Effects of Changing Snow and Ice Cover Conditions on Phytoplankton Chlorophyll‐<i>a</i> and Community Composition in a Mesotrophic Lake

ABSTRACT Ice and snow cover on lakes plays a fundamental role for under‐ice ecology by reducing water column mixing and light availability. Previous studies have shown that such reductions can significantly influence the growth and reproduction of phytoplankton, primarily focusing on changes in ice‐on and ice‐off dates in a warming climate. This study goes beyond studying the effects of ice phenology on phytoplankton by addressing two fundamental questions: (1) how does a snow cover on ice influence below‐ice phytoplankton chlorophyll‐ a and community composition and (2) how do variations in ice phenology influence spring phytoplankton chlorophyll‐ a and community composition after ice‐off? To address these two questions, we assessed long‐term monitoring data collected at least monthly on phytoplankton chlorophyll‐ a and community composition. We combined the phytoplankton data with annual ice phenology and nearby meteorological data on daily snow depth between 1997 and 2019 in a mesotrophic lake (Erken) in Sweden. Snow cover resulted in an exponential decrease of phytoplankton chlorophyll‐ a , with detectable effects during all 3 months studied (January–March). Deeper snow cover changed the community dominance from autotrophs to mixotrophs in two of the months studied (January and March), which we explain by reduced light availability caused by snow cover. In spring, phytoplankton chlorophyll‐ a increased with longer ice periods and delayed ice‐off dates. A wide range of taxa in the spring community increased with delayed ice‐off dates, while delayed ice‐on dates mainly promoted diatoms. Convective mixing is important to keep non‐motile taxa in the photic zone and could explain the increased phytoplankton growth with longer ice duration. Our results highlight seasonal ice and snow cover as key regulators for the timing of growth and reproduction of primary producers below ice, with effects of the ice cover period lasting after ice‐off. Snow on ice causes light constraints, which commonly result in reduced under‐ice primary production and a higher proportion of mixotrophs in the phytoplankton community. Losing high nutritional phytoplankton groups such as mixotrophs following changes in ice phenology and snow cover can have consequences for the trophic transfer and the biogeochemical cycling in lakes.