LB 5-48 - A 20-year record linking bacterial community composition to long-term environmental change and seasonal cycles of stratification in an Arctic lake

Arctic freshwater microbiomes perform critical ecosystem functions including carbon cycling during the short, ice-free summer period when conditions for microbial activity are controlled by environmental drivers. A 20-year record of bacterial community composition in Toolik Lake, a tundra lake sampled by the Arctic Long-Term Ecological Research project in northern Alaska, reveals a remarkably consistent, recurring seasonal pattern of community composition contrasted with a long-term, decadal shift in the relative predominance of taxa. Within a single growing season, epilimnion and hypolimnion bacteria diversity is coincident with the progression of temperature-mediated water column stratification, suggesting the importance of physical lake drivers in determining short-term ecological assemblages. During summer, bacterial communities drifted in relative interannual composition since the beginning of the record in 2000. The summertime proportion of Actinobacteria decreased by over half in recent years, while Bacteroidia and the unicellular picocyanobacterium Cyanobium sp. PCC 6307 concurrently increased in abundance. Some conditions in the lake also shifted across this period, including increases in primary production, conductivity, alkalinity, and temperature, with the largest changes observed in July. These results indicate seasonal drivers of microbial community composition in the lake are highly persistent, while assemblages across longer time scales may respond to broader ecosystem change and disturbance. These bacteria may serve as sentinels of ecological modification as climate change alters the timing and magnitude of Arctic seasonality.