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 in western Lake Superior, and used it to identify food web linkages supporting young fish production. We observed a broad, spatial pattern in the carbon stable isotope ratio (delta C-13); downriver enrichment in particulate organic carbon and aquatic vegetation delta C-13, as well as pelagic, benthic and littoral invertebrate delta C-13, reflected isotope mixing along the river-lake transition zone. Fishes with similarly enriched delta C-13 were used to identify benthopelagic and littoral trophic pathways. River and Lake Superior organic matter (OM) sources contributed to both pathways. Differences between the delta C-13 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 incorporated multiple OM sources from across the watershed.