Filtering impacts of an introduced bivalve (Dreissena polymorpha) in a shallow lake: Application of a hydrodynamic model

Nonindigenous species may exert strong effects on ecosystem structure and f unction. The zebra mussel (Dreissena polymorpha) has been attributed with p rofound changes in invaded ecosystems across eastern North America. We expl ored vertical profiles of water flow velocity and chlorophyll a concentrati on in western Lake Erie, over rocky substrates encrusted with Dreissena, to assess the extent to which mussels influence coupling between benthic and pelagic regions of the lake. Flow velocity was always low at surveyed sites (less than or equal to 2.9 cm s(-1)) and declined in direct proximity to t he lakebed. Mean chlorophyll a concentration was also low (less than 5 mu g L-1) at all sites and depths. Chlorophyll a concentration was positively c orrelated with distance above lakebed and was lowest (0.3 mu g L-1) directl y adjacent to the lakebed. Spatial patterns of zooplankton grazers could no t explain observed vertical gradients in chlorophyll concentration. Hydrody namic modeling revealed that filtering effects of Dreissena in a nonstratif ied, shallow basin depend mainly on upstream chlorophyll concentration, int ensity of turbulent diffusion, feeding efficiency of the mussel colony, and the distance downstream from the leading edge of the mussel colony. In con trast to widespread perceptions that molluscs reduce phytoplankton concentr ation only adjacent to the lakebed, modeling scenarios indicated that deple tion occurs throughout the water column. Depletion was, however, inversely proportional to distance above the lakebed. Simulation results are consiste nt with field-based observations made in shallow water habitats populated b y large Dreissena populations in the Great Lakes and elsewhere. Results fro m this study indicate that zebra mussels strongly enhance coupling between pelagic and benthic regions in shallow lakes. Enhanced coupling between the se regions explains, in part, high population densities of Dreissena and of many benthic invertebrates in ecosystems invaded by zebra mussels.