While the sentinel nature of freshwater systems is now well-recognized, widespread integration of freshwater processes and patterns into our understanding of broader climate-driven Arctic terrestrial ecosystem change has been slow. We review the current understanding across Arctic freshwater systems of key sentinel responses to climate, which are attributes of these systems with demonstrated and sensitive responses to climate forcing. These include ice regimes, temperature and thermal structure, river baseflow, lake area and water level, permafrost-derived dissolved ions and nutrients, carbon mobilization (dissolved organic carbon, greenhouse gases, and radiocarbon), dissolved oxygen concentrations, lake trophic state, various aquatic organisms and their traits, and invasive species. For each sentinel, our objectives are to clarify linkages to climate, describe key insights already gained, and provide suggestions for future research based on current knowledge gaps. We suggest that tracking key responses in Arctic freshwater systems will expand understanding of the breadth and depth of climate-driven Arctic ecosystem changes, provide early indicators of looming, broader changes across the landscape, and improve protection of freshwater biodiversity and resources.

The northern coast of Ellesmere Island in the Canadian High Arctic is undergoing amplified warming that parallels the rapid decline in Arctic Ocean sea ice extent, and many lakes in this region have already shown changes in response to warming. However, biogeochemical data from High Arctic freshwaters are limited, and mostly restricted to the short, ice-free period. We sampled four coastal lakes in Stuckberry Valley (82° 54’ N, 66° 56’ W) before the onset of spring melting in 2017, 2018 and 2019, to assess biogeochemical gradients in their water columns and characteristics of their surface sediments. Despite their proximity, there were large differences in limnological properties. The two shallower lakes closer to the ocean were oxygen deficient while the two deeper, more distant lakes were more oxygenated. There were pronounced vertical gradients in major ions, metals and nutrients that suggested large differences in the extent of anaerobic microbial processes among the lakes. Morphometry and dissolved oxygen were the overriding determinants of biogeochemical differences rather than position along this short ocean-inland gradient. The diversity of limnological conditions, and the sensitivity of these characteristics to changes in ice cover, underlines the need for further study of under-ice processes in extreme northern lakes.

Arctic lakes are experiencing increasingly shorter periods of ice cover due to accelerated warming at northern high latitudes. Given the control of ice cover thickness and duration over many limnological processes, these changes will have pervasive effects. However, due to their remote and extreme locations even first-order data on lake ecology is lacking for many ecosystems. The aim of this study was to characterize and compare the microbial communities of four closely spaced lakes in Stuckberry Valley (northern Ellesmere Island, Canadian Arctic Archipelago), in the coastal margin zone of the Last Ice Area, that differed in their physicochemical, morphological and catchment characteristics. We performed high-throughput amplicon sequencing of the V4 16S rRNA gene to provide inter- and intra-lake comparisons. Two deep (>25 m) and mostly oxygenated lakes showed highly similar community assemblages that were distinct from those of two shallower lakes (<10 m) with anoxic bottom waters. Proteobacteria, Verrucomicrobia, and Planctomycetes were the major phyla present in the four water bodies. One deep lake contained elevated proportions of Cyanobacteria and Thaumarchaeota that distinguished it from the others, while the shallow lakes had abundant communities of predatory bacteria, as well as microbes in their bottom waters that contribute to sulfur and methane cycles. Despite their proximity, our data suggest that local habitat filtering is the primary determinant of microbial diversity in these systems. This study provides the first detailed examination of the microbial assemblages of the Stuckberry lakes system, resulting in new insights into the microbial ecology of the High Arctic.