Characterizing the Seasonal Variability of Hypolimnetic Mixing in a Large, Deep Lake

In this study, we report on turbulent mixing observed during the annual stratification cycle in the hypolimnetic waters of Lake Michigan (USA), highlighting stratified, convective, and transitional mixing periods. Measurements were collected using a combination of moored instruments and microstructure profiles. Observations during the stratified summer showed a shallow, wind-driven surface mixed layer (SML) with locally elevated dissipation rates in the thermocline (epsilon approximate to 10-7W/kg) potentially associated with internal wave shear. Below the thermocline, turbulence was weak (epsilon approximate to 10-9W/kg) and buoyancy-suppressed (Q=epsilon/nu N2 < 8.5), with low hypolimnetic mixing rates (Kz approximate to 10-7m2/s) limiting benthic particle delivery. During the convective winter period, a diurnal cycle of radiative convection was observed over each day of measurement, where temperature overturns were directly correlated with elevated turbulence levels throughout the water column (epsilon approximate to 10-7W/kg; Kz approximate to 10-1m2/s). A transitional mixing period was observed for spring conditions when surface temperatures were near the temperature of maximum density (T-MD approximate to 3.98 degrees C) and the water column began to stably stratify. While small temperature gradients allowed strong mixing over the transitional period (KZ approximate to 10-3m2/s), hypolimnetic velocity shear was overwhelmed by weakly stable stratification (N2 approximate to 10-6s-2; Ri approximate to 0.2), limiting the development of the SML. These results highlight the importance of radiative convection for breaking down weak hypolimnetic stratification and driving energetic, full water column mixing during a substantial portion of the year (>100 days at our sample site). Ongoing surface water warming in the Laurentian Great Lakes is significantly reducing the annual impact of convective mixing, with important consequences for nutrient cycling, primary production, and benthic-pelagic coupling.

Automated summary

This study reports on turbulent mixing observed during the annual stratification cycle in the hypolimnetic waters of Lake Michigan, highlighting stratified, convective, and transitional mixing periods. Results show a shallow, wind-driven surface mixed layer and locally elevated dissipation rates in the thermocline during the stratified summer, while turbulence is weak and buoyancy-suppressed below the thermocline.