Zebra Mussels and Plankton

Introduction Data Collection and Analysis Results The Bottom Line

Introduction

Since their introduction to Lake Champlain in the early 1990s, zebra mussels, Dreissena polymorpha, have firmly established themselves as part of the Lake Champlain ecosystem.  In general, the introduction of zebra mussels to freshwater systems has resulted in decreases in phytoplankton density and an often welcome accompanying increase in water clarity; however, it is unclear whether zebra mussels impact all phytoplankton groups equally.  Some studies, particularly those from western Lake Erie, have suggested that zebra mussels may drive phytoplankton to an alternative species composition, favoring cyanobacteria (blue-green algae) (Vanderploeg et al. 2000).  Whether this shift might be the result of preferential ingestion of other algae, changing nutrient concentrations or alteration of the lower trophic levels is not well understood.

When assessing the impact of zebra mussels on the phytoplankton community, another grazer group, the zooplankton, is often forgotten.  Through preferential grazing, zooplankton may mitigate or exacerbate any shift in phytoplankton community composition due to zebra mussels.  Zooplankton may also be negatively impacted by zebra mussels indirectly through increased competition for food or directly by ingestion of offspring or mechanical damage during filtering.

In addition to impacting phytoplankton, there is some indication that zebra mussels and zooplankton are capable of influencing rotifer populations, potentially causing drastic decreases in rotifer densities (MacIsaac, Lonnee and Leach 1995, Gilbert 1988).  In Lake Champlain, declines in the number of rotifers have been reported since the mid-1990s (Mihuc, personal communication).  The impact of zebra mussels on rotifers is often overlooked, but may have consequences for the phytoplankton community that affect the broader food web.

This study was designed to investigate the effects of grazing by both zebra mussels and the zooplankton on phytoplankton and rotifers in Lake Champlain.  The inclusion of both rotifers and zooplankton in this zebra mussel-plankton interaction study is unique and better reflects the whole natural communities of lakes.  From this study we should gain a better understanding of the broader impacts of zebra mussels on the plankton community of Lake Champlain.


Literature Cited

Gilbert, J.J. 1988. Susceptibilities of ten rotifer species to interference from Daphnia pulex.  Ecology 69: 1826-1838.

MacIsaac, H.J., C.J. Lonnee and J.H. Leach. 1995. Suppression of microzooplankton by zebra mussels: importance of mussel size. Fresh. Bio. 34:379-387.

Vanderploeg, H.A , J. Liebig, W. Carmichael, M. Agy, T. Johengen, G. Fahnenstiel, and T. Nalepa.  2000. Zebra mussels (Dreissena polymorpha) selective filtration promoted toxic Microcystis blooms in Saginaw Bay (Lake Huron) and Lake Erie. Can. J. Fish. Aquat. Sci.  58:1208-1221.

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