum vitreum (Mitchill)

Common name

Abbreviation

alewife

quillback

longnose sucker

white sucker

unident. coregonid

unident. cottid

common carp

johnny darter

black bullhead

channel catfish

burbot

smallmouth bass

largemouth bass

shorthead redhorse

emerald shiner

spottail shiner

rainbow smelt

yellow perch

trout- perch

brown trout

lake trout

walleye

May include both Coregonus artedii Lesueur (lake herring or Cisco)
and Coregonus hoyi (Gill) (bloater) because divers could not
distinguish between these species while underwater.

May include both Cottus cognatus Richardson (slimy sculpin) and
Cottus bairdi Girard (mottled sculpin) because divers could not

distinguish between these species while underwater.

160

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