Warwick F. Vincent

Rapid warming in the High Arctic has induced changes in terrestrial environments that have included major recent shifts in lakes and ponds. Numerous studies exist of spatial trends in water chemistry, bioindicator groups, and paleoenvironmental change from High Arctic lakes; however, little is known about lake thermal stratification regimes at high latitudes beyond assumptions based on generalized classification schemes. Here, we report on the presence in late July 2022 of positive thermal stratification during open-water conditions in two freshwater lakes near Clements Markham Inlet, on northern Ellesmere Island. These lakes are situated in the polar desert at 82.6°N, >1000 km north of the generally accepted northern limit of summer thermal stratification. Given their location approaching the northernmost land on Earth, the thermal profiles of these lakes suggest that the occurrence of summer stratification may no longer be discounted anywhere in the Northern Hemisphere.

Arctic climate change is leading to sea-ice attrition in the Last Ice Area along the northern coast of Canada and Greenland, but less attention has been given to the associated land-based ecosystems. Here we evaluated bacterial community structure in a hydrologically coupled cryo-ecosystem in the region: Thores Glacier, proglacial Thores Lake, and its outlet to the sea. Deep amplicon sequencing revealed that Polaromonas was ubiquitous, but differed genetically among diverse niches. Surface glacier-ice was dominated by Cyanobacteria, while the perennially ice-capped, well-mixed water column of Thores Lake had a unique assemblage of Chloroflexi, Actinobacteriota, and Planctomycetota. Species richness increased downstream, but glacier microbes were little detected in the lake, suggesting strong taxonomic sorting. Ongoing climate change and the retreat of Thores Glacier would lead to complete drainage and loss of the lake microbial ecosystem, indicating the extreme vulnerability of diverse cryohabitats and unique microbiomes in the Last Ice coastal margin.