When sunlight penetrates the ice layer covering lakes in winter, it warms the top water layer and sets up convection, with several potentially contrasting effects on phytoplankton. While convective mixing keeps cells in suspension and prevents sedimentation losses, it may also transport phytoplankton well below the euphotic zone. We investigated diurnal variations in the vertical distribution of phytoplankton under ice and just after ice-off in Lake Onego (Russia), a lake with moderate to high colored dissolved organic carbon (CDOM) levels. We showed that diurnal variation in convection under ice restricts phytoplankton access to light in the morning hours to a narrow euphotic zone, whereas cells are mixed through a deep aphotic layer in the afternoon. After ice-off, low chlorophyll a was found on the open-water side of the thermal bar as convection distributed cells throughout the water column. By contrast, the inshore side had significantly higher concentrations of chlorophyll a (p < 0.001) because the mixing depth brought about by diurnal microstratification was reduced, resulting in greater access to light in the afternoon. Overnight, convective cooling broke down microstratification, which redeveloped the next day. Our work highlights the importance of studying diurnal variation in light availability for photoautotrophic growth, both under ice and after ice-off in lakes characterized by high CDOM.

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