The influence of zebra mussels (Dreissena polymorpha) on phytoplankton abundance is well known, but their community-level impact on energy flow is less clear. Reduced phytoplankton abundance could increase reliance by fish and aquatic invertebrates on alternative energy sources such as epiphyton and benthic algae. We assessed impacts of zebra mussels on energy flow by comparing food webs in two Minnesota, USA, lakes during summer of 2013 and 2014. Lake Carlos had a dense population of zebra mussels, while upstream Lake Ida was free of zebra mussels until this study began and densities remained extremely low throughout our study. We used δ13C to assess patterns of energy flow between lakes as littoral primary production is enriched in δ13C relative to pelagic production. We also used δ15N to assess trophic position as trophic fractionation causes higher trophic levels to be enriched in δ15N. We used baseline-corrected (BC) δ13C to test whether littoral invertebrate primary and secondary consumers were more reliant on littoral carbon in Carlos compared to Ida, and we used an isotope mixing model to determine whether fish species were more reliant on littoral carbon in Carlos, and whether trophic position of fish species differed between lakes. Lastly, we compared isotopic niche space by estimating standard ellipses areas for fish in Carlos and Ida lakes.
Results/Conclusions
Results showed invertebrate secondary consumers had more enriched BC δ13C in Carlos than in Ida, indicating greater reliance on littoral energy. Mixing models indicated 10 of 12 fish species were more reliant on littoral carbon in Carlos, with littoral carbon use in the 10 species 1.5-fold higher in Carlos. Isotopic niche analysis also showed increased littoral reliance in Carlos fish, as the same 10 fish species in Carlos had statistically distinct ellipses that were enriched in δ13C. Lastly, mixing models indicated significant differences in trophic position of 7 of 12 fish species analyzed, and in each case the trophic position was significantly higher in Carlos. Our results indicate zebra mussels have community-wide impacts on energy flow in lakes, with invertebrate predators and many species of fish increasing their reliance on littoral energy sources, and some species of fish shifting to higher trophic positions. A key question is whether increased water clarity associated with zebra mussels can increase littoral production sufficiently to compensate for higher demand. If not, it’s plausible invertebrate and fish production will decline due to increased intra- and inter-specific competition.