Climate change, the invasion of Bythotrephes longimanus, and recent changes in the zooplankton community of Lake Superior

Abstract

In recent decades, average summer surface temperatures in Lake Superior have increased and the invasive predatory zooplankton, Bythotrephes longimanus, became established. While climate warming and Bythotrephes have influenced zooplankton communities in other lakes, it is unclear how either have or will influence the zooplankton community in Lake Superior. A late spring ice out in 2014 provided an opportunity to observe the response of zooplankton in Lake Superior to inter-annual variation in temperature. To evaluate this response, I compared biomass estimates, phenologies, and community compositions of the zooplankton communities in western Lake Superior during the 2014 and 2015 growing seasons. I also compared the community compositions observed in these years to published literature to determine whether the community has changed in response to climate warming or planktivory by Bythotrephes. I evaluated the possible role of Bythotrephes in zooplankton community changes using a bioenergetic model to compare the consumptive demands of Bythotrephes to the production rates of their potential prey. Annual peaks in zooplankton biomass were correlated with peaks in surface temperature. Peak biomass in 2014 occurred approximately 20 days later than in 2015 suggesting that continued warming could have long-term effects on the timing of peak zooplankton biomass in Lake Superior. The amount of biomass at the peak did not differ between years nor did overall community structure suggesting that zooplankton biomass and species composition in Lake Superior may be more constrained by food availability than temperature. However, long-term comparisons of zooplankton community composition indicate that densities of Bosmina longirostris declined and the proportional contribution of Daphnia mendotae to cladoceran biomass increased since the 1970s. These community changes are more consistent with the expected outcome of planktivory by Bythotrephes than the expected outcomes of changes in temperature, primary production, or vertebrate planktivory. The results suggest that Bosmina is the cladoceran species most vulnerable to suppression by Bythotrephes in Lake Superior, which supports the hypothesis that Bythotrephes has altered the cladoceran community in Lake Superior. While consumption by Bythotrephes did not exceed total zooplankton production in Lake Superior during 2014 or 2015, future increases in Bythotrephes density and temperature could cause the top-down effects of Bythotrephes on the zooplankton community to increase. This work helps to clarify how climate warming and Bythotrephes could influence the zooplankton community and energy flow pathway in Lake Superior in the future.