Daphnia vertical position and implications for the impact of the invasive zooplankton predator, Bythotrephes longimanus, on plankton communities in south-central Ontario

Abstract

Species diversity and identity play an important role in determining community structure and regulating ecosystem processes. However, species interactions are often characterized by the mean effect or response, often ignoring intraspecific variation in trait expression. In this thesis, I examined the ecological consequences of intraspecific trait variation in Daphnia vertical position in plankton communities invaded by the presence of the invasive zooplankton predator, Spiny water flea (Bythotrephes longimanus). In the first chapter, I assessed the factors influencing Daphnia vertical position response to Bythotrephes presence using a field survey of invaded and uninvaded lakes across a gradient of abiotic variables in south-central Ontario. In the second chapter, I examined the influence of Daphnia vertical position on Bythotrephes impacts on plankton communities using mesocosm experiments where I manipulated Bythotrephes presence and Daphnia vertical position. In the final chapter, I used mesocosm experiments to determine if differences in the vertical position of dispersing Daphnia influenced recovery in Daphnia populations invaded by Bythotrephes. Results from my field survey show that Daphnia vertical position response to Bythotrephes is species specific, with only D. mendotae and D. longiremis vertical position influenced by Bythotrephes presence. In invaded lakes, D. mendotae vertical position in dependent on water clarity and Bythotrephes density. Results from my mesocosms experiment suggest that Daphnia vertical position influences the type and magnitude of Bythotrephes impacts on Daphnia populations, resulting in indirect impacts on small cladocerans, large cladocerans and copepodids. Furthermore, differences in Daphnia vertical position in invaded mesocosms influenced algal densities, resulting in trophic cascades in mesocosms where more Daphnia were hypolimnetic. There was no effect of differences in vertical position of dispersing Daphnia on Daphnia populations invaded by Bythotrephes. Taken together, these results suggest that Daphnia vertical position response to Bythotrephes is species specific and that differences in Daphnia vertical position result in differences in Bythotrephes impacts on plankton communities in invaded lakes.