Quantifying Lake Michigan coastal wetland-nearshore linkages for sustaining sport fishes using stable isotope mixing models
Coastal wetlands of the Laurentian Great Lakes are highly productive environments that support diverse faunal communities and provide habitat for 90% of Great Lakes fish species. However, these systems have suffered extensive degradation since European settlement with approximately half of coastal wetland area being converted to other land uses. Remaining wetlands have been impacted by invasive species, fragmentation, hydrologic manipulation, and contaminant loading. Movement of energy and organisms between coastal wetlands and adjacent nearshore habitats of Lake Michigan represents a critical, but largely unstudied, ecological linkage. Use of these wetlands as spawning, rearing, and foraging habitat by native sport fishes, including yellow perch (Perca flavescens), walleye (Sander vitreus), northern pike (Esox lucius), and smallmouth bass (Micropterus dolomieu), represents a potentially vital linkage with the nearshore lake. We hypothesized that linkages between open-water nearshore areas and coastal wetlands would contribute to high fish production and diversity found within Lake Michigan nearshore systems, and that wetlands would provide energy to sport fishes throughout their life cycle. We characterized this use of wetland-derived energy by sampling fish and food resources in both zones and then using stable carbon and nitrogen isotope mixing models to reveal energy flow.
We sampled nine paired wetland-nearshore sites in three locations of Lake Michigan representing different wetland geomorphic types. Fifty species of forage and sport fish were sampled, with 17 species collected in both nearshore and wetland habitats. Various size classes of sport fish were collected to determine isotopic differences throughout life history stages. Prey items sampled included phytoplankton, zooplankton, and benthic macroinvertebrates including snails (Gastropoda), midge larvae (Chironomidae), and amphipods (Hyalella and Gammarus). We used isotope mixing models to construct cross-habitat food webs and estimate dietary contributions of potential prey groups to nearshore sport fish. Preliminary data indicate that coastal wetland trophic pathways are isotopically distinct from nearshore pathways, with wetland dissolved inorganic carbon d13C being 4.5‰ more depleted on average than adjacent nearshore waters. Our estimates of linkages between coastal wetland and nearshore habitats will fill a major data gap concerning how native sport fish utilize diverse habitats within Lake Michigan during different periods of their life history. Improved understanding of nearshore fish habitat usage and the origin of energy supporting nearshore sport fishes will provide managers with new tools for prioritizing wetland preservation and restoration efforts in Lake Michigan to enhance sport fish recruitment and production.