Seasonal Variation of <scp> δ ¹³ C </scp> and <scp> δ ¹⁵ N </scp> in a Boreal Lake Fish Community—Year‐Round Patterns and Explanatory Variables

ABSTRACT Seasonal changes in freshwater ecosystems influence fish resource use through both environmental and biological factors, but year‐round studies that include ice‐covered months have been lacking. We investigated seasonal variation in the stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) using year‐round monthly sampling of the fish community in a deep, seasonally ice‐covered boreal lake in southern Finland. We simultaneously examined how month and a suite of biological variables (total length, sex, gonadosomatic index, condition factor, stomach fullness, C:N ratio of fish muscle) varied across the same period and assessed their relationship with isotope ratios of fish. We found seasonal variations in both δ 13 C and δ 15 N, although the patterns differed among fish species. Three percids: pikeperch ( Sander lucioperca ), perch ( Perca fluviatilis ) and ruffe ( Gymnocephalus cernua ) were enriched in 15 N in the winter, while no change was evident in the cyprinid species. Conversely, roach ( Rutilus rutilus ) and pikeperch 13 C values were enriched in autumn and winter, while no change was detected for perch and ruffe. Total length, month and C:N ratio explained most of the variation in isotope ratios in most species. Results confirm that all four abundant species with year‐round samples of muscle stable isotope ratios are subject to seasonal changes in the studied boreal lake, suggesting that this variation should be taken into account in future spatial and temporal studies.