Bottom boundary mixing: the role of near-sediment density stratification

The turbulent dynamics and stratification of bottom boundary layers, as well as the net diapycnal buoyancy flux in the deep water, have been observed to vary strongly among lakes. The most relevant parameters governing the different regimes are the bottom current stress and the rate of release of dissolved solids from the sediment. The ratio of boundary-induced mixing to the density flux associated with the flux of ions from the sediment determines whether the bottom boundary layer is extremely stably stratified or well-mixed. The aim of this contribution is (1) to demonstrate these two boundary phenomena, (2) to give a physical criterion for assessing the two mixing regimes, (3) to present a potential model to quantify the boundary-induced buoyancy flux and the basinwide diapycnal diffusivity, and (4) to test the model with data from two representative lakes with significantly different deep-water mixing characteristics.