Detailed observations of thermal structure over two winters in a large lake reveal the presence of large (10-20 m) overturns under the ice, driven by diurnal solar heating. Convection can occur in the early winter, but the most vigorous convection occurred near the end of winter. Both periods are when our lake ice model suggest thinner ice that would have been transparent. This under-ice convection led to a deepening of the mixed layer over time, consistent with previous short-term studies. During periods of vigorous convection under the ice at the end of winter, the dissolved oxygen had become super-saturated from the surface to 23 m below the surface, suggesting abundant algal growth. Analysis of our high-frequency observations over the entire winter of 2015 using the Thorpe scale method quantified the scale of mixing. Furthermore, it revealed that changes in oxygen concentrations are closely related to the intensity of mixing that could be influenced by climate change or the increased input of salt into the lake. I will briefly present an overview of some of the basics of physical limnology before presenting our results.