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Detailed observations of thermal structure over an entire winter 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 supersaturated from the surface to 23 m below the surface, suggesting abundant algal growth. Analysis of our highfrequency 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.

The mixing and stratification present under ice-covered lakes during winter can potentially influence the oxygen concentrations in the subsequent spring. During winter, plankton rely on updrafts caused by convection to remain in the photic zone and perform primary production. Thus, there is a crucial link in winter between under-ice light levels, circulation, and dissolved oxygen (DO) production. Detailed observations of temperature, oxygen, and incoming solar radiation over three winters between 2015 and 2017 suggested that plankton abundance and DO concentrations were maximal near the end of the winter before the ice melted when there was increased under-ice light and convection. DO was supersaturated by the end of the severe winter of 2015 when the ice cover duration was the longest. In comparison, DO increased but was not saturated in the warmer winters of 2016 and 2017. The spring overturn started when the water column became isothermal and continued until when the mean water column temperature first exceeded 4 C, at which time, hypolimnetic DO depletion started and continued until the fall overturn. The concentration of DO when the water column becomes stratified in spring, along with the timing of the end of spring overturn, are both important initial conditions for summer DO concentrations.

1. The weak diel vertical migration observed in the large cladoceran Bythotrephes longimanus seems contradictory to the predator-avoidance hypothesis that predicts large zooplankton should have long migration amplitudes. However, cold-water planktivores, especially Coregonus spp., are a main source of mortality for Bythotrephes and hence a deeper migration would result in a greater overlap with these hypolimnetic planktivores. We hypothesized that Coregonus artedi (cisco) modifies the normal vertical migration pattern of Bythotrephes, such that the latter stays higher in the water column during the day and thus migrates less extremely at night. 2. The vertical distribution of Bythotrephes during the day was determined from single visits to six lakes in Ontario, Canada, all of which contain warm-water, epilimnetic planktivores but differing in whether they contain cisco. One lake of each fish type was sampled day and night every 2-3 weeks over the ice-free season to examine daytime depths and migration amplitude. 3. The vertical migration of Bythotrephes differed in the presence and absence of cisco. In the lakes with cisco, there were significantly fewer Bythotrephes in the hypolimnion and they were higher in the water column during the day. Migration amplitude was smaller in the cisco than in the non-cisco lake. These observations were not attributable to differences in physical factors, and, although not conclusively attributable to cisco, are consistent with an effect of cisco. 4. We suggest that diurnal depth selection by Bythotrephes in lakes containing cisco is a trade-off between the risk of predation by warm-versus cold-water predators, balanced by the benefits of increased temperature and feeding rates near the surface. Even in lakes without cisco, however, the vertical migration of Bythotrephes was less than expected, suggesting that diurnal depth selection is a balance between the risk from warm-water planktivores and access to sufficient light to feed effectively.

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