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AUTHORS: Ben Turschak, Harvey Bootsma — Univeristy of Wisconsin-Milwaukee, School of Freshwater Sciences
ABSTRACT: Nutrient abatement measures and the invasion of dreissenid mussels in Lake Michigan have coincided with major declines in offshore productivity and a redirection of nutrients and energy to nearshore areas. As a consequence, much greater research and management emphasis has been placed on understanding the role of nearshore areas in Laurentian Great Lake food webs. We used stable carbon and nitrogen isotopes to elucidate differences in primary energetic source and trophic level of Lake Michigan fish species from 2010-2012 across 8 nearshore and 3 offshore study sites. In order to assess diet shifts during the period of dreissenid mussel proliferation and major ecosystem changes, we compared the 2010-2012 dataset with a dataset from 2002-2003—prior to dreissenid proliferation. Results suggest that local productivity processes—measured using remote sensing techniques—as well as regional diet differences correlate strongly with large scale spatial variation in nearshore fish C isotopes. Stable N isotope ratios varied little spatially suggesting that allochthonus N loading plays little direct role in Lake Michigan’s nearshore foodweb structure. Magnitude and direction of the C isotope shift from 2002-2003 to 2010-2012 indicated significantly greater reliance upon nearshore benthic energy sources among nearly all fish taxa as well as profundal invertebrates. We propose that the carbon isotope shift has likely resulted from the offshore transport of nearshore benthic algal production, direct reliance upon nearshore prey items, or some combination of both. N isotopic changes over time were more variable but further illustrate restructuring of the Lake Michigan food web.
