PREDICTING THE CONSEQUENCES OF DREISSENID MUSSELS ON A PELAGIC FOOD-WEB

Although abundant in marine systems, hard substratum fouling organisms with high fecundity and large dispersal capability are rare in freshw ater ecosystems. A noteworthy exception is a recent invader of North A merican lakes and rivers, the zebra mussel, Dreissena polymorpha. Cons equences of this novel ecological type in these systems are not fully understood. Using computer simulations we modelled the potential conse quences of Dreissena abundance for lake planktonic community structure along a nutrient gradient, using Green Bay of Lake Michigan as an exa mple. We constructed a model of food-web interactions that accurately reproduces abundances of Phytoplankton and zooplankton along this nutr ient gradient. To this model we added consumption and filtration rates of Dreissena at varying densities. Model results indicate that Dreiss ena have strong negative effects on large phytoplankton, with chloroph yll reduced up to 80% as compared to the same community without zebra mussels. Results also indicate that Dreissena have relatively small ef fects on nanoplankton and herbivorous zooplankton. Effects are qualita tively similar along the nutrient gradient, and are strongest at the m ost eutrophic site. The effects of zebra mussels on the abundance of s mall phytoplankton and zooplankton were far less than the effects of n utrient levels along the gradient. The effects of zebra mussels on the large phytoplankton, however, were in the same order of magnitude as the effects of the nutrient gradient.