&EPA
United States
Environmental Protection
Agency
Effects of the Vertical Distribution of
Zooplankton on the Estimation of
Abundance and Biovolume using Deep
and Shallow Tows
December 2003
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U.S. Environmental Protection Agency
Great Lakes National Program Office (G-17J)
77 West Jackson Boulevard
Chicago, IL 60604
EPA-905-R-03-006
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Effects Of The Vertical Distribution Of Zooplankton On The Estimation Of
Abundance And Biovolume Using Deep And Shallow Tows
Prepared for:
Marc L. Tuchman
Project Officer
US EPA Great Lakes National Program Office
77 West Jackson Boulevard
Chicago, IL 60604
Prepared by:
Richard P. Barbiero1,
Lori L. Schacht2, and
Mark A. DiMartino2
^yncorp Science and Engineering Programs
6101 Stevenson Avenue
Alexandria VA 22304
2Grace Analytical Laboratory, Inc.
536 South Clark Street, 10th Floor
Chicago IL 60605
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Effecfe of Vertical Distribution of Zooplankton
Foreword
This report was prepared under the direction of Marc L. Tuchman, Project Officer, and Louis Blume,
Work Assignment Manager and Quality Assurance Officer, USEPA Great Lakes National Program
Office. The report was prepared by Richard P. Barbiero, DynCorp Science and Engineering Program,
and Lori L. Schacht and Mark A. DiMartino, Grace Analytical Laboratory, Inc.. A version of this report
was initially circulated in January 2000. The content and conclusions of the present report are identical to
those of the original report.
Disclaimer
The research described in this report was funded by the U.S. Environmental Protection Agency's (EPA)
Great Lakes National Program Office. This document has been subject to the Agency's peer and
administrative review, and it has been approved for publication. Mention of trade names or commercial
products does not constitute endorsement or recommendation for use by the U.S. Environmental
Protection Agency.
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Effecfe of Vertical Distribution of Zooplankton
Table of Contents
Foreword i
Disclaimer i
Introduction 1
Vertical Migration 1
Brief History of the GLNPO Zooplankton Sampling Protocol 1
Methods 3
Sample Collection and Processing 3
Analytical Approach 3
Results 6
Differences in Net Efficiency between 64-^m and 153-^m Mesh Nets 6
Differences in Relative Community Composition Between Deep and Shallow Tows 6
Species-specific Differences in Depth Distribution and Migration Behavior 7
Cladocera: 7
Copepoda: 7
Differences in Animal Lengths Between Deep and Shallow Tows 9
Discussion 10
Migration 10
Depth Distribution 12
Lengths of Migrating Individuals 13
Conclusions 14
Literature Cited 15
List of Tables
Table 1. Summary of Results of Wilcoxon Signed Rank Test Comparison Between Zooplankton
Samples Collected with 64-^m and 153-^m Mesh Nets 19
Table 2. Summary of ANOVA Results Testing for Depth Preferences and Indications of Zooplankton
Migratory Behavior in Lakes Huron (HU), Michigan (MI), and Superior (SU) 20
Table 3. Results of Paired /-Test Comparison of Zooplankton Abundances Estimated from Shallow
and Deep Tows at Stations Sampled During the Day in Lakes Erie and Ontario 21
Table 4. Results of/-Tests or Mann-Whitney Rank Sum Tests Comparing Zooplankton Length in
Deep and Shallow Day Tows 22
Table 5. Percent Differences Between Zooplankton Length Measured from Deep and Shallow Day
Tows 23
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Effects of Vertical Distribution of Zooplankton
List of Figures
Figure 1. Summer 1998 GLNPO Zooplankton Sampling Stations 24
Figure 2. Factor Structure of the ANOVA Analyses 25
Figure 3. Differences in Zooplankton Abundance and Community Composition between Shallow
and Deep Tows (Lake Superior, Summer 1998) 26
Figure 4. Differences in Zooplankton Abundance and Community Composition between Shallow
and Deep Tows (Lake Michigan, Summer 1998) 27
Figure 5. Differences in Zooplankton Abundance and Community Composition between Shallow
and Deep Tows (Lake Huron, Summer 1998) 28
Figure 6. Differences in Zooplankton Abundance and Community Composition between Shallow
and Deep Tows (Lake Erie, Summer 1998) 29
Figure 7. Differences in Zooplankton Abundance and Community Composition between Shallow
and Deep Tows (Lake Ontario, Summer 1998) 30
Figure 8. Average Relative Similarity between Zooplankton Communities from Shallow and Deep
Tows, for Tows Taken at Night and during the Day 31
Figure 9. Bosmina longirostris Abundance for both Shallow and Deep Tows taken during the Day
and Night 32
Figure 10. Eubosmina coregoni Abundance for both Shallow and Deep Tows taken during the Day
and Night 33
Figure 11. Holopedium gibberum Abundance for both Shallow and Deep Tows taken during the Day
and Night 34
Figure 12. Daphnia galeata mendotae Abundance for both Shallow and Deep Tows taken during
the Day and Night 35
Figure 13. Bythotrephes cedarstroemi Abundance for both Shallow and Deep Tows taken during
the Day and Night 36
Figure 14. Tropocyclopsprasinus Abundance for both Shallow and Deep Tows taken during the
Day and Night 37
Figure 15. Diacyclops thomasi and Cyclops Copepodite Abundance in Lake Michigan for both
Shallow and Deep Tows taken during the Day and Night 38
Figure 16. Diacy clops thomasi and Cyclops Copepodite Abundance in Lake Superior for both
Shallow and Deep Tows taken during the Day and Night 39
Figure 17. Diacy clops thomasi and Cyclops Copepodite Abundance in Lake Huron for both
Shallow and Deep Tows taken during the Day and Night 40
Figure 18. Leptodiaptomus ashlandi Abundance for both Shallow and Deep Tows taken during
the Day and Night 41
Figure 19. Leptodiaptomus minutus Abundance for both Shallow and Deep Tows taken during
the Day and Night 42
Figure 20. Skistodiaptomus oregonensis Abundance for both Shallow and Deep Tows taken during the
Day and Night 43
Figure 21. Leptodiaptomus silicis Abundance for both Shallow and Deep Tows taken during
the Day and Night 44
Figure 22. Relative Abundance of Leptodiaptomus silicis, as Percent of Total Crustacean
Abundance, at Sites in Lakes Superior, Michigan, and Huron, Estimated by Shallow
and Deep Tows (Summer 1998) 45
Figure 23. Diaptomid Copepodite Abundance for both Shallow and Deep Tows taken during
the Day and Night 46
Figure 24. Relative Abundance of Limnocalanus macrurus, as Percent of Total Crustacean
Abundance, at Sites in Lakes Superior, Michigan, and Huron, Estimated by
Shallow and Deep Tows (Summer 1998) 47
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Effecfe of Vertical Distribution of Zooplankton
Figure 25. Limnocalcmus macrurus Abundance in Lakes Michigan and Superior for both
Shallow and Deep Tows taken during the Day and Night 48
Figure 26. Senecella calamities Abundance in Lakes Michigan and Superior for both Shallow
and Deep Tows taken during the Day and Night 49
Figure 27. Relative Abundance of Senecella calanoides, as Percent of Total Crustacean Abundance,
at Sites in Lakes Superior and Michigan, Estimated by Shallow and Deep Tows
(Summer 1998) 50
Figure 28. Epischura lacustris Abundance in Lakes Michigan, Huron, and Superior for both
Shallow and Deep Tows taken during the Day and Night 51
Figure 29. Epischura Copepodite Abundance in Lakes Huron and Michigan for both Shallow and
Deep Tows taken during the Day and Night 52
Figure 30. Percent Abundance ofBosmina longirostris, in Lakes Erie and Ontario, in Shallow and
Deep Tows taken during the Day and Night 53
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Introduction
Vertical Migration
Vertical migration by zooplankton is one of the earliest and most commonly observed behavioral
phenomena in both marine and freshwaters (Cuvier, 1817; Leydig, 1862; Russell, 1927). Alternately
referred to as diel or diurnal vertical migration (DVM), this usually involves a migration upward from
deeper waters at dusk, resulting in a population maximum in relatively shallow water at night, followed
by sinking to deeper water at dawn and a mid-day population maximum at depth. Deviations from this
"nocturnal" DVM include "reverse" DVM, with a single surface maximum during the day, and a
"twilight" DVM in which surface maxima form both at dawn and dusk (Hutchinson, 1967; for examples,
see: Dumont, 1972; Cunningham, 1972; Ohman etal, 1983).
The proximal cause of migration has most often been found to be relative changes in light
intensity (Siebeck, 1980; McNaught and Hassler, 1964; Ringelberg, 1964), while the ultimate reason for
this behavior is somewhat more obscure. The most common pattern of nocturnal migration involves
removal of the animals from a warm, food-rich environment at night to a cold, food-poor environment
during the day, at a substantial energetic cost. Most attempts to explain this apparent paradox fall into
one of two categories (Lampert, 1989):
1. Vertical migration provides a metabolic or demographic advantage, or
2. Avoidance of surface waters during the day reduces losses from predators.
The first hypothesis, originally proposed by McLaren (1963; 1974) assumes either that a resting
period in low temperatures confers a long-term energetic advantage, or that low temperatures allow
individuals to reach a larger adult body size, which in turn results in greater fecundity. The second
hypothesis, which is probably more widely accepted, holds that migration to a dark refugium in deep
waters during daylight hours reduces losses due to visual predators (Zaret and Suffern, 1976).
Within a lake, the pattern and amplitude of DVM can vary from species to species (Wells, 1960;
Geller, 1986; Angeli etal., 1995) and within a species with size, age and sex (Stich, 1989). In general,
more visually conspicuous individuals (e.g., larger animals, gravid females) exhibit greater amplitudes of
migration (Wright et al, 1980; Haney and Hall, 1975). The strength and extent of DVM can also vary
seasonally for a given species within the same habitat (Stich and Lampert, 1981; Ringelberg etal., 1991).
Often in deep lakes migratory behavior begins soon after the clear water phase in June (Geller, 1986;
Stich, 1989). Migratory behavior can vary for a given species from year to year, and from lake to lake, in
response to changes in predator abundance. Williamson and Magnien (1982), Stirling et al (1990),
Lehman and Caceres (1993) and Frost and Bollens (1992), have all found interannual variations in the
amplitude of zooplankton migration to be related to changes in the size of predator populations, while
Gliwicz (1986) found a relationship between amplitude of migration of copepods and the age of the fish
populations across a number of lakes that had been stocked with char for different periods of time.
Brief History of the GLNPO Zooplankton Sampling Protocol
The Great Lakes National Program Office (GLNPO) began annual monitoring of the Great Lakes
in 1983 for Lakes Michigan, Huron, and Erie. In 1986 sampling was extended to include Lake Ontario,
and in 1992 sampling of Lake Superior was added. In 1983 and 1984, two zooplankton tows were taken
at each site with a 64-^m mesh net: one from 2 m above the bottom to the surface, and a second from 20
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Effects of Vertical Distribution of Zooplankton
m to the surface, although little analysis was apparently conducted (Makarewicz, 1987; Makarewicz,
1988). In 1985, the deep tow seems to have been discontinued (Makarewicz and Bertram, 1991).
Prior to the summer 1997 cruise, the zooplankton sampling protocol was changed to include 100-
m tows, in addition to the 20-m tows. Unlike previous deep tows, the 100-m tows were taken using a net
with a larger mesh size (153 |^m) to prevent clogging and to reduce the pressure wave created by the net
during sampling. Also, time of day the tows are taken was recorded from 1996 on.
The depth at which zooplankton tows are taken can have a number of possible effects on the
estimation of zooplankton abundance and community composition. In species that exhibit nocturnal
DVM with a maximum depth greater than 20 m, shallow tows taken during the day can result in an
underestimation of abundances. In addition, if larger animals exhibit greater amplitudes of migration than
smaller animals, then 20-m tows will have a disproportionately large percentage of smaller animals during
the day, and thus individual and total biovolumes of migrating animals will be underestimated in day
samples. These two factors can result in the appearance of spurious horizontal spatial patterns in
zooplankton community data where in fact none exist, particularly if sites near to each other are sampled
at similar times of day, as is often the case. Additionally, populations of deeper-living zooplankton that
rarely migrate above 20 m would be consistently underestimated in 20- m tows, whether taken during the
day or at night. On the other hand, the abundances of animals whose maximum depth of occurrence is
substantially above 100 m would be underestimated, on a volumetric basis, by deeper tows, due to
"dilution" of the population by the sampling of deep water which is essentially free of any individuals.
Here we examine the comparability of data collected with the two different tows. In particular,
we sought to address the following questions:
1. Are there differences in the net efficiencies of the two mesh sizes (64 |^m and 153 i^m) to capture
specific zooplankters?
2. Do shallow tows result in differences in relative community composition during the day? If so, what
species' abundances are underestimated during the day by shallow tows?
3. Are there lake to lake or within lake differences in the suitability of shallow tows to estimate
abundances of specific species?
4. Are there species which are underestimated at all times by shallow tows?
5. Do shallow tows result in low bias in length measurements (and hence biovolume estimates) during
the day?
6. Are volumetric estimates of abundance and biovolume underestimated by deep tows?
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Effecfe of Vertical Distribution of Zooplankton
Methods
Sample Collection and Processing
Samples used in this study were collected during the summer, 1998 cruise, during which a total of
72 stations were sampled for zooplankton (Fig. 1). Two sampling tows were performed at each site, using
a 0.5-m diameter conical net (D:L = 1:3). The first tow was taken from 20 meters below the water surface
using a 64-^m mesh net, and the second tow from 2 meters above the bottom of the lake or 100 m,
whichever was less, using a 153-^m mesh net. If the station depth was less than 20 m, both tows were
taken from one meter above the bottom. Triplicate tows of each depth were taken at the Master Stations.
After collection, samples were immediately narcotized with soda water, and were preserved with
sucrose formalin solution (Haney and Hall, 1973) approximately twenty minutes later. Samples were
split in the lab using a Folsom plankton splitter, and four stratified aliquots examined per sample. Length
measurements were made on the first twenty individuals of each species encountered per sample.
Identifications followed Balcer et al. (1984) for adult calanoids, malacostracans, the cladocerans
Leptodora kindtii, Polyphemus pediculus, Holopedium gibberum, and Diaphanosoma birgei; Hudson et
al. (1998) for adult cyclopoids and harpacticoids. Brooks (1957) and Evans (1985) were used for all
Daphnidae, and the remaining cladocerans (Chydoridae, Bosminidae, and Macrothricidae) were classified
according to Edmundson (1959). Members of Cercopagidae (i.e., Bythotrephes cederstroemi, Cercopagis
pengoi) were identified according to Rivier (1998).
Analytical Approach
To determine the comparability of data generated from nets with the two different mesh sizes (64
l^m and 153 |^m), all other factors being equal, data from sites less than 20 m (i.e., where tows with both
nets were taken from the same depth) were examined. There were 12 such sites in the western and central
basins of Lake Erie during the summer, 1998 cruise. For each species, estimates of abundances (# m"3)
generated from the two different tows were compared to determine if there were any changes in collection
efficiency that could be attributed to differences in net mesh size. Comparisons were made using the
Wilcoxon signed rank test, a nonparametric paired-sample test which is used to examine data in which
measures are repeated only once (e.g., replicate tows with different mesh sizes). The use of a paired-
sample test in this case eliminated the influence of inter-site variability on the analysis. Count data tends
to follow a Poisson, rather than a normal, distribution, and thus violates the normality assumption of
parametric tests.
Differences in relative community composition estimates as a result of different tow depths were
assessed using Whittaker's (1952) percent similarity (PSC) index:
/ K
PSC = 100 - 0.5 .£ \a - b\
where a and b are, for a given species, percentages of the total samples A and B which that species
represents. The absolute value of their difference is summed over all species. This number ranges from 0
to 1, with 0 indicating two samples with no species in common, and 1 indicating two samples with all
species present in both samples in the same relative proportions. A relative index, rather than an absolute
index (e.g. Pinkham and Pearson's (1976) Biosim index), was used to disregard effects of dilution in
cases where the majority of individuals were present substantially above 100 m.
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Effects of Vertical Distribution of Zooplankton
If a substantial portion of the zooplankton community underwent diel migration to depths greater
than 20 m, then one would expect similarities between shallow and deep tows to be greater for sites
visited at night than for sites visited during the day. This was tested for each lake by first classifying
samples according to whether they were collected during the day or at night, where day was defined as
being from one hour after sunrise to one hour before sunset. Percent similarities of zooplankton
communities estimated with deep and shallow tows were then calculated for each site (in the case of Lake
Erie, for the eastern basin only), and differences in similarities between day samples and night samples
assessed with a t-test where PSC values were both normally distributed and exhibited homoscedasticity,
and with a Wilcoxon rank sum test where the assumptions of normality and homoscedasticity were not
met.
To determine if individual species were undergoing migration, two-way nested analyses of
variance (ANOVAs) were conducted on species abundances within each of the upper lakes. Here, depth
of tow (depth) was one factor, and time of day (time), e.g., day or night, the other factor, and the response
variable was species abundance m"3 for each species within each lake. To isolate the variation due to site
differences, Site was included as a factor in the analysis. Since each site was only sampled during one
time period (i.e., either day or night), but two tows were taken at each site, factor effects of Site were
crossed with depth of tow, but nested within time of day (Fig. 2). If species were undergoing DVM of an
amplitude that took them substantially out of the upper 20 m during the day, then the magnitude of
difference in numbers of individuals estimated from deep and shallow tows would be dependent on the
time of day the tows were taken. This would show up as a significant interaction effect between the
factors depth and time. Since site was nested within time, and its variance was consequently used as the
denominator in tests of significance for factors including time, high site to site variance for a given
species within a lake would result in reduced power of the test to detect both time effects and migration
(i.e., time x depth interaction) effects. A significant effect of depth, without a significant interaction
effect, could be due to either a preference of the species for depths substantially above the depth of the
deep tow, and hence dilution of the organism in the deep tow, or a preference for depths greater than 20
m, and therefore an underestimation of abundance by the 20-m tow.
Separate ANOVAs were conducted for each species within each lake, and the data were assessed
for conformance to the assumptions of normality and homoscedasticity. Where these assumptions were
not met, one of the following transformations was used, depending on which stabilized variance and
resulted in a normal distribution:
x
' = Jx + 0.5 x' = In (x + 1)
Sites in both the western basin and the central basin of Lake Erie are relatively shallow, with
mean depths of 8.8 m and 25.5 m, respectively. These sites were therefore not included in these analyses.
The eastern basin of Lake Erie is deeper, with an average site depth of 47.5 m. However, there were only
four sites visited in this basin, three during the day and one at night. Similarly, in Lake Ontario species
distributions were uneven enough across the lake to limit the number of sites that could be used for any
one ANOVA. As a result, a different approach was used for these two lakes. Here, differences in the
abundances of individual species between deep and shallow tows for sites visited during the day were
tested for using a paired test. Assumptions of homoscedasticity and normal distribution were met in all
cases, so a one-way paired t-test was used. The null hypothesis tested was:
H0 ' ^20-100 ^ °
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Effecfe of Vertical Distribution of Zooplankton
where: d20_ioo is me average of the differences in abundances between shallow and deep tows at each
station. If abundances were greater in deep tows than in shallow tows at stations sampled during the day,
this would suggest the possibility that animals were migrating below 20 m during the day.
To compare differences in length between animals captured by the shallow tows and animals
captured by the deep tows, average lengths for deep daytime and shallow daytime tows were compared
within each lake using either a t-test, or the Mann-Whitney rank sum test in the event of non-normality or
heteroscedasticity. With this approach, variability due to site to site differences was confounded with
variability due to differences in length as a result of depth. However, the considerable non-normality of
most of the data, and the lack of an appropriate nonparametric method to incorporate site-to-site
differences, particularly given the large differences in sample size within each cell, precluded a better
approach. It should be borne in mind, therefore, that the power of this test is probably quite low.
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Effects of Vertical Distribution of Zooplankton
Results
Differences in Net Efficiency between 64-|_im and 153-(Jtn Mesh Nets
A total of 24 crustacean taxa were found at the 12 sites with depths less than 20 m. Of these, only
one taxon, Mesocyclops copepodites, showed a significant (a = 0.05) difference between the abundance
estimates made with the two different mesh sizes (Table 1). At an a of 0.05, it is expected that one Type I
error will be committed, on average, for every 20 analyses. Therefore, there appears to be no real
difference in the estimates of macrozooplankton abundances collected from a fixed depth using a 64-^m
compared to a 153-^m mesh net.
Differences in Relative Community Composition Between Deep and Shallow Tows
Zooplankton community compositions by major taxa, as estimated using deep and shallow tows,
are shown for all five lakes in Figs. 3 through 7. In Lake Superior, nine of 19 stations were sampled
during the day (Fig. 3). All sites were dominated by immature copepods. While volumetric abundances
differed between shallow and deep tows taken during the day, differences in relative community
composition, at least on the basis of broad taxonomic groups, were not apparent. In both Lakes Michigan
and Huron (Figs. 4, 5), about half of the tows were taken during the day. When shallow tows taken
during the day were compared with those taken at night, clear differences in both species composition and
abundance where apparent, with day tows showing consistently lower volumetric abundance estimates
and a greater proportion of immatures compared to night tows. Deep tows in both lakes showed a
remarkable degree of spatial homogeneity, in terms of both species composition and total abundance. In
Lake Erie, sites in the western and central basins were, by and large, 20 m or less in depth, so not
surprisingly, no substantial differences were noted in community composition between deep and shallow
tows taken during the day (Fig. 6). On the other hand, in the eastern basin, the genus Bosmina, which
was dominant in the deep tows, was almost completely missing from all shallow tows. Lake Ontario, like
Lake Erie, exhibited a notable degree of spatial heterogeneity in community composition, with the
western three sites dominated by Bosmina, and the eastern four sites dominated by Daphnia (Fig. 7). A
central site was intermediate in species composition between these two communities. Half of the sites
were visited during the day, and no systematic differences in community composition were apparent,
although abundances tended to be higher in the shallow tows.
To determine if differences in community composition from shallow and deep tows were
statistically significant for each lake, PSC similarity values between shallow and deep tows for each site
collected during the day were compared to similarity values between shallow and deep tows collected at
night (Fig. 8). In Lake Erie, only sites in the eastern basin were compared, since sites in the western and
central basins were not substantially deeper than 20 m. Statistically significant differences (a = 0.05)
were found for Lakes Huron and Michigan, indicating that shallow and deep tows taken at night were
more similar to each other than shallow and deep tows taken during the day. This implies that a
substantial portion of the zooplankton community was undertaking nocturnal DVM to a depth greater
than 20 m. Although a very large difference in the similarity of deep and shallow tows between day and
night was apparent in the eastern basin of Lake Erie, the small sample size precluded a statistically
significant difference from being detected. No differences in similarity between day and night samples
were detected in Lake Ontario, where similarity values were uniformly high. In Lake Superior,
similarities between shallow and deep tows were comparable for day and night samples; however, overall
similarities between shallow and deep tows were lower than for other the lakes. This implies that there
were differences in the depth distributions of species both day and night.
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Effecfe of Vertical Distribution of Zooplankton
Species-specific Differences in Depth Distribution and Migration Behavior
To test for depth preferences and indications of migratory behavior on a species-by-species basis,
ANOVAs were conducted on individual species from Lakes Huron, Michigan and Superior. The full
results of all ANOVAs are presented in Table 2.
Cladocera:
There were five species of cladocera present in sufficient numbers in at least one of the upper
lakes to permit analysis: Bosmina longirostris, Eubosmina coregoni, Holopedium gibberum, Daphnia
galeata mendotae and Bythotrephes cedarstroemi. Abundances of B. longirostris were high enough in
both Lakes Huron and Michigan to permit analysis. In both lakes there was a significant interaction
effect between time and depth, indicating the likelihood of DVM. This can be seen in box plots of
abundances of this organism, which show that shallow tows taken at night result in significantly higher
abundance estimates than shallow tows taken during the day for both lakes (Fig. 9). Abundances of E.
coregoni in Lake Huron were substantially greater in both deep day and shallow night samples, compared
to shallow day samples (Fig. 10), suggesting nocturnal DVM, although there were no significant factor
effects. Failure to find significant differences could have been a result the low power of the performed
test, however (P = 0.076 for time x depth). H. gibberum, present in Lake Superior, showed significant
depth effects, with abundances estimated from shallow tows consistently higher than those from deep
tows (Fig. 11). This suggests that H. gibberum prefers upper waters, and therefore that deeper tows
consistently underestimate its abundance. There was no evidence of migration in this species. D. galeata
mendotae was present in substantial numbers in all three upper lakes, although its behavior seemed to
differ from lake to lake. In both Lakes Michigan and Huron, significant depth x time interaction effects
were found, suggesting that D. galeata mendotae undergoes DVM in these lakes (Fig. 12). This was
particularly pronounced in Lake Michigan, where mean abundances in shallow night tows were nearly
two orders of magnitude higher than in shallow day tows. In both lakes, deep tows were greater than day
shallow tows, and less than night shallow tows, indicating that though animals were migrating above and
below 20 m, most of the population was well above the depth of the deep tows, and so abundances were
underestimated by the deep tows. In contrast, in Lake Superior, no significant interaction effects were
found, but depth had a significant effect on abundance estimates. Mean abundance estimates from
shallow tows were significantly higher than those from deep tows, indicating that animals were staying
above 20 m both day and night, and consequently deep tows underestimated abundance by diluting the
samples. B. cedarstroemi was common in Lakes Huron and Superior; no evidence of migration below 20
m was found in either lake (Fig. 13).
Paired t-test analyses were conducted on the species abundances estimated from deep and shallow
tows at the stations sampled during the day in Lakes Erie and Ontario. Full results from these analyses
are presented in Table 3. In Lake Erie, a statistically significant difference between abundance estimates
from deep and shallow tows was found only for Bosmina longirostris. Interestingly, a difference was not
found for this organism in Lake Ontario. When the relative abundances of this species in the two lakes
are examined (Fig. 30), dramatic differences in its vertical distribution are apparent. The relative
abundances of B. longirostris estimated from deep and shallow tows at sites in Lake Ontario sampled
during the day are essentially identical, while in Lake Erie this organism is almost completely absent from
the upper 20 m during the day at sites in which it appears to be the dominant organism, judging from
abundances estimated from the deeper tows.
Copepoda:
Of the copepods, there were two species of cyclopoids and seven species of calanoids abundant
enough in at least one lake to test. In addition, immatures (copepodites) were tested. The cyclopoid
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Effects of Vertical Distribution of Zooplankton
Tropocyclops pmsinus mexiccmus was found in Lake Michigan, and both adult and immatures were
significantly more prevalent in the upper 20 m both day and night, with no evidence of migration below
20 m (Fig. 14). Both mature Diacyclops thomasi and its copepodites showed evidence of extremely
strong DVM in Lake Michigan, with adult abundances in shallow night tows about two orders of
magnitude higher than shallow day tows (Fig. 15). The extremely low abundances found in the shallow
day tows strongly suggest that most of the adult population was migrating below 20 m during the day. In
Lake Superior, on the other hand, there was no indication of migration below 20 m during the day for this
species (Fig. 16). In fact, both adults and copepodites were more abundant in shallow day tows than in
shallow night tows, although this difference was not statistically significant. There was a statistically
significant effect of depth, pointing to dilution of the deep tow samples. D. thomasi populations in Lake
Huron showed no statistically significant effects of depth or time (Fig. 17).
Calanoid copepods were the most diverse group of the macrozooplankton, with a total of seven
species found. Four species of the family Diaptomidae were found in the lakes: Leptodiaptomus
ashlandi, Leptodiaptomus minutus, Skistodiaptomus oregonensis, and Leptodiaptomus sicilis.
Abundances of L. ashlandi from shallow tows in both Lakes Huron and Michigan were several times
higher at night than during the day, strongly suggesting DVM (Fig. 18). However, these differences were
not statistically significant. A similar, but stronger, pattern was found in both lakes for L. minutus,
indicating strong DVM below 20 m during the day (Fig. 19). Deep tows were similar both day and night,
and were lower than shallow night tows and higher than shallow day tows. This indicates that most of the
population was probably well above 100 m at all times. Substantial populations of S. oregonensis were
found only in Lake Michigan, and while shallow night tows were much higher than shallow day tows,
there was no statistically significant difference (Fig. 20). Since deep tows were always less than shallow
tows, indicating dilution of these population in deep tows, it appears that both shallow day and all deep
tows underestimate these populations. L. sicilis was present at a substantial number of sites in all three
lakes, and its populations were always significantly greater in deep tows than in shallow tows (Fig. 21).
There was some suggestion of migration into the upper 20 m at night in Lakes Huron and Michigan,
although this was not statistically significant. Percent abundance data for L. sicilis clearly indicate that
shallow tows greatly underestimate abundances of this species in Lake Michigan and Superior (Fig. 22).
Diaptomid copepodites showed evidence of migration in Lake Michigan, and of increased numbers with
depth in Lake Superior (Fig. 23). No patterns were noted in Lake Huron. However, since this group
probably represented a mix of species with different behaviors, conclusions regarding differences
between lakes are of limited value.
Limnocalanus macrurus was found in both Lakes Michigan and Superior, and in both lakes
relative abundances were significantly higher in deep tows, both day and night, than shallow tows taken
at any time (Fig. 24). In addition, animals were nearly absent from the upper 20 m during the day, but not
at night, suggesting migration into the upper 20 m at night (Fig. 25), although a significant interaction
effect was noted only in Lake Superior. The lack of an interaction effect in Lake Michigan was probably
due to the high variability in that lake. In both lakes, abundance estimates were higher from deep night
tows than deep day tows, suggesting that perhaps a portion of the population was residing in waters
deeper than 100 m during the day and migrating up at night. Senecella calanoides, also present in both
Lakes Superior and Michigan, showed an even more extreme distribution, being completely absent from
surface waters both day and night (Fig. 26). Somewhat more individuals were found in deep night tows
than in deep day tows, suggesting again that perhaps some of the population was migrating below 100 m
during the day, but numbers of individuals were too low to permit confident conclusions. As with L.
macrurus, estimates of the relative abundance of S. calanoides made with deep and shallow tows were
very different (Fig. 27).
Epischura lacustris was present at a substantial number of sites in all lakes, and showed some
evidence of migration in Lake Michigan, but not in the other lakes (Fig. 28). Epischura copepodites,
December 2003
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Effecfe of Vertical Distribution of Zooplankton
present both in Lake Michigan and Lake Huron, were significantly more abundant in the upper 20 m in
both lakes (Fig. 29).
In Lake Ontario, diaptomid copepodites showed a slightly significant difference in abundances
estimated from the two tows (Table 3). None of the other copepods tested in the two lower lakes
exhibited any differences in vertical distribution, indicating that DVM below 20 m was not occurring for
these species.
Differences in Animal Lengths Between Deep and Shallow Tows
The full results of length comparisons between shallow and deep day tows are presented in Table
4. Of the 20 species tested, 11 showed significant differences in length between individuals captured in
the deep tows compared to the shallow tows in at least one lake. Only in the case of D. thomasi were
larger individuals found in the shallow tows (Table 5). Differences in length, though significant, were in
most cases relatively minor, often amounting to less than a one percent change in lengths between the two
tows. However, for some organisms, in particular the cladocerans B. longirostris and D. galeata
mendotae, the differences were substantial. In the case of Lake Huron, differences in length of B.
longirostris and D. galeata mendotae between shallow and deep tows would have lead to an
underestimation in biovolume of approximately 50%, even if abundances calculated from the two tows
were identical.
December 2003
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Effects of Vertical Distribution of Zooplankton
Discussion
Migration
A number of different patterns of depth distribution were found for the zooplankton in the Great
Lakes. In general, cladocerans exhibited nocturnal DVM in Lakes Michigan and Huron, with patterns
being stronger in the former lake. B. longirostris, the dominant cladoceran in eastern Lake Erie, also
appeared to be undergoing migration in that lake, being virtually absent from 20-m tows during the day.
In contrast, where sufficient organisms were present to test, migratory behavior was not apparent in
cladoceran populations in Lake Superior or Lake Ontario. In Lake Superior in particular, cladocerans
exhibited a strong tendency to remain in the upper 20 m both day and night.
Finding comparable data in the literature is difficult, because most lakes are much shallower than
the Great Lakes, and therefore migration patterns tend to be much more restricted with respect to depth.
Surprisingly, little work on migratory behavior has been done on the Great Lakes, although some studies
have been conducted which have addressed vertical distribution. Wells (1960), in an extensive study of
the zooplankton of Lake Michigan, found that D. galeata mendotae undertook diel migrations, but that
populations did not descend below 20 m. In contrast to the present study, his data indicated that
migration took place almost entirely between the surface and 10m. McNaught and Hasler (1966), on the
other hand, found that populations of D. retrocurva in Lake Michigan had an amplitude of migration of
over 20 m, with population mean density at a depth of 34 m during the day. Conway et al. (1973), in a
study of zooplankton distribution in Lake Superior, found that abundances of cladocerans declined
notably between 20 and 30 m, which is consistent with the present findings. Wilson and Roff (1973)
observed variable migration in Bosmina in Lake Ontario. During September, there was notable migration
below 20 m during the day, although the majority of the population remained above that depth. In
contrast, D. retrocurva exhibited stronger migration below 20 m in their study. Migration ranges for the
two species in September were about 20 m and 14 m, respectively.
In Lake Geneva, Angeli et al. (1995) found that migration of small individuals of D. hyalina was
limited to the upper 15m, while larger individuals undertook migrations with an amplitude of greater than
30m. Makino et al. (1996), working on a caldera lake in Japan, observed population maxima of D.
longispina between 25 - 50 m during the day in spring, but mostly above 30 m in October. Similar depth
maxima were found for Bosmina (Eubosmina) coregoni.
In contrast to the smaller cladocerans, the large invertebrate predators Leptodora kindtii,
Bythotrephes cedarstroemi and Cercopagis pengoi did not exhibit substantial migratory behavior in any
of the lakes. This seems paradoxical, since the size of these zooplankton would appear to make them
more vulnerable to sight-feeding planktivores. However, Leptodora kindtii is extremely transparent, and
it is possible that it is not visible to fish, even in epilimnetic waters during the day. Both Bythotrephes
and Cercopagis have very long barbed spines which can get caught in the gill rakers of planktivorous fish
(Barnhisel and Harvey 1995), thus reducing their vulnerability to predation. Stich (1989), working on
Lake Constance, also found that migratory behavior of the species Leptodora kindtii and Bythotrephes
longimanus was either slight or absent.
Our study also found evidence of migration in many copepod species, although in some cases site
to site variability prevented this from being statistically significant. Of the cyclopoids, only D. thomasi
showed evidence of migration, and then only in Lake Michigan. On the other hand, most of the
calanoids, and in particular the diaptomids, exhibited depth distributions in Lakes Michigan and Huron
suggestive of migratory behavior, although in some cases these were not statistically significant.
10 December 2003
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Effecfe of Vertical Distribution of Zooplankton
Evidence of migration of these organisms was not apparent in the other lakes. Wells (1960) similarly
found nocturnal DVM, extending to below 20 m, for the diaptomids L. minutus, L. ctshlcmdi and S.
oregonensis. No evidence of migration was found for these organisms in the lower lakes, which agrees
with Wilson and RofTs (1973) observations on Lake Ontario.
There are a number of possible explanations for the differences observed in migration patterns
from lake to lake. The lack of observable migration in Lakes Superior and Ontario could be due to a
smaller amplitude of migration in those lakes. If movements were largely confined to the top 20 m,
migratory behavior would not have been apparent from our analyses. These differences could also be due
to differences in predation pressure by juvenile or small fish. There is a great deal of literature
documenting the influence of the presence of predators on the extent of DVM in freshwater zooplankton.
In field studies of Polish lakes, Gliwicz (1986) found that Cyclops obssorum migrates only where Arctic
char are present, and this behavior can be induced through planktivorous fish stocking. The initiation of
vertical migration in Daphnia hyalina in Lake Maarsseveen has been observed to coincide with the
appearance of active juvenile perch, and to cease when these predators disappeared from the open water
(Ringelberg etal., 1991). Similarly, migration ofDiaptomus kenai in Gwendoline Lake, British
Columbia, ceased when the predator Chaoborous was removed from the lake by an invasion of small
trout, but when Chaoborous were added in an in situ enclosure, DVM resumed within 4 hours (Neill
1990). Stirling et al. (1990) found that DVM of Daphnia galeata mendotae in Lake St. George increased
in amplitude by a factor of 2 during years in which planktivorous fish recruitment was high, while
Daphniapulex responded to Chaoborous additions in Ranger Lake, Ontario by initiating migratory
behavior (Nesbitt and Riessen, 1996).
Laboratory work has provided evidence that the induction of DVM in zooplankton is a response
to the presence of chemical exudates produced by predators. Dodson (1988) first demonstrated that
several different species of Daphnia could be induced to ascend or descend when added to water that was
preconditioned by the presence of a vertebrate or invertebrate predator, and that this response was both
predator- and prey-specific. This initial observation has been corroborated by a number of subsequent
laboratory studies (e.g., Ringelberg, 1991; Loose, 1993; Watt and Young, 1994; Van Gool and
Ringelberg, 1998), and recent attempts have been made to chemically characterize the fish exudates
responsible for this behavior (Von Elert and Loose, 1996). The response of zooplankton exposed to such
predator exudates is typically rapid (Neill, 1990) but short-lived (Dodson, 1988; Loose, 1993). In
general, contact with vertebrate predators, which are mainly visual feeders, induces "normal" DVM,
while invertebrate predators, which are mainly tactile feeders and often undergo normal DVM
themselves, induce reductions in mean population depths or reverse migrations (Leibold, 1990; Nesbitt
and Riessen, 1996; Dodson, 1988; Brancelj and Blejec, 1994). In this way, contact time with the two
predator groups is reduced.
The intensity of the response to predator cues appears to have a genetic component (Young and
Watt, 1994), and it has been supposed that this serves to set a limit on the maximum extent of behavioral
response to predator cues. Therefore, populations from environments historically subject to greater
predation pressure might be genetically predisposed to exhibit greater behavioral responses to predator
fish cues than those populations which have been subject to less intense pressure (i.e., those populations
studied by Young and Watt, 1996). These genetic differences can even be exhibited within a given
habitat between genetically distinct populations of a single species. Field studies have found distinct
migratory behaviors between co-occurring, but genetically distinct, populations of Daphnia longispina
(King and Miracle, 1995), Daphnia galeata x hyalina (Spaak and Ringelberg, 1997; Van Gool and
Ringelberg, 1998) and Daphnia pulex (Weider, 1984).
It is possible that the differences in migratory behavior observed in the present study could be the
result of differences in predation pressure among the lakes. This would suggest that predation pressure by
December 2003 11
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Effects of Vertical Distribution of Zooplankton
zooplanktivorous fish is greater, for example, in Lake Erie, where migration by Bosmina longirostris was
observed, than in Lake Ontario, where such behavior was not apparent. Similarly, zooplanktivory by
vertebrates might be expected to be more intense in Lakes Michigan and Huron than in Lake Superior.
Dorazio et al. (1987) have reported differences in the migratory behavior ofDaphnia species in Lake
Michigan that they attributed to interannual differences in planktivore populations. In 1985, during which
the planktivore Coregonus hoyi was present in large numbers, all zooplankton grazers displayed
pronounced vertical migrations, while in 1983, a year of low planktivore numbers, the dominant
herbivore, D. pulicaria, did not exhibit vertical migration.
The lack of apparent migration in Lakes Superior and Ontario, on the other hand, might be the
result of predation pressure by invertebrates. The main invertebrate predator in the lakes, Bythotrephes
cedarstroemi, was present at most sites in Superior, albeit at very low abundances. However, populations
of this predator were much higher in eastern Erie than in western Ontario, which is the opposite of what
would be expected on the basis of the migration patterns of B. longirostris in these two areas.
Alternatively, nocturnal diurnal migration by planktivores can result in static populations of zooplankton
in shallow waters. Levy (1990) has shown that Bosmina populations remained in shallow waters in
British Columbia lakes where juvenile sockeye salmon migrated vertically, but undertook vertical
migrations where sockeye vertical migrations were reversed and sticklebacks were present. More
concrete conclusions about the possible causes for the differences in vertical distributional patterns
observed in our study will have to await an analysis offish census data in the lakes.
Depth Distribution
A number of calanoid species were found to exhibit depth preferences that resulted in a majority
of the population residing below 20 m at all times. Limnocalanus macrurus, present at a substantial
number of sites in both Lakes Michigan and Superior, was significantly more abundant in the 100-m
tows, both day and night, while Senecella calanoides, also present in both lakes, was virtually absent
from the 20-m tows at all times. Leptodiaptomus sicilis, which was present at most sites in all three upper
lakes, was also always more abundant in the deeper tows, although the difference in abundance between
deep and shallow tows was not as pronounced as for the other two species.
Other studies have documented the deep water preferences of these species. In the Wells (1960)
study of Lake Michigan, L. macrurus was almost completely absent from the top 20 m during a period of
strong thermocline delineation, but when thermal structure was weaker, animals were found in surface
waters at night, suggesting that a sharp temperature gradient could restrict its movements. McNaught and
Hasler (1966) also found evidence that a sharp thermal gradient presented a hindrance to movement of L.
macrurus in Lake Michigan. Wilson and Roff (1973) reported a mean depth of 50-62 m for the L.
macrurus population in Lake Ontario. While usually below 20 m, the animals did reach the surface
waters at night through the summer and fall, although apparently a distinct epilimnion did not form during
their study. Conway et al. (1973), working in Lake Superior, found that L. macrurus was present at all
depths from June through early August, but was restricted to depths below the thermocline when it was
present. L. macrurus appeared to be restricted to a temperature below 12° C. Wells (1960) found that
abundances of S. calanoides in Lake Michigan were usually highest below 20 m. This species was rare in
his collections, but since his tows only extended to 40 m, it is possible that the majority of its populations
were missed. Conway et al. (1973) rarely found this animal above 40 m in Lake Superior. It is clear,
therefore, that shallower tows run the risk of substantially underestimating the abundances of these
species, or missing them altogether.
12 December 2003
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Effecfe of Vertical Distribution of Zooplankton
Lengths of Migrating Individuals
In Lakes Michigan and Huron, where migration occurred, the larger cladocerans appeared to
exhibit greater amplitudes of migration than the smaller organisms. This phenomenon was first suggested
by Wilson and Roff (1973) when they suggested a relationship between zooplankton body weight and
range of migration. Haney and Hall (1975) found that for Daphnia pulex and Daphnia galeata mendotae,
filtering activity at the surface at night was up to 25 times higher due to the presence of larger-bodied
animals, which were absent from surface waters during the day. Wright et al. (1980) used a model to
predict that larger individuals and gravid females of Daphnia parvula would exhibit stronger migrations,
and this has been observed for Daphnia hyalina galeata and Daphnia galeata mendotae, respectively
(Guisande etal, 1991; Lampert, 1992).
Both cyclopoid and diaptomid copepodites, as well as mature Leptodiaptomus minutus, also
showed a significant relationship between body length and depth of migration. Wells (1960) found that
diaptomus copepodites in Lake Michigan did not migrate as strongly as the adults. For the calanoid
copepod Limnocalanus macrurus, both Carter (1969) and Wilson and Roff (1973) found that later
copepodite instars occurred deeper in the water column than earlier instars.
December 2003 13
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Effects of Vertical Distribution of Zooplankton
Conclusions
Members of both the cladocera and the copepoda were found to undertake vertical migrations
below 20 m. These migrations were largely confined to Lakes Michigan and Huron, although the
dominant cladoceran in eastern Lake Erie, Bosmina longirostris, was found to migrate almost entirely out
of the top 20 m during the day. Therefore, care should be taken when interpreting data from these lakes
at sites sampled during the day with 20-m tows. Given the plasticity of migratory behavior documented
in the literature, even data from lakes in which migration was not documented in the present study should
be approached with caution, since differences in predator community structure could have induced
migration in the past in species not presently exhibiting such behavior.
Underestimates of abundances resulting from 20-m tows are likely to be exaggerated when
abundances are converted to biovolumes, particularly for cladocerans, by the tendency of larger animals
to undertake deeper migrations. Previous findings of spatial patterns in zooplankton data generated from
20-m tows should be approached with extreme caution. In cases where the time of sample collection is
known, a possible ameliorative measure is to exclude historical data collected during the day from any
future trends analysis. However, the large copepods Limnocalanus macrurus, Senecella calanoides and
Leptodiaptomus sicilis have depth preferences substantially below 20 m, and have most likely always
been underestimated by 20-m tows, regardless of time of collection.
14 December 2003
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Effecfe of Vertical Distribution of Zooplankton
Literature Cited
Angeli, N., B. Pinel-Alloul, G. Balvay and I. Menard. 1995. Diel patterns of feeding and vertical
migration in daphnids and diaptomids during the clear water phase in Lake Geneva (France).
Hydrobiologia 300/301:163-184.
Balcer, M.D., N.L. Korda and S.I. Dodson. 1984. Zooplankton of the Great Lakes. A guide to the
identification and ecology of the common crustacean species, 174 p. University of Wisconsin
Press, Madison.
Barnhisel, D.R. and H.A. Harvey. 1995. Size specific fish avoidance of the spined crustacean
Bythotrephes: field support for laboratory predictions. Can. J. Fish. Aquat. Sci. 52:768-775.
Brancelj, A. and A. Blejec. 1994. Diurnal vertical migration ofDaphnia hyalina (Leydig, 1860)
(Crustacea: Cladocera) in Lake Bled (Slovenia) in relation to temperature and predation.
Hydrobiologia 284:125-136.
Brooks, J.L. 1957. The systematics of North American Daphnia. Mem. Connecticut Acad. Arts and Sci.
13:1-180.
Carter, J.C.H. 1969. Life cycles of Limnocalanus macrurus and Senecella calanoides, and seasonal
abundance and vertical distributions of various planktonic copepods, in Parry Sound, Georgian
Bay. J. Fish. Res. Bd. Canada 26:2543-2560.
Conway, J.B., O.R. Ruschmeyer, T.A. Olson and T.O. Odlaug. 1973. The distribution, composition, and
biomass of the crustacean zooplankton population in western Lake Superior. Univ. of Minn.
Water Resour. Res. Center Bull. 63.
Cunningham, L. 1972. Vertical migration ofDaphnia and copepods under ice. Limnol. Oceanogr.
17:301-303.
Cuvier, B. 1817. La regne animale. Paris.
Dodson, S. 1988. The ecological role of chemical stimuli for the zooplankton: Predator avoidance
behavior in Daphnia. Limnol. Oceanogr. 33:1431-1439.
Dorazio, R.M., J.A. Bowers, and J.T. Lehman. 1987. Food-web manipulations influence grazer control of
phytoplankton growth rates in Lake Michigan. J. Plankt. Res. 9:891-899.
Dumont, H.J. 1972. A competition-based approach of the reverse vertical migration in zooplankton and
its implications, chiefly based on a study of the interactions of the rotifer Asplanchna priodon ta
(Gorse) with several Crustacea Entomostraca. Int. Rev. Gesamten Hydrobiol. 57:1-38.
Edmondson, W.T. 1959. Freshwater Biology, 2nd Ed., Wiley, New York, 1248 pp.
Evans, M.S., G.J. Warren, D.I. Page and L.F. Flath. 1985. Zooplankton studies at the Donald. C. Cook
Nuclear Plant 1979-1982 investigations, including preoperational (1971-1974) and operational
(1975-1982) comparisons. Great Lakes Research Division Special Report No. 111.
December 2003 15
-------
Effects of Vertical Distribution of Zooplankton
Frost, B.W. and S.M. Bollens. 1992. Variability of diel vertical migration in the marine planktonic
copepod Pseudocalanus newmctni in relation to its predators. Can. J. Fish. Aquat. Sci. 49:1137-
1141.
Geller, W. 1986. Diurnal vertical migration of zooplankton in a temperate great lake (L. Constance): A
starvation avoidance mechanism? Arch. Hydrobiol. Suppl. 74:1-60.
Gliwicz, M.Z. 1986. Predation and the evolution of vertical migration in zooplankton. Nature 320:746-
748.
Guisande, C., A. Duncan and W. Lampert. 1991. Trade-offs in Daphnia vertical migration strategies.
Oecologia 87(3):357-359.
Haney, J.F. and D.J. Hall. 1975. Diel vertical migration and filter feeding activities of Daphnia. Arch.
Hydrobiol. 75:413-441.
Haney, J.F. and D.J. Hall. 1973. Sugar coated Daphnia: a preservation technique for Cladocera. Limnol.
Oceanogr. 18:331-333.
Hudson, P.L., J.W. Reid, L.T. Lesko and J.H. Selgeby. 1998. Cyclopoid and Harpacticoid Copepods of
the Laurentian Great Lakes. Ohio Biological Survey Bulletin NS 12(2).
Hutchinson, G.E. 1967. A Treatise on Limnology Vol. II: Introduction to Lake Biology and the
Limnoplankton. John Wiley and Sons, New York.
King, C.E. and M.R. Miracle. 1995. Diel vertical migration by Daphnia longispina in a Spanish lake:
Genetic sources of distributional variation. Limnol. Oceanogr. 40:226-231.
Lampert, W. 1989. The adaptive significance of diel vertical migration of zooplankton. Fund. Ecol. 3:21-
27.
Lampert, W. 1992. Zooplankton vertical migration: Implications for phytoplankton-zooplankton
interactions. Ergebn. Limnol. 35:69-78.
Lehman, J.T. and C.E. Caceres. 1993. Food-web responses to species invasion by a predatory invertebrate
Bythotrephes in Lake Michigan. Limnol. Oceanogr. 38:879-891.
Leibold, M.A. 1990. Resources and predators can affect the vertical distributions of zooplankton. Limnol.
Oceanogr. 35:938-944.
Levy, D.A. 1990. Reciprocal diel vertical migration behavior in planktivores and zooplankton in British
Columbia lakes. Can. J. Fish. Aquat. Sci. 47:1755-1764.
Leydig, F. 1862. Die Naturgeschichte der Daphniden. Tubingen.
Loose, C.J. 1993. Daphnia diel vertical migration behavior: response to vertebrate predator abundance.
Ergebnisse der Limnologie 39:29-36.
Makarewicz, J.C. 1987. Phytoplankton and zooplankton composition, abundance and distribution: Lake
Erie, Lake Huron an Lake Michigan - 1983 Volume 1 and 2 U.S. Environmental Protection
Agency. EPA-905/2-87-002. 183 p.
16 December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Makarewicz, J.C. 1988. Phytoplankton and zooplankton composition, abundance and distribution: Lakes
Erie, Huron and Michigan - 1984 U.S. Environmental Protection Agency. EPA-905/3-88-001.
Makarewicz, J.C. and P. Bertram. 1991. Phytoplankton and zooplankton composition, abundance and
distribution Lakes Erie, Huron and Michigan - 1985. U.S. Environmental Protection Agency.
EPA- 905/3-85-003.
Makino, W., H. Haruna and S. Ban. 1996. Diel vertical migration and feeding rhythm ofDctphnict
longispina and Bosmina coregoni in Lake Toya, Hokkaido, Japan. Hydrobiologia 337(103): 133-
143.
McLaren, LA. 1963. Effects of temperature on growth of zooplankton and the adaptive value of vertical
migration. J. Fish. Res. Board Can. 20:685-727.
McLaren, LA. 1974. Demographic strategy of vertical migration by a marine copepod. American
Naturalist 108:91-102.
McNaught, B.C. and A.D. Hasler. 1966. Photoenvironments of planktonic Crustacea in Lake Michigan.
Verh. Internal. Verein. Limnol. 16:194-203.
Neill, W.E. 1990. Induced vertical migration in copepods as a defense against invertebrate predation.
Nature 345:524-526.
Nesbitt, L.M. and H.P. Riessen. 1996. Opposing predation pressures and induced vertical migration
responses in Daphnia. Limnol. Oceanogr. 41:1036-1311.
Ohman, M.D., B.W. Frost and E.B. Cohen. 1983. Reverse diel migration: an escape from invertebrate
predators. Science 220:1404-1407.
Pinkham, C.F.A. and J.G. Pearson. 1976. Applications of a new coefficient of similarity to pollution
surveys. J. Water Poll. Control Fed. 48:717-723.
Ringelberg, J. 1964. The positively phototactic reaction of Daphnia magna. A contribution to the
understanding of vertical migration. Netherlands J. Sea Res. 2:319-406.
Ringelberg, J. 1991. A mechanism of predator-mediated induction of diel vertical migration in Daphnia
hyalina.J. Plankton Res. 13:83-89.
Ringelberg, J., B.J.G. Flik, D. Lindenaar and K. Royackers. 1991. Diel vertical migration of Daphnia
hyalina (sensu latiori) in Lake Maarsseveen: Part 1. Aspects of seasonal and daily timing. Arch.
Hydrobiol. 121:129-145.
Rivier, I.K. 1998. The Predatory Cladocera (Onychopoda: Podonidae, Polyphemidae, Cercopagidae)
and Leptodorida of the World. Backhuys Publishing, Leiden, pp 213.
Russell, F.S. 1927. The vertical distribution of plankton in the sea. Biol. Rev. 2:213 pp.
Siebeck, O. 1980. Optical orientation of pelagic crustaceans and its consequence in the pelagic and littoral
zones, pp. 28-38 in W.C. Kerfoot (Ed.) Evolution and Ecology of Zooplankton Communities.
University Press of New England, Hanover, NH.
December 2003 17
-------
Effects of Vertical Distribution of Zooplankton
Spaak, P. and J. Ringelberg. 1997. Differential behaviour and shifts in genotype composition during the
beginning of a seasonal period of diel vertical migration. Hydrobiologict 360:177-185
Stich, H.B. 1989. Seasonal changes of diel vertical migrations of crustacean plankton on Lake Constance.
Arch. Hydrobiol. Suppl. 83:355-405.
Stich, H.B. and W. Lampert. 1981. Predator evasion as an explanation of diurnal vertical migration by
zooplankton. Nature 293:396-398.
Stirling, D.G., D.J. McQueen and M.R.S. Johannes. 1990. Vertical migration in Daphnia galeata
mendotae (Brooks): Demographic responses to changes in planktivore abundance. Can. J. Fish.
Aquat. Sci. 47(2): 3 95-400.
Van Gool, E. and J. Ringelberg. 1998. Light-induced migration behaviour of Daphnia modified by food
and predator kairamones. Animal Behaviour 56:741-747.
Von Elert, E. and C.J. Loose. 1996. Predator induced diel vertical migration in Daphnia: enrichment and
preliminary chemical characterization of a kairomone exuded by fish. J. Chem. Ecol. 22:885-895.
Watt, P.J. and S. Young. 1994. Effect of predator chemical cues on Daphnia behaviour in both horizontal
and vertical planes Animal Behaviour 48:861-869.
Weider, L.J. 1984. Spatial heterogeneity of Daphnia genotypes: Vertical migration and habitat
partitioning. Limnol. Oceanogr. 29:225-235.
Wells, L. 1960. Seasonal abundance and vertical movements of planktonic Crustacea in Lake Michigan.
U.S. Fish. Wildlife Serv. Bull. 60 (172):343-369.
Whittaker, R.H. 1952. A study of summer foliage insect communities in the Great Smoky Mountains.
Ecol. Monogr. 22:6-31.
Williamson, C.E. and R.E. Magnien. 1982. Diel vertical migration inMesocyclops edax: implications for
predation rate estimates. J. Plankton Res. 4:329-339.
Wilson, J.B. and J.C. Roff 1973. Seasonal vertical distributions and diurnal migration patterns of Lake
Ontario crustacean zooplankton. Proc. 16th Conf. Great Lakes Res. pp. 190-203.
Wright, D., W.J. O'Brien and G.L. Vinyard. 1980. Adaptive value of vertical migration: A simulation
model argument for the predation hypothesis. In: Kerfoot, W.C. (Ed.) Evolution and Ecology of
Zooplankton Communities. University Press of New England. Hanover. New Hampshire.
Young, S. and P.J. Watt. 1994. Behavioural and genetic differences in populations of Daphnia subjected
to different levels of predation. Freshwater Biology 32:479-487.
Young, S. and P.J. Watt. 1996. Daily and seasonal vertical migration rhythms in Daphnia. Freshwater
Biology 36:17'-22.
Zaret, T. and J.S. Suffern. 1976. Vertical migration in zooplankton as a predator avoidance mechanism.
Limnol. Oceanogr. 21:804-813.
18 December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Table 1. Summary of Results of Wilcoxon Signed Rank Test Comparison Between Zooplankton Samples
Collected with 64-|jm and 153-|jm Mesh Nets
Species
Bosmina longirostris
Bythotrephes cederstroemi
Cyclops copepodites
Cyclops vernalis
Daphnia galeata mendotae
Daphnia longirostris
Daphnia retrocurva
Diacyclops thomasi
Diaphanosoma birgei
Diaptomid copepodites
Epischura copepodites
Epischura lacustris
Eubosmina coregonii
Eurytemora affinis
Leptodiaptomus ashlandi
Leptodiaptomus minutus
Leptodiaptomus sicilis
Leptodiaptomus siciloides
Leptodora kindtii
Mesocyclops copepodites
Mesocyclops edax
Skistodiaptomus oregonensis
Tropocyclops copepodites
Tropocyclops prasinus mexicanus
W
-28
-4
-6
4
-25
1
-8
4
-13
-14
12
7
-8
-9
6
-8
-15
-9
-9
-56
-28
-17
14
-10
T+
25
12
36
16
15
8
10
16
4
32
45
26
29
3
17
29
3
3
18
11
25
19
21
34
T-
-53
-16
-42
-12
-40
-7
-18
-12
-17
-46
-33
-19
-37
-12
-11
-37
-18
-12
-27
-67
-53
-36
-7
-44
P(est.)
.29
.8
.845
.8
.221
1
.554
.8
.208
.61
.666
.722
.756
.281
.673
.756
.142
.281
.636
.031
.29
.415
.272
.724
P(exact)
.301
.813
.85
.813
.232
1
.578
.813
.219
.622
.677
.734
.765
.313
.688
.765
.156
.313
.652
.027
.301
.432
.297
.733
P<0.05
*
Indicates that the difference was statistically significant
December 2003
19
-------
Effects of Vertical Distribution of Zooplankton
Table 2. Summary of ANOVA Results Testing for Depth Preferences and Indications of Zooplankton
Migratory Behavior in Lakes Huron (HU), Michigan (Ml), and Superior (SU)
Species
Cladocerans
Bosmina longirostris
Bythotrephes cedarstroemi
Daphnia galeata mendotae
Eubosmina coregoni
Holopedium gibberum
Calanoid copepods
Diaptomid copepodites
Epischura lacustris
Epischura copepodites
Leptodiaptomus ashlandi
Leptodiaptomus minutus
Leptodiaptomus sicilis
Limnocalanus macrurus
Senecella calanoides
Skistodiaptomus oregonensis
Cyclopoid copepods
Cyclops copepodites
Diacyclops thomasi
Tropocyclops prasinus
Tropocyclops copepodites
Time
HU
0.015
0.261
0.005
0.736
0.161
0.249
0.692
0.034
0.021
0.258
0.136
0.062
Ml
0.823
0.050
0.121
0.070
0.115
0.055
0.019
0.185
0.239
0.155
0.100
0.002
<0.001
0.383
0.637
SU
0.140
0.655
0.803
0.834
0.715
0.552
0.004
0.369
0.312
0.226
Depth
HU
0.003
0.102
0.009
0.612
0.839
0.192
<0.001
0.104
0.031
<0.001
0.605
0.164
Ml
0.017
0.227
0.435
0.492
0.015
0.407
0.012
0.002
<0.001
0.036
0.054
0.142
0.002
0.007
0.019
SU
0.01
0.014
<0.001
0.097
0.054
0.021
<0.001
<0.001
<0.001
0.004
Time x Depth
HU
0.012
0.932
0.001
0.285
0.566
0.578
0.761
0.121
0.011
0.305
0.460
0.170
Ml
0.015
<0.001
0.015
0.006
0.070
0.058
<0.001
0.366
0.743
0.155
0.117
<0.001
<0.001
0.380
0.568
SU
0.598
0.556
0.108
0.855
0.775
0.609
0.016
0.369
0.555
0.198
All species with an adequate sample size were tested.
Probabilities for factor effects are presented, with boldface denoting significance at a = 0.05.
20
December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Table 3. Results of Paired f-Test Comparison of Zooplankton Abundances Estimated from Shallow and
Deep Tows at Stations Sampled During the Day in Lakes Erie and Ontario
Lake Erie
Lake Ontario
Species
Bosmina longirostris
Bythotrephes cedarstroemi
Cyclops copepodites
Daphnia galeata mendotae
Diaptomid copepodites
Epischura copepodites
Epischura lacustrus
Leptodiaptomus minutus
Leptodiaptomus oregonensis
Leptodora kindti
Mesocyclops copepodites
Mesocyclops edax
Tropocyclops copepodites
Tropocyclops prasinus mexicanus
Bosmina longirostris
Daphnia retrocurva
Diacyclops thomasi
Diaptomid copepodites
t
-3.812
0.571
1.035
-0.082
1.794
2.481
1.335
1.621
-0.244
2.969
1.147
2.179
2.029
2.945
1.486
1.909
1.266
-2.354
P
0.03
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
0.05
The following one-tailed hypothesis was tested:
H0 : 0, with boldface denoting significance at a = 0.05.
December 2003
21
-------
Effects of Vertical Distribution of Zooplankton
Table 4. Results of f-Tests or Mann-Whitney Rank Sum Tests Comparing Zooplankton Length in Deep
and Shallow Day Tows
Species
Bosmina longirostris
Bythotrephes cedarstroemi
Cercopagis pengoi
Cyclops copepodites
Daphnia galeata mendotae
Daphnia retrocurva
Diacyclops thomasi
Diaptomid copepodites
Epischura copepodites
Epischura lacustris
Eubosmina coregoni
Holopedium gibberum
Leptodiaptomus ashlandi
Leptodiaptomus minutus
Leptodiaptomus sicilis
Leptodora kindtii
Limnocalanus macrurus
Polyphemus pediculus
Tropocyclops copepodites
Tropocyclops prasinus mexicanus
Huron
<0.001
0.006*
0.241
<0.001
0.309
<0.001
0.035
0.641
<0.001
0.112
0.355
0.319
0.380
0.686
Michigan
<0.001
<0.001
0.028
0.331
<0.001
0.379
0.308
0.068
0.016
<0.001
0.061
0.694
Superior
0.380
0.262
0.510
<0.001
0.008
0.001
0.080
0.407
0.902
0.340
0.025
Ontario
0.724
0.484
0.620
0.369
0.914
0.152
0.782
0.862*
0.203*
0.020
0.300*
Probabilities of H0\ lengthd - fengf/)sMow are shown, with boldface denoting significance at a = 0.05.
* Indicates f-test used
22
December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Table 5. Percent Differences Between Zooplankton Length Measured from Deep and Shallow Day Tows
Species
Bosmina longirostris
Bythotrephes cedarstroemi
Cyclops copepodites
Daphnia galeata mendotae
Diacyclops thomasi
Diaptomus copepodites
Epischura copepodites
Eubosmina coregoni
Leptodiaptomus minutus
Leptodiaptomus sicilis
Limnocalanus macrurus
Huron
29.9%
-3.3%
23.3%
0.8%
0.4%
0.5%
Michigan
13.6%
0.4%
1.0%
49.2%
4.5%
0.8%
Superior
0.7%
-0.1%
0.3%
3.3%
Ontario
9.2%
Percent difference is calculated as:
length
shallow
Only those species/lake combinations that have statistically significant differences in length between deep and shallow day tows
are shown (see Table 4).
December 2003
23
-------
Figure 1. Summer 1998 GLNPO Zooplankton Sampling Stations
24
-------
Effecfe of Vertical Distribution of Zooplankton
Figure 2. Factor Structure of the ANOVA Analyses
Time of Day
Shallow
Deep
Day
Sitel
Site 2
Site3
Site 4
Night
SiteS
Site 6
Site 7
SiteS
Tow
Depth
Depth of tow (depth) and time of day (time) are the two factors, with species abundance the response variable. Note that sample
site is crossed with depth of tow, but nested within time of day.
December 2003
25
-------
Figure 3. Differences in Zooplankton Abundance and Community Composition between Shallow and Deep Tows (Lake Superior,
Summer 1998)
Bosmina sp.
Calanoida
Cladocerans
Cyclopoida
Daphnia sp.
Immature Calanoida
Immature Cyclopoida
Sampled in Daylight
20,000/rrf
10,000/m3
<2,000/m3
Shallow
26
-------
Figure 4. Differences in Zooplankton Abundance and Community Composition between Shallow and Deep Tows (Lake Michigan,
Summer 1998)
Shallow
30,000/mJ
15,000/m3
<3,000/m3
Bosmina sp.
Calanoida
Cladocerans
Cyclopoida
Daphnia sp.
Immature Calanoida
Immature Cyclopoida
Sampled in Daylight
27
-------
Figure 5. Differences in Zooplankton Abundance and Community Composition between Shallow and Deep Tows (Lake Huron, Summer
1998)
Bosmina sp.
Calanoida
Cladocerans
Cyclopoida
Daphnia sp.
Immature Calanoida
Immature Cyclopoida
Sampled during Daylight
60,000/rrf
30,000/m3
<6,000/m3
28
-------
Figure 6. Differences in Zooplankton Abundance and Community Composition between Shallow and Deep Tows (Lake Erie, Summer
1998)
Shallow
Bosmina sp.
Calanoida
Cladocerans
Cyclopoida
Daphnia sp.
Immature Calanoida
Immature Cyclopoida
Sampled in Daylight
70,000/m3
20,000/m3
<7,000/m3
29
-------
Figure 7. Differences in Zooplankton Abundance and Community Composition between Shallow and Deep Tows (Lake Ontario,
Summer 1998)
Bosmina sp.
Calanoida
Cladocerans
Cyclopoida
Daphnia sp.
Immature Calanoida
Immature Cyclopoida
Sampled During Daylig
200,000/m3
100,000/m3
20,000/m3
Shallow
30
-------
Effecfe of Vertical Distribution of Zooplankton
Figure 8. Average Relative Similarity between Zooplankton Communities from Shallow and Deep Tows, for
Tows Taken at Night and during the Day
o.o
Huron Michigan Ontario Superior Erie-E Erie-CW
Lake
An asterisk (*) indicates a significant difference between day and night comparisons, at a = 0.05.
December 2003
31
-------
Effects of Vertical Distribution of Zooplankton
Lake Huron
Figure 9. Bosmina longirostris Abundance for both Shallow and Deep Tows taken during the Day
and Night
3500
3000 ~
2500 -
CO
"0 2000 -
O
to 1500 H
o
C
E 100° ~
500 -
0 -
1400
1200 H
1000 -
co
-55 800
O
C
CO 600
C
13
.a
400 -
200 -
0 -
Lake Michigan
day/shallow day/deep night/shallow night/deep
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
32
December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Figure 10. Eubosmina coregoni Abundance for both Shallow and Deep Tows taken during the Day
and Night
OO
-------
Effects of Vertical Distribution of Zooplankton
Figure 11. Holopedium gibberum Abundance for both Shallow and Deep Tows taken during the Day
and Night
2500
2000 -
E 1500 -
IB
o
c
03 1000 -
T3
C
U
ff 500 -
0 -
Lake Superior
day/shallow day/deep night/shallow night/deep
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
34
December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Figure 12. Daphnia galeata mendotae Abundance for both Shallow and Deep Tows
taken during the Day and Night
60000
50000 -
40000 -
O 30000 -
co
C 20000 -
3
.O
** 10000 -
0 -
14000
12000
10000
r>
Jj 8000
O
C
CO 6000
C
jjjj 4000 -
2000
0
2500
2000 -
E 1500
"55
o
c
CO 1000 -
T3
C
I 500 H
0 -
Lake Huron
Lake Michigan
Lake Superior
day/shallow day/deep night/shallow night/deep
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
December 2003
35
-------
Effects of Vertical Distribution of Zooplankton
Figure 13. Bythotrephes cedarstroemi Abundance for both Shallow and Deep Tows taken during
the Day and Night
40
30 -
g 20 -\
C
CO
"D
i 10 H
JD
0 -
40
30 -
Q)
O
C
CD
20 -
10 -
0 -
Lake Huron
Lake Superior
i
day/shallow day/deep night/shallow night/deep
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
36
December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Figure 14. Tropocyclops prasinus Abundance for both Shallow and Deep Tows taken during the
Day and Night
300
250 -
200
CD
O 150 -\
CD
T3
13
_Q
100 -
50 -
0 -
180
160 -
140 -
120 -
0) 100
O
c
03
"O
C
13
_Q
80 -
60 -
40
20 -
0 -
Tropocyclops prasinus mexicanus
Tropocyclops copepodites
day/shallow day/deep night/shallow night/deep
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
December 2003
37
-------
Effects of Vertical Distribution of Zooplankton
Figure 15. Diacyclops thomasi and Cyclops Copepodite Abundance in Lake Michigan for both
Shallow and Deep Tows taken during the Day and Night
1600
1400 -
1200 -
E 1000 -
"35
g 800 -
CD
T3
C
600 -
400 -
200 -
0 -
7000
6000 H
5000 -
co
-55 4000
O
CD 3000 H
T3
£ 2000
1000 -
0 -
Diacyclops thomasi
Cyclops copepodites
day/shallow day/deep night/shallow night/deep
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
38
December 2003
-------
Effects of Vertical Distribution of Zooplankton
Figure 16. Diacyclops thomasi and Cyclops Copepodite Abundance in Lake Superior for both
Shallow and Deep Tows taken during the Day and Night
. en
o o o
0 0 0
o o o
o
Diacyclops thomasi
Cyclops copepodites
i
day/shallow day/deep night/shallow night/deep
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
December 2003
39
-------
Effects of Vertical Distribution of Zooplankton
Figure 17. Diacyclops thomasi and Cyclops Copepodite Abundance in Lake Huron for both Shallow
and Deep Tows taken during the Day and Night
5000
4000
Diacyclops thomasi
*>
E 3000 H
ID
0
c
CO 2000 H
T3
C
13
<| 1000 -j
0 -j
1 nnnn
f a
Cyclops copepodites
8000 H
E 6000 -]
"CD
o
c
CD 4000 H
C
zs
2000 H
day/shallow day/deep night/shallow night/deep
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
40
December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Figure 18. Leptodiaptomus ashlandi Abundance for both Shallow and Deep Tows taken during the
Day and Night
4000 -, 1
Lake Huron
3000 -j
o
"g 2000 j
C
03
T3
C
1000 -
0 H
CO
O
C
03
T3
C
£1
lLa
1400 -|
1200 -J
1000
800 H
600 H
400 J
200 H
ke Michigan
0 -j ' '
i i
i i
i i
day 20
day 100
night 20 night 100
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
December 2003
41
-------
Effects of Vertical Distribution of Zooplankton
Figure 19. Leptodiaptomus minutus Abundance for both Shallow and Deep Tows taken during the
Day and Night
2500
2000 -
E 1500
CO 1000
T3
c
3
500
0 -
600
500 -
n 400
g
O 300
c
CO
T3
c 200
3
JD
100 -
Lake Huron
Lake Michigan
day 20
day 100
night 20 night 100
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
42
December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Figure 20. Skistodiaptomus oregonensis Abundance for both Shallow and Deep Tows taken during
the Day and Night
300 -
250
CO
c
bundance/n
->> -* NJ
O U1 O
o o o
50 -
Lake Michigan
L !
day 20
day 100
night 20 night 100
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
December 2003
43
-------
Effects of Vertical Distribution of Zooplankton
Figure 21. Leptodiaptomus silicis Abundance for both Shallow and Deep Tows
taken during the Day and Night
2500
Lake Huron
2000 -
E 1500 -
IB
o
CD 1000 -
T3
C
3
500
CD
O
C
CD
T3
C
1800 -
1600 -
1400
1200 -
1000
800
600 -
400 -
200
0 -
350 -
300
Lake Michigan
Lake Superior
250 -
200
CO 150
"O
100 -
day 20
day 100
night 20 night 100
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
44
December 2003
-------
Figure 22. Relative Abundance of Leptodiaptomus silicis, as Percent of Total Crustacean Abundance, at Sites in Lakes Superior,
Michigan, and Huron, Estimated by Shallow and Deep Tows (Summer 1998)
Shallow
Deep
13%
45
-------
Effects of Vertical Distribution of Zooplankton
Figure 23. Diaptomid Copepodite Abundance for both Shallow and Deep Tows
taken during the Day and Night
§
CO
C
-D
16000
14000
12000
10000
8000
6000
4000
2000
10000 |
8000
Lake Huron
Lake Michigan
6000
co 4000
-a
3
5 2000
0
8000
6000
g 4000
c
co
T3
§ 2000
J3
<
Lake Superior
day 20
day 100
night 20
night 100
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
46
December 2003
-------
Figure 24. Relative Abundance of Limnocalanus macrurus, as Percent of Total Crustacean Abundance, at Sites in Lakes Superior,
Michigan, and Huron, Estimated by Shallow and Deep Tows (Summer 1998)
Shallow
47
-------
Effects of Vertical Distribution of Zooplankton
Figure 25. Limnocalanus macrurus Abundance in Lakes Michigan and Superior for both Shallow
and Deep Tows taken during the Day and Night
250
200 -
E 150
"S
o
c
TO 100
50
0 -
600
500
400 -
300 -
c
CO
C 200 -
13
100 -
0 -
Lake Michigan
Lake Superior
day 20
day 100
night 20 night 100
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
48
December 2003
-------
Effecfe of Vertical Distribution of Zooplankton
Figure 26. Senecella calanoides Abundance in Lakes Michigan and Superior for both Shallow and
Deep Tows taken during the Day and Night
0)
o
c
03
T3
C
U
CO
12
10 H
8
6
4
2
0 -
50
40
E 30 -
o
03 20
T3
C
I Ğ
Lake Michigan
Lake Superior
day 20
day 100
night 20 night 100
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
December 2003
49
-------
Figure 27. Relative Abundance of Senecella calanoides, as Percent of Total Crustacean Abundance, at Sites in Lakes Superior and
Michigan, Estimated by Shallow and Deep Tows (Summer 1998)
Shallow
Deep
1.7%
50
-------
Effecfe of Vertical Distribution of Zooplankton
Figure 28. Epischura lacustris Abundance in Lakes Michigan, Huron, and Superior
for both Shallow and Deep Tows taken during the Day and Night
250
Lake Michigan
200 -|
r}
E 150 1
CO 100 -I
c
5 50 -I
400
Lake Huron
300
g 200
c
ra
T3
§ 100
-D
35
30 H
25 J
55 20
o
(C 15
T3
10
0 -!
Lake Superior
day 20
day 100
night 20 night 100
The solid line in the box represents the median abundance, while the top and bottom edges of the box are the 25th and 75th
percentile values. The whiskers represent the 10th and 90th percentile values, and the solid black dots are outliers.
December 2003
51
-------
Effects of Vertical Distribution of Zooplankton
Figure 29. Epischura Copepodite Abundance in Lakes Huron and Michigan for both Shallow and
Deep Tows taken during the Day and Night
CO
c
-Q
1600
1400 -
1200 -
1000 -
800 -
600 -
400
200 -
0
800
600
-------
Figure 30. Percent Abundance of Bosmina longirostris, in Lakes Erie and Ontario, in Shallow and Deep Tows taken during the Day
and Night
60
50
40
30 -
20 -
10
0
ON 41
Percent
Abundance
------- |