Seasonal variation of ecosystem processes in a large Arctic lake

Despite previous research on under-ice ecology and thermal drivers of productivity, year-round datasets of Arctic lake ecosystem dynamics remain limited. We conducted continuous observations of lake water-column temperatures, water quality and vertical fluxes of organic matter and algal pigments between April 2022 and April 2023 in Lake Kilpisjärvi, Finland. The ultraoligotrophic lake exhibited minor year-round variation in water quality, while distinct seasonal patterns in algal diversity and indicators of production were driven by shifts in thermal structure, including brief spring overturn, weak summer stratification with frequent large-scale wind‑driven mixing, extended autumn overturn, and weak winter inverse stratification. Autumn was the most productive season, with nutrient enrichment from overturn supporting diverse algal communities including diatoms, chrysophytes, cryptophytes, chlorophytes, and cyanobacteria. Spring phytoplankton communities were dominated by siliceous algae. Summer stratification was weak overall with a brief interval of stability that may have limited nutrient availability supporting the development of a deep chlorophyll maximum dominated by motile cryptophytes. Under-ice production was observed in both spring and autumn, but not in winter polar night. The year-round dataset from Lake Kilpisjärvi highlights the key role of thermal processes and wind-driven mixing, offering insights into large Arctic lake ecosystems and informing future monitoring.