Predicted impact of zebra mussel (Dreissena polymorpha) invasion on water clarity in Lake Mendota

Lake Mendota, which is plagued by periodic cyanobacterial blooms, is typica l of many lakes in the Laurentian Great Lakes region that are vulnerable to zebra mussels (Dreissena polymorpha) but have yet to be invaded. We couple d removal estimates with chlorophyll production estimates in a dynamic mode l to predict the likely impact of mussel-mediated removal of phytoplankton on water clarity across a range of hypothetical zebra mussel densities. Mod els were fit to chlorophyll and temperature data collected biweekly from La ke Mendota during 1977-1993. When we assumed daily epilimnetic circulation, the percentage of days when the chlorophyll concentration exceeded 50 mu g .L-1 was decreased threefold at mussel densities as low as 1000 mussels.m(- 2). When we assumed less frequent epilimnetic circulation, the density of m ussels required to substantially improve water clarity increased dramatical ly. We predict that zebra mussel invasion would lead to increased water cla rity in Lake Mendota. Cyanobacterial blooms would be reduced but not elimin ated. Negative impacts on other lake processes following zebra mussel invas ion could outweigh the benefits of lower phytoplankton concentrations.