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Food Webs without Borders? Watershed-coast Interactions Influence Diet Sources of Fish in the St. Louis River
Citation:
HOFFMAN, J., G. S. PETERSON, A. M. COTTER, AND J. R. KELLY. Food Webs without Borders? Watershed-coast Interactions Influence Diet Sources of Fish in the St. Louis River. Presented at American Fisheries Society, Duluth, MN, February 02 - 04, 2009.
Impact/Purpose:
Our goals were to determine whether we can detect a stable isotope gradient along the river-Great Lake transition zone in a coastal river mouth and use it to identify changes across this hydrologic gradient in the food web supporting young-of-year and juvenile fish production.
Description:
Complex ecosystems form where coastal rivers enter the Laurentian Great Lakes. These ecosystems span a river-to-Great Lake transition zone encompassing a mosaic of river channel, drowned river mouth, littoral, wetland and coastal habitats. Our goals were to determine whether we can detect a stable isotope gradient along the river-Great Lake transition zone in a coastal river mouth and use it to identify changes across this hydrologic gradient in the food web supporting young-of-year and juvenile fish production. We characterized the food web along the lower 35 km of the St. Louis River, MN, a coastal tributary that flows into Lake Superior, using carbon and nitrogen stable isotope ratios (d13C, d15N). The d13C of fish were significantly different among stations and increasingly enriched towards the river mouth. The gradient allowed us to describe local food webs. Throughout the river, the food web was supported by two trophic pathways: a bentho-pelagic pathway based on consumption of zooplankton and zoobenthos in the river channel and a littoral pathway based on consumption of macroinvertebrates in submerged vegetation beds. Near Lake Superior, isotopic data indicated that Lake Superior autotrophs (both phytoplankton and periphyton) contributed to production of fish captured in the river. The high diversity of fish in Great Lakes coastal habitat relative to offshore waters may be explained by both the array of potential OM sources which can sustain fish production and the diversity of fish life histories.