Vertical mixing and weak stratification over zebra mussel colonies in western Lake Erie

Zebra mussels ( Dreissena polymorpha) are an invasive species that have been implicated in the reduction of algae stocks in the near-shore environment of western Lake Erie. To determine their basin-wide effects, we applied a two-dimensional hydrodynamic and water-quality model for 1994. The model accurately reproduced lake-wide hydrodynamics and water quality. When modeled as true benthic organisms ( resting on the bottom), the dreissenids grazed 53% of the western basin May through September net algal growth. This grazing resulted in a similar to 0.1 mg L-1 reduction in the pelagic algae concentration relative to the case without dreissenids. In comparison, dreissenids grazed 77% western basin net algal growth when the lake was modeled as a fully mixed water column. We found that the biomass grazed was governed by a balance between the timescales of vertical wind-induced mixing and benthic grazing. During calm conditions, weak diurnal stratification (similar to 1 degrees C between surface and bottom waters) was sufficient to suppress vertical mixing, when the mean daily wind speed 4 m above the lake surface ( U-4) was < 6 m s(-1). These conditions allowed a concentration boundary layer similar to 1 m thick to form, accounting for the reduced grazing effect relative to the fully mixed case. Entrainment of the concentration boundary layer occurred for U-4 > 6 m s(-1) ( associated with the lake's characteristic 10- d storm cycle) facilitating algae supply to the benthos. We formulated the mean daily biomass grazed in terms of the dreissenid areal pumping rate ( a) and U4 and found that because typically U4 is,6 m s21, the western basin is weakly stratified thermally and a concentration boundary layer forms when U-4 < 3 alpha or alpha > 2 m(3) m(-2) d(-1). The dynamics of both wind-mixing and thermal stratification must, therefore, be considered in mixing models applied to shallow weakly stratified lake basins.