Changes in the Lake Michigan Trophic Structure: As Revealed by Stable C and N Isotopes

Abstract

Food web structures which incorporate both slow (nearshore or detrital) and fast (pelagic) energy channels convey stability upon food web biota through asynchrony and multichannel trophic omnivory. Within the Lake Michigan food web, invasive dreissenid mussels have caused rapid changes to food web structure and energy flows. I used stable C and N isotopes and gut content analysis to determine how Lake Michigan food web structure and stability has changed in the past decade, coincident with the expansion of dreissenid mussels and a decrease in pelagic phytoplankton production. Fish and invertebrate samples collected near the port of Milwaukee, WI were analyzed to determine the 13C:12C (d13C) and 15N:14N (d15N) ratios. Sampling took place during three distinct periods 2002-2003, 2005-2006 and 2010-2011, and included nearshore, pelagic and profundal fish and invertebrate taxa. d13C and d15N values were adjusted relative to primary consumers (dreissenid mussels and amphipods) to facilitate meaningful temporal comparison. Enrichment of d13C in most food web components over time suggests increased reliance upon a nearshore basal energy source, although the mechanisms remain unclear. The importance of nearshore energy resources may vary spatially depending upon local nearshore benthic productivity. d15N results were more variable but further illustrate restructuring of the Lake Michigan food web. Dreissenid mussels appear to have had two opposing effects of both severely altering food web structure through depletion of offshore food availability while simultaneously increasing stability though increased slow energy channel production. Results suggest that the persistence of most species in the lake may be a consequence of multichannel omnivory and increased reliance upon nearshore energy channels despite substantial declines in lake-wide biomass. Management of Lake Michigan fish stocks should consider increased dependence upon this energetic pathway at least for local spatial scales.