THE CRUSTACEAN ZOOPLANKTON OF THE SOUTHERN NEARSHORE ZONE
OF THE CENTRAL BASIN OF LAKE ERIE IN 1978 AND 1979:
INDICATIONS OF TROPHIC STATUS
PREPARED FOR
*v
GREAT LAKES NATIONAL PROGRAM OFFICE
U,S, EPA, REGION V
CHICAGO, ILLINOIS
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THE CRUSTACEAN ZOOPLANKTON OP THE SOUTHERN IEARSHORE ZOIE
OF THE CEITRAL BASH OF LAKE IN 1978 AND 1979:
INDICATIONS OF TROPHIC STATUS
by
Kenneth A. Krieger
Water Quality Laboratory
Heidelberg College
Tiffin, Ohio 44883
June 1981
Final Report
Grant No. R 005350012
Contract No. 68-01-5857
Hobert J. Bowden, Project Officer
Great Lakes National Program Office
U.S. EPA, Region V
Chicago, Illinois
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ABSTRACT
The crustacean zoo-plankton community of the southern nearshore zone of
the Central Basin was investigated as part of a two-year intensive
limnological surveillance of Lake Erie conducted by the United States and
Canada. Thirty stations were sampled in 1978 in an area approximately 155 km
long and up to 5 km from shore between Vermilion and Ashtabula, Ohio, Six
stations were sampled in 1979. Four sampling cruises were conducted each year
from April or May through October.
Sampling methods differed between 1978 and 1979 in terms of depth of
tows, net mesh widths (243 and 64 microns), number of stations sampled, number
of samples per station, and sampling dates. The samples collected in 1979
from one meter above the bottom with a 64 micron mesh net provided greater
abundance estimates than those collected in 1978 from two meters above the
bottom with a 243 micron mesh net. Zooplankton abundance was highly variable
between stations and at individual stations during each c-ruise, indicating a
patchy zooplankton distribution; however, definite seasonal patterns in the
abundance of the individual species and the total zooplankton were observable.
Thirty-two crustacean taxa were identified in 1978, including 17 species
of Cladocera, six Cyclopoida, seven Calanoida, one Harpacticoida, and the
order Ostracoda. In 1979, 22 species were encountered, including 11
cladocerans, four cyclopoids, six calanoids, and one ostracod. The smaller
number of taxa seen in 1979 probably resulted from sampling at only six
stations, only one of which was close to shore, thereby eliminating
semiplanktonic or tychoplanktonic species. Highest crustacean abundances were
encountered during the June, July, and October cruises, and the lowest
abundances were found in April, May, and August. In 1979 the nauplii usually
constituted the most abundant group and the majority of the crustaceans. The
Cladocera were more numerous in July, however, at four of the six stations.
In descending order of abundance, the major species during the four 1979
cruises were Daphnia galeata mendotae, I), retrocurva, Eubosmina coregoni,
Chydorus sphaericus, Eubosmina with pseudomucro, Cyclops bicuspidatus thornas_4,
Bosmina sp., Tropocyclops prasinus mexicanus, Mesocyclops edax, Diaptomus
ashlandi, and ]). oregonensis. All of the species reported as occurring in the
middle of the Central Basin in 1967 and 1968 were encountered in the nearshore
zone in 1978 and 1979.
In order to evaluate whether or not major changes have taken place in the
crustacean zooplankton composition of the Central Basin over the past 30
years, the present results were compared with previous Central Basin studies.
Comparison of these studies was complicated by the different sampling areas;
different sampling methods, apparatus, and mesh sizes; varying sampling
dates; and taxonomic uncertainties. No one species has consistently been the
predominant species in all studies, probably due largely to these variables,
although several cladocerans and copepods have remained numerically important
in the lake since at least 1928. Some species appear to have disappeared or
to have increased or decreased in relative abundance, and this may be related
to an increase in the eutrophication of the lake.
A trophic index based on the ratio of the abundance of calanoid copepods
to the abundance of cyclopoid copepods plus cladocerans was applied to this
ii
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and several earlier studies of Lake Brie. No difference in this ratio between
the present and previous studies was detectable. Compared to values obtained
in parts of the upper Great Lakes, however, the ratio indicates a relatively
eutrophic status for all three basins of Lake Erie. Further investigation of
the effectiveness of this ratio in measuring the trophic status of lakes is
necessary, especially with regard to the effects of the inclusion or exclusion
of naupliar stages of copepods and the dates of sample collection.
111
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CONTENTS
Page
Abstract ii
List of Tables v
List of Figures vii
INTRODUCTION I
STUDY AREA 2
METHODS 4
RESULTS AND DISCUSSION 6
Environmental and Analytical Influences
on the Data 6
Abundance Patterns at JO Stations in 1978
(243 Micron Mesh Samples) 11
Comparison of Abundances between 1978 and
1979 (243 Micron Mesh Samples) 18
Abundance Patterns at Six Stations in 1979
(64 Micron Mesh Samples) 18
Cladocera 21
Cyclopoida 24
Calanoida 29
Trophic Status of the Nearshore Zone 31
ACKNOWLEDGMENTS .39
LITERATURE CITED 40
IV
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LIST OF TABLES
Table Paje
1 Dates encompassed by the sampling cruises
in 1978 and 1979 5
2 Similarities between duplicate samples and
duplicate aliquots of the crustacean
zooplankton in 1978 ................ 7
3 Comparison of total crustacean zooplankton
estimates from duplicate samples collected
in 1979 with a 243 micron mesh net . 8
4 Estimates of total crustacean zooplankton and
total cladoceran abundance from duplicate
samples obtained on three consecutive dates
in 1979 with a 64 micron mesh net 8
5 Comparison of total crustacean zooplankton
abundance estimates from samples obtained
with a 64 micron mesh net at depths of 10 m
and 1 m from the bottom 11
6 Comparison of the abundance estimates of total
crustacean zooplankton, total cladocerans,
and total cyclopoid copepods provided by
paired samples, one employing a 243 micron
mesh net, the other a 64 micron mesh net 12
7 Mean densities of crustacean zooplankton taxa
during May 1978 .................. 13
8 Mean densities of crustacean zooplankton taxa
during June 1978 ...» ....'..... 14
9 Mean densities of crustacean zooplankton taxa
during August-September 1978 . . 15
10 Mean densities of crustacean zooplankton taxa
during October 1978 16
3
11 Comparison of the numbers per m of
cladocerans, cyclopoids, and calanoids
computed from 243 micron mesh samples
obtained each year at three stations . 19
12 Abundance of cladoceran species at each station
each cruise in 1979 ................ 22
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LIST OF TABLES (Continued)
Table
13 Species of immature copepods identified at each
station each cruise in 1979 .......... . . 27
14 Abundance of nauplii and cyclopoid copepods
at each station each cruise in 1 979 ........ 28
15 Abundance of calanoid copepod species at each
station each cruise in 1979 ............ 30
16 Comparison of sampling areas, methods, and
dates as well as the most abundant crustacean
species in nine major Lake Erie studies ...... 33
17 Ratios of Calanoida to Cyclopoida plus Cladocera
in areas of the southern nearshore zone of the
Central Basin in 1978 . .............. 36
18 Ratios of Calanoida to Cyclopoida plus Cladocera
at six stations in the southern nearshore zone
of the Central Basin in 1 979 ........... 36
1 9 Ratio of calanoids to cyclopoids plus cladocerans
in studies on Lakes Erie and Lanao ........ 37
VI
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LIST OF PIGUEES
Figure Page
1 Map of study area showing sampling stations in
1,978 and 1979 3
2 Total zooplankton density at each station during
each cruise in 1978, estimated with a 243 micron
mesh net ...................... 9
Percent of zooplankton abundance contributed by major
crustacean groups during each cruise in 1978 .... 17
Mean number per m of each major crustacean
zooplankton group at six stations during each
cruise in 1979 ............. .20
Mean number per m of Daphnia retrocurva
and D. galeata. mendotae at six stations
during each cruise in 1979 .....23
Mean number per m of Bosmina sp.,
Eubosmina spp., and Chydorus sphaericus
at six stations during each cruise in 1979 ..... 25
3
Mean number per m of immature Cyclopoida
and immature Calanoida at six stations during
each cruise in 1979 26
Vli
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IITRODUCTION
The crustacean zoo-plankton community of the St. Lawrence Great Lakes
forms an essential component of the food base for fishes (Wilson 1960), and
its species structure provides an indication of the trophic status of the
lakes (Gannon and Stemberger 1978), More fundamentally, knowledge of this
community is important to understanding the inter-relations between the "biotic
and abiotic components of these large lake ecosystems.
Although detailed studies of the crustacean zooplankton community of Lake
Erie date "back at least to 1928 (Fish 1929), the composition and dynamics of
the community are not well documented. All of the studies have been conducted
on limited areas of one or more of the three basins, have employed a variety
of sampling devices and methods, have been conducted during varying months of
the'year for varying durations, and have often been beset with a lack of
taxonomie certainty. Yet probably the main factor precluding a detailed
history of zooplankton composition and abundance in Lake Erie has been the
lack of continuing studies in specific areas of the lake over the years.
The few investigations of the southern nearshore zone (defined for this
report as less than 5 km offshore) of the Central Basin have been conducted
during the past 30 years by Davis (1954, 1962) and Czaika (1978). A few other
Central Basin studies, primarily by Davis (1968, 1969) and Patalas (1972),
characterized the crustacean zooplankton of the open lake and included few if
any nearshore stations. In this study, estimates of total crustacean
zooplankton abundance and individual species contributions are presented and
are related to the past and present trophic status of Lake Erie,
This study formed part of a two-year intensive surveillance program
conducted by the United States and Canada in which the major physical,
chemical, and biological components of the water column and sediments of Lake
Erie were measured. The program conformed to a planning study performed by
the Center for Lake Erie Area Research (CLEAR) (Herdendorf 1978), which
reflected the general objectives of the Surveillance Subcommittee, Great Lakes
Water Quality Board, International Joint Commission. The objectives of the
surveillance plan, as stated by Herdendorf (1978), were (1 ) to search for,
monitor, and quantify violations of the existing Agreement objectives, the IJC
recommended objectives, and jurisdictional standards, criteria and objectives;
(2) to monitor local and whole lake response to abatement measures and to
identify emerging problems; (?) to determine the cause-effect relationship
between water quality and inputs in order to develop the appropriate
remedial-preventative actions and predictions of the rate and extent of local
and whole lake responses to alternative abatement proposals. With
consideration of key issues specific to Lake Erie, the surveillance plan
additionally focussed on (1) determining the long-term trophic state of Lake
Erie and determining to what degree remedial measures have effected
improvements; and (2) assessing the presence, distribution, and impact of
toxic substances.
In addition to this report, other reports on the southern nearshore zone
of the Central 'Basin assess the physical and chemical components (Richards
1981), the bacteriology (E. [Stanford] McMahon, in preparation), the
phytoplankton (P. A. Kline, in preparation), and the aoobenthos (Krieger
1981). Additional reports are being produced for the nearshore zones of the
Eastern and Western Basins and the open lake of all three basins.
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STUDY AREA
The sampling stations were located in an area approximately 155 km long
from northwest of Vermilion, Ohio, to east of Ashtabula, Ohio, at distances as
much as 5 km from shore (Figure 1). The exact location and the rationale for
the selection of each station are provided by Herdendorf (1978). Stations
were generally clustered around river mouths and harbors. Station depths at
the time of sampling ranged from about 5 to 15 in-
Curing the four cruises in 1978, 30 stations were sampled throughout the
study area, representing areas both within and outside of harbors. During the
four 1979 cruises, six stations were sampled, and none of these was within
harbors (Figure 1).
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76 • 80 «PWT 87
«****
Rocky River 1 Lakewood
_.
56- A 59
Cuyahoga R,
4
" I
1
0
K M
5 10
i I
l
5
MILES
15
I
1
10
139
Geneva-On-
The-Lake Ashtabula R
,,Cowles Cr.
Wheeler Cr.
Arcola Cr.
115
114
113 .120^
lllA ^ ^
110*5.
1978 «
1979 A
'Chagrin R,
Figure 1. Map of the study area, showing stations sampled in 1978 and 1979,
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METHODS
Sampling methods differed between 1978 and 1979 in ways which directly
affected the data. Both years the zooplankton was sampled with a 0.5 m
diameter cone net with a detachable straining bucket. In 1978, vertical tows
at a constant rate were made beginning at two meters from the bottom. In
1979, vertical tows were made from one meter off the bottom. In July, August,
and October 1979, at stations with a depth of over 11 m, an additional tow was
made beginning at 10 m. The samples were obtained between 0700 and 1650 hours
local time during four cruises each year. The sampling dates are shown in
Table 1 .
In 1978, a net with mesh openings of 245 microns was used, thereby
retaining only the larger immature and adult crustacean zooplankters. In
1979, the 245 micron mesh net was employed except in October to obtain samples
for comparison with the 1978 samples. Most samples in 1979 were obtained with
a net having 64 micron mesh openings in order to ensure collection of all of
the life stages of the crustacean zooplankters.
In 1978, a single sample was taken at most of the 50 stations each
cruise, although duplicate samples were obtained at a few of the stations.
The duplicated stations varied from cruise to cruise. In 1979S only six
stations were sampled for zooplankton (Figure 1). During the April, July, and
August 1979 cruises, one 64 micron sample was collected with the 245 micron
net, and a second sample was collected with the 64 micron net. Duplicate
samples were collected with each net at two of the stations each cruise.
During the July, August, and October 1979 cruises, one 64 micron sample (or
duplicates) was obtained at each station on each of three consecutive days.
The volume of water strained during each tow was measured with a
calibrated General Oceanics digital flowmeter attached to the net bridle.
After each tow the net was thoroughly "backwashed with a stream of water to
rinse all zooplankters into the straining bucket. Each sample was poured into
a 500 ml bottle, and about 15 ml of carbonated water were added to anesthetize
the zooplankters. After several minutes the sample was preserved with 25 ml
of yi% formalin, yielding approximately 2% formalin in the sample.
In the laboratory, samples with few zooplankters were concentrated to a
workable volume. A 1.15 ml or 2.52 ml aliquot was withdrawn from the
thoroughly mixed sample with a calibrated Hensen-Stempel pipet and was placed
in a Ward's zooplankton counting wheel for identification and enumeration of
the specimens. A minimum of 200 non-naupliar crustaceans was enumerated, with
additional aliquots taken as necessary. Duplicate aliquots were enumerated
from 5.2^ of the 1978 samples, and 7.5^ of the 1978 samples were obtained in
duplicate. Duplicate aliquots were omitted from the 1979 sample analysis, but
approximately 55$ of the 1979 samples were obtained in duplicate.
Cladocerans and adult copepod crustaceans were identified to species and
sexed when possible. The systematics of the cladoceran family Bosminidae is
unsettled, and variant forms occur in the Great Lakes (Watson 1974, Gannon and
Stemberger 1978). Thus, bosminid specimens were identified as jtosmina
sp. (with mucrones, probably B_. longirostris), Eubosmina _coregoni (without
mucrones or pseudomucrones), and Eubosmina with pseudomucrones. There was
often a gradation in the form an3"exT;ent ~of development of the pseudomucrones
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Table 1. Dates encompassed by the sampling cruises in 1978 and 1979.
1978
Cruise 1 18-31 May
Cruise 2 15-27 June
Cruise 3 28 August - 9 September
Cruise 4 8-20 October
1979
Cruise 1 11 - 25 April
Cruise 2 11-25 July
Cruise 3 18 August - 1 September
Cruise 4 2-18 October
in the specimens of a given sample. Sources for identification included
Brooks (1957), Czaika and Robertson (1968), Deevey and Deevey (1971),
Edmondson (1959)? Pennak (1978), and Torke (1974). The samples are on deposit
at Heidelberg College,
Zooplankton densities were calculated as:
lumber per m =
H[v(d)/V(a)]
Q
where
total zooplankters in aliquot
total sample volume (ml)
N
V(d)
V(a) = combined volume of aliquots (ml)
3
Q = quantity of water strained (m ).
A detailed analysis of the 1978 samples was reported previously (Krieger
1980). Because the 243 micron mesh size employed in obtaining those samples
allowed a large proportion of the crustaceans to pass through the net (see
below), the data should be considered to be qualitative; estimates of
zooplankton numbers made from these samples are too low, particularly for
samples obtained when a large proportion of the crustaceans consisted of small
immature forms.
Analyses of variance, t-tests, regressions, and correlations were
performed using the package Minitab (Hyan et al. 1980),
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AID DISCUSSION
Detailed data on the species enumerated from each sample collected in
1978 and 1979 are provided in several appendices available under separate
cover.
Environmenta_1 and Analytical Influences on jhe Data
The data obtained from duplicate samples and duplicate aliquots from the
1978 cruises were compared to determine the amount of field (environmental and
equipment) and analytical (subsampling) variation. Selected data are
presented in Table 2. A t-test revealed a highly significant difference
[t(9,6) = 27.42, p < .01] in the agreement between duplicate sample densities
and duplicate aliquot densities. Estimates of total crustacean zooplankton
density were less variable between aliquots (mean similarity of estimates =
0.92) than between samples (mean similarity = 0.74). The high similarity of
the aliquots indicates that the samples were thoroughly mixed prior to
subsampling. The lower similarity between replicate samples is expected from
the general patchiness of zooplankton distributions in lakes (Wetzel 1975).
In contrast to total density (Table 2), there was essentially no
difference in the extent of variation between replicate aliquots and replicate
samples for total taxa, taxa shared, and percent Cladocera. In these
instances, the variation of both aliquots and samples would have been reduced
by counting more than 200 non-nauplii, although this would have represented a
compromise in terms of increased time for analysis.
Similarly, the 1979 data from 243 micron mesh tows were analyzed to
determine the amount of variation between replicate samples (Table 3)« The
range of similarities between duplicate samples for total abundance was very
similar to the 1978 range, with a mean similarity of 0.77.
Duplicate samples obtained with the 64 micron mesh net for three
consecutive days were analyzed to determine whether total zooplankton
abundance differed between days, F ratios obtained by one-way analyses of
variance (Table 4) indicated that there was a significant difference in total
abundance between days in only one of six sample sets, and in the abundance of
cladocerans (water fleas) in two of the six sets. Thus? it appears that, in
general, temporal changes in crustacean zooplankton dynamics cannot be
detected within a three-day time interval, at least not without increasing the
number of samples obtained each day (i.e., increasing precision).
Nevertheless, as the 1978 data showed (Figure 2), zooplankton dynamics can be
observed over a 15 day period even by sampling only once at many widely spaced
stations.
Paired tows from the 10 m depth and from one meter above the bottom were
accomplished on eight occasions in 1979 at three stations (Table 5). For six
of the eight sample pairs, the "bottom" tow yielded a greater estimate of
total zooplankton abundance, as would be expected if the zooplankters are
distributed throughout the water column. The two pairs in which the 10 m tow
yielded a greater estimate were at two stations where "bottom" tows produced
greater estimates on other dates. Samples were collected at about the same
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Table 2. Similaritiea (s) between duplicate «s«»ple« (S) and duplicate aliquota (A) of the ometacean
zooplankton in 1978. Hatio« axe ths smaller nunb*r divld«fl by the larger number.
Station/
Date
55
780518
55
781008
56
780615
61
780518
61
781008
63
780615
63
780828
69
780518
69
781008
76
780618
76
780831
84
780621
90
780525
113
780624
118
78-528
122
780906
Mean Sag
No. /m and Similarity
3402
3894 (R)
14915
13872 (R)
5076
5759 (A)
8351
8802 (R)
10304
11536 (A)
37106
41117 (R)
22081 (A)
44057
56612 (A)
6155
6069
11592
12105 (R)
48279
90095 (A)
15785
14481 (R)
87233
99587 (R)
5258
4004 (A)
95517
75-85 (A)
9348
12459 (A)
8366
19828 (A)
>le similarity
Mean subsample similarity
s - 0.87
s - 0.93
B - 0.88
s - 0.95
a - 0.89
s - 0.90
B = 0.60
s-0.78
s - 0.99
s = 0.96
e - 0.54
s - 0.92
s - 0.88
s » 0.76
s = 0.79
B - 0.75
a - 0.42
- 0.74 ±
" 0.92 i
Humber of Taxa total
and Similarity Taxa
8
.9 fl "
13
13 8'
S •-
10
11 8 "
13
13 s "
13
10 S -
10 S -
10
13 8 "
13 8 -
12
13 8 "
14
10
8 8"
9
B S"
11
11
9 *•
12
12
10 8 "
0.17 (S.D.)
0.03 (S.D.)
0.89 U
1.00 «
0.93 16
0.91 U
1.00 13
0.77 15
0.77 14
0.77 «
0.85 14
0.92 13
0.79 15
0.80 12
0.89 12
1.00 14
0.82 13
0.92 13
0.83 13
0.87 ± 0.09
0.88 ± 0.08
Shared
6 55%
12 86%
11 69%
10 91%
13 100%
8 53%
9 64%
9 64X
10 71%
12 92%
10 67%
6 50%
5 42%
8 57%
7 54%
10 77%
9 69%
69 ± 13X
67 * 22%
Percent Cladocera
and Similarity
59.5
49,0
39.8
42.4
65,1
58.2
59.9
59.8
43.2
45.7
33.2
35.2
33.8
75.2
74.3
39.4
37.4
52.5
51.3
53.2
65.8
79.9
81.5
82.9
83.2
37.5
32,7
73.6
75.9
25.2
20,3
83,5
85,9
s • 0.82
a - 0,94
8-0,89
a - 1.00
s - 0.95
a - 0.94
a - 0.98
s-0,99
B • 0.95
s • 0,98
s - 0.81
8-0,98
s - 1.00
8 » 0.87
s - 0.97
8-0.81
B • 0.97
0.97 ± 0,07
0.95 t 0.06
time of day each day. Prom these results, it appears that considerable
variation in the abundance estimates can arise from the sampling process
itself, perhaps being affected by such factors as the amount of drift of the
sampling craft during the tow, causing an oblique tow which is taken at a
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Table 3- Comparison of total crustacean zooplankton estimates
(lo./m3) from duplicate samples collected in 1979
with a 243 micron mesh net.
Station
59
69
72
96
111
139
Date
790830
790723
790827
790417
790714
790411
Sample 1
1813
25479
1481
4219
18399
4663
Sample 2
3260
32260
1145
5325
15126
5212
Similarity
.56
.79
.77
.79
.82
.89
Table 4, Estimates of total crustacean zooplankton and total claetoeeran abundance CNo./cu.m.) from
duplicate samples obtained on three consecutive dates in 1979 with a 64 micron mesh net.
F ratios derived by one-way analyses of -variance were either highly significant (p <.01,**S»
significant (p <.05,*J or not significant.
Station
59
69
69
72
111
139
790830
30629
24196
790723
66325
94722
791016
54007
65098
790827
14175
14209
790714
193013
174596
791002
151766
84458
Total
Dates
798031
26780
35920
790724
52872
68677
791017
67941
52986
790828
22094
24823
790715
96690
110798
791003
124200
72934
Zooplankton
F Ratio
790901
52671 8.89
45349
790725
52755 0.89
74584
791018
53643 0.51
53137
790829
24608 3.62
16430
790716
97621 49.45**
90519
791004
138563 0.17
96712
790830
5328
4535
790723
37372
41814
791016
14266
19959
790827
2011
2091
790714
16355
15142
791002
51503
21239
Total
Dates
790831
5091
6318
790724
32044
27109
791017
20551
15646
790828
3417
3875
790715
22784
36208
791003
29239
20710
Cladocera
F Ratio
790901
8086 7,33
7112
790725
22867 12.52*
26313
791018
20007 0.04
15775
790829
1098 66.90**
753
790716
33086 5.54
33120
791004
29396 0,39
30911
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70POO-
50,000 -
2 iqOOO-
z 5pOO-
iono_
MAY IS
III
)78
*
10,000 •
5,000 •
IOOO
AUG- SEP 1978
*,« ,*«,«.- „ -3 o 'w-te-^^i's. —S5 to °^^lr'S ?fS SJ Slw
KW ^^~S-S*s-J»-J1T--tr»"'°B3?»o«'^r-^r-»»«- ^^ ^ ^ ^ ^3 VX ^<*
STATION
zoqooo-n
locpoo -
sopoo-
iqooo-
1
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1
1
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1
JUNE 1978
!tss-s.a^
1
, SS;<£
-------
shallower depth than the apparent sampling depth; the extent of rolling of
the sampling craft during the tow, again influencing the effective depth of
sampling; whether or not the plankton net opens immediately upon beginning
the tow; and the natural horizontal patchiness of zooplankton distribution,
as two successive hauls are never at exactly the same location in the water
column due to drifting of the craft and lake currents.
In April, July, and August 1979, paired tows were accomplished once at
each of the siz stations, one tow employing a 243 micron mesh net, and the
other a 64 micron mesh net. Total zooplankton, cladoceran, and cyclopoid
copepod abundance (number per m ) as estimated by each net is shown in Table
6. Except for the samples obtained at station 96 in July, every pair of
samples showed much larger abundance estimates in all three categories for the
64 micron mesh than for the 243 micron mesh. Mesh width (pore size) has been
shown to directly affect filtration efficiency (Tranter and Smith 1968): A
mesh width of 0.27 mm (270 microns) and a porosity of 0.44 demonstrated a 97!?
filtration efficiency, whereas a mesh width of 0.06 mm and 0.26 porosity
demonstrated an 88^ efficiency. It is also known, and was often very apparent
during our study, that nets with smaller mesh widths clog more readily than
those with larger mesh widths, and In this way filtration efficiency is
further reduced in nets with smaller mesh widths.
Despite the reduced efficiency of the 64 micron mesh net, the data In
Table 6 Indicate that a much larger number of crustaceans was retained In that
net than in the 243 micron net. Nauplii present in the 64 micron samples but
absent in the 243 micron samples account for much of the difference in total
zooplankton estimates. Similarly, the much less numerous young copepodid
stages of the copepods were retained in the 64 micron net but usually escaped
through the 243 micron net. However, the cladocerans were generally larger,
and most should have been retained by both mesh sizes. As Table 6 shows, the
243 micron mesh estimates for cladocerans were also much lower than the 64
micron mesh estimates, even in July, when many of the cladocerans were large
daphnias measuring well over 0.5 mm in height and over 1.0 mm in length. The
greater filtration efficiency of the 243 micron net was expected to increase
the estimates of those samples relative to the 64 micron samples. Thus, it
seems possible that many of the larger crustaceans also were forced through
the larger openings of the 243 micron net. Other factors which could have
influenced the sample estimates, such as tow speed, filtering area, and depth
of tow, were constant between the two mesh sizes. Different flowmeters were
used on the two nets, and each was factory calibrated; therefore, erroneous
readings should not have occurred. Because the 64 micron mesh net was known
to retain all life stages of the crustacean zooplankters, despite its greater
tendency to clog, and assuming that both flowmeters were accurate, the 64
micron mesh estimates are probably more correct than are the 243 micron mesh
estimates. This smaller mesh size should be used in all future studies of the
crustacean zooplankton of the Great Lakes. Few, if any, published studies of
Lake Erie crustaceans have employed a mesh size greater than 119 microns (see
Table 16).
10
-------
Table 5« Comparison of total crustacean zooplankton abundance
estimates (lo./m ) from samples obtained with a 64 micron
mesh net at depths of 10 m and 1 m from the bottom.
Station
59
59
72
72
72
96
96
96
Date
791016
791018
?91 01 2
791014
791015
791008
791010
791011
Actual
Depth (m)
12
15.5
13.5
14
14
12.5
12
12
10 m
Tow
49406
75385
75472
1146J1
91468
255432
84299
91610
"bottom"
Tow
77655
64117
91126
124199
98186
184518
163468
102377
Similarity
.64
.85
.83
.92
.93
.72
.52
.89
Paired-t = -0.315, U.S. at alpha = 0.05 with 12 degrees of freedom.
Abundance Patterns at 30 Stations in 1978
[243 Micron Mesh Samples)
Thirty-two crustacean taxa were identified from the Central Basin
nearshore zone in 1978 and included 17 species of Cladocera, 14 species of
Copepoda (six Cyclopoida, seven Calanoida, one Harpacticoida), and the order
Ostracoda (not identified further) (Tables 7 - 10).
Zooplankton densities, as estimated by the 243 micron mesh net, varied
greatly from station to station during each sampling periods but even more so
at each station between sampling periods (Figure 2). In June 1978 some
zooplankton densities were more than ten times higher than those recorded in
May and October. The highest densities encountered (164,000/nP) were in June
at stations 80 and 114. However, the stations with the highest zooplankton
densities were different for each sampling period.
The lowest densities were found during the August-September cruise. At
that time densities were severely depressed in and around Cleveland Harbor,
with as few as 341 crustaceans/in-^ (figure 2). A slight depression in total
density in this vicinity was also apparent in May and October 1978.
The total densities obtained during each sampling period were strongly
affected by the seasonal dynamics of the zooplankton community. During May a
general increase in total abundance was apparent from west to east (Figure 2),
and a gradual decline in numbers was reflected in August-September and
October, again from west to east, which was the progressive direction of the
cruises. Indeed, each set of histograms in figure 2 continues at the
approximate density at which the preceding set ends. Thus, differences in the
crustacean densities between longitudinal areas of the basin probably are
expressions more of temporal changes than instantaneous spatial differences.
11
-------
Table 6. Comparison of the abundance (lo./rrP) estimates of total
crustacean zooplankton, total cladocerans, and total
eyclopoid copepods (excluding nauplii) provided by paired
samples, one employing a cone net with 243 micron mesh
openings, the other a cone net with 64 micron mesh openings*
Station Date
Total Zooplankton
64 245
Total Cladocera
64 243
Total Cyclopoida
64 243
59 79042J
790723
790830
69 790423
790723
7908JO
72 790421
790720
790827
96 790417
79071 7
790824
111 7904H
790714
790821
139 790411
79071 1
79081 8
52020
61947
30626
32057
66325
18549
21648
77472
14174
33405
45HO
21835
27686
1 9301 5
49983
39426
1 63078
18454
3425
29311
1813
1600
25479
4563
2111
20292
1481
4219
49347
504
4477
18399
7576
4663
21313
731
2871
45645
5327
1083
37372
2915
1162
50100
2010
1524
37495
1329
897
16355
9302
225
44249
1915
633
28122
1256
565
22727
3192
229
15586
578
518
41057
276
230
7964
4074
119
17510
576
2134
5356
2683
1118
11293
1255
1419
10264
2060
6741
5734
4506
4783
32063
7060
6038
27477
1711
406
1020
137
208
2674
67
734
4156
219
1800
6316
5
1987
10002
471
2855
3602
50
The number of taxa encountered was similar for the western, central, and
eastern areas of the Central Basin nearshore zone during each sampling period.
The harbors revealed a possible reduction in species richness when compared to
open stations except in August-September, when both areas possessed the same
number of taxa. The inshore stations consistently revealed'more taxa than the
offshore stations, due largely to the presence shoreward of several
semiplanktonic or tychoplanktonic species which do not occur in the open lake.
Seasonal changes in the abundance of each crustacean group (Cladocera,
Calanoida, Cyclopoida) followed a similar pattern in most parts of the study
area, with lowest densities in Hay and October, and peak densities in June.
The changes correspond very well with the historical density fluctuations
demonstrated by Watson (1974). Likewise, Davis (1968, 1969), who sampled the
open lake in July and October 1967 and January 1968, noted much greater
densities of all groups in July. Britt et al. (1973) sampled the Western
12
-------
Table 7. Mean densities (No./m ) of crustacean zooplankton taxa during May 1978. Means were computed using
all replicates and subsawples. Station areas are Lorain-Vermilion (L-V), Cleveland SCLEV),
Pairport Harbor-Ashtabula (FH-A); harbor, open! inshore (IN), offshore (OFF).
OSTRACODA
CLADOCERA
Alona guttata Sars
Alona quadrangularis O.F. Muller
Ceriodaphnia lacustris Birge
Ceriodaphnia reticulata (Jurine)
Chydorus sphaericus (O.F. Muller)
Daphnia ambigua Scourfield
Daphnia galeata mendotae Birge
Daphnia parvula Fordyce
Daphnia retrocurva Forbes
Diaphanosoma leuchtenbergiana Fischer
Bostninidae
Holopedium gibberum Zaddach
Ilyocryptus spinifer Herrick
Leptodora kindtii (Focke)
Leydigia quadrangularis (Leydig)
L-V
0
5
0
0
0
10
2
22
73
1,115
0
1,665
0
0
13
0
CLEV
0
0
0
1
0
38
47
27
15
918
18
3,437
0
0
23
0
FH-A
0
0
0
0
0
210
6
5
0
836
5
5,586
0
0
5
0
HARBOR
0
0
0
1
0
18
46
24
36
478
0
1,976
0
0
2
0
opm
0
2
0
0
0
125
4
9
24
1,199
12
4,483
0
0
20
0
IN
0
0
0
0
0
46
27
19
19
901
11
2,704
0
0
10
0
OFF
0
5
0
0
0
180
0
15
49
1,061
0
5,642
0
0
21
0
COPEPODA
HARPACTICOIDA
Carithocamptus sp.
25
12
13
CYCLOPOIDA
immature Cyclopoida
Cyclops bicuspidatus thomasi S.A, Forbes
Cyclops scutifer Sars
Cyclops vernalis Fischer
Eucyclops speratus (Lilljeborg)
Mesocyclops edax !S.A. Forbes)
Tropocyclops prasinus mexicanus Kiefer
CALANOIDA
immature Calanoida
DlapComus ashlandi Marsh
Diaptomus mlnutus Lilljeborg
Dlaptotnus oregonensis Lilljeborg
Diaptomus sieilis S, A. Forbes
Diaptomus aiclloldes Lilljeborg
Epischura lacuatris S, A, Forbes
Eurytanora affinis (Poppe)
Mean Total Individuals Per m
790
548
0
111
3
12
0
2,134
160
37
16
0
5
0
350
7,118
4,086
469
0
86
0
73
0
1,196
29
19
1
0
6
0
372
11,065
7,526
2,177
3
181
6
114
0
820
157
89
0
3
0
0
9
25,699
2,229
820
0
154
9
82
0
1,093
81
13
1
0
6
0
563
8,325
5,291
1,217
2 '
112
0
60
0
1,500
131
67
8
2
2
0
72
18,284
3,312
815
2
96
4
52
0
1,172
81
32
3
2
1
0
76
11,464
6,275
1,666
0
193
0
103
0
1,773
188
a?
10
0
10
0
162
22,337
Basin twice monthly from mid-June to mid-September 1961 and obtained
contrasting results. They found a small peak in the Copepoda in early July
and a slightly higher peak in September, whereas the Cladocera demonstrated a
single, large increase in density in late August and September. Studies in
the Bass Islands region in 1939, 1949, and 1959 (Bradshaw 1964) demonstrated
two periods of maximum density of Cladocera, in June and .September, although
the actual densities and the dates of their occurrence varied considerably
from year to year.
13
-------
Table 8. Mean densities (No./m ) of crustacean zooplankton taxa during June 1978. Means were computed
using all replicates and subsamples. Station areas are Lorain-Vermilion (L-V), Cleveland (CLEV),
Fairport Harbor-Ashtabula (FH-A) ,• harbor, open; inshore (IN), offshore (OFF).
OSTRACODA
CLADOCERA
Alona guttata Sars
Alona quadrangularis O.F. Muller
Ceriodaphnia lacustris Birge
Ceriodaphnia recticulata (Jurine)
Chydorus sphaericus (O.F. Muller)
Daphnia ambigua Scourfield
Daphnia galeata mendotae Birge
Daphnia parvula Fordyce
Daphnia retrocurva Forbes
Diaphanosoma leuchtenbergiana Fischer
Bosrainidae
Holopediun gibberum Zaddach
Ilyocryptus spinifer Herrick
Leptodora kindtii (Focke)
Leydigia quadrangularis (Leydig)
L-V
0
0
0
26
0
67
0
60
57
24,126
332
4,113
0
0
192
0
CLEV
0
0
0
29
0
0
0
274
0
53,738
517
6,416
21
0
124
0
FH-A
0
0
0
28
55
10
29
492
0
54,011
1,303
3,558
31
0
209
0
HARBOR
0
0
0
33
0
29
0
247
52
50,867
713
5,077
9
0
213
0
OPEN
0
0
0
24
29
24
15
292
0
40,054
720
4,479
22
0
154
0
IN
0
0
0
28
13
16
0
347
26
47,859
745
4,242
14
0
210
0
OFF
0
0
0
26
33
52
35
90
0
33,816
647
5,873
26
0
85
0
COPEPODA
HARPACTICOIDA
Canthocamptus sp.
CYCLOPOIDA
immature Cyclopoida
Cyclops bicuspidatus thomasi S,A. Forbes
Cyclops scutifer Sars
Cyclops vernalis Fischer
Eucyclops speratus (Lillueborg)
Mesocyclops edax (S.A. Forbes)
Tropocyclops prasinus raexicanus Kiefer
CALANOIDA
immature Calanoida
Diaptoraus ashlandi Marsh
Diaptomus minutus Lilljeborg
Diaptomus oregonensis Lilljeborg
Diaptomus sicilis S.A. Forbes
Diaptomus siciloides Lilljeborg
Epischura lacustris S.A. Forbes
Eurytemora affinis (Poppe)
3
Mean Total Individuals per m
9,256
3,630
0
1,556
0
282
0
2,721
268
203
364
19
330
38
453
48,093
13,862
3,047
0
123
0
617
0
1,017
136
198
254
15
617
0
128
81,324
10,032
2,279
0
77
0
1,034
0
1,897
161
1,853
1,975
19
1,746
147
545
81,501
11,089
2,363
0
182
0
512
0
786
101
378
612
0
635
9
194
74,136
11,028
3,338
0
813
0
719
0
2,496
238
962
1,00.7
28
1,046
91
478
68,142
11,811
2,993
0
412
0
756
0
1,087
129
722
817
19
837
68
252
73,496
9,071
2,966
0
1,036
0
353
0
3,936
344
827
988
15
1,055
45
695
62,014
In 1978, the high June densities of Cladocera were maintained in the
August samples in the western area of the basin, but not in the other areas,
whereas the Cyclopoida gradually declined between June and October in all
areas. The Calanoida demonstrated minimum densities in May and August, with a
slight recovery in some areas in October. Britt et al. (1973) found minimum
densities of Calanoida in June 1961 followed by a gradual increase in density
throughout the sampling season.
14
-------
Table 9. Mean densities (No./m ) of crustacean zooplankton taxa during August-September 1978. Means were
computed using all replicates and subsamples. Station areas are torain-Vermilion (L-V), Cleveland
(CLEV), Fairport Harbor-Ashtabula (FH-A); harbor, open; inshore (IH), offshore (OFF),
OSTRACODA
CLflDOCBRA
Alona guttata Sars
Alona quadrangularis O.F. Muller
Ceriodaphnia lacustris Birge
Ceriodaphnia reticulata (Jurine)
Chydorus sphaericus (O.F. Muller)
Daphnia ambigua Scourfield
Daphnia galeata mendotae Birge
Daphnia parvula Pordyce
Daphnia retrocurva Forbes
Diaphanosoma leuchtenbergiana Fischer
Bosrainidae
Holopedium gibberxm Zaddach
Ilyocryptus spinifer Herrick
Leptodora kindtii (Foche)
Leydigia quadrangularis (LeydigS
L-V
0
0
0
0
0
122
0
9,191
12
9,744
5,927
8,173
275
0
200
0
CLEV
0
0
0
0
0
10
0
215
0
814
195
2,311
80
1
10
0
FH-A
3
0
0
0
0
6
0
1,599
0
379
673
3,843
31
0
94
3
HARBOR
0
0
0
0
0
26
0
1,551
11
1,322
1,086
2,131
16
1
114
3
OPEN
2
0
0
0
0
51
0
4,382
0
4,382
2,571
5,901
174
0
90
0
IN
1
0
0
0
0
52
0
2,259
5
2,325
2,214
4,298
58
1
107
2
OFF
0
0
0
0
0
17
0
6,583
0
6,127
1,706
5,566
289
0
76
0
COPEPODA
HARPACTICOIDA
Canthocamptus sp.
CYCLOPOIDA
immature Cyclopoida
Cyclops bicuspidatus thomasi S.A. Forbes
Cyclops scutifer Sars
Cyclops vernalis Fischer
Eucyclops speratus (Lilljeborg)
Mesocyclops eclax (S.A. Forbes)
Tropocyclops prasinus mexicanus Kiefer
CALANOIDA
immature Calanoida
DiapComus ashlandi Marsh
Diajitiomus rninutus Lilljeborg
Diaptomus oregonensls Lilljeborg
DiapComus sicilis S. A. Forbes
Diaptomus siclloides Lilljeborg
Epischura lacustria S. A, Forbes
Eurytemora afflnis (Poppe)
Mean TotaJ Individuals per m
2,872
116
0
103
0
2,574
0
2,055
0
0
1,290
23
104
0
0
42,855
248
5
0
15
0
28
0
132
0
0
21
0
10
0
0
4,474
996
83
0
9
0
328
7
295
0
0
363
0
45
3
0
8,891
908
18
0
30
0
489
0
375
0
0
402
0
4
3
0
8,534
1,518
92
0
44
0
1,118
4
973
0
0
594
11
75
0
0
22,299
1,200
76
0
20
0
785
3
732
0
0
588
10
60
2
0
15,074
1,622
44
0
90
0
1,238
0
885
0
0
374
0
28
0
0
24,739
The relative contribution of each crustacean group in the large mesh 1978
samples varied with the sampling period (Figure 3). These samples undoubtedly
would have revealed someifhat different group ratios had the nauplii and
smaller immature stages been retained. In May the Cladocera were present in
relatively low numbers, but in June, August-September, and October they
comprised the predominant group in all areas, becoming most important in June
and August, The cyclopoid copepods were relatively most abundant in May in
15
-------
Table 10. Mean densities (No,/tn ) of crustacean zooplankton taxa during October 1978. Means were computed
using all replicates and subaamples. Station areas are Lorain-Vermilion (L-V), Cleveland (CLBV),
Fairport Harbor-Ashtabula (PH-A); harbor, open; inshore (IN), offshore (OFF),
OSTRACODA
CLADQCERA
Alona gtittata Sars
Alona quadrangularis O.F, Muller
Ceriodaphnia lacustris Birge
Ceriodaphnia reticwlata (Jurine)
Chydorus sphaericus (O.F. Muller )
Daphnia ambigua Scourfield
Daphnia galeata mendotae Birge
Daphnia parvula Fordyce
Daphnia retrocurva Forbes
Diaphanosoma leuchtenbergiana Fischer
Bosrainidae
Holopedium gibberum Zaddach
Ilyocryptus spinifer Herrick
Leptodora kindtii (Focke)
Leydigia quadrangularis (Leydig)
L-V
0
0
3
0
0
574
0
390
3
1,372
457
2,647
5
0
163
0
CLEV
0
0
1
8
0
380
0
95
0
923
129
2,186
4
0
34
1
FH-A
0
0
0
21
15
48
0
20
0
519
53
2,452
6
0
42
6
HARBOR
0
0
1
10
0
178
0
41
3
671
163
2,244
0
0
82
6
OPEN
0
0
2
9
7
439
0
256
0
1,120
262
2,546
8
0
86
0
IN
0
0
2
12
5
314
0
156
1
815
140
2,279
7
0
70
3
COPEPODA
HftRPACTICOIDA
Canthocamg-tus sp.
CYCLOPOIDA
immature Cyclopoida
Cyclops bicuspidatus thomasi S.A. Forbes
Cyclops scutifer Sars
Cyclops vernal is Fischer
Eucyclops speratus (Lilljeborg)
Mesocyclops edax (S.A. Forbes)
Tropocyclops prasinus rnexicanus Kiefer
CALANOIDA
immature Calanoida
Diaptomus ashlandi Marsh
Diaptotnus minutus Lilljeborg
Diaptomus oregonensis Lilljeborg
Diaptomus sicilis S.A. Forbes
Diaptomus siciloides Lilljeborg
Bpischura lacustris S.A. Forbes
Eurytemora affinis (Poppe)
3
san Total Individuals per m
1,216
688
0
442
0
424
0
1,408
3
0
1,321
0
416
0
24
12,101
496
97
0
6
0
111
0
1,025
0
0
505
0
63
0
2
6,421
247
13
2
9
0
8
2
774
0
0
36
0
4
0
48
4,370
457
215
0
70
0
73
0
1,025
0
0
553
0
79
0
48
6,070
804
334
1
223
0
260
1
1,124
2
0
722
0
229
0
12
8,873
462
179
1
117
0
125
1
989
2
0
551
0
136
0
28
0
0
3
4
416
0
232
0
1,252
395
2,754
2
0
113
0
1,119
509
0
273
0
331
0
,283
0
0
881
0
256
0
18
6,634 10,341
most areas but showed no distinct pattern during the remainder of the season.
The calanoid copepods also presented an unclear pattern, but their
contribution appeared to be least in most areas in June and August, when their
numbers were especially reduced in the Cleveland area.
16
-------
S OTHER
• CALANOIDA
HCYCLOPOIDA
DCLADOCERA
100 r
60 r
60
40 --
20
00
80
60
40
20
MJ AO
L-V
M J AO MTTAO
CLEV FH-A
MJAO
HARBOR
Figure 3. Percent of zooplankton abundance contributed by major
crustacean groups during all cruises in 1978 (top) and
during the May, June, August-September, and October
1978 cruises (bottom), estimated with a 243 micron mesh
net. The "Other" category includes large rotifers of
the genus Asplanchna, which were consistently retained
in the net, as well as harpacticoid copepods and ostracods.
17
-------
Comparison of Abundances between 1978 and __
(243 Micron Mesh Samples")
Of the 30 stations sampled in 1978, only three were again sampled in
1979- Table 11 provides a comparison of the numbers per m3 of cladocerans,
cydopoids, and calanoids computed from 243 micron mesh samples obtained each
year at the stations. These data represent only the relative numbers each
year, as the naupliar and smaller immature copepodid and cladoceran stages
were not retained in the samples.
The sampling dates for the stations each cruise were from three to 13
days apart. These differences in sampling dates make comparison of stations
within a single cruise of questionable value because of the continual seasonal
shifts in abundances noted above. Furthermore, because only a single sample
was obtained at each station on nrost dates, the degree of variability of
sample estimates and hence an accurate measure of the similarity of the three
stations is not available. On occasions when replicate samples were obtained
at a station, the variability was large; thus, the average abundances of
crustaceans at the stations probably would have been more similar than were
the one-sample estimates in Table 11.
At only one station on one cruise were the sampling dates similar in 1978
and 1979. Therefore, it is impossible to compare in detail the similarity of
seasonal abundances between the two years. In general, June and July appear
to be the months of peak zooplankton abundance in the Central Basin nearshore
zone. In April and early May, calanoid copepods outnumbered cladocerans and
cyclopoid copepods, and in late May through October the cladocerans were the
most abundant group, especially in June and July.
Abundance Patterns at Six Stations in 1979
(64 Micron Mesh SamplesJ~
Twenty-two crustacean zooplankton species were encountered in the 64
micron mesh samples in 1979; including 11 cladocerans, 4 cyclopoids, 6
calanoids, and one ostracod. Total zooplankton abundance in April averaged
34,370/m . In July the average abundance was 92,030/m3 with the largest
sample estimate for the study of 193>000/m3. By August the average abundance
at the six stations had decreased to only 23,890/m3, but in October an average
of 99,960 crustacean zooplankters/m with a maximum of 185,000/m^ was
obtained. By comparison, Czaika (1978), in samples obtained near Cleveland
Harbor in 1973 and 1974, found a maximum mean abundance in June of 158,000/m3,
and the second highest mean abundance in August of 110,000/m3, whereas her
lowest numbers were in November-December, with 27,600/m3, followed in April
with 47,300/m . Figure 4 presents the mean number per m3 +_ one standard error
of each major group. Samples were collected only one day at each station in
April, but on three days during the remaining cruises.
The nauplii (cyclopoid and calanoid) constituted the largest group of
crustaceans throughout the year except in samples obtained in July at the four
westernmost stations, where the nauplii were outnumbered by the Cladocera.
The nauplii usually comprised well over half of the total zooplankton except
in October, when they nevertheless were the most abundant group.
-------
Table 11. Comparison of the numbers per m3 of cladocerans,
cyclopoids, and calanoids computed from 243 micron mesh
samples obtained each year at three stations.
Cladocera 2423
Cyclopoida 1139
Calanoida 2593
Total
6155
1978
21778
10322
1247
62827
8591
4027
Station 69
18 May 15 Jun 28 Aug 8 Oct
6081
2443
2500
33347 75445 11024
565
208
826
1600
1979
23 Apr 23 Jul 30 Aug
22727
2674
79
25479
3192
67
1304
4563
Station 111
28 May 24 Jun 6 Sep 17 Oct
Cladocera 17935 17167 1440 2629
Cyclopoida 30422 5129 191 100
Calanoida 1135 4292 99 800
Total 49492 26588 1730 3529
14 Apr 14 Jul 21 Aug
230
1987
2259
4477
7964
10002
432
18399
4074
471
3030
7576
Station 139
31 May 27 Jun 9 Sep
Cladocera 5250 64623 9386
Cyclopoida 2788 14914 2805
Calanoida 398 9717 972
Total 8436 89254 13163
11 Apr 11 Jul ' 18 Aug
119
2855
1689
4663
17510
3602
200
21313
576
50
104
731
The presence of large numbers of nauplii throughout the four cruises
indicates a continuous production of new copepods during that period, although
the species probably varied. The cruises showing the greatest productivity
were in April and October, except at the two easternmost stations, where the
most nauplii occurred in July and October (Figure 4).
As in the 243 micron samples, non-naupliar calanoids outnumbered the
cladocerans in April but were themselves outnumbered by non-naupliar
cyclopoids at three of the six stations during that month. In July and
October the calanoids were much less abundant than either the cyclopoids or
the cladocerans, but in August the three groups were present in approximately
equal proportions. The total abundance of calanoids appeared to remain about
19
-------
90
80
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Figure 4. Mean number per cubic meter jf one standard error of each major
crustacean zooplankton group at six stations during each cruise
in 1979.
20
-------
the same throughout all four cruises except at the easternmost station in
October, when unusually high numbers were encountered.
The non-naupliar cyclopoid copepods usually maintained intermediate
abundances between the cladocerans and the calanoids throughout the sampling
year. Their numbers were somewhat higher at the three easternmost stations
during all cruises. They demonstrated two peaks, the first in July and the
second, usually larger one in October.
The Cladocera increased sharply in abundance from the least numerous
group in April to the most abundant group in July. Their numbers declined
again by August but during that cruise were somewhat higher than the April
abundances. A second peak was encountered in October which was somewhat
smaller or about the same as the July peak.
In each group, the species contributing most to the abundance of the
group changed throughout the sampling season. The more abundant species, and
their ecological implications, are discussed below.
Cladocera. The abundance of each species at each station during each
cruise is presented in Table 12. The two most abundant species, as in 1978,
were Daphnia retrocurva and I), galeata mendotae (Figure 5), which had attained
approximately equal abundances in July. Both species were generally absent in
April. In October a second increase in abundance was noted for both species,
although JO. galeata was less abundant and was even absent at two of the
stations at that time.
In the 1978 (243 micron) samples, JD. retrocurva had completely dominated
the crustaceans in June, whereas _D. galeata did not reach high densities until
the August cruise. Both species were found to be less abundant in May and
October 1978 than in June and August 1978.
Previous studies throughout Lake Erie have presented similar but slightly
varying abundance patterns for these two daphnid species. Britt et al. (1973)
reported that in the shallow Western Basin in 1969 _D« _galgata mendotae
"... was the dominant cladoceran species, becoming abundant at the end of
July and continuing so through the end of the summer. JD. retrocurva was
always present too, but showed two obvious pulses, one in early July and the
other at the end of August and continuing into September." Davis (1969) found
in his open lake samples that these two species formed most of the cladoceran
biomass and that JD. galea_ta was the more abundant. He found both species in
relatively small numbers in October, and in still smaller numbers in January.
Davis (1968) noted for his July 1967 samples that JD. galeata occurred in
similar abundances in all three basins, whereas _D» retrocurva occurred
sporadically except in the Western Basin, where it exceeded J). galeata at five
of the eight stations. However, he reported (Davis 1962) that D_. retrocurva
was "... the most common daphnid in the Cleveland Harbor area of the Central
Basin in 1957."
Males of I), retrocurva were often encountered in the June 1978 samples
throughout the Central Basin nearshore zone, less commonly in October, and
infrequently in May and August. In 1979, by contrast, males were encountered
infrequently and only in October. A few males of D. galeata_ were found in May
21
-------
Table 12. Abundance (No./m + one standard error) of cladoceran species at each station during each cruise in 1979.
STATION
TAXOM CRUISE
Bositiina sp .
Eubosmina
ooregoni
Bubosmina witii
pseudomucro
Diaphanosoma
leuchtenbergiana
Holopedium
gibberum
Chydorus
sphasricus
Laptodora 9
kindtii ¥
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Daphp.ia 9
refcrocurva 9
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59 69
72 96 111 139
Figure 5. Mean number per cubic meter + one standard error of Daphnia
retrocurva and D. galeata aendotae at six stations during
each cruise in 1979.
23
-------
and October 1978 but none were found in the 1979 samples. Previous
investigators have not recorded the occurrence of male cladocerans.
Species of Bosmina and Eubosmina were major contributors to the Cladocera
throughout the 1979 sampling period (Figure 6), as they were in 1978, although
their abundance never approached that of the daphnids. They were the most
numerous cladoeerans in October both years. Bosmina was present in relatively
low numbers in April and August 1979 and was entirely absent from the samples
in July. It was collected in greatest numbers in October 1979. Eubosmina
coregoni was present every cruise, showing a continual increase in abundance
from April through October, Eubosniina coregoni attained about five times the
abundance of Bosmina. Intermediate forms of Eubosmina (some of which may
actually be cyclomorphic forms of Bosmina•, see Kerfoot 1980) showed an
increase similar to that of jB. coregoni, although their abundance was
declining during the October cruise.
Davis (1969) reported that the Bosminidae, along with ^hy^[0£HS,
sphaericus^ were the important crustacean zooplankters in October and January
in the open lake. He reported (Davis 1968) a maximum of 238,000 bosminids/m3
in the Western Basin in July, with an average of 92,000/m , whereas in his
open lake samples from the Central and Eastern basins in October (Davis 1969)
they were considerably less numerous than in the October 1979 samples.
Of the remaining cladoceran species, only Chydorus sphaericus contributed
over 10,000 individuals/m during the 1979 sampling period(Table 12). It
showed inconsistent but relatively low abundances at the six stations except
in October, when it reached peak abundances of about 5,000 to 15>000/m
(Figure 6). In 1978 Chydorus was present in all areas of the Central Basin
nearshore zone during all cruises, and it also demonstrated an October peak as
in 1979. Thus, this species along with the bosminids comprised the
predominant cladoeerans in Lake Erie during the fall. Britt et al, (1973)
stated that Chydorus was the predominant cladoceran in the Western Basin in
late August 1961. Davis (1968) reported it to be present in very low numbers
in the Central Basin in July 1967 and to be apparently absent in the Eastern
Basin. He later found it to be common and actively reproducing in the Central
Basin in October 1967 (Davis 1969). Earlier counts have shown densities of
Chydorus as high as 24,000/m in October 1967 in the Cleveland vicinity (Davis
1962*3~, an abundance very similar to that encountered in 1979 at the three
stations nearest Cleveland (Figure 6), and 8,500/m in October 1967 near the
center of the Central Basin (Davis 1969). Vorce in 1882 (cited by Davis 1969)
reported the species to be common in Lake Erie.
Cyclopoida. Immature cyclopoid copepods were present throughout the 1979
sampling period. Relatively low numbers were encountered in April and August,
and the greatest abundances were recorded in October (except in July at the
easternmost station), when densities approached 31,000/in3 (Figure 7). It was
possible to identify the larger stages of the immature cyclopoid copepods to
species (fable 13). Immature specimens of three of the four species were
found during each cruise. Immature TropoCyclops prasinus mexicanus were
observed during all except the April cruise.
Of the four cyclopoid species identified in 1979, Cyclops bicuspidatus
thomasi occurred in the greatest numbers, with a maximum abundance of up to
24
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Figure 6. Hear, number per cubic meter + one standard errot of
sp>' B*225i™ SPP-» and Chydorug ephaerleus at six '
25
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139
Mean nxanber per cubic meter + one standard error of Immature
Cyclopoida and immature Calanoida at six stations during
each cruise in 1979.
26
-------
Table 13. Species of immature copepods identified at each station each cruise
in 1979. P indicates that the species was present; U indicates
that species were not identified for the samples.
TAXON
Cyclops
bicuspidatus
thomasi
Cyclops
vernalis
Me socy clops
edax
Tropoeyclops
prasinus
mexicanus
Diaptomus spp .
Eurytemora
affinis
CRUISE
APR
JUL
AUG
OCT
APR
JUL
AUG
OCT
APR
JUL
AUG
OCT
APR
JUL
AUG
OCT
APR
JUL
AUG
OCT
APR
JUL
AUG
OCT
59
U
U
P
P
U
U
P
P
P
U
P
P
U
U
P
U
U
P
P
U
U
P
P
69
P
P
P
P
P
P
P^
P
P
P
P
P
P
P
P
P
P
STATION
72
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
96
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
111
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
139
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P,
P
P
P
P
P
P
P
5,000 adults/m in July (Table 14). A somewhat lower abundance was
encountered in April, and adults of this species were uncommon in August and
October. Mesoeyelops edax also was most abundant in July (less so than
_C. bicuspidatus), was entirely absent in April', and occurred infrequently in
August and October. Tropoeyclops prasinus mexicanus was generally absent in
April 1979 but the number of adults increased each cruise to a maximum in
October. Cyclops vernalis was the least abundant adult cyclopoid, being
generally absent in April and present in minimal numbers during the other
cruises. This species was not found in the samples from the easternmost
station.
The ratio of adult males to adult females of each cyclopoid species
27
-------
Table 14. abundance (No,/in — one standard error) of nauplii and cyclopoid copepods at each station during each cruise in 1979.
STATION
TAXOM
Nauplii
Immature
Cyclops
bicuspidatus
fchomasi
Cyclops vernalis
Mesocyclops
edax
Tr opocy c lop s
prasinus
mexicanus
CRUISE
¥
9-
?
tf
S
tf
9
-------
varied considerably from station to station and from cruise to cruise (Table
14). Ovigerous (egg-bearing) females of _C_. bicuspidatus were observed in the
April, July, and August samples, but only at the four easternmost stations,
Ovigerous females of M_. edax occurred in July at two Cleveland area stations,
and ovigerous Tropocyclops were observed in July, August, and October, but not
at the westernmost station. Only one ovigerous female of _C_. vernalj.g was
seen, in July near Pairport Harbor.
Within the limitations of the cruise dates, the periods of maximum
abundance of Cyclops bi_cus_pMatus and M.. edax agree with earlier studies.
Davis (1968) reported a mean density of C_. bicuspid atus in July 1967 in the
middle of the Central Basin of 5,950 adults/m with a maximum of 11,900/m .
He reported (Davis 1962) a June 1957 maximum of 20,400/m-^ in the Cleveland
Harbor area. Davis (1968) found that M_. edax was distinctly less abundant
than _C. bieuspidatus in July 1967, as was true in the present study, and
reported a maximum abundance of 5,730/m , as compared to a maximum of 4,138 in
July 1979 in the Cleveland Harbor area (station 96). Davis (1962) reported a
similar August 1957 maximum for this species of 4,300/m^ in the Cleveland
Harbor area.
The data of Britt et al. (1973) show that _£. vernalis was generally more
abundant than M. edax in the Western Basin in 1961. However, Davis (1962)
found _£. vernalis only in October 1956 in his Cleveland Harbor area study, and
in 1967-1968[Davis 1968, 1969) he found only very small numbers of
jC. vernalis in the Central Basin, which corresponds to the present results for
the nearshore zone.
Calanoida. Immature calanoid copepods were present in minimal numbers in
April 1979 but during July, August, and October maintained relatively high
numbers, with a very large abundance (15,800/m ) in October at the easternmost
station (Figure 7). They were usually much less abundant than the immature
cyclopoids. Jive of the six calanoid species found in 1979 belonged to the
genus Diaptomus. Adults were present in low numbers during every cruise,
although a few samples yielded over 1,500/m (Table 15). Immature diaptomids
were found in the samples every cruise at every station (Table 13), but
D. oregonensis was the only species for which adults were found during all
four cruises. It occurred only sporadically in April and July, and never
attained large numbers except at the easternmost station in October.
Adult Dia_ptomus siciloides were found in July, August, and October 1979,
but always sporadically and in small numbers. Both D_. ashlandi and J3. si oil is
were found in April at all stations and in July were represented by females at
a single station. _D. ashlandi in April attained the largest abundance of all
the diaptomid species. Adult J). minutus were seen only in the April samples.
The period of greatest diaptomid reproduction appeared to be in April, as
ovigerous females of three of the five species were observed only at that time
(Table 15). Ovigerous females of I), oregonensis were seen in the August and
October samples, and no ovigerous females of D. siciloides were observed.
The only other calanoid species observed in the 1979 samples was
Eurytemora affinis. Immature Eurytemora were observed sporadically in the
April, July, and August samples but consistently in the October samples (Table
29
-------
Table 15. abundance (No./m +_ one standard error) of calanoid copepod species at each station during each cruise in 1979.
STATION
TRXON
Immature
Diaptomus
ashlandi
Diaptamus
minutus
Diaptomus
oregonensis
Diaptomus
sicilis
Diaptomus
siciloides
Earytemora
affinis
CRUISE
$
tf
?
j
ef
s
rf
?
-------
13); and adults were found occasionally in small numbers in July and October
(Table 15). No egg-bearing females were observed.
Previous reports on the calanoid copepods compare favorably with the
present results, Britt et al. (1973) found that mature and immature calanoids
were most numerous in late August 1961, although peak abundance in 1979
appeared to occur from July through October. Davis (1968) found a July 1967
maximum of _D, oregonensis of 1,640/m^ at a station in the middle of the
Central Basin, with an average of only 700/m . He reported (Davis 1962) as
many as 2,700/m in the Cleveland Harbor area in September 1957.
Davis (1968) found that J). siciloides was more abundant than
D. oregonensis at several open-lake' Central Basin stations in July 1967 and
obtained a maximum abundance there of 1,570/m^ at two stations, as opposed to
a lake-wide maximum of 6,470/m^ in the Island Region, In the present study,
the average October abundance of I), siciloidea was slightly greater than that
of D. oregone_nsj. s at one station.
Although we found ]). minutus only in April 1979 and in May and June 1978,
Britt et al. (1973) found it in June and July in the Western Basin in numbers
fewer than 100/m , and Davis (1968, 1969) only rarely recorded the species in
the western Central Basin, in July and January.
Both Davis (1968, 1969) and Britt et al. (1973) found only the five
diaptomid species reported in the present study. Maptomus pallidus Herrick,
though not found in these studies, has been reported from Lake Erie (Watson
1974), most recently from the Eastern Basin in 1976 but previously only from
the Central Basin (Cap 1979).
In this study, _D. ashlandi was found in April and July 1979 and in May,
June, and October T978. The studies by Britt et al. (1973) and Davis (1962,
1968, 1969) described similar abundances and temporal patterns. JD. sioilis
was infrequently found in all of these studies.
The euryhaline calanoid J3. affinis was first recorded in the Great Lakes
system in 1959 from Lake Ontario and has subsequently invaded the other Great
Lakes (Watson 1974). It was first reported in Lake Erie in 1962 in the
Western Basin (Faber and Jermolajev 1966). Davis (1968, 1969) found adult
—' affi^s in July and October, In the present study, adults were found
throughout the nearshore zone in May, June, and October 1978 and July and
October 1979, being absent entirely in August both years.
The calanoid Epischura lacustris was found in low numbers in June and
August 1978 but did not appear in our 1979 samples. This species has been
reported previously in low numbers at sporadic locations (Britt et al. 1973;
Davis 1962, 1968, 1969).
Trophic Status _of the Nearshore Zone
All except two of the crustacean zooplankton species listed by Watson
(1974) as occurring in Lake Erie, as well as all those reported by Davis
(1968, 1969) for the middle of the Central Basin, were recorded in the present
study. Watson (1974) also listed the calanoids Limnocalanus macrurus, usually
found in cold, deep water, and Diaptomus pallidus. Britt et "all0973), who
-------
sampled the shallow Western Basin, also reported L_. macrurus, as well as the
cladocerans Daphnia pulex and Eurycerus lamellatus, neither of which was found
in the present study.
Several cladoceran species appeared rarely in the 1978 samples from the
Central Basin nearshore zone which have not often been reported for Lake Erie.
One of these, Ilyocryptus spinifer, was previously found in the Islands Region
in 1960 and 1 '§62 (Bradshaw"1964). Leydigia quadrangularis apparently has not
been reported earlier from Lake Irie. These two species appear to be
characteristically benthic (Pennak 1978) or tychoplanktonic and probably only
occasionally venture into or become suspended in the water column.
In order to evaluate whether or not major changes have occurred in the
crustacean zooplankton composition of the Central Basin since the beginning of
detailed studies, the present results were compared with earlier
investigations. Table 16 summarizes nine major studies which have been
conducted on the crustacean zooplankton of Lake Erie from 1928 to 1979 and
lists in descending order the most abundant species encountered in each study.
Comparison of the studies is difficult because different areas of the lake and
often different stations within areas were sampled, sampling methods and mesh
sizes varied, and sampling dates were often considerably different. As
demonstrated above, the month of sampling may have an important influence on
which species are encountered, and of those encountered which ones are
abundant. Taxonoruic uncertainties in most of the analyses further complicate
attempts at comparison.
With the many variables involved, the overall similarity of the
predominant species in many of the investigations is remarkable. No one
species has consistently been the most abundant species in all studies,
probably due largely to these variables. Daphnia galeata mendotae, often
referred to earlier by various synonyms (see Cap 1980), has been among the
most important crustaceans since the 1928 study of Fish and associates (1929)
and has ranged from the most abundant species to the eighth most abundant
species. Daphnia retrocurva has ranked among the three most abundant species
at least since 1950-1951[Davis 1954), although it was not abundant in Fish's
study. Cyclops bicuspidatus has been an important species in most studies
except in 1928, whereas .M. edax has been shown to be abundant from 1928 to the
present. Both of these cyclopoids have consistently been relatively less
abundant in the Western Basin than in the Central and Eastern Basins.
Diaptomus ashlandi, whose appearance in the samples is very seasonal, has
occasionally been encountered in relative abundance from the 1928 study to the
present time.
Changes in the relative abundance of several species seem to have
occurred since 1928, and these may relate to changes in the trophic status of
the lake. Gannon and Stemberger (1978) have noted that because most
crustacean zooplankton species occur in a wide variety of lake types, their
utility as indicators of the trophic status of lakes is limited to conditions
of extreme oligotrophy or eutrophy. They emphasize, however, that the
relative abundance of species can serve as a sensitive indicator of subtle
differences in physicochemical characteristics. Such factors as toxic
substances, eutrophication, and imbalance between planktivorous and
piscivorous fishes, can cause shifts in the composition and relative
abundances of crustacean species. Mutrient loading and pollution by toxic
32
-------
Table 16. Comparison of sampling areas, methods, and dates as well as the most
abundant crustacean species in nine major Lake Erie studies-
Author
Pisb, 1929
(cited by
Cap 1980)
Areas and
No. of
Stations
Eastern
Basin ,
No . not
stated.
Tow Type ,
Net Sisse,
Mesh Size
horizontal,
1 m diam. f
112 pi and
119 u
Cruise
Dates
July? Aug. ,
Sep. 1928
Most M>imdant Species,
in Descending Order
Daphnia pulex, Diaptoraus ashlandi,
Daphnia galeata mendotae, Mespcyclops
edax, *Sida crystalline, *Diaptomus
sicilisr *Bpisehura lacustris
Davis, 1954 Cleveland 1C 1 Juday Approx.
Harbor plankton trap, biweekly,
area surface and Sep. 1950
9 (NS)** 6.5 m Sep. 1951
Daphnia retrocurva, D. pulex, Daghnia
lgnging£ina (= JX c£. mendotae?),
R- ashlandi, M. edax, D. sicilis,
Cyclops b icu sp id atus_ thomasi
Davis, 1962 Cleveland 10 1 Juday Biweekly,
Harbor plankton trap, Sep. 1956
area surfacef 6m, Oct. 1957
3 (NS) and 12 m
Botswana Iqngirpstris, D. retrocurva,
C- bicusgIdatujst Chydorus_ s^haericus_,
M- edax, Dlaphanosoma leuchtenbergianum,
Di^a^tomus oregonansi_s, D^. M^onqlspina" ,
Pia£tomu3 ^s^ci 1 oides^, Tropocy_cl,Qps
prasinus
Davis, 1968
Eastern
Basin, 8
Central
Basin, 11
vertical from
2 m above
bottom,
0.5 m diam.,
No. 20
bolting silk
(76 u)
July 1967 D. g. rneridotae, C. bicuspidatus,
D. retrocurva, £. oregonensis,
Hqlopedium gibberuin, Leptodpra
kindtii, D. _sic_iloides
D* 3_. me^ndotae, C. bicusp^idatus,
Bosmina coxegQnj^, M. edax,
E>. Ojggggnensis, D_, siciloides,
H. gibberuM
Western
Basin, 8
retrocurva,
Cyclpps; vernalis,
k ind t i i,
Davis, 1969
Eastern
Basin, 8
Central
vertical from
2 m above
bottom,
0.5m diam.,
Ho. 20
bolting silk
Basin, 11 £76 p)
Western
Basin, 11
Oct. 1967 , J3. 1. Qn:n£i£Q st£ is, &_* coregoni ,
Jan. 1968 C_. b i c ugp i datus t C_. sph ae r i cu s f
—• HfSi^iS' ™" JgtroSHEys, T, prasinus,
*M. edax, *D. oregonensis
^
iB. IqnqlroBtris, C. bicuspidatus,
D. galeata, JD^ oregonensist
JD, re t r o_curya, M. gflax
B. longi ro s t ri. s, B. coregoni ?
C_. s£haericias, Diaptpmus minutus,
^* ashlandi|? C. bicuspidatusn,
T. prasinus, D, retrocurva
Patalas, Eastern vertical 50 m 29 July -
1972 Basin, 11, to surface or 3 August
and Central bottom to 1968
Basin, 21 surface,
(NS)
Western
Basin, 3
CMS?)
12 cm diatn*,
90 |i
—' Mi£H£Ei^^HS, thcunasi, D. g-
mendgtae^, p. r e t r o c ur va,
jB. n]:on^ir os_tr is , JD^ gregonensi s ,
M. edax:, D, longiremis, I), siciloides
C. vernalis, C. b. thomasi,
?* cpregon^i coregoni, _
D. g. mendotae, M. edax
33
-------
Table 16 continued.
Author
Czaika,
1978
Cap, 1980
Areas and
No. of
Stations
off
Cleveland
Harbor ,
5 (NS)
Eastern
Basin,
No . not
stated.
Tow Type ,
Net Size,
Mesh Size
vertical
just off
bottom,
0.5m diam. ,
64 n
vertical.
0.5m diam, ,
64 p
Cruise
Dates
June , Aug • ,
Nov. , 1973,
Apr. 1974
June , July ,
Aug . , Sep . ,
1974
Most Abundant Species,
in Descending Order
Bostninids with ntucro, Eubosrnina
coregoni, D, retroeurva, C. b.
thomasi, D. g. mendotae, C. verhali
Cerlodaphnia lacustris, M. edax
D. g» mendotae/ D. retrocurva,
C. b. thomasi, D. oregonensis,
M. edax, mucronate bosminids,
*noninucronate bosminids, *D.
longiremis, *D, leuchtenbergianum
this study Central vertical from May, June,
Basin, 2 in above Aug.-Sep.,
30 {NS) bottom, Oct. 1978
0.5m diam,,
243 p
5.' retrocurva, E. goreggni,
—' k> thomasi, D. g. njfjjArtfLe,
mucronate bosininids,
D. leuchtenbergiana, D. oregonensis,
D. siciloides, Eubosmina with
pseudomucro, C« vernalis
this study
;ral
Ln,
*S)
vertical from
1 m above
bottom ,
0.5m diara. ,
64 ft and
243 u
Apr , , June ,
Aug. , Oct.
1979
D. g. mendotae, D. retrocurva,
E. coregoni , C. sphaericus,
Eubosmina with pseudomucro, C. b.
thomasi , Bostnina sp. , T. prasinus
mexicanus, M. edax, D. ashlandi,
D. gregonensis
* Present in approximately equal proportions.
** Study area included nearshore (NS) stations.
substances, which influence zooplankton species in various ways, often occur
at the same locations, and it may thus be difficult to separate the influence
of the types of pollutants (Gannon and Steraberger 1978). These complex
interactions probably are especially operative in the harbors of the southern
nearshore zone of the Central Basin, where the zooplankton abundance in 1978
differed noticeably from harbor to harbor.
Gannon and Stemberger (1978) have summarized the literature concerning
indicator crustacean zooplankton species in the Laurentian Great Lakes. The
calanoid copepods Limnocalanus macrurus and Senecella calanoides appear to be
reliable indicators of oligotrophic conditions. Both are rarely encountered
in waters warmer than 15 C and below 0.6 mg/L dissolved oxygen. Because of
their stringent environmental requirements, changes in the populations of
these two species may serve as an early indicator of changes in lake
conditions. Limnocalanus, which was relatively abundant in Lake Erie during
the 1920's, became rare by the late 1950's (Gannon and Beeton 1971, cited "by
Gannon and Stemberger 1978) and early 1960's (Britt et al. 1973)« Diaptomus
sicilis, another cold stenotherm, appears to be a characteristic oligotrophic
species in the Great Lakes. It has only been found in low numbers at sporadic
locations in Lake Erie in recent years (Cap 1980, Britt et al. 1973, Davis
-------
1968, 1969), including the present study, when it was found during both cool
and warm water periods. However, it was an important species numerically in
the 1928 study (Pish 1929, Wilson 1960).
The cladocerans Bosmina and Eubosmina have been considered to be trophic
indicators (Watson 1974, Deevey and Deevey 1971, Gannon and Stemberger 1978),
demonstrating a species shift during eutrophication from the oligotrophic
Eubosmina coregoni to the eutrophic Bosmina longirostris. However, as
reviewed by these same authors, the continuing uncertainty as to the
systematics and ecological tolerances of the several morphotypes in the Great
Lakes greatly reduces their role as indicator organisms. Nevertheless, this
species shift seems to have occurred to some degree in Lake Erie within the
past 30 years (Table 16). Although Bosmina longirostris ("mucronate
bosrainids") was relatively scarce in the summer of 1928, it was the most
abundant species in the Cleveland area in 1956-1957 and ranged from most
abundant to fifth most abundant in later studies. Eubosmina coregoni
("nonmucronate bosminids") first appeared as an important component of the
zooplankton in the July 196? study of the open lake, generally ranging from
first to third in abundance through the present study.
Chydorus sphae_ricu£ has also been employed as an indicator of eutrophy.
Based on the reports in Table 16, this species first appeared in relatively
large numbers in 1956-1957, but its abundance has been inconsistent since that
time, due possibly to the varying sampling dates.
Diaptomus siciloides and Cyclops vernalis may be good indicators of
eutrophic conditions, although _C_. vernalis may be leas- reliable because of
morphological variability and cryptic speciation. Both species were found
only at eutrophic locations in western Lake Erie in 1930 but were found
throughout the lake by 1967. In the upper Great Lakes these species are
mainly restricted to the most eutrophic areas (Gannon and Stemberger 1978).
Table 16 reveals that I), siciloides was an abundant species only after the
early 1950's, whereas I), sicilis was no longer one of the most abundant
species after that time. Davis (1962) noted that I). sic_:Qp_idjgg_ had been
absent or nearly so in Lake Erie in 1929 but by 1956 had become one of the two
most abundant diaptomids in all three basins. He postulated that this change
may indicate eutrophication or a gradual warming of the lake reported by other
investigators. In the present study, I), siciloidea was the second most
abundant calanoid in 1978, but was relatively scarce in 1979'
Because the calanoid copepods appear to be the crustacean group best
adapted for oligotrophic conditions, several investigators (Patalas 1972,
Gannon and Stemberger 1978) have considered the ratio of calanoid copepods to
cyclopoid copepods plus cladocerans as a measure of the extent of eutrophy.
Gannon and Stemberger (1978) successfully demonstrated the application of this
ratio in several areas in Lake Michigan and in the Straits of Mackinac region
where water masses demonstrated only subtle differences in water quality. In
the latter study they obtained ratios between 0,2 and 6.6 (with one value of
13.2). They did not, however, suggest ranges of values which would indicate
the particular trophic status of a lake.
This ratio was applied to the semiquantitative data from the 1978 samples
(Table 17) and to the quantitative data from the 1979 samples (Table 18). The
1978 data produced ratios for the different areas between 0.03 and 0.62, with
35
-------
Table 17. Ratios of Calanoida to Cyclopoida plus Cladocera in areas
of the southern nearshore zone of the Central Basin in
1978.
Area Cruise
May June August October All
Lorain-Vermilion .62 .10 .09 .58 .14
Cleveland .18 .03 .04 -36 .06
Fairport Harbor-
Ashtabula .07 .11 .09 .25 .11
Harbor
Open
.30
.14
.04
.10
.10
.08
.40
.33
.07
.12
Table 18. Ratios of Calanoida to Cyclopoida plus Cladocera at six
stations in the southern nearshore zone of the Central
Basin in 1979-
Cruise
April
July
August
October
59
.85
.03
.26
.03
69
.74
.03
.14
.05
72
.72
.07
.61
.08
Station
96
.27
.04
.19.
.08
111
.39
.11
.44
.14
139
.40
.06
.34
.29
a station maximum of 1.06. Similarly, the 1979 data produced ratios at the
individual stations of 0.03 to 0.85. By grouping the 30 stations sampled in
1978 into several geographical areas, it was possible to compare different
parts of the nearshore zone (Table 17). The ratio did not reveal any
consistent differences from cruise to cruise between alongshore areas nor
between harbor and open areas. When the data for all of the cruises were
combined, however, the ratios for the Cleveland and harbor station groupings
36
-------
Table 19. Ratio of calanoids to cyelopoids plus cladocerans
in studies on Lakes Erie and Lanao.
Author
Lake
Ratio
Comments
Lewis, Lanao
1979 (Phillipines)
0.14
Based on total copepodids
collected weekly and monthly
Sep. 1970 to Nov. 1971.
Lewis,
1979
Lanao
0.073 As above, except ratio
includes nauplii.
Davis,
1968,
1969
Erie
Eastern Basin
Central Basin
Western Basin
Based on copepodid stages.
Stations on midlake transect,
0,067 July 1967
0.054 October 1967 and January 1968
0.098 July 1967
0.066 October 1967 and January 1968
0,024 July 1967
0.10 October 1967 and January 1
Britt Erie
et al.,
1973
Based on copepodids,
station 4, east of South Bass
Island.
0.13 13 June 1961
0.0097 11 July 1961
0.027 14 August 1961
0.13 12 September 1961
Britt
et al.,
1973
Brie
As above, station 6, n.w. of
Pelee Island.
0.12 13 June 1961
0.091 11 July 1961
0,30 14 August 1961
0.27 21 September 1961
indicated that the abundance of calanoid copepods was relatively suppressed,
which may reflect adverse environmental conditions in those areas.
The ratio was further applied (Table 19) to other studies on Lake Brie
and, for general comparison, to a detailed study on the zooplankton of Lake
Lanao, a highly productive but nitrogen limited tropical lake in the
Phillipines (Lewis 1979). The Lake Lanao study reveals the effect that
37
-------
inclusion or exclusion of the naupliar stages can exert on the ratio, although
a two-fold difference may be irrelevant to its interpretation. The fact that
both ratios are low compared to those reported by Gannon and Stemberger (1978)
in the Straits of Mackinac confirms the association of low ratios with
eutrophic and highly productive waters. The studies of Davis (1968, 1969)
along a midlake transect of the three basins of Lake Erie also yielded low
ratios for each basin. Differences in the ratio between the basins were not
consistent, however, indicating, as in the present study, that small
differences in the ratio are not meaningful. The same conclusions derive from
the ratios computed from the data of Britt et al. (1973) for two stations in
the Western Basin.
Compared to the high ratios found by Gannon and Stemberger (1978) in the
Straits of Mackinac, all regions of Lake Erie appear to be relatively
eutrophic. It is obvious that this ratio is heavily influenced by the
seasonal dynamics of the cladoceran populations: The ratios in the present
study were highest in April and August, when the Gladocera were present in
relatively low numbers. A similar pattern is evident in the ratios for the
data of Britt et al. (1973). A more accurate indication of the trophic status
of a lake by use of this ratio might be obtained by deriving an average ratio
from the individual ratios calculated from samples gathered at regular
intervals (e.g., biweekly or monthly) during the ice-free months, or by some
other compositing technique. Such a method would smooth out the effects of
population pulses of particular species at the individual sampling times.
Additional application of the ratio to studies in the Great Lakes and
elsewhere is necessary before its utility in assessing the extent of
freshwater eutrophication can be established.
-------
ACKNOWL1DGMEITS
This report culminated from the efforts of many collegues. Numerous
students from Heidelberg College and coworkers from the Water Quality
Laboratory assisted in the collection, preservation, and processing of
samples, and in data entry. Joan Ferguson wrote a computer program for much
of the summary data which appears in this report. Thanks are due also to
Roberta Cap at the Great Lakes Laboratory of the State University College at
Buffalo, Donna Larson at the Center for Lake Erie Area Hesearch of the Ohio
State University, and Doug Grothe, EPA Region ¥, for taxonomic assistance
early in this study. June Hatoor typed several of the tables and assisted in
report production. Heidi Goetg and Marie Cole produced most of the figures.
David Baker, Phillip Kline, and Peter Richards provided valuable insights into
the overall scope of the Intensive Study and reviewed all or portions of the
draft of this report.
39
-------
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island area of western Lake Erie. Bull. Ohio Biological Survey.
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Brooks, J. L. 1957. The systematics of North American Daphnia. Memoirs
Connecticut Acad. Arts and Sci. 13: 1-180,
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41
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