A Lakewide Comparison of Zooplankton Biomass and Its Species Composition in Lake Erie, 1983–87

J. Great Lakes Res. 19(2):275-290 Internat. Assoc. Great Lakes Res., 1993

A Lakewide Comparison of Zooplankton Biomass and Its Species Composition in Lake Erie, 1983-87 Joseph C. Makarewicz Department of Biological Sciences State University ofNew York at Brockport Brockport, New York 14420

ABSTRACT. From 1983 to 1987, 118 species representing 53 genera from the Calanoida, Cladocera, Cyclopoida, Rotifera, and Harpacticoida comprised the offshore zooplankton community of Lake Erie. Twenty-eight common species plus their juvenile stages accounted for 94.4% of the total biomass and 92.5% of the total abundance. The eutrophic indicators Brachionus caudatus, B. calyciflorus, B. angularis, Filinia longiseta, Trichocerca multicrinis, and Trichocerca cylindrica had abundances restricted to or significantly higher in the western basin. Between 1983 and 1987, essentially smaller species (Daphnia galeata mendotae, Daphnia retrocurva, Eubosmina coregoni, Bosmina longirostris, and Diaphanosoma leuchtenbergianum) were dominant as in 1948 and 1970. In 1984 a large cladoceran new to Lake Erie, Daphnia pulicaria (mean = 755 individualslm 3, maximum abundance of 3,752Im3 ), became prevalent. Another cladoceran, Bythotrephes cederstroemi, was first observed in Lake Erie in 1985 and continued to be present in 1986 and 1987. Cyclops vernalis, present throughout the lake in 1967, was observed only in the western basin during 1983 to 1987 similar to the distribution in the 1930s. Rotifer composition in 1967 and the 1983-87 period was similar. Cladocera abundance during the 1983-87 period was comparable to abundances observed in the '30s and '40s. Zooplankton abundance and biomass decreased from the western basin to the eastern basin except in 1985. This decrease correlated with the decrease in phytoplankton biomass (r = 0.81). INDEX WORDS:

Lake Erie, zooplankton, species composition, population density, trends.

for complementing phytoplankton data to assess the effects of water quality (Gannon and Sternberger 1978) and fish populations on biota (e.g., Brooks and Dodson 1965). In this investigation, the abundance, biomass, and composition of the offshore zooplankton community were determined from 1983 to 1987 in all basins of Lake Erie. The primary objectives were: 1) to characterize the composition, abundance, and biomass of the offshore zooplankton community from 1983 to 1987 for comparison with past conditions to the extent they are known; 2) to provide firm documentation with which future assessment of the changes in water quality of the lakes can be made; and 3) to characterize the present water quality by studying the abundance and autecology of zooplankton.

INTRODUCTION Changes in ecosystem health can be detected by monitoring changes in the biotic community (Nicholls et al. 1980). Typical perturbations include the addition of nutrients stimulatory to phytoplankton production, toxic substances, and exotic fishes. The zooplankton community in such systems respond particularly to changes in food resources and to selective predation by changes in community structure. Whether aquatic ecosystems are perturbed from the top downward or stimulated from the first trophic level upward, the zooplankton are sensitive integrators of such changes (McNaught and Buzzard 1973). Thus zooplankton have value as indicators of water quality and the structure of the biotic community. They have also proved useful

275

276

J. C. Makarewicz

METHODS A Wildco Model 30-E28 conical style net (62-J..UIl mesh net; D:L ratio = 1:3) with 0.5-m opening (radius = 0.25m) was used to collect a vertical zooplankton sample at several stations during 33 cruises during the spring, summer, and autumn from 1983 to 1987 ( See Table 1 and Fig. 1 in Makarewicz 1983 for cruise dates and for locations of sampling stations). Generally, only data from the unstratified spring period and the stratified summer period are presented here. Vertical tows were taken from 20 m to the surface. At stations where water depth was less than 20 m, the toW was taken from 1 m above the bottom to the surface. Filtration volume was determined with a Kahl flow meter (Model 00SWA200) mounted in the center of the net. Following collection, the net contents were quantitatively transferred to 500-mL sample bottles, narcotized with club soda, and preserved with 5% formalin. Enumeration of zooplankton follows Gannon (1971) while identification follows Sternberger (1979) and Edmondson (1959). The volume of each rotifer species was computed by using the geometrical shape that most closely resembled the species (Downing and Rigler 1984). For each cruise, length of at least 20 specimens of each rotifer species was measured. Width and depth were also measured on one date for each lake to develop length-width and length-depth ratios for use in the simplified formulas of Bottrell et aI. (1976). Assuming a specific gravity of one, volume was converted to fresh weight and to dry weight assuming a ratio of dry to wet weight of 0.1 (Doohan 1973) for all rotifer species except AspIanchna spp. A dry weight/wet weight ratio of 0.039 was used for AspIanchna spp. (Dumont et aI. 1975). Because of the considerable variability in length and thus weight encountered in the Crustacea, the dry weights of Crustacea were calculated using length-weight relationships (Downing and Rigler 1984, Makarewicz and Likens 1979). Average length of crustaceans (maximum of 20 for each station) was determined for each station of each cruise. A comparison of calculated weights to measured weights of individual Crustacea in Lake Michigan suggested good agreement at the minimum weight range (Makarewicz 1988). The weight of the Copepoda nauplii followed Hawkins and Evans (1979). The zooplankton data were computerized. Statistical evaluations and other data manipulations were conducted within the INFO (Henco Software, Inc.,

100 Fifth Avenue, Waltham, Mass.) data management system. To allow east to west comparisons, data from stations on a north south axis were averaged to give one point. For example, data from Stations 36, 37, and 38 (Fig. 1 in Makarewicz 1993) were averaged to form one point on an east to west transect reported as Station 37 in Figures 1-5.

RESULTS AND DISCUSSION Annual Abundance and Geographic Distribution of Zooplankton Groups Mean weighted density and biomass for 1983 1987 (spring and summer) were 322 organisms/L ± 16 (mean ± S.E.) and 87 mglL ± 5.5 (mean ± S.E.) (Tables 1 and 2). From 1983 to 1987, 118 species representing 53 genera from the Calanoida, Cladocera, Cyclopoida, Rotifera, Amphipoda, and Harpacticoida comprised the offshore zooplankton community of Lake Erie (Table 3). The number of species was greatest in the western basin and decreased to the eastern basin (Table 3). Twenty eight common species plus their juvenile stages accounted for 94.4% of the total biomass and 92.5% of the total spring-summer abundance for the 1983-87 period (Table 4). Statistics for common species, averaged by basin, are presented in Tables 5-7. The Rotifera contained the greatest number of species (65, Table 3) and the highest relative abundance (81.2%, Table 8), but accounted for only 16.4% of the biomass. However, this relative percentage was much higher than that observed in Lake Huron (2.5%) and Michigan (2.7%) (Makarewicz et aI. 1992). The Calanoida, Cyclopoida, and the nauplius stage of the copepod represented 16.6% of the total zooplankton abundance in Lake Erie (Table 8). The Cladocera (30.6%), the Cyclopoida (19.0%), and the Calanoida (18.5%) contributed the most biomass to the zooplankton community. Considerable variability in abundance and biomass of various groups occurred during the 1983-87 period, and no obvious trends were observed during this time. For example, Rotifera biomass and abundance were considerably higher, and Cladocera abundance and biomass were lower in 1986 than in other years (Tables 1, 2, and 8). Zooplankton abundance (Fig. la) and biomass (Fig. Ib) decreased from the western basin to the eastern basin, except in 1985 when no obvious pattern existed (Fig. 1b). This decrease was correlated with a similar decrease in phytoplankton biomass (r = 0.81) observed by Makarewicz (1993). The higher abundance of Rotifera and the higher biomass of Cladocera in the western basin are the

Zooplankton Species and Biomass in Lake Erie, 1983-87

277

TABLE I. Time trends in zooplankton abundance of selected groups in Lake Erie, 1983-1987 (spring and summer data only). ND = No data. CAL = Calanoida, CLA = Cladocera, CYC = Cyclopoida, COP = Copepoda nauplii, ROT = Rotifera. **Weighted mean that considers number of stations sampled and all taxonomic groups (e.g., Mysidacea). Mean abundance is ofall taxonomic groups.

- - - .----

---.

CAL #/L

CLA

COP

CYC

%

#/L

19&~

5" .'-1

1984 1985 1986 1987 Mean

3.2 4.2 3.8 4.4 4.2

0.8 1.0 0.8 1.4 1.0

17.7 8.7 9.2 6.2 16.7 10.7

2" 2.1 2.2 1.3 5.4 2.7

394 Tq 28.6 6.9 47.7 1l.5 34.8 7.1 39.1 12.7 37.9 9.2

ht) 4.0 10.1 5.7 6.9 6.7

I ~ 1.0 2.4 1.2 2.3 1.6

J431 I 371.8 343.3 442.6 240.6 367.1

89.3 82.8 89.8 78.2 85.4

50.14 416.4 415.0 493.1 307.7 **420.5

CENTRAL BASIN 1983 1984 1985 1986 1987 Mean

11.3 6.7 14.8 7.2 5.6 9.1

3.6 6.1 5.4 1.5 2.1 3.7

13.3 4.1 9.2 1.4 2.8 6.2

4.3 3.7 3.4 0.3 1.0 2.5

65.1 17.3 42.0 38.0 35.8 39.6

20.8 15.7 15.4 7.8 13.5 14.6

21.4 6.9 13.1 7.6 7.1 11.2

6.8 6.3 4.8 1.6 2.7 4.4

201.7 75.1 193.4 435.8 214.4 224.1

64.5 68.3 71.0 88.9 80.7 74.7

312.8 110.0 272.4 490.0 265.7 **310.6

EASTERN BASIN 1983 1984 1985 1986 1987 Mean

13.0 5.6 16.2 5.6 9.8 10.0

6.8 3.3 8.1 1.4 6.7 5.3

21.5 11.2 5.7 3.4 8.1 4.1 0.3 0.1 2.9 2.0 7.7 4.2

36.1 18.7 44.6 23.6 18.2 28.2

18.8 11.0 22.3 5.8 12.4 14.1

12.9 6.7 10.1 5.8 8.9 8.9

6.7 108.7 56.6 3.9 132.9 78.4 5.1 120.6 60.4 1.4 371.9 91.3 107.1 72.9 6.0 4.6 147.0 71.9 Mean for the entire lake

%

#/L

%

#/L

Mean Abundance

ROT

%

#/L _--_ #/L % _-_.----•..

..

WESTERNBAS1N

cause of this geographic pattern (e.g., Figs. 2 and 3). From 1983 to 1987, a number of species, especially rotifers, had higher abundances in the western basin (Fig. 4). Cyclops bicuspidatus thomasi and Diaptomus oregonensis had a maximum abundance in the central basin during the study period (Fig. 5). Daphnia pulicaria was prevalent in the central and eastern basins, while Holopedium gibbe rum and Ceriodaphnia lacustris were more prevalent in the eastern basin. Five species, Cyclops bicuspidatus thomasi, Conochilus unicornis, Daphnia galaeta mendotae, Daphnia pulicaria, and Diaptomus oregonensis had low abundances in the western basin relative to the rest of the lake. Numerous researchers have documented similar differences in species composition and abundance from the central, western, and eastern basins of Lake Erie (e.g., Davis 1969, Watson 1974, Patalas 1972, Gannon 1981).

~1

I

192.2 169.6 199.7 407.0 146.9 **227.1 **321.8

Historical Trends in Abundance Several historical studies of zooplankton in the western basin of Lake Erie are comparable with the 1983-87 work. Collections in 1939 (Chandler 1940; 49 collections), 1949 (Bradshaw 1964; 30 collections), and 1959 (Hubschmann 1960; daily collections July and August) were taken with a lO-liter Juday trap equipped with a 64-!J1l1 mesh net in the western basin. A 1961 study by Britt et al. (1973) included two collections per month from mid-June to mid-September, also with a 64-!J1l1 mesh net, while Davis (1968) used a 76-!J1l1 mesh net in July of 1967. Watson and Carpenter (1974) sampled all three basins in 1970 using a 64-!J1l1 mesh net in vertical hauls. A 1970 study by Nalepa (1972) is not included in this analysis because it is from the far western end of the basin and may not be representative of the entire western basin. A comparison of the April-October Crustacea

J. C. Makarewicz

278

TABLE 2. Time trends in zooplankton biomass (Ilg/L) of selected groups in Lake Erie, 1983-1987 (spring and summer data only). ND = No data. CLA = Cladocera, CAL = Calanoida, CYC = Cyclopoida, COP = Copepoda nauplii, ROT = Rotifera. **Weighted mean that considers number of stations sampled and all taxonomic groups. Mean abundance- is for all taxonomic groups. CAL

CLA %

f.lglL

%

ND

ND

ND

ND

7.2 9.2 10.1 10.0 9.1 14.1 8.9 5.9 8.8 9.8

27.2 31.7 8.1 94.7 40.4

34.8 31.4 12.5 62.9 35.4

f.lglL

WESTERN BASIN 1983 1984 1985 1986 1987

Mean CENTRAL BASIN 1983 1984 1985 1986 1987

Mean

COP

CYC

ROT %

f.lglL

ND

ND

ND

24.9 16.0 21.2 9.3 17.9

31.9 15.8 32.8 6.2 21.7

%

f.lglL

%

ND

ND

ND

ND

11.4 19.1 13.9 15.6 15.0

14.7 18.9 21.5 10.4 16.8

7.3 24.3 12.4 21.8 16.5

9.4 24.0 19.1 14.5 16.8

ND

f.lglL

Mean Abundance

f.lglL

ND

ND

ND

ND

ND

ND

ND

ND

ND

15.6 39.7 13.8 11.3 10.2

23.1 27.9 19.5 18.0 22.1

27.9 44.0 5.6 10.5 22.0

41.2 30.9 7.9 16.8 24.8

6.9 16.8 15.2 14.3 13.3

10.2 11.8 21.5 22.9 16.6

13.5 34.9 17.1 9.6 18.8

20.0 24.5 24.1 15.4 21.0

3.8 5.6 6.8 4.8 19.1 27.0 16.8 26.9 11.6 16.1

78.1 101.1 64.8 150.4 **100.9

ND 67.7 142.1 70.8 62.5 **87.2

EASTERN BASIN 1983 1984 1985 1986 1987

Mean

ND

ND

ND

ND

ND

ND

ND

ND

ND

1l.5 33.9 9.0 15.4 17.5

15.7 29.2 17.8 38.6 25.3

37.0 38.3 1.2 6.5 20.8

50.2 32.9 2.4 16.3 25.5

7.5 17.8 9.4 7.3 10.5

10.2 15.3 18.7 18.3 15.6

11.7 22.8 9.4 7.1 12.8

15.9 19.6 18.7 17.7 18.0

6.0 8.1 3.5 3.0 21.4 42.4 3.6 9.1 8.6 15.7

ND

Mean for the entire lake

ND 73.7 116.3 50.4 39.8 **70.1 **87.1

TABLE 3. Number of species and genera observed in each taxonomic group of zooplankton, Lake Erie, 1983-87. AMP = Amphipoda, CAL = Calanoida, CYC = Cyclopoida, H~R = Harpacticoida, ROT = Rotifera. NUMBER OF SPECIES AMP CAL CLA CYC HAR ROT Total NUMBER OF GENERA

1983 0 9 15 4 I

30 59 1983

AMP 0 CAL 5 CLA 11 CYC 4 HAR 1 ROT 15 Total 36 NUMBER OF SPECIES BY BASIN WESTERN AMP 1 CAL 9 CLA 26 CYC 9 HAR 4 ROT 59 Total 108

3 1 43 73

1985 0 8 15 7 1 45 76

1986 0 8 19 0 4 39 70

1987 0 8 13 3 2 44 70

1983-87 1 9 30 9 4 65 118

1984 1 4 10 3 1 16 35

1985 0 5 9 6 2 17 39

1986 0 5 12 4 2 18 41

1987 0 4 9 3 2 20 38

1993-87 1 5 15 6 2 24 53

CENTRAL

EASTERN

0 7 20 4 3 43 70

0 8 16 3 0 35 62

1984 1 8 17

Zooplankton Species and Biomass in Lake Erie, 1983-87

279

TABLE 4. Summary of common zooplankton species occurrence in Lake Erie during 1983-87. Biomass values are for 1984-1987 only. Summary is based on spring and summer cruises only. Species were arbitrarily classified as common if they accounted for ~l.O% of the total biomass, with the exception of rotifers. Rotifer species were considered common if they accounted for ~1.0% of the total abundance. Maximum Density (#/m 3)

Taxon

- -..

_,._-"-'~-'----~-

~-~----.~~--

..

ROTIFERA Ascomorpha ovalis Asplanchna priodonta Conochilus unicornis Gastropus stylifer Kellicottia longispina Keratella cochlearis Keratella crassa Keratella earlinae Keratella hiemalis Nothoica foliacea Notholca laurentiae Notholca squamula Polyarthra dolichoptera Polyarthra major Polyarthra vulgaris Synchaeta sp. Total

.. _._---,,-

%of Total Abundance

Mean Biomass (flg/m3)

%of Total Biomass

_ - - " . _ - ~ - - _

COPEPODA Copepoda-nauplii Cyclopoida Cyclopoid-copepodite Cyclops bicuspidatus thomasi Mesocyclops-copepodite Mesocyclops edax Tropocyclops-copepodite Tropocyclops prasinus mexicanus Calanoida Calanoid-copepodite Diaptomus-copepodite Diaptomus oregonensis Total CLADOCERA Bosmina longirostris Chydorus sphaericus Daphnia galaeta mendotae Daphnia pulicaria Daphnia retrocurva Diaphanosoma sp. Eubosmina coregoni Leptodora kindtii Total

Average Density (#/m 3)

270,395

36,383.1

11.31

11,779

15.66

27,348 17,980 14,157 14,584 14,065 5,697

4,559.4 1,608.1 916.9 876.2 522.4 425.1

1.42 0.50 0.29 0.27 0.16 0.13

2,789 5,642 1,252 3,899 159 426

3.71 7.50 1.66 5.18 0.21 0.57

39,549 42,227 19,032

865.6 4,257.7 2,013.2

0.27 1.32 0.63 16.30

0 5,566 6,954

0.00 7.40 9.24 51.13

9,029 50,919 60,152 3,752 35,662 21,621 20,735 5,341

691.7 383.0 2,198.0 115.6 1,588.7 648.9 1,177.1 103.9

0.22 0.12 0.68 0.04 0.49 0.20 0.37 0.03 -2.15

699 275 8,007 1,918 7,172 976 2,386 946

0.93 0.37 10.64 2.55 9.53 1.30 3.17 1.26 -29.74

97,548 52,038 805,564 83,185 37,007 206,149 253,562 127,297 126,708 120,055 93,031 1,181,725 154,581 102,788 1,215,683 568,620

5,834.4 2,170.3 32,982.8 3,342.4 4,399.6 19,438.9 14,803.0 3,249.8 6,091.2 7,088.2 4,262.3 30,311.7 5,890.4 6,327.6 54,649.0 37,525.5

1.81 0.67 10.25 1.04 1.37 6.04 4.60 1.01 1.89 2.20 1.32 9.42 1.83 1.97 16.98 11.66 74.08

108 2,885 558 88 50 63 679 96 149 226 334 685 204 627 2,250 1,174

0.14 3.83 0.74 0.12 0.07 0.08 0.90 0.13 0.20 0.30 0.44 0.91 0.27 0.83 2.99 1.56 -13.52

Total ~-~~-----~---~--------~~-

~-'---

--

--

92.52

94.39

280

J. C. Makarewicz

TABLE 5. Summary of common zooplankton species occurrence in the western basin of Lake Erie during 1983-87. Biomass values are for 1984-1987 only. Summary is based on spring and summer cruises only. Species were arbitrarily classified as common if they accounted for '2.0.1% of the total abundance or '2.1.0% of the total biomass, with the exception of rotifers. Rotifer species were considered common if they accountedfor '2.l.0% of the total abundance.

Taxon COPEPODA Copepoda-nauplii Cyclopoida Cyclopoid-copepodite Cyclops bicuspidatus thomasi Cyclops vernalis Mesocyclops-copepodite Mesocyclops edax Calanoida Calanoid-copepodite Diaptomus-copepodite Diaptomus ashlandi Diaptomus siciloides Total CLADOCERA Bosmina longirostris Chydorus sphaericus Daphnia galaeta mendotae Daphnia retrocurva Diaphanosoma sp. Eubosmina coregoni Leptodora kindtii Total ROTIFERA Ascomorpha ovalis Asplanchna priodonta Brachionus sp. Conochilus unicornis Keratella cochlearis Keratella crassa Keratella earlinae Nothoica foliacea Notholca laurentiae Notholca squamula Polyarthra dolichoptera Polyarthra major Polyarthra vulgaris Synmchaeta sp. Total Total

(~g/m3)

%of Total Biomass

8.95

12,778

14.80

3,620.7 717.4 382.3 858.6 782.5

0.86 0.17 0.09 0.20 0.19

2,484 2,504 2,173 1,336 5,489

2.88 2.90 2.52 1.55 6.36

13,630 11,409 3,717 9,302

442.4 1,522.3 539.9 763.8

0.11 0.36 0.13 0.18 -11.24

0 2,102 1,395 1,650

0.00 2.44 1.62 1.91 -36.96

9,029 50,919 21,410 35,662 17,318 20,735 5,341

915.9 1,381.9 1,106.5 3,433.5 1,123.2 2,263.2 336.5

0.22 0.33 0.26 0.82 0.27 0.54 0.08 -2.51

819 989 5,086 19,606 1,375 6,234 2,864

0.95 1.15 5.89 22.71 1.59 7.22 3.32 -42.83

73,545 52,038 540,369 97,065 110,636 253,562 127,297 120,055 93,031 373,879 154,581 37,145 1,215,683 468,620

6,203.9 2,535.7 10,115.5 4,742.3 18,041.7 14,102.3 5,407.1 17,972.0 8,661.8 50,780.7 10,777.3 4,428.8 85,110.7 84,657.7

1.48 0.60 2.41 1.13 4.29 3.35 1.29 4.27 2.06 12.08 2.56 1.05 20.24 20.13 76.95 --

119 2,639 177 85 52 587 156 571 712 1,106 363 404 3,365 2,409

0.14 3.06 0.21 0.10 0.06 0.68 0.18 0.66 0.82 1.28 0.42 0.47 3.90 2.79 -14.76 --

Maximum Density (#/m3)

Average Density (#/m3)

124,140

37,650.9

16,588 6,631 8,151 9,152 11,751

% of Total Abundance

----

90.71

Mean Biomass

94.57

281

Zooplankton Species and Biomass in Lake Erie, 1983-87

TABLE 6. Summary of common zooplankton species occurrence in the central basin of Lake Erie during 1983-87. Biomass values are for 1984-1987 only. Summary is based on spring and summer cruises only. Species were arbitrarily classified as common if they accounted for ?0.1% of the total abundance or -;d .0% of the total biomass, with the exception of rotifers. Rotifer species were considered common if they accountedfor -;d.0% of the total abundance. ._---------_._------Taxon COPEPODA Copepoda-nauplii Cyclopoida Cyclopoid-copepodite Cyclops bicuspidatus thomasi Mesocyclops-copepodite Mesocyclops edax Tropocyclops-copepodl te Tropocyclops prasinus mexicanus Calanoida Calanoid-copepodite Diaptomus-copepodite Diaptomus oregonensis Total CLADOCERA Bosmina longirostris Daphnia galaeta mendotae Daphnia pulicaria Daphnia retrocurva Diaphanosoma sp. Eubosmina coregoni Total ROTIFERA Ascomorpha ecaudia Ascomorpha ovalis Asplanchna priodonta Collotheca sp. Conochilus unicornis Gastropus stylifer Kellicottia longispina Keratella cochlearis Keratella crassa Keratella earlinae Keratella hiemalis Keratella quadrata Nothoica foliacea Notholca laurentiae Notholca squamula Ploesoma lenticulare Polyarthra dolichoptera Polyarthra major Polyarthra remata Polyarthra vulgaris Synchaeta sp. Trichocerca cylindrica Trichocerca multicrinis Trichocerca similis Trichocerca sp. Total Total

Maximum Density (#/m 3)

Average Density (#/m 3)

%of Total Abundance

Mean Biomass (llg/ m3 )

%of Total Biomass

270,395

39,207.4

12.62

12,367

16.26

25,087 17,980 14,157 14,584 7,077 2,266

5,262.7 2,356.4 1,035.5 917.4 487.4 383.2

1.69 0.76 0.33 0.30 0.16 0.12

3,054 8,263 1,356 3,497 151 388

4.01 10.86 1.78 4.60 0.20 0.51

39,549 31,110 19,032

950.2 4,707.1 2,968.8

0.31 1.52 0.96 18.76

0 6,085 10,527

0.00 8.00 13.84 60.06

4,976 32,381 3,752 17,916 21,621 14,584

560.2 2,473.9 149.4 667.2 555.2 758.3

0.18 0.80 0.05 0.21 0.18 0.24 1.66

681 10,022 2,526 2,151 979 948

0.89 13.17 3.32 2.83 1.29 1.25 22.75

64,418 97,548 21,365 59,214 805,564 83,185 37,007 206,149 247,145 42,167 126,708 39,641 103,484 43,064 1,181,725 34,790 88,623 70,770 34,024 543,460 337,159 46,969 52,521 6,380 27,744

2,130.5 6,490.7 1,972.8 3,493.8 38,423.2 3,999.3 5,894.4 22,883.3 15,666.5 2,227.3 10,372.3 3,070.7 4,234.3 3,330.1 31,551.6 2,532.6 5,169.3 6,042.2 859.7 45,391.8 25,393.1 1,009.7 1,847.4 330.5 1,115.3

0.68 2.09 0.64 1.12 12.37 1.29 1.90 7.37 5.04 0.72 3.34 0.99 1.39 1.07 10.16 0.82 1.66 1.95 0.28 14.61 8.18 0.33 0.59 0.11 0.36 79.05 99.47

3 123 3,506 8 670 99 66 79 737 66 249 197 139 240 737 375 179 632 18 1,892 908 68 123 9 33

0.00 0.16 4.61 0.01 0.88 0.13 0.09 0.10 0.97 0.09 0.33 0.26 0.18 0.32 0.97 0.49 0.24 0.83 0.02 2.49 1.19 0.09 0.16 0.01 0.04 14.66 -97.47

---_._---------

282

J. C. Makarewicz

TABLE 7. Summary of common zooplankton species occurrence in the eastern basin of Lake Erie during 1983-87. Biomass values are for 1984-1987 only. Summary is based on spring and summer cruises only. Species were arbitrarily classified as common if they accounted for 'CO.1% of the total abundance or ?l.O% of the total biomass, with the exception of rotifers. Rotifer species were considered common if they accountedfor 'CI.O% of the total abundance.

Taxon COPEPODA Copepoda-nauplii Cyclopoida Cyclopoid-copepodite Cyclops bicuspidatus thomasi Mesocyclops-copepodite Mesocyclops edax Tropocyclops-copepodite Tropocyclops prasinus mexicanus Calanoida Calanoid-copepodite Diaptomus-copepodite Diaptomus oregonensis Diaptomus siciloides Total CLADOCERA Bosmina longirostris Daphnia galaeta mendotae Daphnia pulicaria Daphnia retrocurva Diaphanosoma sp. Eubosmina coregoni Holopedium gibberum Total ROTIFERA Ascomorpha ovalis Asplanchna priodonta Collotheca sp. Conochilus unicornis Gastropus stylifer Kellicottia longispina Keratella cochlearis Keratella crassa Keratella earlinae Notholca squamula Plyaarthra major Polyarthra vulgaris Synchaeta sp. Total Total -------

Maximum Density (#/m 3)

Average Density (#/m 3)

% of Total Abundance

Mean Biomass (f..lg/m3)

%of Total Biomass

127,691

28,314.8

12.47

9,204

15.40

27,348 10,832 6,864 7,260 14,065 5,697

4,081.5 967.4 714.1 895.5 1,166.4 892.4

1.80 0.43 0.31 0.39 0.51 0.39

2,549 3,420 911 2,886 350 876

4.26 5.72 1.52 4.83 0.59 1.47

19,187 42,227 8,130 13,334

1,186.8 6,559.0 2,047.0 246.6

0.52 2.89 0.90 0.11 -20.73

0 8,594 6,393 95

0.00 14.38 10.70 0.16 -59.02

4,066 60,152 3,398 34,225 3,730 7,550 807

721.5 2,893.3 177.5 1,464.8 286.4 818.2 70.9

0.32 1.27 0.08 0.65 0.13 0.36 0.03 -2.83

593 6,921 2,828 3,586 482 1,008 767

0.99 11.58 4.73 6.00 0.81 1.69 1.28 -27.08

57,498 44,904 14,120 419,754 51,737 25,136 82,517 136,186 36,538 60,997 102,788 448,063 105,427

3,868.3 2,180.4 2,542.1 54,891.9 4,451.4 3,003.3 13,191.5 13,664.7 2,977.4 2,465.1 9,304.1 38,841.0 8,008.3

1.70 0.96 1.12 24.18 1.96 1.32 5.81 6.02 1.31 1.09 4.10 17.11 3.53 -70.20

59 1,752 6 877 131 37 40 655 90 51 888 1,718 281

0.10 2.93 0.01 1.47 0.22 0.06 0.07 1.10 0.15 0.09 1.49 2.87 0.47 -11.01 --

--

93.76

--

97.11

283

Zooplankton Species and Biomass in Lake Erie, 1983-87

TABLE 8. Relative abundance and biomass of zooplankton in Lake Erie. Spring and summer data only. The 1983-87 mean is weighted by the number of samples each year. Biomass was not calculated in 1983. - - -

Percent Abundance (Biomass) 1984 1985

1983 Rotifera

73.0

Cladocera

4.5

Copepoda nauplii

15.0

Cyclopoida

4.5

Calanoida

3.0

Harpactiocoida

<0.1

Amphipoda

0.0

1986

1987

89.6 (31.0) 0.6 (8.5) 7.2 (21.1) 1.4 (21.8) 1.2 (17.6) <0.1 «0.1) 0.0 «0.1)

78.9 (14.3) 2.7 (40.9) 13.1 (15.9) 2.9 (15.1) 2.4 (13.7) <0.1 «0.1) 0.0 (<0.1)

~ - - - _ . _ - _ . _ - - -

81.2 (14.2) 2.7 (41.6) 9.9 (11.5) 2.9 (15.7) 2.6 (16.9) <0.1 «0.1 ) 0.0 (0.0)

72.4 (6.0) 3.2 (31.4) 15.6 (13.6) 4.2 (23.4) 4.7 (25.6) <0.1 «0.1) 0.0 «0.1)

- - ' - - , - - - - - - _.... ,

40

800

o

LAKE ERIE

CAL

83-87

~------

81.2 (16.4) 2.2 (30.6) 11.3 (15.5) 2.8 (19.0) 2.5 (18.5) <0.1 «0.1) <0.1 «0.1)

LAKE ERIE

30

20 10 Q)

o

400

~

C

o -0

c

::J

0-+--+--+---+-+--l......L...jf--+-+-+-+---+JL+--+--I 60 57 91 55 92 42 73 37 78 31 79 18 15

o

1984. 1986. 1987

•

1985

9

20

UJ UJ

o

E o m

200

10

O-+--+--+-+----jf---+-I-+--+--+-+----j-+-L+--+--, Western

40

-........-. 01

:::t

150

CIl CIl

100

COP

::.t

"-./

<{

250

o

........... 30 (J)

200

.0

-::J

0 +--+--+---+--l-f.-4-+--+--+-+--l-.J-I-+-+----j

--.J

o

Central

Eastern

CYC

30

........."

o

20 10

E o

m

50

O-+--+--+-+----jf----H-+--+--+-+----jf---+-L+--+----j Central

Eastern

O-+--+--+--+-+-t-l--l-l--+-+--+--+-~-+--I

60 57 91 55 92 42 73 37 78 31 79 18 15

9

STATION

FIG. 1. Geographical distribution (abundance, biomass) of zooplankton of Lake Erie.

6057 91 55 92 42 73 37 78 31 79 18 15 9

STATION

FIG. 2. Geographical distribution (biomass) of zooplankton groups in Lake Erie from 1983-87. CAL = Calanoida, CYC = Cyciopoida, and COP = nauplii.

284

J. C. Makarewicz

75 50 r------.J

25

'-.... 0'

:::i.

0

0

Cyclops bicuspidalus thomasi

• V

Dioptomus - copepodite Diaptomus oregon~

~

~""

0

Eastern

Central

~

2

L

/""'..

--.J 40

E 0

o

4

Western

'--" (f) (f)

6

Htt1

LAKE ERIE

8

LAKE ERIE

100

30

m 20 10

o

~

ROT

~~T-N

::::J:l:::: '--.-/

Q)

U

::J

«

0.0

75

LAKE ERIE

""/0,,,,-° 0-.-0

o

Synchaela sp.

•

Kerotello eorlinae

"

00'"0'0

'""OOC'O

"

\

;

I\;\,V/! ~ \v"

+-.....--.--ll~ . . . . . . ~=l~~~~:.--.e-HP=:qr::=i Western

Central

Eastern

80 60

o •

Daphnia galaeta m e n d : l \ t a e

Conochilus unicornis

40 20

o

Holopedium gibberum

o -.0

100

Ceriodaphnia lacustris

•

0.2

--0 C

605791559242733778317918159

FIG. 3. Geographical distribution (biomass) of zooplankton groups in Lake Erie from 1983-87. CLA =Cladocera, ROT =Rotifera.

o

C

O+--+-+-----1f--+---1-l-t----f-t--+--+---+---'-1r--+----l STATION

0.4

.--

• 60579155924273377831791815 9

'V Notholco foliacea

STATION

50

FIG. 5. Geographical distribution of selected zooplankton species in Lake Erie. Values are the mean from 1983 to 1987.

25

/""'..

0

--.J

~

8

::::J:l:::: '--.-/

6

Q)

U

o

Diaphonosomo sp.

•

Chydorus sphaericus

V

Brochionus budapestinen

... Polyarthra remota

C

4

0 --0

2

C ::J

-.0

«

0

Weslern

Easlern

Central

2.0

o

1 .5

V D. siciloides

•

Cyclops vernalis Diaplomus minulus

... Eurytemora -

o

copepodite

E. offinis

1.0 0.5

6057915592427337783179 18 15 9 STATION

FIG. 4. Geographical distribution of selected zooplankton species in Lake Erie. Values are the mean from 1983 to 1987.

means of 1939, 1949, and 1983 through 1985 in the western basin suggests an increase in zooplankton abundance from 1939 to 1949, although the difference in mean abundance between 1984 and both 1985 and 1986 is of the same magnitude as that between 1939 and 1949 (Fig. 6). The mean abundance for just the month of August from 1939 to 1961 suggests the increase in zooplankton abundance continued into the 1950s and 1960s (Fig. 7). The cause of the increase was a major increase in Cladocera abundance, particularly in the late 1950s and the 1960s. Both Bradshaw (1964) and Gannon (1981) concluded similarly. Average ice-free abundances of Copepoda and total Crustacea from 1949 to 1985 suggest a decreasing but insignificant downward trend (Fig. 6). However, Cladocera abundance in August during the 1983-87 period decreased markedly from their 1961 and 1967 maxima and were comparable to abundances observed in the 1930s and 1940s (Fig. 7). An increase in

285

Zooplankton Species and Biomass in Lake Erie, 1983-87 100

o • ~

~

50

c

o

"D

f

Copepodo Cladocera Total Crustacea

800

I - - - -__ 1 I ~

T1

I

25

«

1961

"

1984

o 1985

600

•

"'" ::::t*=:

1986

l:J. 1987

'---" Q)

C 0 -0 C

.~

1948

"i1

400

:::l

..0

0+-r-.--.--r-+-....--.-.-1--.--r--.-r+--.-.-.-.--t--.-r-r--r-l

1938

"

0

c

:::J -0

1939

•

"i1 1983

r-...

---l

o

1958

1968

1978

«

1988

FIG. 6. Crustacean zooplankton abundance since 1939 in the western basin of Lake Erie. Values are the mean ±S.E. 1939 data are from Chandler (1940; April-September). 1949 data are from Bradshaw (1964; April-October). 1983-85 (This Study; April-October); 1986-87 (This Study; spring and summer only).

200

A

M J

J

A

SON

0

FIG. 8. Seasonal flucuation of Rotifera in the western basin of Lake Erie from 1938-87. Sources: 1939-Chandler (1940); 1961-Britt et al. (1973).

150 • '<7

~

Cladocera Adult Copepoda Total Copepoda

250 Q) ()

c

o

o

"D

c

200

50

•

1970 1983

"i1 1984

:::J -0

«

~

1985

o

1986

•

1987

O-+=,.-.--.--r+-....--.-.-+--.--r--.--r+--.-.---.-.-+--.--.......,..,

1938

1948

1958

1968

1978

1988

FIG. 7. August abundance of Cladocera and Copepoda in the western basin of Lake Erie. Data are from Chandler (1940), Bradshaw (1964), Hubschman (1960), Britt et al. (1973), Davis (1969), and this study. The number of Copepoda in 1939 and 1959 follow Bradshaw's calculations. The 1967 data are from late July, not August (Davis 1969).

spring and late summer Rotifera abundance in the western basin is suggested since 1939 (Fig. 8). A seasonal comparison of weighted lake-wide means suggests little change in zooplankton abundance during the early spring, summer, and autumn from 1970 to 1983-87 (Fig. 9). Within each basin, no trends in the spring and summer abundance are

Q)

o c o

100

-0

c

:::l

..0

«

50

o-J=-~~-+---1f--+-+---+-+---+--+---1

A M J

J

A SON

D J

FIG. 9. Seasonal flucuation of weighted mean Crustacea (nauplii excluded) abundance in 1970 and 1983-87, Lake Erie. 1970 data follow Watson and Carpenter (1974). 1983-87 values are corrected using the areal weighting factors of 15.6%, 59.6%, and 24.6% for the western, central, and eastern basins (after Munawar and Munawar 1976).

J. C. Makarewicz

286

obvious from 1983 to 1987, except for significantly higher numbers in the central and eastern basins in 1986 (Fig. 10). Spring and summer abundance was also significantly lower in the central basin in 1984 than during other study years.

Changes in Species Composition Brooks (1969) suggested that a shift in the Lake Erie cladoceran assemblage was evident by 194849 with smaller cladocerans, such as Daphnia galeata mendotae, D. retrocurva, and Diaphanosoma sp., being more abundant than in 1938-39. In 1970 the most commonly found Daphnia species were D. retrocurva, D. galeata mendotae, and D. longiremis although Bosmina longirostris and Eubosmina coregoni were also abundant (Watson and Carpenter 1974). The most abundant Cladocera (lake-wide) from 1983-87 were Daphnia galeata mendotae, Daphnia retrocurva, Eubosmina coregoni, Bosmina longirostris, and Diaphanosoma leuchtenbergianum (Table 4). Between 1983 and 1987, essentially the same smaller species were dominant as in 1948 and 1970. One exception was the abundant larger D. pulicaria in 1984. In 1984, abundance of D. pulicaria averaged 755/m3 with a maximum abundance of 3,752/m 3 at

800

o

.........

•

Western Central

v

Eastern

600

-.J

""'II::: '--"'

ClJ U C

400

0 -0 C :::J

..0

«

200

0-+------1---+---+---+--+------1

1983 1984 1985 1986 1987

FIG. 10. Zooplankton abundance versus time for the western, central, and eastern basins of Lake Erie. Values are the mean ±S.E. Spring and summer data only.

Station 73 on 5 August. On a biomass basis, Daphnia pulicaria (mean biomass = 11.9 mg/m 3 ) was also the dominant Cladocera (16.6% of the total biomass) for the lake in 1984. Although D. pulicaria was observed in 1985, 1986, and 1987, abundance was relatively low (45, 2 and 3/m3 , respectively) (Makarewicz et al. 1992). A rare species in the offshore waters of the western basin in 1929-30 (Tidd 1955), Chydorus sphaericus was a prominent constituent in the 1950s (Davis 1962) and in 1970 with a higher abundance in the western basin (Watson and Carpenter 1974). From 1983 to 1987, this species contributed 0.12% (average abundance = 383/m 3) of the total abundance for the lake (Table 4). Abundance was considerably higher in the western basin (Fig. 4, Table 5). Although not a common species, the discovery of Bythotrephes cederstroemi in Lake Erie has attracted considerable attention from Great Lakes researchers (Bur et al. 1986, Berg and Garton 1988). Its large size (>10 mm) and its potential to effectively crop down Daphnia populations (Lehman 1988) and thus affect lower trophic levels make it a species of interest. Bur et al. (1986) first reported this organism in Lake Erie in the stomachs of yellow perch and walleye and from vertical zooplankton hauls. The November vertical zooplankton hauls cited in Bur et al. (1986) were part of the study presented here. Average density in 1985 was 4.5/m3 with a maximum density of 72/m 3. B. cederstroemi was not observed in August but was observed throughout the central and eastern basins during the October cruise in 1985 (Fig. 11). In 1986 and 1987, B. cederstroemi was observed in August at relatively high densities (mean = 21 and 20 individuals/m 3 ) in the central and eastern basins, but again not in the western basin. No samples were taken in October of 1986 and 1987. Similarly in Lake Michigan, B. cederstroemi was initially observed in the autumn, and in the second year the first occurrence was in June with highest abundance in July and August (Evans 1988, Lehman 1988). In Europe, Bythotrephes sp. is found typically in the plankton from May to December (Andrew and Herzig 1984, Nauwerk 1963, Hakkari 1978, de Bernardi and Canali 1975). The prevalence of Cyclops vernalis has changed over the past 50 years. In the 1930s, C. vernalis was found only in the extreme western end of Lake Erie at the mouth of the Detroit and Maumee rivers (Tidd 1955). By 1967 it had spread throughout the lake (Davis 1969). Similarly, Patalas (1972) and

Zooplankton Species and Biomass in Lake Erie, 1983-87 50

Bythotrephes cederstromi

~

1'0

E

~1985 m1986 EHE1987

40

~

~

30

Q)

U

C

o

20

"U C ::J

.0

10

«

0

A

M

J

J

A

S

0

N

D

J

75 ~1985 ~1986

m1987 50

Q)

U C

o

"U C

25

::J

.0

«

o 605791559242733778317918159

STATION

FIG. 11. Seasonal and geographical distribution of Bythotrephes cederstroemi from 1985 to 1987. This species was not observed in 1983 and 1984.

Watson (1976) reported it as numerous in the western basin of Lake Erie during the late '60s and '70s. This species was not common in the central and eastern basins from 1983 to 1987, but it was a common species in the western basin, averaging 382 organisms/m3 for the period (Table 5). The lakewide, dominant cyclopoid copepod in 1970 was Cyclops bicuspidatus thomasi with Mesocyclops edax common in the summer (Watson 1974). Cap (1980) documented a shift in predominant copepods in the eastern basin from calanoids in 1928 to cyclopoid copepods, mainly Cyclops bicuspidatus thomasi, in 1974. Tropocyclops prasinus was present in low numbers (Watson and Carpenter 1974). From 1983-87, the same three species (c. bicuspidatus thomasi, M. edax, and T. prasinus)

287

predominated in the entire lake (Table 4) with the addition of C. vernalis from the western basin. Abundance of Diaptomus siciloides has increased in Lake Erie (Gannon 1981). It was most prevalent in the western basin and western portion of the central basin in the late '60s and '70s (Patalas 1972, Watson 1976). Abundant diaptomids in the eastern and central basins in 1970 were Diaptomus oregonensis and D. siciloides, which were also the most abundant calanoids in Lake Erie from 1983 to 1987 in the eastern and western basins (Tables 5 and 7) Differences in dominant calanoids were observed between basins. In the western basin, Diaptomus siciloides was the dominant, and D. ashlandi was common (Table 5). Diaptomus oregonensis was present but was neither dominant or common. Although D. ashlandi did occur in the central and eastern basins, it was never a common species (Tables 6 and 7). When abundance of D. ashlandi was high in the western basin, as in 1986 and 1987, this species became a common species for the entire lake (Makarewicz et al. 1992). Davis' studies (1968, 1969) of the zooplankton of Lake Erie included rotifers. Because a #20 net was employed, the samples were not quantitative for rotifers, and certain soft-bodied rotifers were not identified. However, it is apparently the only historical lake-wide study of the offshore that included the rotifers. Species observed to be abundant in 1967 were Brachionus angularis, B. calyciflorus, Conochilus unicornis, Keratella cochlearis, K. quadrata, Kellicottia longispina, Synchaeta stylata, and Polyarthra vulgaris. From 1983 to 1987, a similar group of rotifers was found. In particular, Polyarthra vulgaris, Synchaeta sp., Conochilus unicornis, and Keratella cochlearis were abundant in the 1967 and the 1983-1987 period (44.9% of the total zooplankton from 1983 to 1987) (Table 4). Notholca squamula, a species not considered to be abundant in 1967, was an abundant species in the 1983-87 period accounting for 9.4% of the total abundance for the entire lake. Indicators of Trophic Status The geographical distribution patterns of zooplankton in Lake Erie (Figs. 4 and 5, Table 9) probably reflect environmental factors unique to the various basins. Thus zooplankton may be used as assessors of trophic status. Rotifers, in particular, respond more quickly to environmental changes than do the crustacean plankton and appear to be sensitive indicators of changes in water quality

288

J. C. Makarewicz

TABLE 9. Occurence (mean number/m 3 ) of eutrophic zooplankton indicator species in Lake Erie, 1983-87.

Brachionus angularis B. budapestinen *

Western 2,350 2,456

B.oo~~~

I~I

B. caudatus Filinia longiseta Keratella cochlearis f. tecta Trichocerca cylindrica T. elongata * T. multicrinis T. pusilla

1,675 2,383 661 2,266 197 3,263 8

Basin Central Eastern 0 0 0 0 0 0 0 0 28 4 0 0 1,010 10 0 0 1,847 2,196 0 0

*Not listed as a eutrophic species by Gannon and Sternberger (1978).

(Gannon and Sternberger 1978). Brachionus angularis, B. calyciflorus, Filinia longiseta, and Trichocerca multicrinis are four rotifer species indicative of eutrophy. Other species in the genus Brachionus are also good indicators of eutrophy in the Great Lakes (Gannon 1981). The lack of dominance of eutrophic indicator species for the entire lake from 1983 to 1987 suggests that Lake Erie, as a unit, was not eutrophic. The four most abundant rotifer species were Polyarthra vulgaris, Notholca squamula, Conochilus unicornis, and Synchaeta sp. Polyarthra vulgaris is a eurytopic species while Notholca squamula is a cold stenotherm often associated with oligomesotrophic lakes (Gannon and Sternberger 1978) during the summer but are often encountered in eutrophic lakes during the winter or early spring (as in Lake Erie in 1985). Conochilus unicornis is often associated with oligo-mesotrophic conditions while some species of Synchaeta are eutrophic indicators (Gannon and Sternberger 1978). The conclusion that Lake Erie, as a unit, was not eutrophic would agree with the conclusion derived from phytoplankton indicator species and the algal biomass classification of trophic status (Makarewicz 1993). However, the eutrophic indicators Brachionus caudatus, B. calyciflorus, B. angularis, Filinia longiseta, Trichocerca multicrinis, and Trichocerca cylindrica had abundances restricted to or significantly higher in the western basin (Table 9). Total zooplankton abundance was also higher in the western basin. As with phytoplankton biomass and species composition (Makarewicz 1993), both

rotifer abundance and species composition indicated a greater degree of eutrophy in the western basin than in the central or eastern basins. Another measure of trophic status is the calanoid/cylopoid plus cladoceran ratio (plankton ratio, Gannon and Sternberger 1978, McNaught et al. 1980, Krieger 1981). Calanoid copepods generally appear best adapted for oligotrophic conditions, while cladocerans and cyclopoid copepods are relatively more abundant in eutrophic waters (Gannon and Sternberger 1978). Except for 1983 in Lake Erie, when the ratio for the western and central basins were essentially equal (Table 10), the ratio was always the lowest in the western basin and generally increased from the western basin to the eastern basin, indicating a more eutrophic status for the western basin. The increase in the weighted mean ratio for the entire lake from 1983 to 1986 suggests an improvement in trophic status. However, the decrease in 1987 to levels observed in 1984 compromises this observation. The zooplankton community of Lake Erie has changed. August crustacean abundance (1983-87) has decreased from the 1960s and is comparable to abundances observed in the 1930s and 1940s. This may be related to decreases in phytoplankton abundance ( Makarewicz 1993). Changes in species composition, such as the appearance of the large cladocerans, is more likely a response to a decrease in the forage fish base (Makarewicz and Bertram 1991). The higher algal biomass of the western basin relative to the central and eastern basins (Makarewicz et al. 1992) was reflected in the abundance of zooplankton, species composition, and the plankton ratio. Compared to Lakes Huron and Michigan in 1983 and 1984, abundance of zooplankton was greatest and the plankton ratio was lower in Lake Erie (Makarewicz et al. 1992), indicating the more eutrophic status of Lake Erie. TABLE 10. Ratio of Calanoida abundance to Cladocera plus Cyclopoida abundance in Lake Erie, 1983 to 1987. Spring and summer data only. Nauplii are not included in the ratio.

1983 1984 1985 1986 1987 Mean

Western Basin 0.34 0.31 0.22 0.31 0.17 0.27

Central Basin 0.32 0.60 0.65 0.95 0.38 0.58

Eastern Basin 0.44 0.46 0.82 0.85 0.89 0.69

Weighted Mean 0.36 0.48 0.63 0.75 0.46 0.54 -~_._--~-

Zooplankton Species and Biomass in Lake Erie, 1983-87 ACKNOWLEDGMENTS I thank T. Lewis, P. Bertram, D. Klarer, and an anonymous reviewer for their constructive comments on this manuscript. This work was supported by a grant from the Great Lakes National Program Office of the Environmental Protection Agency. Species identifications and enumeration were performed by The Bionetics Corporation under contract to the U.S.E.P.A. REFERENCES Andrew, T. E., and Herzig, A. 1984. The respiration rate of the resting eggs of Leptodora kindtii (Focke 1884) and Bythotrephes cederstroemi Leydig 1860 (Crustacea, Cladocera) at environmentally encountered temperatures. Oecologia 64:241-244. Berg, D. J., and Garton, D. W. 1988. Seasonal abundance of the exotic predatory cladoceran, Bythotrephes cederstroemi, in western Lake Erie. J. Great Lakes Res. 14:479-488. de Bernardi, R., and Canali, S. 1975. Population dynamics of pelagic cladocerans in Lago Maggiore. Mem. 1st. Ital. Idrobiol. 31 :362-392. Bottrell, H. H., Duncan, A., Gliwicz, Z. M., Grygierek, E., Herzig, A., Hillbricht-Ilkowska, A., Kurasawa, H., Larsson, P., and Weglenska, T. 1976. A review of some problems in zooplankton production studies. Norw. J. Zool. 24:419-456. Bradshaw, A.S. 1964. The crustacean zooplankton picture: Lake Erie 1939-49-59, Cayuga 1910-51-61. Verh.lnternat. Verein. Limnol. 15:700-708. Britt, N. A., Addis, J. T., and Angel, R. 1973. Limnological studies of western Lake Erie. Bull. Ohio BioI. Surv.4. Brooks, J. L. 1969. Eutrophication and changes in the composition of zooplankton. In Eutrophication, Causes, Consequences, Correctives, pp. 236-255. National Academy of Sciences. Wash., D.e. _ _ _, and Dodson, S. I. 1965. Predation, body size, and composition of plankton. Science 150:28-35. Bur, M. T., Klarer, D. M., and Krieger, K. A. 1986 First records of a European cladoceran, Bythothrephes cederstroemi, in Lake Erie and Lake Huron. J. Great Lakes Res. 12:144-146. Cap, R. K. 1980. Comparative study of zooplankton from the eastern basin of Lake Erie in 1928 and 1974. Ohio J. Sci. 80:114-118. Chandler, D. C. 1940. Limnological studies of western Lake Erie. 1. Plankton and certain physical-chemical data of the Bass Islands region, from September 1938 to November 1939. Ohio J. Sci. 40:291-336. Davis, e. e. 1962. The plankton of the Cleveland Harbor area of Lake Erie in 1956-1957. Ecol. Monogr. 32:209-247. _ _ _. 1968. The July 1967 zooplankton of Lake Erie. In Proc. 11th Conf. Great Lakes Res., pp. 61-75. Inter. Assoc. Great Lakes Res.

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Submitted: 27 July 1992 Accepted: 10 January 1993