Zooplankton trophic structure in Lake Michigan as revealed by stable carbon and nitrogen isotopes

Within the last few decades Lake Michigan's plankton community has undergone substantial changes. Oligotrophication of the pelagic zone has led to a decrease in zooplankton biomass, while species composition has shifted, with copepod species becoming more dominant. Although these observations have been relatively well documented, their implications for the food web have not been well addressed. To define feeding relationships within the zooplankton community, we measured the stable carbon and nitrogen isotope composition and biomass of major zooplankton species, three seston size classes, and seston collected from three depths in southeastern Lake Michigan. Nitrogen stable isotope ratios suggest a complex plankton food web with zooplankton appearing to occupy at least three distinct trophic positions. The large calanoid copepod Limnocalanus macrurus occupied a tertiary consumer trophic position, more than one trophic position above all other zooplankton species sampled. Carbon stable isotopes revealed significant differences among zooplankton species. Though the mechanisms responsible for these differences are not clear, these results suggest that there is significant partitioning of resources at the base of the food web.