Long-term dreissenid impacts on water clarity in Lake Erie

Since shortly after their introduction, dreissenid mussels have been thought to have improved water clarity in Lake Erie, particularly in the western basin. However, long-term monitoring (1982-2004) has found no evidence of persistent, basin-wide increases in water clarity in either the western or the central basin of Lake Erie since the Dreissena invasion. In fact, spring water clarity in both of those basins has exhibited statistically significant declines in the post-dreissenid period. In contrast, chlorophyll a levels in the western basin have declined by about 50% since the Dreissena invasion during both spring and summer. The discrepancy in the responses of water clarity and chlorophyll a is probably a consequence of both the large sediment loads entering the western basin and resuspension of unassimilated non-algal particulates. In the eastern basin, spring transparency has increased substantially and turbidity has decreased since Dreissena colonization, in spite of the much greater depth of this basin. This is probably due to higher mussel densities and the lack of major sources of turbidity in that basin. Summer turbidity, has also decreased markedly in the eastern basin, although thermal stratification during this period would probably preclude direct filtration effects. Instead, we hypothesize that reductions in whiting events due to calcium uptake by dreissenids have contributed to the summer decreases in turbidity, seen in the eastern basin.