Seasonal depletion of resources intensifies trophic interactions in subarctic freshwater fish communities

Summary Climate change in recent decades has resulted in an increase in both the density and diversity of consumers in subarctic freshwater ecosystems. Despite this, harsh winter conditions in the region limit productivity and may serve as a bottleneck driving trophic interactions within the fish community, potentially determining the outcome of climate change for resident biota. However, due to the difficulties in sampling during winter months, few studies have assessed seasonal variation in the ecological interactions between native and range‐expanding species in subarctic regions. We examined seasonal variation in activity, diet (stomach content) and resource use (δ 13 C and δ 15 N stable isotope ratios) of two fishes: the resident cold‐water‐adapted European whitefish ( Coregonus lavaretus L.) and the range‐expanding cool‐water‐adapted ruffe ( Gymnocephalus cernua L.) in lakes with low and high consumer density. Results were contrasted with seasonal availability of benthic and pelagic resources and indirect measures of individual fitness, that is condition, growth and estimated lipid content (muscle elemental C : N ratio) of both species. The effects of fish density were apparent in both the diet and resource use of whitefish, which altered their diet and displayed reduced stomach fullness during winter in high‐density lakes. This was associated with an overall reduction in whitefish growth, condition and estimated lipid content in high‐density lakes, which was especially pronounced during winter. Ruffe utilised a greater proportion of profundal resources than whitefish in both summer and winter, potentially exploiting a vacant niche. Ruffe maintained condition and lipid reserves throughout the winter, highlighting the potential for further northward range expansion of the species. Winter acts as a seasonal bottleneck in subarctic lakes, but assumptions that this bottleneck primarily selects against warmer‐water‐adapted invasive species such as ruffe may be too restrictive. The effects of seasonal resource depletion on fish condition and invertebrate density were most severe in lakes with a high density of fish, and in these conditions, the cold‐water‐adapted resident whitefish was most adversely affected.