Logotyp: till Uppsala universitets webbplats

At northern latitudes, lakes are ice-covered for several months every year, and winter has long been viewed as an ecologically dormant period. However, biological processes continue beneath the ice, and the conditions during ice cover can shape lake food webs into spring. This thesis examines how ice cover and winter conditions structure lake food webs, with a particular focus on the drivers of phyto- and zooplankton growth under ice. Through a combination of literature synthesis, long-term monitoring, and experimental and field studies, the work investigates how ice cover, winter precipitation and runoff, and habitat-specific processes. influence productivity and trophic interactions below ice. One main finding was that snow cover and ice quality (black versus white ice) are critical predictors of under-ice phytoplankton growth (paper I-II). Reduced light transmission through snow limited phytoplankton biomass and shifted community composition toward mixotrophic taxa. Further, the duration of ice cover influenced spring dynamics, where shorter ice-covered periods reduced the magnitude of the spring phytoplankton bloom (paper I-II). These findings highlight that both the duration and composition of the ice cover set the conditions for winter and spring productivity in lakes. Furthermore, the timing and magnitude of runoff were found to shape under-ice food webs. Experimental manipulations showed that early nutrient inputs under low-light conditions promoted mixotrophs, while later inputs supported stronger spring phytoplankton blooms (paper III). Field observations indicated that terrestrial organic matter inputs reduced the nutritional quality of basal resources during winter. In lakes with high terrestrial organic matter availability, copepods incorporated more terrestrial and bacterial resources but maintained nutritional quality (paper IV). Finally, spatial analyses revealed pronounced habitat heterogeneity in winter resources, with littoral habitats enriched in terrestrial and bacterial organic matter, and pelagic habitats dominated by algal-derived organic matter. These differences were reflected in copepods (paper IV), demonstrating that resource heterogeneity under ice is transferred to higher trophic levels. Together, the findings of this thesis highlight winter as a dynamic period in lake ecosystems. Climate-driven shifts in ice phenology, ice quality, and winter runoff may increase heterotrophy and reduce resource quality, weakening energy transfer to higher trophic levels and altering lake carbon cycling.