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

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Abstract

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 ((Formula presented.)) potentially associated with internal wave shear. Below the thermocline, turbulence was weak ((Formula presented.)) and buoyancy-suppressed ((Formula presented.) < 8.5), with low hypolimnetic mixing rates ((Formula presented.)) 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 ((Formula presented.); (Formula presented.)). A transitional mixing period was observed for spring conditions when surface temperatures were near the temperature of maximum density (TMD (Formula presented.) 3.98 (Formula presented.)) and the water column began to stably stratify. While small temperature gradients allowed strong mixing over the transitional period ((Formula presented.)), hypolimnetic velocity shear was overwhelmed by weakly stable stratification ((Formula presented.); (Formula presented.)), 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.

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Cannon, D. J., Troy, C., Bootsma, H., Liao, Q., & MacLellan-Hurd, R. A. (2021). Characterizing the Seasonal Variability of Hypolimnetic Mixing in a Large, Deep Lake. Journal of Geophysical Research: Oceans, 126(11). https://doi.org/10.1029/2021JC017533

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