568 / 2024-08-02 22:32:46

The role of under-ice oxygen dynamics in setting the spring-summer dissolved oxygen levels in a large dimictic lake (Lake Simcoe, Ontario)

Winter oxygen dynamics; Under-ice physical processes; Ecological memory; Ice-covered lake

Subtheme 3: Eco-hydraulics and Environment > Topic 2: Water Eco-environment Monitoring and Simulation

In dimictic lakes, it was often assumed that the spring and fall periods of full water-column mixing act as a “reset” on dissolved oxygen (DO) conditions. Recent works, however, suggests an “ecological memory” from winter conditions may influence spring-summer DO dynamics. Testing this hypothesis is challenging partly due to the logistical difficulties of multi-year winter monitoring. We present a rich 6 year-dataset of high-frequency measurements from Lake Simcoe, a large and deep dimictic lake, including temperature and DO datasets from 2015-2017 and 2022-2024. Our observations indicate that under-ice DO dynamics are active and variable with the surface mixed layer, where solar radiation drives both convection and the growth of phytoplankton. Late-winter DO levels set the initial conditions for hypolimentic oxygen before early summer stratification. These end-of-winter DO levels correlated to varying ice durations. Warmer winters with shorter ice durations exhibited greater surface DO variations, while developing smaller hypoxic zones in the hypolimnion. In contrast, colder winters with longer ice durations exhibited large benthic hypoxic zones with intense inverse stratification and supersaturation of DO in the surface mixed layer following a prolonged convective period. Next-step research on analyzing the correlations of hypolimnetic temperatures and DO across winter, spring, and summer will be finished, to further validate whether under-ice convection and spring turnover reset hypolimnetic temperatures, and to determine whether winter and spring oxygen dynamics have a carryover effect into late summer, influencing late summer DO concentrations. Our findings establish a critical connection between winter and summer hydro-ecological processes in the dimictic lakes, essential for lake ecosystem and fisheries management in a warming climate.