Phenological Shifts in Autumn Drive Changes in Ice‐on Timing and Under‐Ice Water Temperature in Dimictic Finnish Lakes

Abstract Autumn is a pivotal but understudied season in limnology. As climate change alters the timing and duration of autumnal cooling, the implications for winter thermal structure and long‐term ecosystem function in dimictic lakes remain uncertain. To address this gap, we synthesized 37–50 years of data from 47 Finnish lake sites, providing the first comprehensive long‐term assessment of how autumn conditions indirectly govern winter thermal regimes. Furthermore, we examined how autumn surface warming, winter under‐ice temperatures, and ice phenology affect maximum summer surface water temperature. We found that autumn surface waters warmed by 0.37°C per decade ( n = 8), delaying ice‐on by 4.1 days per decade ( n = 47) over the last 50 years. Interestingly, later ice formation correlated with colder under‐ice bottom water temperatures ( r = −0.54), with a marked regime shift in 2002 toward later freezing and cooler bottom water conditions. Additionally, our structural equation model linked stronger autumn winds, higher autumn shortwave radiation, and lakes with larger areas to colder under‐ice water temperatures, likely due to their effect on lake water cooling and ice formation. Furthermore, later ice‐on and earlier ice‐off led to higher maximum summer surface water temperature; however, under‐ice water temperature had no direct effect. As climate warming prolongs open water periods and delays ice‐on, extended water column mixing and atmospheric exposure can cool under‐ice water temperatures. These shifts have potential consequences for oxygen and nutrient dynamics, winter habitat quality, and overall ecosystem resilience, emphasizing the need to integrate autumn dynamics into future limnological research and lake monitoring efforts.