The effects of Bythotrephes longimanus and calcium decline on crustacean zooplankton communities in Canadian Shield lakes

Declining calcium concentrations and invasion by Bythotrephes longimanus are two important, often co-occurring, stressors affecting Canadian Shield lakes. However, there has been no experimental examination of how they might jointly influence zooplankton communities. We conducted a 6-week field mesocosm experiment in Havelock Lake, Haliburton, Ontario, Canada to examine the individual and joint effects of Bythotrephes and calcium along a gradient ranging from 1.2 to 2.6mg/l on zooplankton communities. Although densities of Bythotrephes in our study are unknown, it significantly reduced total zooplankton abundance in invaded compared to uninvaded treatments by 46%, with the greatest impacts on small cladocerans and daphniids. Low calcium reduced total zooplankton and cladoceran abundances. Although Havelock Lake has the lowest calcium concentration among invaded lakes in the Muskoka/Haliburton region (1.2mg Ca/l), an effect of calcium on individual species abundances was not detected. Additionally, we did not detect an interactive effect of both stressors. Our results suggest that lake calcium concentration may not yet be low enough to effect a strong response. However, as Bythotrephes continue to invade low calcium lakes, and as calcium concentrations further decline, we may see larger impacts on cladocerans as calcium thresholds for reproduction and growth are reached.