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
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                                                                          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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-------
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

-------
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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 ¥
?
d"
Daphp.ia 9
refcrocurva 9
S
9

-------
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Figure 5.  Mean number per cubic meter + one standard error of Daphnia
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           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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                                        25

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                                 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

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    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

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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.

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                               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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                               LITERATURE CITED
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                                       41

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