Using Stable Isotope Mixing in a Great Lakes Coastal Tributary to Determine Food Web Linkages in Young Fishes

We characterized stable isotope mixing along a river-Great Lake transition zone in the St. Louis River, an important fish nursery inwestern LakeSuperior, and useditto identify food web linkages supporting young fish production. We observed a broad, spatial pattern in the carbon stable isotope ratio (δ 13 C); downriver enrichment in particulate organic carbon and aquatic vegetation δ 13 C, as well as pelagic, benthic and littoral invertebrate δ 13 C, reflected isotope mixing along the river-lake transition zone. Fishes with similarly enriched δ 13 C were used to identify bentho- pelagic and littoral trophic pathways. River and Lake Superior organic matter (OM) sources contributed to both pathways. Differences between the δ 13 C in fishes and invertebrate prey revealed that fish production was supported at multiple spatial scales. The result was that the food web specific to any location along the transition zone incorporat- ed multiple OM sources from across the watershed.