THE Damn-amen or" zoommon _ '
ALONG. THE : . -. -  — SHORE.
or LAKE ERIE

Thesis for the Degree of M. S.
MICHIGAN STATE UNIVERSITY
THOMAS F. NALEPA
1972

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ABSTRACT
THE DISTRIBUTION OF ZOOPLANKTON
ALONG THE WESTERN SHORE
OF LAKE ERIE
by

Thomas F. Nalepa

ZooPIankton distributions in the near—shore areas of western
Lake Erie were studied in relation to six environmental variables -
temperature, oxygen, particulate organic carbon, primary productivity,
suspended solids, and fish predation. Samples were collected with a
Van Dorn water bottle at 2 week intervals from 1 May 1970 to 7 November
1970. ZOOpIankton density, biomass and composition were compared in
four different habitats: near the shore of western Lake Erie, a
man-made discharge canal, a shallow creek embayment, and a polluted
river.

Distributions were generally uniform within the lake but different
from the inshore areas. The lake was intermediate in density but
highest in biomass. The discharge canal and the shallow embayment had
relatively high densities but were intermediate in biomass. The
river was lowest in both density and biomass. Species composition was
essentially similar in all the areas but density composition of the major
taxa of 200plankton (rotifers, copepods, caldoceran) differed widely.
The discharge canal and the shallow embayment had a comparatively high
density of rotifers while the discharge canal also had a high density
of copepod nauplii. The lake had the highest density of Cladocerans.
The river had the lowest density of all major taxa of zooplankton taxa

except in June. Mean size of individual plankters was greatest in

Thomas F. Nalepa

the lake, intermediate in the river, and lowest in the discharge
canal and the shallow embayment.

These basic differences in zooplankton distributions were
attributed mainly to variations in oxygen, food availability, and
fish predation. Therefore, where abiotic conditions are tolerable,
food availability and predation appeared to be the most influential
regulators of zooplankton in the near—shore areas of western Lake

Erie.

THE DISTRIBUTION OF ZOOPLANKTON
ALONG THE WESTERN SHORE

OF LAKE ERIE

By

Thomas F. Nalepa

A THESIS

Submit ted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Fisheries and Wildlife

1972

ACKNOWLEDGEMENTS

I wish to express my appreciation to all those that helped
make this thesis a reality: R. A. Cole, for his guidance and inspir—
ation; C. B. McNabb, N. R. Kevern and W. T. Porter, for serving on my
committee; W. T. Edmondson and C. C. Davis, for helping with the
identification of species; B. R. Parkhurst and M. D. Marcus, for
assisting in the field work and for the use of their data. Use of
Michigan State University computing facilities was made possible

through support, in part, from the National Science Foundation.

ii

TABLE OF CONTENTS

Page

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . 1
DESCRIPTION OF STUDY AREA. . . . . . . . . . . . . . . . 5
METHODS AND MATERIALS. . . . . . . . . . . . . . . . . . 24
Experimental Design and Analysis. . . . . . . . . . 24
Field and Lab Techniques. . . . . . . . . . . . . . 25
RESULTS. 0 I O O O O I O I I O O O I O 0 O O O O O O O I 29
Horizontal Distributions. . . . . . . . . . . . . . 29
ZOOpIankton Density. . . . . . . . . . . . . . 29
ZooPlankton Biomass. . . . . . . . . . . . . . 35
Zooplankton Composition. . . . . . . . . . . . 41

Vertical Distributions. . . . . . . . . . . . . . . 76
200plankton Density. . . . . . . . . . . . . . 76
Zooplankton Biomass. . . . . . . . . . . 81

Diurnal Change in Vertical Distribution. . . . 86

Predation . . . . . . . . . . . . . . . . . . . . 86
Chemical and Physical Regulators. . . . . . . . . . 90
DISCUSSION . . O O O O O O O O O O O O O O O O O O O O O 98
Regulation of Zooplankton Distributions . . . . . . 98
Changes in Western Lake Erie. . . . . . . . . . . . 102

LITERATURE CITED . . . . . . . . . . . . . . . . . . . . 110

iii

LIST OF TABLES

Estimated dry weights and lengths of
Species. . . . . . . . . . . . . . .

200plankton

Multiple range comparison of numbers of zooplank-

ton at the sampling stations . . . .

Multiple range comparison of biomass
zooplankton at the sampling stations

Mean seasonal biomass per individual
dual) 0 o o o o o o o o o o o o o o 0

Numbers of 200p1ankton per liter at different

depths . . . . . . . . . . . . . . .

(mg/liter) of

(pg/indivi-

Biomass (mg/liter) of 200plankton per liter at

different depths . . . . . . . . . .

Number and lengths of fish used for stomach

analysis . . . . . . . . . . . . . .

Selectivity for certain zooplankton species

expressed as the ratio between the per cent each
species made of the total stomach contents and
the per cent of the total density it comprised

in the water . . . . . . . . . . . .

Relative relation of zooplankton groups to
variation in oxygen concentration, inorganic
suspended solids, particulate carbon and

temperature. 0 O O O O O D O O O 0

iv

Page

27-28

32-34

38-40

44

77—80

82—85

88a

89

94

LIST OF FIGURES

 

 

 

 

 

 

 

 

Figure
1. Map of western Lake Erie and specific area under
Study. 0 O O O O O O O O O O O O 0 O O O O O O O O
2. Map of the study area with the location of sampling
StatiOnS O O O O O O O O O O I O 0 O I O O O O O O
3. Mean seasonal variation of temperature and oxygen.
4. Mean diurnal variation of temperature and oxygen
on July 1, 1970 at four representative stations
in the study area. . . . . . . . . . . . . . . . .
5. Mean seasonal variation of particulate organic
carbon C C I O O C C O O O I O O O O O O O O O O C
6. Mean seasonal variation of suspended solids. . . .
7. Mean seasonal variation of total ZOOplankton
dEHSity. o o o o o o o o o o o o c o o o o o 0 o o
8. Mean seasonal variation of total zooplankton
biomass. O O O O O O O C O O C O O O O O O O O O 0
9. Mean seasonal variation of total 200plankton
biomass in the lake. . . . . . . . . . . . . . . .
10. Mean seasonal variation of the total density of
Brachionus calyciflorus and Keratella cochlearis .
11. Mean seasonal variation of the total density of
Pomplyx sulcata and Polyarthra spp. . . . . . . .
12. Mean seasonal variation of the total density of
rotifers O O O 0 O O O O I O O O I O O O O O O O O
13. Mean seasonal variation of the total density of
Keratella quadrata and Synchaeta stylata . . . . .
14. Mean seasonal variation of the total density of
Daphnia retrocurva and Chydorus shaericus. . . . .
15. Mean seasonal variation of the total density of

mature and immature Daphnia retrocurva in the
lake and in discharge canal. . . . . . . . . . . .

 

V

Page

l6

19

31

37

43

47

51

53

55

57

Figure Page

16. Mean seasonal variation of the total density

.‘ ‘3 ”A. ‘ \v. .- - . 1. s ' l 1 I , ’ I '9 “ ‘. "
of PoemTHa sp. and anhhlg gale? mendot.e. . . 59
—. __ --—-—.. _- ‘1‘ '

..,. _.

 

17. Mean seasonal variation of total density
and total biomass of Cladocerans . . . . . . . . 62

18. Mean seasonal variation of total density
and total biomass of c0pepods. . . . . . . . . . 64

19. Mean seasonal variation of the total density
of copepod nauplii and juvenile cyclopoids . . . 66

20. Mean seasonal variation of the total density
of Cyclops vernalis and Tropocyclop§_

prasinus . . . . . . . . . . . . . . . . . . . . 68

 

 

21. Mean seasonal variation of the total density
of juvenile calanoids and Diaptomus siciloides . 70

 

22. Mean seasonal variation in the composition of
density of the major zooplankton groups. . . . . 72

23. Mean seasonal variation in the composition of
biomass of the major 200plankton groups. . . . . 74

24. Comparison of the daywnight vertical distri—
butions at three representative stations . . . . 88

25. Mean seasonal variation in the composition of
zooplankton in the stomachs of young perch

and white bass . . . . . . . . . . . . . . . . . 92

26. Seasonal relationship between the density of
200p1ankton in the river and river discharge . . 97

27. Changes in the total number of Cladocerans
in Lake Erie from 1939 to 1970 . . . . . . . . . 104

28. Changes in the total number of copepods in Lake
Erie from 1939 to 1970 . . . . . . . . . . . . . 106

29. Changes in the total number of rotifers in Lake
Erie from 1939 to 1970 . . . . . . . . . . . . . 108

vi

INTRODUCTION

The purpose of this research was to determine the seasonal vari-
ation in the horizontal and vertical distributions of ZOOplankton
along the western shore of Lake Erie in relation to factors that hypo-
thetically regulate their distribution. Much of the resource value of
the Great Lakes, including Lake Erie, is derived from the use of its
near-shore waters. Any resource use or management program must be
based on a sound knowledge of the ecosystem function that influences
these resources. Although the zooPlankton are not directly utilized
as'a resource, they are the main trophic link between the algae and the
fisheries resource and are likely to have a direct influence on their
dynamics. Western Lake Erie is the most thoroughly studied of the
Great Lakes, but near-shore variability in the 200plankton, particularly
in relation to environmental variation, has not been well-assessed.

The distributions of zooplankton populations are influenced by
a complex of chemical, physical, and biological variables. Hypo—
thetically, these variables act in consort upon the plankton and
determine any variation in time and space. Of all the variables that
could potentially influence 200plankton dynamics, temperature, food,
competition and predation are thought to have the most impact. Temp—
erature increases, below critically high levels, enable potential
increases in zooplankton densities by reducing maturation time and
therefore increasing the production of young animals (Hall, 1964).

Considering differential metabolic and reproductive rates, changes in

2
temperature also contribute to the seasonal progression of species
through competitive selection. Certainly there are eurythermal and
stenothermal species in most large phylogenetic groups with widely
differing temperature tolerances and optimums (Odum, 1959). Since
the zooplankton differ widely in mean size, variation in species biomass
is also potentially important. The larger forms (Cladocera, COpepoda)
seem to be positively related to temperature (Hazelwood and Parker,
1961) while the much smaller rotifers can become abundant at various
times at greatly differing water temperatures (Davis, 1962). However,
attempts to explain large seasonal changes in terms of temperature
differences alone have largely met with failure (Davis, 1962).

Two of the more important factors to consider are food and pre-
dation. Z00plankton reproductive rates can be highly dependent upon
food availability (Hall, 1964; Edmondson, 1964). In general, increased
food leads to pOpulation increases through increased brood number.
Given a moderate, constant supply of food at a moderately low temperature,
an increment in the pOpulation can be caused by increasing the temp-
erature which, in turn, increases the rate of molting and brood pro—
duction, or by increasing the food supply which stimulates the produc-
tion of eggs per brood (Hutchinson, 1967).

The composition and size of available food is also very important.
Pennak (1955) suggests that seston is the main source of food for the
ZOOplankton but Davis (1958) found phytOplankton to be more important
in western Lake Erie. Total particulate organic carbon is a measure
of organic seston and gross primary production is a measure of total
phytoplankton availability, as long as phytoplankton respiratory

efficiencies are not widely variant.

3

Competition for food among the zooplankton is very intense (Brooks
and Dodson, 1965). Hypothetically, the larger plankters (Cladocerans,
calanoid copepods) can outcompete the smaller forms (rotifers, small
Cladocerans) for the same-sized food through increased efficiency of
food collecting and greater metabolic efficiency (Brooks and Dodson,
1965). They can also utilize larger particles than the smaller
200plankton. The smaller forms should then be effectively eliminated.
However, through size—selective feeding by fish, predation on the
larger plankters is much more intense than on the smaller forms (Brooks
and Dodson, 1965; Galbraith, 1967; Hall, Cooper and Werner, 1970).

In fact, Hall (1964) found that the population size of Daphnia galeata

 

mendotae, a relatively large plankter, was probably regulated more by

predation than by food supply during the summer. Therefore, when pre-
dation is intense, the smaller plankters that escape predation should

become dominant.

Oxygen concentrations and inorganic suspended solids can also
influence zooplankton distributions. Low oxygen concentrations (1-2
mg/liter) definitely have an adverse effect upon the larger plankters
(Pacur, 1939; Hazelwood and Parker, 1961) while rotifers can better
withstand low oxygen concentrations (Ruttner, 1952). Inorganic
suspended solids should have an indirect effect on the zooplankton
by inhibiting the production of food. Turoidity has been found to
reduce light penetration and thus reduce algal photosynthesis (Chandler
and Weeks, 1945). Also, high concentrations of inorganic suspended
solids in western Lake Erie are often indicative of stormy conditions
which could also affect zooplankton distributions (Andrews, 1948).

Reduction of all these variables to a minimal number is necessary

before any generalities can be made about the planktonic system. This

4

was accomplished by comparing the density, biomass, and composition of
zooplankton in near-shore areas that were highly variable in all the

above environmental factors purported to regulate zooplankton.

DESCRIPTION OF STUDY AREA

The western basin of Lake Erie receives 95% of all the drainage
water entering Lake Erie, yet it comprises only 5% of the total Lake
Erie volume. These tributaries carry various industrial, municipal
and agricultural wastes into the western basin. The Detroit River,
with a mean discharge of 173,000 ft3/sec, delivers 95% of all water
entering the basin. The Maumee River, in the southwest corner of the
basin, contributes 3%. The interaction of these two main tributaries,
along with the influence of the prevailing southwesterly winds, causes
the water along the western shores to circulate counterclockwise
(Andrews, 1948). However, circulation patterns can change dramatically
during storms.

The western basin averages 8 meters deep. This is particularly
true along the western shores where the 6.4 meter contour line extends
8-11 kilometers from the beach (Wright, 1955). Because of this extreme
shallowness, the western basin is usually homothermous and thermal
stratification occurs only occasionally. However, when stratificatbmndoes
occur, rapid oxygen depletion of bottom waters follows (Carr, gt_§l.,
1965). Persistant wind generated mixing permits the resuspensiOn
of bottom sediments, which contributes to high turbidities. Large
quantities of suspended solids are also contributed by tributaries
(Chandler and Weeks, 1945).

The specific near shore region studied centers on the mouth of
the Raisin River near Monroe, Michigan (Figure l). Stations were

5

Figure 1. Map of western Lake Erie and Specific area
under study.

 

     
    

     

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located as given in Figure 2. Stations positioned in the lake (1-6)
were oriented parallel to the prevailing north-easterly currents and
north and south of the mouth of the Raisin River. The stations are
about 1-2 km offshore and about 1 1/2 km apart except Station 6 which
is 3 km offshore and 5.5 km southwest of Station 5. Water depth is
5-6 m at all the lake stations except Station 4 (3-4 m). Bottom sedi—
ments are quite variable. Stations 1 and 2 are composed of silt, sand,
and gravel, Stations 3, 4, and 5 are predominantly sand and Station 6
is predominantly silt.

Plum Creek (Station 7) is a relatively shallow (1-2 m), broad
(1 km) embayment that fosters the growth of scattered macrOphytes.
Bottom sediments consist mainly of silt and clay with particulate
plant debris. The creek enters into the discharge canal (Station 8)
with a discharge that is usually less than 1 m3/sec.

The discharge canal was recently constructed (1969-1970) to
carry the discharged cooling water from a new stream-electric plant.
Its waters are influenced by both Plum Creek and Lake Erie. water
depth is 6-7 m (dredged) with a bottom composed of silt, clay, and
plant debris. The canal is 2 1/2 km long and 150 m wide.

The polluted River Raisin (Station 9) receives municipal wastes,
heavy metals, and paper mill wastes from Monroe, Michigan. Water
depth is 6-7 m (dredged) with a bottom of putty-like silt combined with
paper fiber and traces of oil. In 1970, the mean discharge into the
lake was 17 m3/sec.

Temperature varied uniformly among the six lake stations, while
differences at Plum Creek, the discharge canal and the river were
more discernable (Figure 3). In early May, temperatures were 12.8° C

to 15° C in the lake, 17° C at Plum Creek and the discharge canal, and

Figure 2. Map of the study area with the location
of sampling stations.

 

 

 

é Prevailing current

e Plankton, benthos, and
chemistry station

P Primary productivity
H Trawling Station

11

13.90 C in the river. By July 24, all stations reached 22° C other
than the discharge canal, where it was slightly warmer at 24.5° C.
Peak seasonal temperatures occurred in August and early September.
The differences that occurred among the stations at this time were
probably due to diurnal variations incurred by sampling over an eight
to ten hour period. On July 1, when temperatures were measured through-
out a 24—hour period at four of the stations, the diurnal range at
each station was similar to the range found among all the stations on
each late summer sampling date (Figure 4). After early September,
temperatures decreased at all stations until, on November 7, the
average was 9.5° C. Ice began to form along shore in early December.

All the stations showed seasonally similar oxygen concentrations
except for the river, which was always lower (Figure 3). Highest con-
centrations occurred in early May and ranged from 9 mg/liter in the lake
to 5.5 mg/liter in the river. Concentrations decreased uniformly until
early August when some station variation occurred, but, like temperature,
part of this variability was due to the different times of day the
recordings were made. After August, concentrations generally decreased
until, on October 11, the oxygen content was 4 mg/liter at all stations
except in the river, where it was only 0.5 mg/liter. Concentrations in
the discharge canal and the river began to rise sharply in late October,
probably in delayed reapcnse to the seasonal cooling.

The diurnal variations in temperature and oxygen profiles for a
24—hour period (July 1, 1970) are given in Figure 4. At the lake
stations (1, 3) there was a diurnal temperature variation of 2° C
while oxygen concentrations varied between 2-3 mg/liter. Thermal
stratification was strongest during the afternoon hours, with the

thermocline between 3-4 m deep at Station 1 and 2—3 m deep at Station

12

Figure 3. Mean seasonal variation of temperature and
oxygen. Lake; - - - Plum Creek;
.... Discharge Canal; _,_3_, River.

 

 

 

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16

3. The maximum change in one meter was a decrease of 3.5° C in tempera-
ture and 6 mg/liter in oxygen at Station 1.

The discharge canal surface temperatures varied from 25.5° C to
28.0° C while oxygen concentrations varied between 8.5 mg/liter to
11.0 mg/liter during the course of the day. A sharp thermocline occurred
at 2-3 m during the period of stratification from early afternoon to
late evening.

A thermocline occurred in the river at 3—4 m in early afternoon
with temperatures drOpping from 24.5° C to 21.5° C in 1 m. Oxygen
concentrations were nearly uniform at all depths until early afternoon.
At that time, oxygen concentrations increased only at the 3-5 m depth
and lasted until late evening. Perhaps the best explanation for this
phenomena is the upstream movement of oxygenated lake water under a
downstream surface flow of river water during a seiche in the lake.

Data obtained from the Corps of Engineers indicates a rise of 1.5 ft
in the lake from 12 noon to 9 p.m. on this date.

A potential indicator of available food, the seasonal variability
of particulate organic carbon (> .45u) at the 0.5 m depth is summarized
in Figure 5. Concentrations of particulate matter at 2.5 m were usually
similar (within 10%) to concentrations at 0.5 m at all stations. The
lake stations showed similar seasonal concentrations but averaged
consistently lower than Plum Creek, the discharge canal, and the river.
Highest concentrations in the lake occurred in late May with a maximum
of 12 mg/liter. Concentrations then decreased and remained low until
early August with another peak on September 1, 1971. This peak, how-
ever, was lower than that in the Spring, with a maximum of llmg/liter.
All the lake stations recorded stable concentrations of 6-8 mg/liter

during the fall months.

17

Figure 5. Mean seasonal variation of particulate organic
carbon. Lake; —-— Plum Creek; ....Discharge
Canal; _3_3_, River.

 

 

18

 

 

 

103:9): coauou umcumLO
93.39250."—

Months

l9

Particulate organic carbon at Plum Creek, the discharge canal,
and the river averaged 34H)mg/1iter higher than the lake on any one
sampling date. As in the lake, concentrations were high in late May
with a maximum of 251ng/liter at the river. The late summer peak at these
three stations began in early August and remained high until September 1.
Each of these areas peaked at different times. General decreases in
concentrations were recorded during the fall months with the river
having significantly higher concentrations than the other areas.

Suspended solids (seston) consist mainly of plankton, detritus,
and resuspended bottom sediments and is an indicator of total particu—
late material in the water. Seasonally, three definite peaks occurred
at the lake stations: May 15 — maximum of 63 mg/liter; August 4 -
maximum of 36 mg/liter; and September 15 — maximum of 33 mg/liter
(Figure 6). The lake concentrations varied very little among stations
except in early June. At this time, stations north of the river
(Stations 1-3) were significantly lower than those south (Stations 4-6).

Except for an additional peak on July 7, Plum Creek, the discharge
canal, and the river showed seasonal trends comparable to the lake.
However, levels were usually twice as high, with a maximum of 111.3
mg/liter at Plum Creek on May 15 and a minimum of 18 mg/liter at the
discharge canal on October 25. The river had significantly higher
concentrations during the fall months.

During 1970, Parkhurst (1971) demonstrated that fish were unevenly
distributed throughout the study area. Fish numbers and biomass were
highest in the discharge canal, intermediate in the lake, and low in
the river. Total values for numbers and biomass (kg) were: 2423 and
143.6 in the discharge canal; 2058 and 85.0 in the lake; and 173 and

3.3 in the river. The river was devoid of most fish species throughout

20

Figure 6. Mean seasonal variation in suspended solids.
Lake; —-—-Plum Creek; ....Discharge Canal;
. 2_2‘. River.

 

(MG/LITER)

SUSPENDED SOLIDS

”0'

100 '

 

 

21

 

 

22

the sampling period except in late fall when oxygen concentrations
approached saturation. The more important species included: yellow

perch (Perca flavescens), white bass (Roccus chry50ps), carp (Cyprinus

 

 

carpio), goldfish (Crassius auratus) and gizzard shad (Dorosoma cepedianum).

 

 

As an indicator of available algal food, gross primary productivity
measurements were taken at two week intervals throughout the sampling
period (M. Marcus, unpublished data). The discharge canal had signif-
icantly higher values (p < .05) than the lake or river, which were similar
although the lake averaged slightly higher. The seasonal means (spring,
summer, fall), given as mgC/mzlday, for the three areas were: 2447,

3302, 2636 in the discharge canal; 1031, 2097, 828 in the lake; and
935, 1860, and 839 in the river. Species composition was uniform at

the three areas.

METHODS AND MATERIALS

Experimental Design and Analysis

 

Zooplankton samples were taken at two—week intervals from May 1,
1970 to November 7, 1970. Incomplete sets of samples were taken on
April 18, 1970 and February 18, 1970. Triplicate samples were taken
from all the stations at each of two depths, 0.5 and 2.5 meters,
except the shallow Plum Creek station which was only sampled at 0.5 m.
From July 7 to October 11 a single sample from each station at 5.5 meters
was added. The various stations were marked by buoys and the replicate
samples were taken about 500 ft. east and west of this buoy. At Plum
Creek (Station 7), the discharge canal (Station 8) and the river (Station
9) replicate samples were taken about 100 ft. apart.

The extent of vertical day—night movement was measured by taking
night samples on 6 dates and continuous 24—hour sampling on one date
(1 July 1970). The night samples consisted of one sample each from 0.5
and 2.5 m depths at 3 stations (1, 3, 9) while the 24-houruanalysis
consisted of taking one sample from 0.5, 2.5 and 5.5 m depths at 4
stations (1, 3, 8, 9) every 4 hours.

Variance among stations and depths was analyzed on each sampling
date for numbers, biomass and percent composition of the major taxa of
zooplankton. This was followed by Tukey's multiple range comparison of
stations and the least significant difference between depths. Plum

Creek was excluded from the analysis of variance (only 1 depth) but

0

included in the multiple range comparisons. Sing-e samples taken from

24
5.5 m depths were not included in the analysis. All calculations were
made on the raw data before conversions to numbers /liter or mg/liter.
Day-night comparisons were made with the Chi—square test. Arcsin
transformations were performed on all data expressed as percentages

before statistical tests were applied.

Field and Lab Techniques

 

Samples were collected with an 8—1iter Van Dorn water bottle.

The animals in four liters of the sample were concentrated in a #25
Wisconsin plankton bucket and immediately preserved in 5% formalin.
Water from the remaining four liters was analyzed for particulate
organic carbon and total suSpended solids (seston).

In the laboratory, each sample was adjusted to a known volume of
concentrate. This ranged from .05 to .015 of the total sample size,
depending upon the plankter abundance. Two 1-ml aliquots from each
sample were placed in a Sedgewick—rafter counting cell where, under
a binocular zoom sc0pe, the plankters were counted and identified to
species. When possible, life—history stage, sex, and number of eggs
per individual were also recorded. No attempt was made to identify the
nauplii or c0pepedites.

The length of each plankter other than rotifers and nauplii was
measured with a Whipple micrometer. The Cladoceran species were
placed into .25 mm size categories, including the carapace and helmet,

with the exception of Bosmina sp. and Chydorus §phaericus (0.1 mm

 

intervals) and Leptodora kindtii (immature or mature). The calanoid

 

and cyclopoid juveniles were also categorized according to length
(1 mm intervals). The size of maturity of Qaphnia sp. was set at 1.00

mm (Hall, 1964).

25

Volume measurements were made by randomly choosing 20 individuals
of each species size category throughout the sampling dates and measur—
ing their individual areas- Volume was then calculated by assuming
the plankters to be either a cone, cylinder, sphere, or ellipsoid
(Davis, 1958). PrOportions between the body volume of one species and
a similar one (Ravera, 1969) were used as a guideline.

Total volume was calculated by summing the products of the mean
volume and the number of each Species (Naucerk, 1964). Dry weight was
assumed to be 15% of wet weight and constant with a specific gravity of
1.00 (Ravera, 1969). The Species list and their corresponding dry
weights are given in Table l.

The stomach contents of yellow perch (Perca flavescens) and white
bass (Roccus chrvs0ps) were analyzed by taking l—ml aliquots from
the entire contents and calculating th percentage each species made of
the total.

Methods employed for the determination of suspended solids and
particulate organic carbon were basically those outlined by the EPA (1969).

Temperature and oxygen profile readings were made with a YSl
oxygen meter which was periodically standardized againsthfinkler
determinations of oxygen. Duplicate readings were usually made for

each depth at each station.

TABLE 1:

26

Species

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Rotifera (14 species) LEEEED:
Asplanchna.§p.
Brachionus calyciflorus
Brachionus diversicornis
Brachionus EBB:
Conochilus unicornis
Euchlanis sp,
Filinia longiseta
Keratella cochlearis
Keratella_guadrata
Kellicottia longicornis
Polyarthra spp.
memes. ELL-1.98.33
Synchaeta stylata
Trichocera §Bn

Cladocera (7 species)
Bosmina spp. .25
Bosmina spp. .35
Bosmina_spp. .50
Ceriodaphnia_§p. .50
Ceriodaphnia_sp. .75
9:333:11ddp‘rznja ::1".- >1-0
.2 1111991112 17h €3,531: :92
.Dath 1 a 55113955: 9:63:92? 8:: -5
Daphnia galcata mendgtge .75
Daphnia galeata mendotae 1.00
Daphnia galeata mendotae 1.25
Daphnia galeata mendotae 1.50
Daphnia galeata mgndotae 1.75
Daphnia galeata mendotae >2.00
Daphnia retrocurva .5
Daphnia retrocurva .75
Daphnia retrocurva 1.00
Daphnia retrocurva 1.25
Daphnia retrocurva 1.50
Daphnia retrocurva 1.75
Daphnia retroeugga 32.00
Diaphanosoma leuchtenbergiana .50
Diaphanosoma lcuchtcnbergiana .75
Diaphanosoma leuchtenbergiana >1.00
Leptodora kindtii

Copepgda (10 Species)
Nauplii
Juvenile cyclopoid ~25
Juvenile cyclopoid -::

I

Juvenile cyclOpoid .3

Estimated Dry Weights (pg.) and Lengths (mm) of ZOOplankton

Weight

2.1
.06
.01
.01
.01
.05
.005
.005
.01
.005
.01
.005
.03
.01

1.95
3.90

25.3
40.5
1.35
2.8

4.3

10.8
16.5
35.0
50.0
55.0

uJedua-
23‘013'd

27

TABLE 1: (con' t.)

Length Weight
Juvenile diaptomid .25 .
Juvenile diaptomid .35
Juvenile diaptomid .50

Canthocamptus rohertcokeri
Cyclops vernalis

Cyclops bicuspidatus
Diaptomus ashlandi (female)
Diaptomus ashlandi (male)
Diaptomus minutus (female)
Diaptomus minutus (male)
Diaptomus oregonensis (female)
Diaptomus oregonensis (male)
Diaptomus sicilis (female)
Diaptomus sicilis (male)
Diaptomus siciloides (female)
Diaptomus siciloides (male)
Eurytremora affinis
Tropocyclops‘prasinus

 

 

 

 

 

 

 

 

 

 

 

 

 

 

memNmNmMMNmmmet-‘H
OU‘IOUIOU'IOU'IOUIOUIO‘CNOO‘OO

 

RESULTS

Horizontal Distributions

 

ZOOp1ankton Density

 

Inconsistent and relatively minor differences in density occurred
among the lake stations throughout the sampling period but densities
at inshore stations were consistently and often greatly different from
densities in the lake (Figure 7). Lake densities were usually less
than in the discharge canal and Plum Creek but more than in the river.
Most lake stations clearly exhibited two density peaks, a relatively
minor peak in late spring and a major one in late summer. The greatest
mean density, 1,330/1iter, occurred at Station 1 on September 1.

Plum Creek and the discharge canal had significantly (p < .05)
higher densities than all or most of the other stations from May 1 to August
23, excluding May 27 (Table 2). Differences were not detected in the
fall. Throughout the sampling period, the discharge canal usually had
higher densities than Plum Creek, being significantly so on four dates.
The greatest difference, 1,470/liter, occurred on June 24 when the
discharge canal had a density of 3,150/liter. This was the highest
density recorded for any station on any one sampling date.

Zooplankton in the river were consistently less dense than in any
other station. Multiple range tests indicated that the river was
significantly (p < .05) lower than all or most of the other stations on
about half of the dates sampled. Only on June 23 and October 25 were

plankters at any stations lower in density than in the river. Densities

29

Figure 7. Mean seasonal variation of total zooplankton
density. .Sm; .... 2.5m.

 

 

30

th202

20m<qn2 20n¢-£ ZOn¢nnz

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

01 x 831” ISHBGWHN 'lVlOl

C-

31

Table 2: Numbers of zooplankton per liter arranged in increasing
order of abundance at the sampling stations.

2 May 1970
Station 9 5 6 3 2 l 4 7 8
Mean 88 144 168 195 241 249 304 1122 1149

Multiple
Range

 

 

15 May 1970

 

Station 9 6 1 5 3 4 2 8 7

Mean 179 202 363 409 414 420 427 693 855

 

Multiple
Range

 

 

27 May 1970
Station 9 2 8 5 3 4 7 1 6
Mean 87 422 504 555 577 598 613 751 849

Multiple -——
Range

 

 

 

10 June 1970

 

 

Station 9 l 6 3 4 2 5 8 7
Mean 342 359 435 459 480 585 667 728 771
Multiple

Range

 

23 June 1970

 

 

Station 1 2 9 3 4 5 6 7 8
Mean 326 348 352 454 684 738 912 1680 3150
Multiple

Range __~__

32

Table 2 : (con't.)

7 July 1970

 

 

 

 

 

 

 

 

 

 

Station 9 l 3 2 4 5 6 7
Mean 286 333 334 334 391 429 777 1080
Multiple
Range
21 July 1970
Station 9 6 l 2 3 4 5 7
Mean 97 354 387 453 582 588 628 1011
Multiple
Range ——-—-
4 August 1970
Station 9 6 2 3 l 5 8 4
Mean 174 336 394 504 657 714 925 949
, 1
Multiple
Range
23 August 1970
Station 9 2 4 3 5 l 7 8
Mean 54 483 663 705 708 915 927 1575
Multiple ———-
Range
1 September 1970
Station 9 6 4 7 3 5 2 8
Mean 321 571 736 825 960 1023 1159 1248
Multiple1

 

 

Range

 

 

1530

2032

1372

1309

 

 

33

Table 2 : (con't.)

15 September 1970

 

 

 

Station 9 3 7 2 8 4 5 6 1
Mean 66 392 417 561 523 511 582 678 990
Multiple --

Range

 

27 September 1970

 

 

 

 

Station 9 1 2 4 3 6 8 5 7
Mean 105 181 288 321 342 394 405 438 660
Multiple

Range

10 October 1970

 

Station 9 7 6 8 4 3 5 2 1
Mean 63 138 156 174 264 264 271 274 288
Multiple ——-——-—-~——— ‘

Range

 

 

25 October 1970

 

 

 

Station 2 8 l 4 3 7 5 6 9
Mean 120 135 148 157 168 232 235 237 262
Multiple

Range

7 November 1970

 

 

 

Station 9 1 6 2 5 4 3 7 8
Mean 55 67 90 120 141 193 202 241 255
Multiple

Range

 

 

1“ Interaction (p <.05) exists between depths and stations.

 

34

weresfignificantly lower than the discharge canal on all dates and

lowertfimn Plum Creek on all but two dates (October 11 and 27).
The mean (t .95 confidence) density in numbers per liter throughout

the~smmfldng period was 464 i 135 in the lake, 797 i 222 at Plum Creek,

1005 iIMK)in the discharge canal, and 170 i 57 in the river. In

summary, Plum Creek and the discharge canal had relatively dense

concentrations of zooplankton, the lake was intermediate, and the river

was lowest.

ZOOplankton Biomass
No obvious trends or differences appeared to exist in 200plankton

biomass at the lake stations (Figure 8). However, subtle differences

indicate that all stations did not change uniformly in biomass over the

Only 3 of the 6 lake stations were ever significantly

Of

sampling period.

different from the majority (3 of 5) of the remaining stations.

the seven dates when these stations were higher, Station 4 was signifi-

cantly higher on three dates, Station 6 on three dates, and Station 1

on one date. Only Stations 1 and 6 were ever significantly lower than

the remaining stations, both on four dates each (Table 3). This indi—

<3ates that Stations 2, 3, 4, and 5 tended to change in phase, with

Statirni 4 having inconsistently greater concentrations than all the
othenr.lake stations; that Station 6 was the most out of phase (Station
6 anus about 5 km from the closest station while all the other lake

statixnaS‘were within 3 km of each other); and that Station 1 was somewhat

out of phase, with tendencies toward lower concentrations.

Zocqxlankton biomass in Plum Creek and the discharge canal was

lower than in the lake but higher than in the river. In the spring,

biomass here was highest of all areas. Significant biomass differences

between Plum Creek and the discharge canal occurred on only two dates,

 

 

 

 

35

 

Figure 8. Mean seasonal variation of total zooplanktort
biomass. .5m; .... 2.5m.

 

 

 

 

36

 

 

 

9.3202

2 O n 4 q

 

 

1

d 11 1

 

 

 

 

 

 

 

 

 

 

 

 

 

HBlllIOW

37

Biomass (mg/liter) of zooplankton arranged in increasing

Table 3 :
order of abundance at the sampling stations.

2 May 1970

Station 3 6
Meal .02 .02 .02 .03 .03 .03 .08 .10 .12

 

 

 

Multiple
Range

 

15 May 1970

Station 9 1

Mean .03 .05 .06 .15 .16 .20 .20 .24 .37

 

 

 

Multiple
Range

27 May 1970
Station 9 l 2 5

Mean .03

 

 

Multiple
Range

 

 

10 June 1970

Station 5

.42 .51 .53 .66 .68 .81 .82 .84 .96

Me an

 

Multiplel
Range

 

23 June 1970

 

 

Station 7 9 2 6 3 l 5 4 8
Phean. .25 .45 .70 .82 .88 .93 1.09 1.22 1.49
Multiple

 

Range

 

38

Table 3: (con't.)

7 July 1970

 

 

Station 2 1 7 4 3 9 5 8 6
Mean .23 .30 .40 .42 .45 .61 .68 .70 .85
Multiple

Range

21 July 1970

 

 

 

 

Station 9 6 7 8 1 2 3 4 5
Mean .05 .29 .39 .49 .57 .70 .79 .81 1.07
Multiple

Range

4 August 1970

Station 9 6 2 7 8 3 l 5 4

 

Mean .18 .58 .58 .74 .75 .78 1.00 1.57 2.36

 

. . . 1
Multiple
Range

 

 

23 August 1970

[\J
U1
to

Station 9 l 7 8 4

Mean .02 .38 .51 .63 .73 .83 1.11 1.24

 

Multiple
Range

 

 

 

1 September 19fl)
Station 9 7 8 6 4 3 2 5 l

bflean. .03 .32 .44 .56 .92 1.17 1.39 1.41 1.54

 

Multiple1
Range

 

 

 

39

Table 3: (con't.)

15

27

10

25

September 1970

Station 9 3 4 7
Meal .02 .17 .21 .22 .35 .36

 

Multiple1

.52

 

.76

.77

 

Range

September 1970

Station 9 l 8

Mean .10 .17 .21 .21 .26 .32

 

Multiple

.32

.34

.34

 

Range

October 1970

Station 9 7 6

Mean .02 .05 .05 .06 .07 .07

 

Multiple

 

Range

October 1970

Station 2 1
Mean .06 .07 .08 .08 .09 112

.14

 

.14

 

.18

 

Multiple
Range

7 November 1970

L.

Station 1 9 6

Mean .02 .03 .03 .05 .10 .14

 

 

Multiple

.14

 

.25

 

.20

 

Range

Interaction exists between depths and stations.

.24

 

 

 

 

40

May 27 and June 24. Biomass in the river was lowest of all the stations

exceptchning the late spring peak (June 10 to July 7) and in late

fall (October 27).

The mean (i .95 confidence) biomass (mg/liter) throughout the

samplhugperiod was .49 i .23 in the lake, .43 i .22 in the discharge

canal, .34 i .12 in Plum Creek, and .18 i .14 in the river. Seasonal

shifts in mean lake biomass are given in Figure 9.

In summary, area differences in biomass were more difficult to
detect than differences in density simply because of the greater vari-

ability in biomass throughout the sampling period and within the station

replicates. However, it is apparent from the relative values of biomass

and density that mean biomass per individual differed considerable

among stations. Individuals in the lake averaged twice the biomass of

those in the discharge canal or Plum Creek (Table 4). The river was

intermediate. Why these differences exist is partly revealed by a

detailed examination of the distribution of the major taxonomic zooplank-

ton groups i.e., rotifers, Cladocerans, and copepods.

Zooplankton Composition

Rotifers: Little variation occurred in rotifer densities or species

(umnposition.among the lake stations. However, the temporal variability

in denusity appeared greatest at Station 1 and Station 6. At Station 1,

spring densities of E. calyciflorus and E. cochlearis reached maximums

that vuare respectively 12 and 3 times greater than the average of the

other lake stations (Figure 10). During the summer, _l_’_. sulcata and

Polyarthra Spp. reached densities three times greater at Station 6 than

 

the cathen: lake stations (Figure 11). Peak densities occurred in late

May and in mid—September at all stations.

 

 

41

Figure 9. Mean seasonal variation of total zooplanktoxfl
biomass in the lake.

 

 

 

 

42

A
MONTHS

 

A u u

d
o a a. 4. 2 o
‘0 I

1,4-
12~

5:: as.

43

 

lable 4: Average biomass per individual (pg/individual) at the
various stations.
STATION
DATE 1 2 3 4 5. 6 7 8 9
1.May‘n) .11 .10 .10 ..26 .20 .14 .08 .10 .27
15 May 70 15 .14 .35 .59 .38 .77 .42 .35 .17
27 May 70 .07 .18 .45 .42 .37 .51 1.06 .72 .46
10 June 70 2.40 1.92 1.09 1.27 .67 1.56 .86 1.21 2.37
24 June 70 3.17 2.01 1.93 1.77 1.47 .90 .15 .47 1.27
7 July 70 .89 .67 1.35 1.06 1.61 1.09 .37 .45 2.12
21 July 70 1.48 1.55 1.36 1.38 1.70 .87 .38 .25 .53
4 August 70 1.52 1.47 1.55 2.48 2.19 1.72 .53 .80 1.01
24 August 70 .41 1.68 1.88 1.09 1.57 - .56 .39 .30
1 September 70 1.20 1.19 1.22 1.24 1.37 .98 .38 .34 .12
15 September 70 .76 58 .44 .41 .86 1.09 .52 .67 .25
27 September 70 .97 1.11 1.08 .65 .77 .82 .39 .45 .94
10 October 70 .53 .49 .22 .27 .27 .26 .33 .38 .24
25 October 70 .46 .47 .45 .55 .59 .75 .50 .56 .96
7 November 70 .27 .42 1.00 1.19 .70 .36 .59 .56 .51
2 .90 .87 .90 .91 .92 .79 .45 .48 .72

 

 

 

44
Plum Creek and the discharge canal had the highest rotifer den-

sities of all the areas (Figure 12). Spring densities peaked two weeks

earlier than in the lake, with g. calyciflorus peaking at densities two
times greater than Station 1 and 24 times greater than the rest of the

lake. Also contributing to high spring densities (Figure 13) were lg.

guadrata (twice as high as the lake) and S. stylata (3 times greater

than the lake). During the summer, total densities remained, in contrast
to the lake, relatively high. At that time Polyarthra sp. maintained

densities that were 5—8 times greater at Plum Creek and the discharge

canal than in the lake. Also, two additional species peaked here but

not in the lake during the summer, Brachionus sp. and Keratella sp.

Individual species densities were always lowest in the river, but

composition was similar to the other areas. Increases of _§_. stylata

are primarily responsible for the peaks that occurred in the river

during May and late October. In summary, rotifer densities were

highest in Plum Creek. and the discharge canal, intermediate in the

lake, and lowest in the river.

Cladocerans: The lake stations exhibited little difference in
density as well as in composition. The two peaks that occurred (during

June and late August) can be attributed mainly to increases of 2 species;

 

D. retrocurva in the spring and Q. Sphaericus and E. retrocurva (to a

The pulses of R. retrocurva

 

lesser extent) in the fall (Figure 14).
consisted mainly of immature individuals (Figure 15). The dramatic

rise and subsequent peaking of 9. sphaericus in late August was most

On August 4 it was virtually non-existent yet on

evident in the lake.
Seasonal densities of

September 1 densities of 300/liter were observed.

:ome other Cladoceran species are given in Figure 16.

 

 

 

Figure 10.

45

Mean seasonal variation of the total densit:)7
of Brachionus calyciflorus and Keratella_
cochlearis. Station 1; -——- mean of
the other—5 lake stations; .... mean of
Plum Creek and the discharge canal;

. . ._. River.

mu...—

 

 

 

46

3601

Kerateilg cochlearis

 

U
C)
9

N
h
9

IBOJ

120*

Number/Liter

 

o
9

 

 

 

Months

3607 W
‘ Brachionus calxciflorus

u
0
9

N

b

O
1

l80'i

Number/Liter

o
9

 

 

Menths

47

Figure 11. Mean seasonal variation of the total density

of Pomplyx sulcata and Polyarthra spp.

Station 6; -—-—mean of the other 5 lake
stations; ....mean of Plum Creek and the
discharge canal; _3_,_,_, River.

 

420-1

180*

Number/ L

0
Q

 

360-1

a
O
9

N

h

0
1

Number/Liter

o.
O
1

.430.

48

{8°
*5
E-
1;

Months

Edam spp.

 

 

 

Months

 

 

49

 

Figure 12. Mean seasonal variation of the total density

of rotifers. mean of lake stations;

————Plum Creek; ....Discharge Canal;
_3_3 River.

NUMBER I ll'l'Ell

14007

1200‘

1000' 1

800- '-

600'

400'

200‘

 

50

 

 

RO'I'IFERS

 

 

MONTHS

 

Figure 13.

51

Mean seasonal variation of the total density of
Keratella Quadrata and Synchaeta stylata. For
the former species: Station 1; -—--mean of

5 other lake stations; ....mean of Plum Creek and
the discharge canal; _3_3_3_,River. For the
latter species: mean of lake stations;
....mean of Plum Creek and the discharge

canal; _3_3_3_3River.

 

 

52

 

fixnsheetqztxlm

 

 

 

 

360-

300-

mum
2

emittoncmaz

.
.0
6

Months

WW

 

 

360-

3001

40.
0-5
0

60-

5...: \mmmfiaz

Months

53

 

Figure 14. Mean seasonal variation of the total density of
Daphnia retrocurva and Chydorus sphaericus.
____mean of lake stations; ....mean of Plum Creek
and the discharge canal; _3_3_3_3River.

 

 

 

54

s haericus

Ch doru

 

 

 

 

360‘

a
O
O
3

4o~

mu 0
8 2

cows—\LWQEDZ

60-4

Months

W

 

 

180-

\\\|)
\\
5. Ali...
h 1.11.:
o 0 mo mu mu
5 2 9 .o 3

.. cmmgxuonEDZ

Months

 

Figure 15.

55

 

Mean seasonal variation of the total density
of mature and immature Daphnia retrocurva in

the lake (upper graph) and in the discharge

canal (lower graph). mature Daphnia

retrocurva; ....immature Daphnia retrocurva.

 

56

 

/ \

 

105-

75-

4s~
15~

 

 

M

duh—.— ~zmeDZ

 

 

IOSW

75.

153

M

MQNTMS

 

57

 

Figure 16. Mean seasonal variation of the total density of
Bosmina sp. and Daphnia galeata mendotae.
mean of lake stations; ....mean of Plum
Creek and the discharge canal; _3_J_,_,River.

 

 

58

300-
Dophnia galeatg mendotoe

300*

ter

240*

d
on
9

u
9

Number/ L

o
9

 

 

 

Months

360*
Bosmina spp.

300-

ter

.- 240‘

Number/L

 

 

 

Months

59

The discharge canal and Plum Creek had densities comparable to

the lake except during the fall when densities of C, sphaericus were

 

six times lower. Plum Creek had lower densities than the discharge
canal during Spring. The most variation in cladoceran densities
occurred in the river where the lowest densities also occurred from
late July to September (Figure 17).

Except in the spring and fall when concentrations were comparable,
Cladoceran biomass was greatest in the lake, intermediate in the discharge
canal and Plum Creek, and lowest in the river (Figure 17).

Copepods: Total densities of c0pepods (nauplii, juveniles, adults)
were highest in the discharge canal followed in decreasing abundance by
Plum Creek, the lake, and then the river (Figure 18). At both the
discharge canal and Plum Creek, densities peaked in early summer while
the lake and river peaked during June and early September. The density
differences between the various areas can be attributed to the nauplii
and juveniles, since the adults had similar abundances at all the
stations. The two extreme cases were the discharge canal and the river.
The former area had a disproportionately larger number of nauplii
and juveniles to adults and the latter a higher prOportion of adults
to nauplii and juveniles (Figures 19, 20, 21).

Compositional Variation: The relative densities and biomass
(expressed as perwcent) of the major zooplankton taxa varied little
within the lake (Figures 22 and 23). Stations 1 and 6 tended to exhibit
minor differences from the rest of the lake stations just as they did
for biomass. At Station 1, rotifers comprised a higher pr0portion of
both density and biomass in late May and October. At Station 6, a
higher preportion of copepods occurred in the fall (mainly nauplii).

Since mean size of the members of each taxa differ considerably, these

 

 

‘1

. Mean seasonal variation of total density (upper
graph) and total biomass (lower graph) of
Cladocerans. ____mean of lake stations; ----
Plum Creek; ....Discharge Canal, _3_3_3_3River.

h-J

Figure

61

 

 

 

 

180- .63 ’
5
.-
:1
I
III
g 120
3
z _.
...".I\‘.
/
60- 1 l \
\l
\/
. /.\ I
.:..T‘ .' o l ( l \. r
- M J J A
1.06
I
u.‘
'—
S
O
E
'\

 

 

 

MONTHS

62

 

Figure 18. Mean seasonal variation of total density (upper

graph) and total biomass (lower graph) of
copepods. mean of lake stations; --—-
Plum Creek; ....Discharge Canal; _, . ._,River.

 

 

NUMBER I UTER

MO I LITER

14004
1200-
1000-
800-
cool

400$

200-J .'

 

.IOT

 

63

'. 1725

 

 

MONTHS

 

64

 

Figure 19. Mean seasonal variation of the total density of
copepod nauplii and juvenile cyclopoids.
mean of lake stations; ....mean of Plum Creek
and the discharge canal; _,_,_3_3River.

NUMBER I ”TE!

1050]
see,
750«
600-
450-

300‘

 

Isoe

65

Copepod
Ncuplli

 

180'

150

120*

90-“

 

 

Ju vonilo
C opepods

 

 

66

Figure 20. Mean seasonal variation of the total density of
Cyclopg vernalis and Tropocyclops prasinus.
mean of lake stations; ....mean of Plum Creek
and the discharge canal; _3_3_,_3River.

 

 

 

45*

M
U
.2995»
9 w 9 «c

Number/ Liter

N
U!
l

 

67

Museum

 

451

37.54

L.)
O
1

15*

Number/Liter

N
U
1

 

 

 

Months

to

Tropegrlops erosion;

/ if. A

/.’ 1\.'-/ I“

l, 74 \2’41

V \~.
.\ .,

k

‘l I

J'JjA'SON

fiAonths

Figure 21.

 

68

Mean seasonal variation of the total density of
juvenile calanoids and Diaptomus siciloides.
mean of lake stations; ....mean of Plum
Creek and the discharge canal; _3_3_3_3River.

 

 

NUMBER 1 UTE!

69

 

 

 

 

45].
3°. Juvenile
Colanoid
15‘
o "' \‘4 -
M J .l A 5
451
Bigptomus

301 stciloides
154

o ' . - \

 

70

Figure 22. Mean seasonal variation in the composition

of density of the major 200plankton groups.
Top stipling - rotifers; middle clear area -
c0pepods; bottom stipling - Cladocerans.

 

71

 

 

 

 

 

 

 

 

72

Figure 23. Mean seasonal variation in the composition

of biomass of the major zooplankton groups.
Top stipling - rotifers; middle clear area -
copepods; bottom stipling - Cladocerans.

 

 

 

 

 

 

 

 

 

 

 

 

73

 

 

 

 

 

 

 

 

74

Table 7 : Number and Lengths of the Two Fish Species Used for
Stomach Analysis

 

 

 

Perch White Bass
Number of Mean (cm.) Number of Mean (cm.)

Date Stomachs Length Stomachs Length
21 May 70 5 9.6 - -
12 June 70 10 11.8 — -
25 June 70 6 3.4 - -
7 .luly 70 23 3.8 — -
22 July 70 11 4.9 7 4.2
4 August 70 5 6.7 5 4.4

16 September 70 6 8.6 14 7.2

 

75

compositional differences contributed to the variations in total
biomass and density observed at these stations.

During the summer, cladocerans dominated the biomass in the lake
(70-90%) but comprised only 30% of the total lake density. In turn,
rotifers comprised <1% of the biomass yet 20—30% of the total density.
This contrasted greatly with the inshore areas where cladocerans
comprised a significantly lower percentage of the density and biomass
and both rotifers and copepods were consistently more important. Cope-
pods were consistently more common than rotifers in the river during
the summer. Cladocerans were virtually non—existent.

It is apparent that the distributions of total density and biomass
are greatly affected by composition of the major taxa. This, in turn,
is assumed to be influenced by the major physical or biological charac-

teristics of the area.

Vertical Distributions

 

Zooplankton Density

 

In the lake, significant density differences (p < .05) between
depths (0.5 m and 2.5m) occurred on two dates, August 4 and September 1
(Table 5). A difference of at least 30% between the depth means was
usually required to detect a significant difference (p < .05). Nearly
every lake station had a greater density at the lower depth during late
summer but the difference was usually too small to detect with any
confidence. The inshore stations did not have any significant depth
differences.

When compared to .5m, there was a strong tendency for higher den-
sities at 5.5m in the lake during July and August but this was not

apparent during the fall. The discharge canal had lower densities

76

I mmm mqm I om cm I mmH mmH I mm em m

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I I Hun I I mam I I mmw I I NNHH m
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mzoaoo mcuaon mzumoo mcuaoa COfiHMum

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.somm xenon ecu cw mcowumom mom msummm ucowmwwflm um mouwa you couscmHQOON mo mumnfisz "m oanmw

77

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78

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. A.u.coov um. mH£mH

79

.mfiummv am.N mam m.o cmmBumn Amo.v av moocoumWMHp osmonwcmea

I we no I cum mmm mm mm mm a

I How own I NMH wMH mMH maH qu w

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I qm qu I qHH NNH com com mom N

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mfiuaoo onumon msuaom QOHHMum

ommH ponao>oz n ohmH Hoaouoo mm ONmH HoAOuoo OH

A6253 ”m oHan

80

at 5.5m on all eight dates this depth was sampled. Although sampling
was not replicated at the 5.5m depth, the values obtained were more
than 30% less than the means at .5m on four dates. When compared to
2.5m, 5.5m densities were inconsistently different in the lake and
river. However, again the discharge canal showed a strong tendency for

lower densities at 5.5m.

Zooplankton Biomass

 

ten significant differences between .5m and 2.5m occurred, the

lower depth always had greater biomass concentrations. A difference of
at least 30% was usually enough to detect significance. At the majority
(: 4) of the lake stations, 2.5m had significantly greater concentrations
from June 11 to September 15 with the greatest difference occurring on
August 4. At Plum Creek and the discharge canal, significant depth
differences occurred less frequently than in the lake (only 4 dates)
and in the river it was negligiable (1 date) (Table 6).

When compared to .5m, biomass concentrations were greater at 5.5m
for the majority of the lake stations (4 of 6) on all the eight dates
the latter depth was sampled. The discharge canal had trends similar
to the lake and the river had inconsistent but usually higher concen-
trations at 5.5m. When compared to 2.5m, biomass concentrations in the
lake were greater at 5.5m in the summer but not in the fall and the
discharge canal and the river were inconsistent.

The fact that significant differences in biomass between depths
(.5m and 2.5m) was much more common than differences in density (9 dates
to 2 dates), indicates that differences in the vertical distribution
of a few large Species were primarily responsible for the depth differences

encountered. Indeed, Q, retrocurva accounted for 60-100% of the biomass

 

81

I mm. on. I NC. co. I so. No. I No. no. m

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I om.H me. I oo. OH. I mo. no. I no. No. N
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am.m Em.N am.o Em.m am.N am.o am.m am.N Em.o 8m.m Em.N am.o

msunmn mnumoo mnummn mfiumma aOHumum

23 65s S 22 so: 8 23 sex 3 23 .6: N

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82

OH. mH. ON. mo. co. co. HO. {HO. Om. mm. Nm. m
mm.H aqO.H mq. HO.H on. He. mO.H an. mo. Nm.H mq.H m
I I «m. I I mm. I I Oq. I mN. 5
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on.H «NN.N mm. mO.H ¥ON.H Om. MN.H «HO. «O. «Nm.H mm. m
mm. «em.m OH.H mm.H «HN.H Nq. OO. «Hm. NN. «OO.H am. q
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qO.H *NH.H mO. OH.N «HN.H ON. mm. wH. ON. ON. On. N
Om.H «mm.H HN. mm.H «so. ON. OO.H mN. «m. emu. NH.H H

Em.m Em.N 8m.O Em.m Em.N Sm.O Bm.m Em.N Em.O 8m.N Em.O

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A.u.coov "O oHan

83

00.

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84

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85

difference on 6 of the 9 dates when depth significance occurred. On
two of the remaining dates it comprised 30-50%. Also contributing

somewhat were 9, vernalis in late spring and C, sphaericus in the late

 

summer .

Diurnal Change in Vertical Distribution

Depth differences between day and night determined by a single set
of samples at three stations suggest that limited migration occurs in the
shallow waters of the study area (Figure 24). The most obvious changes
in vertical distribution from day to night occurred in June when the

larger species, such as Q, retrocurva and g, vernalis were most abundant.

 

The tendency was for movement from the lower depths to the upper depths
in the evening. If anything, rotifers and copepod nauplii tended to

move downward (or disperse) at night.

Predation
Fish predation on 200p1ankton has been accorded to be an important
regulator of zooplankton composition. To appraise the potential effect
of fish predation upon the 200plankton, the stomach contents of two

common species (young of the year) were examined (Perca flavescens,

 

Roccus chrysops) (Table 7). Table 8 consists of ratios computed as

 

the percent each species made of the total stomach contents divided by

the percent that it made up in the water, considering only the species

found in the stomach. Therefore, a ratio of >1 indicates species

selectivity by the fish and a ratio of <1 indicates the Opposite.
Analysis of these fish feeding habits indicates a selection for

zooplankton that is dependent upon the relative density and size of

the organisms. Comparisons between species density graphs and percentage

of stomach contents (Figure 25) reveals that the highest stomach

+2!

 

"tJ

 

Figure 24.

86

Comparison of day — night vertical distributions
at three representative stations. Light bar —
day samples; dark bars - night samples; top

pair of light-dark bars - night samples; top pair
of light-dark bars — .Sm; middle pair of light—
dark bars - 2.5m; bottom pair of light-dark

bars — 5.5m.

87

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Table 7: Number and Lengths of the Two Fish Species Used for

88

Stomach Analysis

 

 

 

Perch White Bass
Number of Mean (cm.) Number of Mean (cm.)

Date Stomachs Length Stomachs Length
21 May 70 5 9.6 - -
12 June 70 10 11.8 - -
25 June 70 6 3.4 - —
7 July 70 23 . 3.8 - -
22 July 70 11“ 4.9 7 4.2
4 August 70 5 6.7 5 4.4
16 September 70 6 8.6 14 7.2

 

4 fl
.L- was; '

 

 

 

 

 

 

mo. H ooo.HA H H. monsouaom OH

 

Ho. Hm N N we. uuewsa m
No. HN N. . H N. NHes NN
mmmm ouHLB

Hoo.v Hoo.v coo.HA H Hoo.v eH uoonueom
H. m. s. m m. m uoawsa
H. mH N. N N. NN NHos
N. s NH coo.HA m. N NHss
so. m a. coo.H. m.N mN ooze

9 llllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
R. No. m. N. oso.H m.e NH ores
Hoo.v ooc.H N. H o.sH HN No2
.Om mcHEmom moOHOHHon .O mHHmcuo> .O mmuomme .w .0 m>msoonuou .O moon

moHooOm aeoxcmHOoom

 

mzomEOom comma

.mmuma oxu :H OomHuOEou uH
RoncoO HmuoH ego mo ucoO pom mzu mom mucmucou nomEoum Hmuoy ecu O0 mom: moHomOm Loom
ucoO use map cooauom oHomm map on pommoumxm onooOm COuxcdeoom :Hmuwmu mo RuH>HuuwHom "O OHOOH

90
percentages of particular prey species are associated with peak den-
sities in the water. However, species size is also a factor. Selec—
tivity ratios indicate a high preference for D. g, mendotae (large
size) and low for Bosmina sp. (small size). No rotifers were found

in any of the stomachs. Leptodora kindtii was the most selected of

 

all 200plankton species. Preference for the other species was depen-

dent upon each of their seasonal densities, although 2. siciloides

 

appeared to be differentially selected; especially by the White Bass.
In the fall, age 0 perch were 8.6 m long and fed almost exclusively
from the bottom (chironomids). In the spring of the following year,

‘ age I fish were feeding heavily on Q, retrocurva (90%). This suggests

 

shifts in feeding habits that are not entirely based on fish size alone
but also upon the density of preferred prey species.
Other predators upon the zooplankton are zooplankton themselves;

Leptodora kindtii, Cyclops vernalis, and Asplanchna sp. The first

 

species was heavily selected lnrthe fish thus probably reducing

its effect on the z00p1ankton. Little is known of the feeding habits

of g, vernalis, but since it had similar densities at all stations it is
unlikely to be responsible for any major station differences in total

biomass or density. The large rotifer, Asplanchna sp., preys on the

 

smaller rotifers, but ratios between the densities of the two (Asplanchna

 

sp./total rotifers) were similar at all areas, therefore, it was not

responsible for any compositional differences between the areas.

Chemical and Physical Regulators of Zooplankton Density

 

Multiple regression was employed to estimate the relationship
between various variables and zooplankton densities. Dependent

variables were expressed as total number per liter of either rotifers,

 

 

91

 

Figure 25. Mean seasonal variation in the composition
of 200p1ankton in the stomachs of young
perch and white bass. ___ Daphnia spp.;
...Cyclops vernalis; _,_3_,_,Diaptomus siciloides.

 

 

 

92

 

 

 

 

 

 

\
.\.
.\
.\
o
7 n
w
.i E5
/ .. In
9 a I
6 s H
.9: w
u L h L H h .h b h b
mww m u o ”.0. w w m
thuhZOU 29(20hm

._<._.Oh 50 m¢<h2uusmn

MON THS

93

cladocerans, copepods, or total zooplankton. Independent variables
included temperature, oxygen, particulate organic carbon, and inorganic
suSpended solids. Regression coefficients for each dependent variable
were significantly different from zero (p < .001). However, the contri-
bution of all four independent variables to the regulation of zooplankton
densities accounted for less than half of the variation encountered. 1v
Partial regression coefficients suggest which factors were more impor- j?

tant to the total relationship within each dependent variable (Table 9).

 

Densities of cladocerans, copepods and total 200plankton were

 

highly correlated with temperature but not rotifers. However, the

differences in temperature among stations at any particular time were

 

negligible so that the spatial variation noted in these groups should
not be caused by thermally regulated processes. The relationship be-
tween rotifers and inorganic suspended solids appears coincidental.
Rotifers happened to be most abundant in the spring and fall when
inorganic seston had high concentrations. Also, when utilizing this
relationship to explain spatial differences, inconsistencies arise.
The river had high inorganic seston concentrations but low rotifer
densities.

Particulate carbon contributed little to the total relationship
in any of the dependent variables. Some relationship would be expected
simply because the zooplankton themselves contribute to particulate
carbon values. Either particulate carbon is a poor estimator of food
availability or food is relatively unimportant in regulating the 200plank—
ton distributions in the study area.

Oxygen concentrations were correlated more with Cladoceran densities
than with the other dependent variables. Oxygen concentrations were lowest

in the river, particularly during the summer and early fall. Cladoceran

 

 

 

mam. wee. emm. Hem. mom. couxcmeaooa Hmuoe

 

Noo.- mee. mom. was. mom. maoawaoo
MN ©NN.- Noe.u amm. was. com. msmpmooemeu
mom. awe. nae. NmN. NNm. mummauom
weeeom zomm<u
amazmamsm qu<omozH onaoao meaqzuHem<m zmoexo ampeammmzme a<eoe mmamaem<> ezmazmmmo

mmgmaflyfig HZMDmemQZH

.mmanmwum>
unopcmmmpcfl paw ucmpsmamp mDOflum> msu sufl3 pmumfioommm muswflowwwooo coammmsmmw Hmwupmm "m maan

95

densities were also low in the river at this time while they were
abundant at the other areas where oxygen was plentiful. Although
densities of rotifers and copepods were also low in the river at this
time, the overall difference in their densities was less than that

of the Cladocerans. River discharge could also conceivably be the cause
of low river densities by washing individuals out of the area. Indeed,
in the spring, there is an inverse relation between river discharge

and densities (Figure 26). However, during the summer, discharge

rates were similar to water movement in the lake and should be

discounted as a cause of low densities at that time.

 

96

 

Figure 26. Seasonal relationship between the density of
200p1ankton in the river and river discharge.

total zooplankton density; ....river
discharge.

 

 

97

Uill'l IHEQWON

 

 

 

G O o

o 0 o

n a ..
,1/7'.

C5 rs uh

m n N
(aestgwrasua aaAIa lewu

MONTHS

DISCUSSION

Regulation of ZOOpIankton Distributions

 

Variation in the density and biomass of zooplankton along the
western shore of Lake Erie can be related to the variation in three
parameters particularly; primary productivity, fish biomass and
density, and oxygen concentrations. Relatively subtle and inconsis-
tent distributional patterns occurred in the lake but the inshore areas;
a newly dug discharge canal, a shallow creek embayment, and a polluted
river all varied from the lake and from each other.

Hypothetically, zooplankton biomass is linearly dependent upon
food supply (Slobodkin, 1954; Hall, Cooper and werner, 1970). The
discharge canal had the greatest potential source of food while the
river and lake had lower food potentials. However, zooplankton biomass
in the discharge canal was lower than in the lake (but higher than
in the river) except in the Spring when rotifers were dominant through-
out the study area. Also, although the river had food potentials
similar to the lake, it had much lower biomass concentration except
in June and late October. Apparently, food is a regulator of zooplankton
biomass only in conjunction with other regulators. Other potential
regulators include temperature, predation and oxygen.

Temperatures changed uniformly throughout the study area. There-
fore, the spatial differences in density and biomass encountered cannot
be explained by any differential Species response to thermal variation.

However, seasonal changes in the zooplankton were definitely related

98

99

to thermal changes. Densities of cladocerans and copepods particularly
appear to be temperature dependent as indicated by Hazelwood and
Parker (1961). Rotifer densities, on the other hand, were much less
related to thermal change which agrees with the observations of Davis
(1962). But even in the larger forms, seasonal relations to temperature
could explain less than half of the variation found in the study area.

Size-selective fish predation can have a substantial effect on
200plankton densities and biomass (Brooks and Dodson, 1965; Hall,
Cooper and Werner, 1970). Assuming that the larger zooplankton are
more efficient at converting food to biomass (Odum, 1959) intense
predation on the larger forms should decrease total biomass. Removal
of large, efficient zooplankton would allow the small, less efficient
forms to become more dense and perhaps increase the total density,
but they would realize a smaller biomass than zooplankton standing crops
dominated by the larger forms. Since predation, as indicated by fish
abundances, was greatest in the discharge canal, intermediate in the
lake, and negligible in the river, observed differences between these
areas fit this pattern well. Mean size of zooplankton was lowest in
the discharge canal where fish predation was high and greatest in the
lake where fish predation was lower. However, these predator-prey rela-
tionships do not fit the expected pattern in the river. If no other
environmental factor is of consequence, mean size of zooplankton in
the river should have been the highest of all areas, for fish predation
was negligible, and total biomass should have been equal to the lake,
for food availability was similar in the two areas. This variation
from the expected could potentially result from: (1) proportionately
'more zooplankton predation in the river, (2) differences in the quality

of food produced, (3) differences in habitat quality which includes

 

100

oxygen concentrations, water velocities and toxins.

Differences in ZOOplankton predators are not likely to be respon-
sible. With the exception of Q, vernalis, none of the potentially
important 200plankton predators were disprOportionately more abundant
in the river. 9, vernali§_was relatively more abundant only in June.

Compositional data on phytOplankton are unavailable but although
food quality may contribute to the distributional pattern observed, it
is likely that the river environment alone can account for much of the
variation. In the Spring, when river discharges were relatively high
and unstable, 200plankton densities were negatively related to the
discharge. However, during the summer and early fall, no correlation
of river discharge and density was noted, yet the river had the lowest
biomass of all areas. Also, the fact that biomass concentrations were
comparable to the discharge canal and the lake in late spring and
late October suggests a factor operating only during the summer months.
This factor is very likely low oxygen concentrations directly or some
toxic by-products associated with anaerobic conditions. Low oxygen
concentrations do inhibit the larger plankters, cladocerans and c0pepods
(Hazelwood and Parker, 1961). Although nothing is known about the
relation of size to oxygen requirements in 200plankton it is conceivable
that larger plankters are more susceptible to low oxygen concentrations
than the smaller ones. This could explain why the mean size was less
than in the lake even though predation was much lower.

Differences in the structure of particular ZOOplankton pOpulations
can also be explained by variations in food, predation and oxygen. The
densities of copepod nauplii and juveniles, in relation to the densities
of adults, was diSproportionately high in the discharge canal and low

in the river when compared to the lake. Less obviously but similarly,

101

the ratio of immature and mature D, retrocurva was lowest in the river.

 

For the copepods, this phenomena could be attributed to: (1) selective
predation on the adults in the discharge canal but not in the river,
(2) higher reproductive rates in the discharge canal than in the river.
Selective predation by fish on c0pepod adults probably was density
dependent. In June and early July, when the greatest difference in
population structure was noted between the two areas, adult c0pepods
were very abundant. Since predation was relatively great in the dis—
charge canal and low in the river, the obvious conclusion is that
predation is the primary factor for the differential population struc-
ture. However, this does not account for the tremendous density of
nauplii found in the discharge canal or low numbers found in the river
(when compared to the lake). Edmondson, §£_§l, (1962) found that copepod
reproductive rates were positively related to food supply. This accounts
for the high number of nauplii in the discharge canal. However, since
nauplii densities were lower in the river than in the lake although
both had similar food supplies, this would indicate that reproductive
capacities in the river were reduced by some abiotic factor, such as
oxygen availability.

The subtle variations in the vertical distribution of zooplankton
in the study area also seem to be related to the mean plankter size.
Larger species accounted for most of the difference in biomass found
at the different depths. If fish predation strongly selects larger
forms of 200plankton, the evolutionary advantage would be for the
large plankters to avoid the illuminated surface waters and thus avoid
sight-feeding fish (McLaren, 1965).

In summary, in western Lake Erie, except where abiotic conditions

are intolerable, the distribution of zooplankton appears to be regulated

102
by the variation in food availability and predation as hypothesized

by Brooks and Dodson (1965).

(‘1 ,-

unanges in Western Lake Erie

 

Data comparisons between this study and previous ones on western
Lake Erie are assumed valid Since similar netting was employed. Several
significant changes have apparently occurred within the last several

decades. The peak of Q, sphaericus in late summer can be associated

 

with a tremendous bloom of blue—green algae (Microcystis, Aphanozomenon)
that occurred at the same time. Historically, this is not unusual

(Hutchinson, 1967). However, peak densities of g, Sphaericus found

 

in this study were the highest ever reported for Lake Erie. Chandler
(1940) did not even record this species from the western basin while
maximums during this study were six times greater than those reported
by David (1962) in the central basin.

Other zooplankton have shown various degrees of increase through-
out the years. Most data is summarized from Bradshaw (1964). The
trend for density increases in cladocerans and c0pepods is quite obvious
(Figures 27, 28), but not in rotifers (Figure 29). Also, the spring
and fall peaks of all these major groups has increased in intensity.
Comparisons of biomass to Davis' (1958) data were made by converting
his volume measurements to mg/liter. Indications are that zooplankton
biomass, from June to August, have increased 5-fold in 14 years.
Although his study was conducted on the east end of the basin, it is
unlikely that there is much difference in phytoplankton between the
two areas (Hartley and Potos, 1971) and therefore zooplankton densities
are likely to be similar. In summary, the western basin has increased

in standing crop of 200p1ankton with apparently most of this production

103

 

Changes in the total number of cladocerans in Lake
Erie from 1939 to 1970. ....Chand1er (1940);
----Verduin (1949), Hubschman (1960); _3._3._J._3.
Davis (1962); this study.

Figure 27.

 

 

104

463

 

 

 

240«

180‘

120~

60‘

an»: ~ Cumin-Z

MONTHS

105

Figure 28. Changes in the total number of copepods in
Lake Erie from 1939 to 1970. ....Chandler

(1940); --——Verduin (1949); _3_3_3_3 Davis
(1962); this study.

106

 

 

‘ 320*

240~

80‘

O
6
1

sub—.— ~ awniaz

MONTHS

107

Figure 29. Changes in the total number of rotifers in
Lake Erie from 1939 to 1970. ._,_3_3_Chandler
(1940); ....Davis (1954); ----Davis (1962);
this study.

./' man

NUMBER

700*

500-

300~

100'1

 

108

 

 

MONTHS

109

being channeled through the larger forms (cladocerans and c0pepods).

Most studies of the diet of young perch in the Great Lakes have
been concerned with samples taken during a limited period in the summer
(Ewers, 1933; Turner, 1920; Tharatt, 1959). Only Price (1963) has
followed the seasonal changes in perch feeding habits. However, his
data on young-of—the-year fish is very limited. This study is unique
in following age 0 perch from spring to fall.

As found in previous studies, the young perch in this study fed
heavily upon the entomostraca, cladocerans and copepods. However,
in addition, species composition of the diet changed dramatically
during the sampling period, shifting to the particular species most
abundant at the time. Therefore, species preference tables, such as
those given by Ewers (1933), are not meaningful unless prey densities
are also given. This is especially true in areas where seasonal

composition is quite variable, as in western Lake Erie.

L ITERATURE C ITED

LITERATURE CITED

Andrews, T. F. 1948. Temporary changes of certain limnological
conditions in western Lake Erie produced by a windstorm.
Ecology, 29: 501-505.

Bradshaw, A. S. 1964. The crustacean zooplankton picture: Lake Erie
1939-49—59, Cayuga 1910-51—61. Verh. Int. Ver. Limnol., 15:
700—707.

Brooks, J. L. and Dodson, S. I. 1965. Predation, body size, and
composition of plankton. Science N.Y. 150: 28—35.

Carr, John F., Applegate, V. C. and Keller, Myrl. 1965. A recent
occurrence of thermal stratification and low dissolved oxygen
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Chandler, D. C. 1940. Limnological studies of western Lake Erie, I.
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Chandler, D. C. and Weeks, 0. B. 194 . Limnological studies of western
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Davis, C. C. 1958. An approach to the problem of secondary production
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Davis, C. C. 1962. The plankton of the Cleveland harbor area of
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Edmondson, W. T. 1960. Reproductive rates of rotifers in natural
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Edmondson, W. T., Comita, G. W., and Anderson, G. C. 1962. Repro-
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110

111

Ewers, L. A. 1933. Summary report of crustacea used as food by the
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FHPCA, 1969. FWPCA methods for chemical analysis of water and wastes.
280 p.

Galbraith, b. F. Jr. 1967. Size selective predation of Daphnia by
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Hall, D. J. 1964. An experimental approach to the dynamics of a
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Hall, D. J., Cooper, W. E., and Werner, E. E. 1970. An experimental
approach to the production dynamics and structure of freshwater
animal communities. Limnol. Oceanogr. 15: 839-928.

Hartley, R. P. and Potos, C. P. 1971. Algae—temperature-nutrient
relationships and distribution in Lake Erie 1968. EPA Water
Quality Office Region V report. 87 p.

Hazelwood, D. H. and Parker, R. A. 1961. Population dynamics of
some freshwater zooplankton. Ecology, 42: 266-274.

Hubschman, J. H. 1960. Relative daily abundance of planktonic

crustacea in the island region of western Lake Erie. Ohio J.
Sci., 60: 335—340.

Hutchinson, G. E. 1967. A treatise on limnology, V. 2. Introduction
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McLaren, Ian A. 1963. Effects of temperative on growth of 200plankton
and the adaptive value of vertical migration. J. Fish. Res.
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Nauwerk, A. 1963. Die Beziehungen zwischen 200p1ankton und phytOplankton
um see Erken. Symb. Bot. Upsal. 17(5): 163 p.

Odum, E. P. 1959. Fundamentals of Ecology. W. B. Saunders Company,
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Pacaud, A. 1939. Contribution a'l'Ecologie des Cladoceres. Bull.
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Parkhurst, B. R. 1971. The distribution and growth of the fish pop-
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Pennak, R. W. 1955. Comparative limnology of eight Colorado mountain
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112

Price, J. W. 1963. A study of the food habits of some Lake Erie
fish. Ohio Biol. Surv., Bull. N.S., 2(1): 1-89.

Revers, O. 1969. Seasonal variation of the biomass and biocoenotic
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Verh. int. Ver. Limnol., 17: 237-254.

Ructner, 1963. Fundamentals of Limnology. University of Toronto
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