Climate-driven extreme winter thaw triggers summerlike microbial activity in a subarctic stream

Winter remains the most understudied season across northern latitudes, despite its growing importance in the context of rapid warming of the north. Winters are changing, for example, via processes related to the Arctic water cycle, such as increased rainfall and extreme temperature fluctuations, which disrupt the previously more predictable hydrological regime of northern stream systems. We show how a large panarctic scale climate-driven winter event triggered biological activity in a typical subarctic stream at the Pallas Atmosphere-Ecosystem Supersite in northern Finland. By using simultaneous high-frequency measurements of water vapor, precipitation, and stream water isotopes, along with water chemistry, discharge, and bacterial community attributes, we captured two exceptionally warm Atlantic air intrusion events in early winter 2020–2021. These two closely spaced rain-on-snow events caused complete snowpack melt and triggered a high discharge and dissolved organic carbon pulse in our study stream, which in turn reactivated the already receding summer-like aquatic bacterial community and initiated their primary production despite the prevailing cold and dark conditions. Our findings reveal how large-scale climate anomalies can abruptly disrupt local-scale hydrology and trigger biological activity in wintering stream ecosystems, highlighting the sensitivity of aquatic ecosystems and biogeochemical processes to shifting weather and climate patterns. Capturing this event underscores the need for continuous high-frequency observations that span seasons and years. Overall, this study reinforces winter as a critical season for ecological research and reveals the vulnerability and responsiveness of northern stream ecosystems to climate change.