Predation and cyanobacteria jointly facilitate competitive dominance of small-bodied cladocerans

The hypothesis that the joint effects of predation and cyanobacteria favor the performance of small-bodied cladocerans relative to large-bodied species was tested using three competition experiments in a two-factor full-factorial design. The largebodied Daphnia pulex outcompeted the medium-bodied Simocephalus vetulus when fed a green alga Chlorella pyrenoidosa. However, the presence of the toxic cyanobacterium Microcystis aeruginosa and the coelenterate predator Hydra oligactis reversed the competitive outcome. Their joint effect was significant and contributed to the strongest dominance of S. vetulus. Similar dominance shifts were also observed in the competition between D. pulex and the small-bodied Bosmina longirostris, and in the competition between S. vetulus and B. longirostris. Life table and predation experiments showed that the order of sensitivity of three cladoceran species to Microcystis and to Hydra was the same as the order of competitive superiority in the absence of cyanobacteria. Therefore, cyanobacteria, predation and their joint effects are important drivers for competitive outcomes among cladocerans, potentially creating new opportunities for small-bodied species to dominate in a community. The present study underscores the importance of considering the combined effects of dietary palatability and predation on competitive dominance among herbivores, when predicting zooplankton responses to frequent cyanobacterial blooms.