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

1. 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 commu-nity, 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. 2. We examined seasonal variation in activity, diet (stomach content) and resource use (d13C and d15N stable isotope ratios) of two fishes: the resident cold-water-adapted European whitefish (Coreg-onus 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 ben-thic 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. 3. 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 conten