Development of picoplankton during natural and enhanced mixing under late-winter ice

We studied the development of autotrophic picophytoplankton and heterotrophic bacterioplankton during the transition from winter ice cover to open water under natural and manipulated mixing conditions in eutrophic Lake Vesijarvi. During the melting of the snow and ice cover, a convection layer developed which eventually met the chemocline at the interface between the oxic and anoxic water masses. However, in the years with mechanically enhanced mixing, the whole water column remained well oxygenated and the deepening of penetrative convection was facilitated. Stochastic variations in weather, primarily the thickness of the snow cover, likely determined the timing of picophytoplankton growth. Mechanical mixing slightly increased the biomass of picophytoplankton, especially the eukaryotic fraction, probably because eukaryotic picophytoplankton coped well with changing light conditions. Bacterial biomass was not notably affected, although their mean cell size was significantly smaller in the oxic deep water than in anoxic conditions during the natural mixing regime. Our results show that, when snow melts and solar radiation increasingly penetrates into lake water, picophytoplankton biomass and numbers already start to increase weeks before ice-break and may reach abundances typical of early summer under the ice. A similar development of bacterioplankton suggests a trophic relationship between heterotrophic and autotrophic producers.