Ice cover and thaw events influence nitrogen partitioning and concentration in two shallow eutrophic lakes

The frequency and duration of lake ice cover is rapidly declining in the Northern Hemisphere. Limited research in oligotrophic and mesotrophic lakes suggests that extended periods of ice cover influence nitrogen (N) cycling by promoting nitrate (NO3−) accumulation. However, ice cover impacts on N cycling in shallow, high-nutrient, eutrophic lakes remains poorly understood. To understand drivers of under-ice water column N concentrations, we examined concurrent under-ice N concentration, hydro-meterological, and physicochemical time series from two shallow eutrophic systems during sustained cold and thaw periods. We compared data from both lakes during a historically cold winter to assess how different lake-watershed physical configurations and algal biomass affected under-ice N cycling. We also used time series data from consecutive winters to assess the influence of a mild versus a historically cold winter on under-ice N cycling in one of the lakes. We found that ice cover promoted NO3− depletion when sustained cold disconnected lakes from watershed loading, but the amount of depletion varied between lakes and was enhanced during the colder winter. Thaw events increased NO3− concentrations compared to late winter and altered the concentration and distribution of N species in the water column, but the degree and nature of increased NO3− varied with thaw severity, the source of meltwater, timing, and lake-watershed morphometry. Our results suggest that winters with shorter ice duration and more thaw events may result in less NO3− depletion and higher peak NO3− concentrations in shallow eutrophic lakes, with potential implications for N cycling and phytoplankton ecology.