J. Great Lakes Res. 30(3):390–396
Internat. Assoc. Great Lakes Res., 2004
Refugia and Local Controls: Benthic Invertebrate Dynamics in Lower
Green Bay, Lake Michigan following Zebra Mussel Invasion
Tara Reed1,*, Sarah J. Wielgus2, Alyssa K. Barnes2, Jeremiah J. Schiefelbein3, and Amy L. Fettes4
1Natural
and Applied Sciences
University of Wisconsin-Green Bay
Green Bay, Wisconsin 54311
2Department
of Natural and Applied Sciences
University of Wisconsin-Green Bay
Green Bay, Wisconsin 54311
3Environmental
Science and Policy
University of Wisconsin-Green Bay
Green Bay, Wisconsin 54311
4Integrated
Paper Services, Inc.
P.O. Box 446
Appleton, Wisconsin 54912
ABSTRACT. In many aquatic ecosystems benthic invertebrate abundance has increased following
zebra mussel (Dreissena polymorpha) invasion. We examine the impact of zebra mussel density on the
abundance and distribution of benthic invertebrates and postulate refuge from predation as a mechanism
for the increases we found in some taxa. Benthic invertebrates in zebra mussel druses and in adjacent
sediment samples were surveyed at sites in six locations representing various trophic conditions in lower
Green Bay. Mean invertebrate density and taxa richness were significantly higher in the druses than in
the adjacent sediment. Species diversity in the druses was inversely correlated to turbidity over the study
area. Sediment samples were dominated by oligochaetes and chironomids. Amphipods were the most
abundant taxa in most, but not all, of the druse samples. Other taxa present included leeches, hydra,
mayflies, and caddisflies. The effectiveness of druses as refuge from predation for amphipods was investigated under laboratory conditions with various predators (perch, round goby, and rusty crayfish). In
mesocosms, predation losses averaged 75% lower where zebra mussels were present. In the absence of
mussels, predation loss to perch and round goby was 100% and 66% to crayfish. We conclude that the
increased abundance of other invertebrates in druses in lower Green Bay may be due to increased refugia, however the assemblage composition at any given site varies with local conditions.
INDEX WORDS:
Zebra mussel, benthic invertebrate, Green Bay.
INTRODUCTION
Zebra mussel invasion can cause dramatic
changes throughout a lake (Nalepa and Fahnenstiel
1995). Perhaps the most dramatic changes occur in
the benthos, where zebra mussels colonize in large
numbers and substantially modify the physical
character of the bottom. The change in habitat
structure can lead to greater macroinvertebrate den-
sity (Stewart et al. 1998, Horvath et al. 1999). Benthic invertebrate densities increase on rocky substrates where complex habitat is already available
(Strayer and Smith 2000) and in soft sediments
where free standing zebra mussel colonies (druses)
provide refuge from predation (Stewart et al. 1999),
habitat and a constant food source in the form of
feces and pseudofeces (Silver Botts et al. 1996). In
some systems benthic invertebrate densities have
not increased (i.e., Mercer et al. 1999), but zebra
mussel colonization has been associated with large
*Corresponding author. E-mail: [email protected]
390
391
Benthic Invertebrates in Green Bay
increases in particular taxa such as Gammarus amphipods (Dermott and Barton 1992, Ricciardi et al.
1997). Even in systems where there has been no
overall gain in invertebrate abundance, amphipod
biomass has increased (Kuhns and Berg 1999).
Predation is a strong structuring variable for amphipod distribution and abundance (Russo 1991,
Corona et al. 2000). Researchers have suggested
that zebra mussel druses may offer a refuge to invertebrates from predation by fish (Gonzalez and
Downing 1999, Stewart et al. 1999). Amphipod activity outside of mussel beds can be reduced by
10–95% in the presence of fish (Kolar et al. 2002).
Round gobies can significantly reduce overall invertebrate biomass (Kuhns and Berg 1999). Yellow
perch consume significantly more benthos when
zebra mussels are present (Thayer et al. 1997)
which may reflect a greater overall invertebrate
density when zebra mussels are present. In mesocosm experiments perch were able to capture amphipods from within a continuous single layer of
zebra mussels, although at somewhat reduced rates
compared with sandy substrates (Cobb and Watzin
2002). In areas dominated by soft sediments, such
as Green Bay, zebra mussel druses form rolling
spheres that are likely to provide a different architectural structure than layers of mussels on hard
substrate. This increased architectural structure may
provide increased refugia and may serve as a mechanism for increased invertebrate abundances in
druses.
While the abundance of benthic invertebrates increases following zebra mussel invasion, community composition may not change substantially
(Hayes et al. 1999) because other factors may be
controlling community composition. In lower
Green Bay there is a strong trophic gradient, ranging from relatively nutrient poor waters along the
western shore, to a hypereutrophic area at the
mouth of the Fox River, which contributes a large
sediment load to the bay, as well as a more
mesotrophic region moving up along the eastern
shore (Sager and Richman 1991). This makes Green
Bay a unique system within which to study the role
of druses versus other environmental factors, such
as temperature, substrate type, and trophic status,
on the distribution of benthic invertebrates within
and adjacent to zebra mussel colonies.
Here we present the results of a survey of selected sites in lower Green Bay designed to compare the impact of druses with other environmental
variables on the distribution of non-zebra mussel
invertebrates (hereafter referred to as invertebrates)
colonizing zebra mussel druses. We also present the
results of a laboratory experiment designed to investigate the effectiveness of zebra mussel druses
as refugia for amphipods from fish predation.
METHODS
Field Survey
Six sites were chosen around Lower Green Bay
(Fig. 1) representing the variety of trophic conditions in the bay. Green Bay has a strong trophic gradient driven by the circulation patterns (Sager and
Richmond 1991). Relatively oligotrophic water enters the bay from Lake Michigan and flows south
down the west shore and toward the inner bay
(Miller and Saylor 1985). The Fox River delivers a
heavy sediment load to the inner bay making those
waters hypereutrophic. As the current moves north
along the eastern shore it carries this sediment, the
concentration of which gradually drops off creating
a strong trophic gradient that goes from hypereutrophic to mesotrophic conditions (Sager and Richmond 1991). Sites include relatively nutrient poor
FIG. 1. Map of sampling sites on lower Green
Bay. Arrows indicate the direction of current
which flows from the more oligiotrophic waters of
Lake Michigan to the hypereutrophic waters at the
mouth of the Fox River. As water moves up the
east shore the sediment load drops off, creating a
strong trophic gradient (Sager and Richmond
1991).
392
Reed et al.
TABLE 1. Environmental variables and non-zebra mussel invertebrate densities and taxa richness for
several sites in Green Bay.
Site
West shore
24 (1 m)
24 (3 m)
17 (1 m)
17 (3 m)
Dissolved
Oxygen
Turbidity
(mg/L) Secchi (m) (NTU)
Substrate
Density of non
zebra mussel
invertebrate
per cm3 druse
Density of non
zebra mussel
invertebrates per
l cm3 sediment
Taxa
richness
druse
samples
Taxa
richness
sediment
samples
11.5
10.7
10.6
9.2
2
2
2
2
2.23
2.23
1.86
1.86
Sand
Sand
Sand
Sand
2.333
1.543
1.923
0.433
0.009
0.261
0.034
0.010
10
11
9
8
2
7
2
3
Fox inlet
1 (1 m)
9 (1 m)
12.3
10.2
0.5
0.5
15.1
9.75
Silt
Silt
0.227
3.137
0.085
0.003
4
4
3
1
East shore
14 (3 m)
4E (1 m)
4E (3 m)
10.2
8.8
11.2
0.5
3
3
14.53
0.97
0.97
Silt
Silt
Silt
2.171
0.124
0.509
0.044
0.025
0.018
11
8
7
3
3
3
sites (17 and 24), very nutrient rich, highly turbid
inner bay sites (1 and 9), and east shore sites where
the nutrient load declines as the current moves up
the bay (14 and 4E). All sites had either sand or silt
substrate (Table 1).
Druse samples were taken by divers who collected all zebra mussels within a 20 m2 swath along
a 10 m transect. To collect sediment samples divers
took three sediment cores (depth 10 cm, diameter
7.6 cm) from within a square meter quadrat haphazardly thrown from the boat. These different sampling methods were necessary because the zebra
mussel distribution was extremely patchy and at
most sites a randomly thrown quadrat would not
have captured a druse. Samples were preserved in
95% ethanol for later processing. Wherever possible we sampled 1 m and 3 m depth sites at each location. Depth at sites 1 and 9 was insufficient to
collect a 3 m sample. Site 14 had no zebra mussels
at the 1 meter site so only the 3 meter site was sampled. Other limnological measures taken at each site
included Secchi depth, a dissolved oxygen/temperature profile, and turbidity. In the laboratory invertebrates were picked from the samples using sugar
flotation technique (Wetzel and Likens 2000) and
identified to the appropriate taxonomic level using
available keys (Hilsenhoff 1975, Thorp and Covich
1991, Merritt and Cummins 1996).
Invertebrate abundances in the druses sample series and the sediment sample series were based on
volume. While 10 cm of sediment were collected in
each core to insure that all invertebrates were collected, in calculating the sediment volume we assumed that the majority of invertebrates were
confined to the first 5 cm based on observed depth
of chironomid activity (Mermillod-Blondin et al.
2002). This gave each sediment core a volume of
226.8 cm 3 . Mussels in druses were separated,
counted, and sorted into 5-cm size classes. Volumes
for each size class of mussel were calculated by
measuring water displacement by mussels in each
size class (based on 20 mussels of each size class).
The volume of a druse was therefore the sum of the
average volume of the mussels in each size category. The density of invertebrates within the druses
was compared to that in the sediment using a paired
t-test. We also investigated correlations between
density within druses and the sediment, taxa richness and environmental variables using a correlation matrix.
Predation Experiment
A controlled laboratory experiment was used to
test the effectiveness of zebra mussel druses as
refuge from predators. We subjected amphipods to
predation pressure far more extreme than would be
found in natural environments by placing them in
barren plastic mesocosms with individual predators
for 24 hours. By weighting the experiment in favor
of the predators we insured that we were testing the
effectiveness of the druses as refuge rather than
testing the effectiveness of the predators.
Benthic Invertebrates in Green Bay
Three predators were used; yellow perch (Perca
flavescens), average length 8.6 cm (± 2.1), rusty
crayfish (Orconectes rusticus), average length 13.4
cm (± 1.7), and a single round goby (Neogobius
melanostomus), length 5.6 cm. When individual
predators were used more than once, these predators were starved for 24 hours before each trial. For
each predator type we ran five trials of the following experiment. Within mesocosms filled with 9.5 L
spring water we placed twenty Gammarus amphipods and allowed them to acclimate for 24 hours
at room temperature. Washed druses (mean weight
114.2 ± 12.2 g) were randomly assigned to half the
containers. Predators were randomly assigned to
each mesocosm and left for 24 hours (because we
had only one goby we randomly assigned the goby
to either a druse or a non-druse container and when
that treatment was completed we starved the goby
for 24 hours before completing the trial with the
other treatment). The druses were subsequently removed and washed to collect the remaining amphipods, which were then counted. To test the
efficiency of the washing procedure a druse and 20
amphipods were placed in a mesocosm without a
predator. All individuals were recovered after colonizing the druse.
All predators were collected from lower Green
Bay or the surrounding watershed and allowed to
acclimate to captivity in aquaria at room temperature for four months before the study began. Zebra
mussel druses were collected from lower Green
Bay, washed under high-pressure cold water for 30
seconds, and then soaked for 1 hour to ensure the
removal of all invertebrates. The washing process
was repeated 3 weeks later before the experiment
began. Druses were starved and, because we were
primarily interested in the druses as structure, we
did not measure mussel mortality. Gammarus amphipods were purchased from Jones Biomedicals.
The experimental data were analyzed using analysis
of variance (ANOVA) with two treatments (druse
vs. no druse) and two factors, predator presence and
predator type.
RESULTS
Field Survey
The study sites reflected the trophic gradient of
this system. The sites with the deepest Secchi reading were along the western shore (17 and 24) and
the northernmost site along the eastern shore (4E)
(Table 1). The highest turbidity was found at the
two Fox River inlet stations (sites 1 and 9). Turbid-
393
FIG. 2. Log-log regression of the number of
amphipods and other benthic invertebrates collected in zebra mussel druses in Green Bay, Wisconsin vs. the volume of zebra mussels (in cm3) in
the druses.
ity was negatively correlated with Secchi depth (r =
–0.91). For all sites dissolved oxygen on the bottom
varied from 8.8–12.3 mg/L (Table 1).
Invertebrate density was significantly lower in
sediment cores than in zebra mussel druses (t-test, p
= 0.003). This finding was robust when sediment
core volume was calculated assuming 2 cm depth
(p < 0.01) or 10 cm depth (p < 0.01) as well as calculating density based on surface area of the mussels and sediment (p = 0.03). On average 0.05 ±
0.08 invertebrates cm –3 were found in sediment
cores compared to 1.38 ± 1.08 invertebrates cm–3 in
the druses (Table 1). Within the druses amphipod
and total invertebrate abundance were significantly
positively correlated with zebra mussel volume
(Fig. 2, r = 0.77 and 0.98 respectively). There were
no significant correlations between invertebrate
density and dissolved oxygen, Secchi depth, turbidity, substrate, depth, or between the druse and sediment samples.
Taxa richness in the druse samples (8.0 ± 2.6)
was significantly higher than in the sediment samples (3.0 ± 1.7, p = 0.0001). Within the druses amphipods were the dominant organism (Table 2).
Other taxa included Hydra, Hydracarina, Caenis
mayflies, and four familes of Trichoptera (Polycentropidae, Hydroptilidae, Leptoceridae, and
394
Reed et al.
TABLE 2. Percent non-zebra mussel invertebrate taxa composition from zebra mussel druses collected
from Green Bay.
Site
West
shore
24 (1 m)
24 (3 m)
17 (1 m)
17 (3 m)
Oligochaete Chironomid Amphipod
Leech
Hydra
Isopod Hyracarina Ephemeroptera
Trichoptera
22.5
12.7
10.4
9.9
4.2
3.7
8.7
16.8
56.6
20.3
40.5
49.6
3.2
4.0
27.8
4.6
0.5
53.9
9.1
15.0
0.7
2.2
0.2
2.1
10.4
2.6
3.0
1.0
0.7
0.4
0.1
0.3
1.1
0.2
0.3
0.6
Fox inlet
1 (1 m)
9 (1 m)
16.3
40.0
4.1
13.3
75.5
13.3
4.1
0.0
0.0
0.0
0.0
0.0
0.0
33.3
0.0
0.0
0.0
0.0
East shore
14 (3 m)
4E (1 m)
4E (3 m)
9.6
37.0
11.3
13.2
7.4
8.2
44.4
22.2
72.2
22.2
14.8
0.0
8.3
0.0
1.0
0.1
3.7
4.1
2.0
3.7
2.1
0.3
7.4
1.0
0.0
3.7
0.0
TABLE 3. Percent non-zebra mussel invertebrate taxa composition of sediment core samples taken from
Green Bay.
Site
West shore
24 (1 m)
24 (3 m)
17 (1 m)
17 (3 m)
Oligochaete
Chironomid
Amphipod
Nematode
Leech
Hydra
Isopod
50.0
52.8
47.8
71.4
50.0
4.5
52.2
14.3
0.0
10.1
0.0
0.0
0.0
1.7
0.0
14.3
0.0
1.1
0.0
0.0
0.0
28.7
0.0
0.0
0.0
1.1
0.0
0.0
Fox inlet
1 (1 m)
9 (1 m)
96.6
0.0
1.7
100
1.7
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
East shore
14 (3 m)
4E (1 m)
4E (3 m)
80.0
41.1
5.0
10.0
47.1
90.0
10.0
11.8
5.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
Hydropsychidae. Sediment samples were dominated by oligochaetes and chironomids (Table 3).
Taxa richness in the druses was weakly negatively
correlated with turbidity (r = –0.35) and positively
correlated with depth over the study area (r = 0.44).
Sediment sample richness was correlated with
depth (r = 0.57) but not correlated with any other
measured environmental variables. Druse and sediment sample taxa richness was lowest in the Inner
Bay (Table 1).
Predation Experiment
In mesocosm experiments more benthic invertebrates survived under predation pressure from all
three predators when zebra mussel druses were present (Fig. 3). Without zebra mussels, perch and
goby consumed all 20 amphipods within 24 hours,
usually within an hour of introduction. In the presence of zebra mussel druse, fish consumed an average of 4.2 (perch) and 4.0 (goby) amphipods after
24 hours. Rusty crayfish consumed fewer amphipods than either species of fish (ANOVA p <
0.01), capturing an average of 13.2 amphipods
without druses and 2.6 with druses present. For all
predators the number of amphipods consumed was
significantly greater in the absence of druses
(Anova p < 0.00001).
Benthic Invertebrates in Green Bay
FIG. 3. Number of Amphipods surviving 24
hours in a 9.5 L mesocosm subject to predation by
perch, goby, or crayfish. The two treatments indicate presence or absence of a zebra mussel druse.
DISCUSSION
Zebra mussel invasion has resulted in increased
invertebrate abundance (Stewart and Hayes 1994,
Karatayev et al. 1997, Ricciardi et al. 1997). This
has been explained by increased habitat structure
and food availability (Silver Botts et al. 1996, Gonzalez and Downing 1999). By increasing the benthic physical complexity zebra mussels create more
substrate available for invertebrate colonization.
There may be many other factors influencing the
presence and abundance of any given taxa including the presence of a colonization source, water
quality, and competition and predation pressures.
In Green Bay invertebrate taxa were not evenly
distributed throughout the study system. Taxa richness was higher in the druses than in the sediment,
indicating that zebra mussel druses increase the diversity of the invertebrate community. Although not
strong, the negative correlation between richness
and turbidity in druse samples suggests that the impact of sediment loading from the Fox River, at the
south end of the bay, may be a structuring factor for
the invertebrate community in this system.
In lower Green Bay, invertebrate density was significantly higher in the zebra mussel druses than in
adjacent sediment samples, possibly because druses
provide refugia from predation. This was demonstrated in the mesocosm experiments where, under
extremely heavy predation pressure, zebra mussel
druses provided amphipods with refuge from large
predators. Similarly, the refuge provided by druses
may be important to amphipods and other invertebrates in the field. Other researchers have shown
the importance of predation pressure in structuring
395
amphipod distribution (Russo 1991, Corona et al.
2000).
Zebra mussels shift the secondary energy production from pelagic to benthic in invaded systems by
filtering large amounts of phytoplankton and transferring that material into feces and pseudofeces.
This fuels secondary production in the benthos
while decreasing the amount of food available to
zooplankton and thereby decreasing pelagic secondary production (Nalepa and Fahnenstiel 1995).
Theoretically this shift could provide increased
food for benthivorous fish while decreasing the material available to planktivorous fish. However, our
results indicate the possibility that invertebrates
may be able to escape this predation by taking
refuge within zebra mussel druses as shown in the
mesocosms experiments.
In summary, our results indicate that the presence
of these detached zebra mussel druses on soft substrates in lower Green Bay dramatically impacts the
abundance, distribution, and diversity of benthic invertebrates. Turbidity and trophic status also appear
to be important variables in overall benthic community composition. Further study is needed to assess
the magnitude of the impact of zebra mussels on the
Green Bay food web.
ACKNOWLEDGMENTS
We are very grateful to the large number of people who participated in this study. Mike Draney and
Titus Seilheimer assisted with field sampling.
Amber Agamaite, Laura Anderson, Jim Daeschler,
Jen Dzim, Rebecca Ely, Aaron Fettes, Kate Horkman, Rusty Japuntich, Ronda Liebmann, Titus Seilheimer, and Adam Woerpel all helped with
laboratory processing. Dave Rades, Dave Dolan,
and Mike Draney provided many insightful study
design suggestions. David Barton, Marlene Evans,
Trefor Reynoldson, Pamela Silver, and Jerry
Woolpy gave us many thoughtful suggestions that
improved this manuscript immensely. This work
was funded in part by the University of Wisconsin
Sea Grant Institute under grants from the National
Sea Grant College Program, National Oceanic &
Atmospheric Administration, U.S. Department of
Commerce, and from the State of Wisconsin. Federal grant number NA86RG0047 project number
R/LR-86-PD and in part by a grant from the University of Wisconsin- Green Bay Research Council.
396
Reed et al.
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Submitted: 2 September 2002
Accepted: 28 June 2004
Editorial handling: Trefor Reynoldson