Changes in the cladoceran community of Lake Superior and the role of Bythotrephes longimanus

Introductions of Bythotrephes longimanus have resulted in reduced cladoceran species richness and biomass in the Laurentian Great Lakes and many inland lakes. Bythotrephes was first observed in Lake Superior in 1987 but its effect on the cladoceran community has been unknown. We compared the composition of the offshore cladoceran community of Western Lake Superior during 2014 and 2015 to zooplankton surveys from 1971-2001 to determine whether changes in the cladoceran community have occurred. Monthly comparisons show that the contribution of Bosmina longirostris to offshore cladoceran numbers was generally twice as much in the 1970s than during 2014-2015 while the relative contribution of Daphnia mendotae increased after the 1970s. These community changes are consistent with changes due to Bythotrephes observed in other lakes. To evaluate evidence for the role of Bythotrephes in these community changes, we used data from 2014-2015 to analyze patterns in spatial and vertical overlap between Bythotrephes and its cladoceran prey species (Bosmina, Daphnia, and Holopedium) and compared estimates of consumption by Bythotrephes to production of these potential prey. Bosmina was the species whose vertical position and rate of production made it most vulnerable to suppression by Bythotrephes. Of the potential cladoceran prey species, Bosmina densities were also the most negatively correlated with Bythotrephes densities. These findings support a hypothesis of top-down effects on Bosmina by Bythotrephes in Lake Superior. This work informs future zooplankton research in Lake Superior and furthers our understanding of the effects of Bythotrephes on the Lake Superior food web.