Recent climate change in the Arctic is driving permafrost thaw, which has important implications for regional hydrology and global carbon dynamics. Permafrost is an important control on groundwater dynamics and the amount and chemical composition of dissolved organic matter (DOM) transported by high‐latitude rivers. The consequences of permafrost thaw for riverine DOM dynamics will likely vary across space and time, due in part to spatial variation in ecosystem properties in Arctic watersheds. Here we examined watershed controls on DOM composition in 69 streams and rivers draining heterogeneous landscapes across a broad region of Arctic Alaska. We characterized DOM using bulk dissolved organic carbon (DOC) concentration, optical properties, and chemical fractionation and classified watersheds based on permafrost characteristics (mapping of parent material and ground ice content, modeling of thermal state) and ecotypes. Parent material and ground ice content significantly affected the amount and composition of DOM. DOC concentrations were higher in watersheds underlain by fine‐grained loess compared to watersheds underlain by coarse‐grained sand or shallow bedrock. DOC concentration was also higher in rivers draining ice‐rich landscapes compared to rivers draining ice‐poor landscapes. Similarly, specific ultraviolet absorbance (SUVA254, an index of DOM aromaticity) values were highest in watersheds underlain by fine‐grained deposits or ice‐rich permafrost. We also observed differences in hydrophobic organic acids, hydrophilic compounds, and DOM fluorescence across watersheds. Both DOC concentration and SUVA254 were negatively correlated with watershed active layer thickness, as determined by high‐resolution permafrost modeling. Together, these findings highlight how spatial variations in permafrost physical and thermal properties can influence riverine DOM.

Climate change is acting to reallocate biomes, shift the distribution of species, and alter community assemblages in Alaska. Predictions regarding how these changes will affect the biodiversity and interspecific relationships of small mammals are necessary to pro-actively inform conservation planning. We used a set of online occurrence records and machine learning methods to create bioclimatic envelope models for 17 species of small mammals (rodents and shrews) across Alaska. Models formed the basis for sets of species-specific distribution maps for 2010 and were projected forward using the IPCC (Intergovernmental Panel on Climate Change) A2 scenario to predict distributions of the same species for 2100. We found that distributions of cold-climate, northern, and interior small mammal species experienced large decreases in area while shifting northward, upward in elevation, and inland across the state. In contrast, many southern and continental species expanded throughout Alaska, and also moved down-slope and toward the coast. Statewide community assemblages remained constant for 15 of the 17 species, but distributional shifts resulted in novel species assemblages in several regions. Overall biodiversity patterns were similar for both time frames, but followed general species distribution movement trends. Biodiversity losses occurred in the Yukon-Kuskokwim Delta and Seward Peninsula while the Beaufort Coastal Plain and western Brooks Range experienced modest gains in species richness as distributions shifted to form novel assemblages. Quantitative species distribution and biodiversity change projections should help land managers to develop adaptive strategies for conserving dispersal corridors, small mammal biodiversity, and ecosystem functionality into the future.

Changing climate conditions are causing global distribution shifts, resulting in altered food webs and novel species assemblages in terrestrial systems. How diets of sympatric small mammals overlap and whether this may translate into competitive exclusion among new species interactions remains largely unknown. Monitoring niche overlap in changing arctic and boreal communities can assist in forecasting interspecific competition and species turnover. We quantified the isotopic niche spaces of small mammals, which may reflect dietary niche spaces, at study sites along two megatransects spanning Alaska. Field sampling resulted in the capture of 724 small mammals belonging to 12 species of rodent (10 Arvicolinae and 2 Sciuridae) and 6 species of shrew (genus Sorex L., 1758). We created dietary mixing models based on hair samples for four rodent species using stable isotope (δ15N and δ13C) analyses in R. We also modeled isotopic niche ellipses and quantified niche overlap among species at small and large scales. A varied combination of fungi and herbaceous plants composed the diets of most species. Fundamental niche spaces overlapped considerably between sympatric species statewide, but realized niche spaces were largely segregated at individual sites. We conclude that some degree of dietary plasticity served to partition niche spaces and minimize interspecific competition, allowing sympatric species to co-exist.

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