RECENT CHANGES IN LAKE ERIE (NORTH SHORE) PHYTOPLANKTON - CUMULATIVE IMPACTS OF PHOSPHORUS LOADING REDUCTIONS AND THE ZEBRA MUSSEL INTRODUCTION

Analyses of phytoplankton samples collected weekly and year-round at m unicipal water supply intakes in Lake Erie have shown a response to lo ng-term changes in phosphorus loading and the more recent invasion of zebra mussels. Total phytoplankton densities in the western basin of L ake Erie declined from about 5,000 Areal Standard Units (A.S.U.)/mL du ring the late 1960s to less than 1,000 A.S.U./mL by the mid-1980s and coincided with declines in total phosphorus inputs to the western basi n of about 50%. Similar declines were not observed at sites in the cen tral and eastern basins; however, a further dramatic decline (>90%) at all four Lake Erie sites during 1988-1990 coincided with the invasion of the lake by zebra mussels (Dreissena polymorpha). Similar declines in phytoplankton were not observed at reference sites in southern Lak e Huron, where zebra mussels were not abundant. In the western basin, chlorophyll a concentrations of < 1 mug/L for most of the April to Sep tember period of 1991 were typical of oligotrophic areas of the upper Great Lakes. The proportional representation by the dominant classes o f algae remained unchanged after the zebra mussel invasion indicating that all classes of algae were equally affected, including the large c olony-forming diatoms that dominated the phytoplankton of the western basin in previous years.