Bernard Yang, Mathew Wells, Jingzhi Li, Joelle Yang

The mixing and stratification present under the ice during winter can have a profound influence on the following summertime hypolimnetic oxygen levels. During winter, plankton rely on updrafts caused by convection to remain in the photic zone in ice-covered lakes, thus there is a crucial link in winter between light levels, under-ice circulation and dissolved oxygen (DO) production. Detailed observations of temperature and oxygen over 3 winters suggest that radiatively driven convection is correlated to oxygen increases in the mixing layer. Both plankton abundance and dissolved oxygen were maximum near the end of the winter before the ice melted. Oxygen became supersaturated by the end of the severe winter of 2015 when the ice cover duration was the longest, whereas DO was slightly below saturation in the warmer winters of 2016 and 2017. After ice-off, the combination of high-frequency measurements through winter and bi-weekly sampling in spring through to summer suggests that decreases in DO started when spring overturn ended and the water column became weakly stratified, which was very close to the timing of when the mean water column temperature first exceeded 4oC. The implication of this work is that the winter oxygen dynamics are important for the hypolimnetic oxygen concentrations when the water column becomes stratified, which in turn sets initial conditions for the degree of any late summer hypoxia.