Benthic and pelagic contributions to Mysis nutrition across Lake Superior

Quantifying nutritional sources for Mysis diluviana will help to clarify the basis for production in lakes with Mysis and improve models of migration-driven nutrient and contaminant transport. We sampled Mysis, plankton, and benthos across Lake Superior using a stratified-random design that provided a statistically valid representation of the lake across depths. We then estimated nutritional contributions to Mysis using stable isotope ratios of Mysis, zooplankton, Bythotrephes, Diporeia, oligochaetes, and detritus in a multiple-source, dual-isotope mixing model. Lake-wide, small (<1.0 cm) mysids relied almost exclusively upon plankton, whereas large mysids occupied a higher trophic position and obtained nutrition among sources. Model estimates of mean benthic contributions to large Mysis ranged from 27% to 58%. We predicted the importance of benthos to Mysis to track declining benthic biomass with depth. Model results indicated that if Diporeia were the only benthic food eaten, benthic contributions would decline to 40% with depth, but inclusion of detritus in the model resulted in consistent importance of benthic food across depths. The importance of benthos to mysid nutrition suggests strong benthic-pelagic coupling at all lake depths and might limit the ability of Mysis to support fisheries in systems that have lost Diporeia.