Impacts of climate and reduced ice cover on trophic structure and community dynamics in a high latitude lake

Climate-driven environmental change can alter both the structural and dynamic properties of ecological communities. With ongoing warming, high-latitude lakes experience reduced ice cover, higher growing-season temperatures, and altered hydrology, potentially triggering cascading effects across lake ecosystems. We investigated long-term variation (1994-2022) in ice phenology, temperature, magnitude of spring floods, and pelagic communities in an oligotrophic subalpine lake in Norway. We analysed the structure and dynamics of phytoplankton, zooplankton, and fish communities, as well as synchrony across trophic groups. Long-term records indicated declining ice-cover duration and increasing growing-season temperatures, although only the reduction in ice-cover duration was significant within the 1994-2022 study period. Phytoplankton richness and biovolume increased, zooplankton communities increased in diversity but declined in temporal stability, and fish community structure changed, with brown trout becoming more abundant at the expense of arctic char. We also observed shifts in synchrony between trophic groups, suggesting changes not only in community organisation but potentially in interspecific interactions. Given the observational, correlative nature of the study, causal attribution remains limited. Although some biological metrics showed significant associations with climate-related variables, several metrics that changed over time were not related to the measured abiotic drivers, indicating partial coupling between environmental variability and community structure and dynamics. This limited coupling may reflect the relatively modest magnitude of environmental change that occurred over the study period. Overall, our results document cross-trophic restructuring of pelagic communities in a high-latitude oligotrophic lake and underscore the importance of long-term monitoring for documenting and anticipating future shifts.