The influence of zebra mussels (Dreissena polymorpha) on reducing phytoplankton abundance is well known, but their impact on overall energy flow in deep, mesotrophic lakes is less clear. We assessed impacts of zebra mussels on energy flow by comparing two Minnesota lakes: Lake Carlos has a dense population of zebra mussels, while upstream Lake Ida was free of zebra mussels until 2014 and densities are still extremely low. We used the stable isotope 13C to assess patterns of energy flow as δ13C values are lower in organisms supported by phytoplankton production relative to littoral energy sources such as periphyton and benthic algae. We sampled 13C in the tissue of multiple species in each lake to quantify energy sources for the overall food web. We analyzed eight taxonomic groups for this analysis (three fish species, five invertebrate groups), and calculated the difference in δ13C between each taxonomic group and filter-feeding mussels in each lake (hereafter corrected δ13C). Mussels rely on phytoplankton as a primary energy source, so larger corrected 13C values in our eight taxonomic groups would indicate greater reliance on littoral energy sources.
Results showed corrected δ13C values differed between Lake Carlos and Lake Ida for seven of the eight taxonomic groups, and all seven groups had higher corrected δ13C values in Lake Carlos. The presence of widespread, higher corrected 13C values in Lake Carlos indicate a greater reliance on littoral energy sources relative to the Lake Ida food web, likely driven by high zebra mussel densities reducing phytoplankton abundance. If littoral and benthic production do not compensate for reduced pelagic primary production, overall availability of energy to native secondary consumers and fish may be lower in Lake Carlos compared to Lake Ida. Increased importance of littoral energy sources could also cause shifts in invertebrate and fish communities towards species better adapted to littoral niches.