Depth variation in isotopic composition of benthic resources and assessment of sculpin feeding patterns in an oligotrophic Alaskan lake

Stable isotopes of carbon (C) and nitrogen (N) are commonly used to track resource flows through lake food webs. However, there remains a weak understanding of the spatial variation in isotopic composition of benthic resources and how this variation affects inference about energy flows among species. Boundary layers at the interface between benthic substrates and the overlying water column restrict diffusion of nutrients and carbon from the water column to benthic algae and may affect the isotopic composition of benthic algae as nutrient and CO₂ concentrations can become locally depleted in the benthic boundary layer. We quantified the variation in C and N stable isotope composition of benthic resources along a depth gradient in a large oligotrophic lake to assess the magnitude of change in stable isotope composition. Snails were increasingly depleted in ¹³C with depth, by about 10 ‰ from 0 to 20 m, while ¹⁵N in snails showed only subtle enrichment over this depth range. Sculpin (Cottas aleuticus) δ ¹³C and δ ¹⁵N signatures did not significantly change with depth and were more enriched in ¹⁵N than would be expected from consumption of snails alone. A comparison of δ ¹³C and δ ¹⁵N values from sculpins relative to shallow and deep snails, and alternative prey (marine-derived salmon resources), within a mixing model suggested sculpins feed selectively on deep grazers in this system in addition to marine-derived resources provided by migrating sockeye salmon. This study illustrates the importance of accounting for depth-related variation in isotope patterns when assessing benthic resource contributions to food webs using stable isotope data.