Hypoxia and Turnover in a Small Ice-covered Temperate Lake

During summer, temperate lakes often exhibit recurrent eutrophication and hypoxia and there has been significant research into the causes and effects. Phytoplankton blooms and low oxygen have also been observed in these lakes under winter ice; however, detailed studies coupling winter stratification to biogeochemistry are lacking. In this study, four years of continuous high resolution temperature and dissolved oxygen observations are analyzed from a small temperate lake in Ontario, Canada. The lake exhibits near-bed hypoxia and anoxia 2/3 and 1/3 of the time, respectively, and so a low oxygen concentration state should be considered typical for the lake. The watercolumn resets to atmospheric oxygen levels only during fall turnover (63-79 d) and long spring turnover (17-19 d) events. Short spring turnover events (1-8 d), which occur during anomalously warm and calm weather conditions, have uniform temperature profiles, through direct heating of the entire watercolumn with short wave solar radiation. The watercolumn subsequently stratifies and wind and convection induced mixing are insufficient to fully mix oxygen over the depth.