Reclaiming Autumn in Limnology: Evidence of Shifting Autumn Phenology and Impacts on Winter Conditions and Cross-Seasonal Lake Dynamics

Abstract

Autumn remains understudied in limnology, despite its recognized ecological importance in other disciplines. In this thesis, we addressed the need for greater integration of autumn into limnological research (Chapter 1) and demonstrated how climate-driven shifts in autumn phenology affect ice formation and winter thermal conditions (Chapter 2). Using 37–50 years of data from 47 Finnish lakes, we found that autumn surface waters have warmed significantly, resulting in widespread delays in ice formation. Delayed ice formation was associated with colder bottom under-ice temperatures. Stronger autumn winds and larger lake size were also associated with colder under-ice conditions through their influence on pre-freeze mixing and heat loss. Furthermore, higher maximum summer surface water temperatures were observed in years with later ice-on dates. As climate change continues to disrupt seasonal patterns, a greater focus on autumn will be critical to effectively predict the impacts of warming on ecosystem functioning of northern lakes.