Vertical mixing and weak stratification over zebra mussel colonies in western Lake Erie
Limnol. Oceanogr., 53(3), 2008, 1093-1110 | DOI: 10.4319/lo.2008.53.3.1093
ABSTRACT: 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 ~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 (~1ºC between surface and bottom waters) was sufficient to suppress vertical mixing, when the mean daily wind speed 4 m above the lake surface (U4) was ~6 m s-1 . These conditions allowed a concentration boundary layer ~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 U4>6 m s-1 (associated with the lakes 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 s-1 , the western basin is weakly stratified thermally and a concentration boundary layer forms when U4<3α or α>2 m3 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.