Depth-specific benthic specialization of Arctic char in an oligotrophic subarctic lake

Fishes consume prey from across both benthic and pelagic habitats, thereby stabilizing the food webs of freshwater lakes. How fish exploit vertical and horizontal heterogeneity within the benthic environment, however, remains unclear. We characterized spatial variation in the C and N stable isotopes of a dominant benthic grazer (snails) along a water depth gradient in an oligotrophic, western Alaska lake to assess the effects of water depth on isotope characteristics in the benthic food web. Importantly, carbon stable isotopes in snails become substantially more depleted in C-13 with increasing water depth. We compared this distribution of snail isotope values to those found in Arctic char (Salvelinus alpinus) diets to estimate the vertical location of char foraging on benthic resources. We found that Arctic char tended to prey on snails at greater than 10 m depth; however, individual fish specialized at a narrower range of water depths spanning from 5 to > 20 m. We used an isotope mixing model to assess the validity of using only shallow-water benthic invertebrates to represent all benthic resources in food web reconstructions and found that mixing models that only used shallow benthic prey underestimated the importance of benthic resources by about 50%. These results can help refine our understanding of how benthic prey contribute to the stability of aquatic food webs and highlight the need to account for spatial variation in the isotope composition of benthic resources in models of lake food webs.