Winter plankton dynamics in a boreal lake: community structure, vertical distribution and reproduction under ice

Lakes are understudied during ice-covered periods; yet physical, hydrodynamical and biological processes continue under the ice. Ice cover reorganizes lake ecosystems by altering thermal stratification and solar radiation, creating conditions that support different organisms, food webs, and potential trophic cascades. We investigated seasonal shifts in biomass and vertical distribution of heterotrophic and autotrophic micro-organisms (heterotrophic bacteria, picoautotrophs, colorless and pigmented nanoflagellates) and zooplankton across winter in relation to limnological characteristics and hydrodynamics. Boreal Lake Simoncouche was sampled at five depths on six dates between autumn overturn (November 2020) and spring overturn (April 2021) for temperature, dissolved oxygen, chlorophyll-a, and plankton. Low chlorophyll-a and high heterotroph-to-autotroph biomass ratios indicated dominance of the heterotrophic energy pathway under ice. Heterotrophic micro-organisms also dominated during well-lit overturns, emphasizing the role of the microbial loop in all seasons. Zooplankton richness remained high under ice (18-22 taxa), with most species favoring deeper, warmer layers. Several species of rotifer and cladoceran sustained egg production through winter, ceasing only briefly in February. These findings highlight strong vertical and seasonal heterogeneity in winter-active plankton communities, shaped by stratification, light, and resources, and support the view that winter is biologically dynamic, with consequences for the subsequent open-water season.