Stable isotope patterns in lake food webs reflect productivity gradients

Stable isotopes C-13 and N-15 are often used in lake ecosystems to assess energy sources and trophic positions, respectively. However, delta C-13 and delta N-15 are also influenced by internal biogeochemical processes in epilimnetic and hypolimnetic habitats in lakes, but the extent to which biogeochemical processing mediates isotope values between these two habitats, and whether these patterns are influenced by lake productivity is not known. We sampled delta C-13 and delta N-15 in epilimnetic mussels, Chaoborus, cisco (Coregonus artedi), and seston and zooplankton in the epilimnia and hypolimnia of 22 Minnesota (USA) lakes ranging from oligotrophic to eutrophic. We also measured lake temperature-oxygen profiles and light levels to assess factors influencing isotope patterns. isotope samples were baseline-corrected using epilimnetic mussels in each lake (sample-mussel) to control for watershed-level differences in isotope values. Results showed delta C-13 in epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco became more depleted in delta C-13 relative to epilimnetic mussels in low-productivity lakes where light penetrated into the hypolimnion, while epilimnetic seston delta C-13 stayed similar to mussel delta C-13 in all lakes. This pattern was likely due to hypolimnetic phytoplankton in Clearwater lakes incorporating more respired CO2, which is depleted in delta C-13, and subsequently passing depleted delta C-13 values up the food chain. Results also showed habitat differences in delta N-15 with epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco becoming more enriched relative to epilimnetic mussels in low-productivity lakes with higher O-2 levels in the hypolimnion, while epilimnetic seston delta N-15 remained similar to mussel values. The delta N-15 pattern is consistent with the idea that denitrification and microbial degradation enriched hypolimnetic seston relative to epilimnetic seston in low nutrient lakes, while enhanced epilimnetic primary production enriched epilimnetic delta N-15 seston relative to hypolimnetic seston in high nutrient lakes. Our results indicate isotopic differences between epilimnetic and hypolimnetic organisms that change along productivity gradients and suggest that microbial processes and the light regime are important drivers.