Fish-Mediated Nutrient and Energy Exchange between a Lake Superior Coastal Wetland and its Adjacent Bay

Abstract

Little has been done to quantify fluxes of organisms, nutrients, and energy between freshwater coastal habitats and adjacent offshore waters or to evaluate the ecological implications of these exchanges on a whole-lake basis. To test the hypothesis that fish-mediated transport might play an important role in the flux of nutrients and energy between coastal wetlands and adjacent lake waters, net carbon, nitrogen, phosphorus, and energy fluxes were estimated in forage fish between a Lake Superior coastal wetland and an adjacent bay. This was accomplished by sampling fish at the inlet/outlet of Bark Bay Slough for 1 week per ice-free month in 1995. Average carbon, nitrogen, and phosphorus content of the 20 species analyzed was 45.1, 11.3, and 2.45% of dry-weight, respectively. Estimates of organism, nutrient, and energy movement revealed a net export from the slough to the adjacent Bark Bay, due largely to emigration of YOY yellow perch, Perca flavescens (> 40,000 individuals), YOY northern pike, Esox lucius (> 600 individuals), and yearling emerald shiner, Notropis atherinoides (> 8,000 individuals) in June and July, and YOY brown bullhead, Ameiurus nebulosus (> 500 individuals) in October. Since these fish movements resulted in relatively small fluxes of nutrients (1,376 g carbon, 335 g nitrogen, 73 g phosphorus) and energy (65,100 kJ) from the wetland to the lake, the most significant influence of forage fish emigration on Lake Superior may be through subsequent trophic interactions in nearshore habitats. However, assessment of the significance of the nutrient and energy results awaits a more complete budget for these ecosystems.