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.

北半球湖冰覆盖的频率和持续时间正在迅速下降。对贫营养和中营养湖泊的有限研究表明，长时间的冰盖通过促进硝酸盐 (NO ₃ ^-) 积累。然而，冰盖对浅水、高营养、富营养化湖泊中氮循环的影响仍然知之甚少。为了了解冰下水柱 N 浓度的驱动因素，我们检查了持续寒冷和解冻期间来自两个浅层富营养化系统的并发冰下 N 浓度、水文气象和物理化学时间序列。我们在历史上寒冷的冬季比较了两个湖泊的数据，以评估不同的湖泊流域物理配置和藻类生物量如何影响冰下 N 循环。我们还使用连续冬季的时间序列数据来评估温和冬季与历史上寒冷的冬季对一个湖泊中冰下 N 循环的影响。我们发现冰盖促进了 NO ₃ ^-持续寒冷时的枯竭使湖泊与流域负荷分离，但湖泊之间的枯竭量有所不同，并且在较冷的冬季加剧。与晚冬相比，解冻事件增加了 NO ₃ ^-浓度，并改变了水体中 N 物种的浓度和分布，但 NO ₃ ^-增加的程度和性质随解冻严重程度、融水来源、时间和湖-分水岭形态测量学。我们的结果表明，冰期较短和解冻事件较多的冬季可能会导致 NO ₃ ⁻消耗减少和 NO ₃ ⁻峰值升高^。 浅层富营养化湖泊中的浓度，对氮循环和浮游植物生态有潜在影响。