Analysis of a Lake Superior coastal food web with stable isotope techniques

Food-web components of a Lake Superior coastal wetland and adjacent offshore waters were examined with stable isotope ratio techniques for carbon and nitrogen. We found distinct carbon isotope ratio signatures for organisms collected in the wetland and from offshore. Both food-web groups seemed to be based on carbon fixed by phytoplankton. Compared to offshore organisms, the wetland food web was depleted in ¹³C. We found the nitrogen isotope ratio signatures to be enriched in ¹⁵N by ∼3‰ at each succeeding trophic level in both wetland and lake samples. No evidence of a direct energy link between the abundant macrophyte biomass in the wetland and the fisheries food web was found. The carbon ratio of rainbow smelt (Osmerus mordax) and walleye (Stizostedion vitreum) exhibited a shift from a wetland signature in young-of-the-year to an offshore signature in juveniles and adults. Yellow perch (Perca flavescens) young-of-the-year exhibited a planktivorous δ¹⁵N signature, while adults were enriched in ¹⁵N. Both examples illustrate the utility of stable isotope ratio techniques in confirming feeding shifts associated with growth and habitat change.