Environmental determinants of chironomid assemblages across a forest-tundra ecotonal permafrost gradient

While temperature is an important driver of the distribution of freshwater biota in the Arctic, within-region environmental variability may pose challenges to interpreting underlying drivers of community assembly. In particular, lakes across the forest-tundra ecotone can have large differences in catchment-mediated inputs that govern the concentration and quality of dissolved organic carbon (DOC) inputs, which can influence water colour, clarity, biogeochemistry, and subsequently freshwater biota. The source of DOC varies depending on the hydrologic connectivity in the catchment, which is known to be susceptible to increasing permafrost degradation associated with climate change. Here, we classified 60 lakes that cross the treeline in the Mackenzie Uplands region north of Inuvik, Northwest Territories, Canada, according to gradients in water chemistry and chironomid morphotype assemblages (Diptera: Chironomidae). We found that the taxonomic composition of chironomid assemblages varied significantly along gradients of conductivity, total phosphorus, and especially DOC among the sampled lakes. While climate likely remains the overarching control that defines ecology of freshwater systems across the transition from forest-tundra, within-region variability of lake water quality was an important factor for the diversity of chironomids. Future research using chironomids as biological indicators should consider the full range of possible environmental changes that could drive species assemblage shifts, and our results highlight that this is especially true at forest-tundra ecotone boundaries, or wherever temperature variation is small and other environmental factors may vary more.