Stratification and turbulent mixing in small strongly stratified lakes with implications for planktonic disease dynamic

Estimates of dissipation of turbulent kinetic energy were made from temperature gradient microstructure profiles collected in five small, dimictic temperate lakes under a range of atmospheric forcing. Buoyancy frequencies exceeded 0.02 s-1 at the seasonal thermoclines of these dimictic kettle lakes. Turbulence was dampened by strong stratification as average dissipation rates were low (10-9 m2s -3) below the surface mixing layer. Average dissipation in excess of 10-7 m2s-3 was only found in the upper meter following an episodic thunderstorm with wind speeds of 5 m s-1. Persistent epilimnetic stratification formed on most summer days. There was no evidence of deepening of the surface mixing layer in response typical afternoon winds of 2-3 m s-1. At a diel timescale, deeper mixing resulted from overnight heat loss (buoyancy flux ∼1x10-7m2s-3) than from afternoon wind stress. The passage of episodic storm fronts resulted in deeper epilimnetic mixing with moderate dissipation. The strong stratification and lack of mixing has implications for pelagic plankton with limited or no mobility. In particular, strong stratification inhibited the resuspension of infective fungal spores settled into the lake sediments through mid-summer. Outbreaks of infection in populations of zooplankton occur synchronously across lakes following late summer cold fronts. The enhanced mixing resulting from storm fronts could resuspend and transport infective spores.