A 25-year record linking bacterial community composition to long-term environmental change and seasonal cycles of stratification in an Arctic lake

A 25-year record from Toolik Lake, a tundra lake in Northern Alaska, shows a strikingly consistent, recurring seasonal pattern of bacterial community composition. Diversity in the epilimnion and hypolimnion tracks thermal stratification, underscoring the importance of physical drivers in shaping short-term assemblages. Over winter, the hypolimnion serves as a reservoir of diversity, producing similar pre-thaw spring communities annually. Since 2000, summer bacterial communities showed greater seasonal than interannual variability, although several taxa showed long-term directional shifts: Alphaproteobacteria declined, while orders within Bacteroidia, Actinobacteria, and Gammaproteobacteria, and members of the unicellular picocyanobacterial genus Cyanobium increased. Over the same period, lake conductivity increased, while temperature, stratification, and bacterial production showed no significant trends. The persistent seasonal reassembly of microbial communities over 25 years suggests that the absence of long-term directional change in the lake’s physical regime, despite interannual variability, maintains these recurring patterns even as other environmental conditions change. These results show how strong seasonal drivers of microbial communities can maintain and support persistent community composition across decades. However, the rise in cyanobacteria reflects global trends and underscores the value of long-term microbial records in detecting early ecological change before it becomes apparent in routine aquatic monitoring.