1. We investigate whether an increase in the occurrence of cyanobacterial blooms affects zooplankton-parasite interactions. Cyanobacteria are expected to be of poor food quality for zooplankton hosts and are therefore expected to increase parasitism. Nevertheless, simultaneous exposure to both stressors may lead to different results, given the antibacterial secondary metabolites of cyanobacteria.
2. We exposed the zooplankter Daphnia magna to the cyanobacterial species Microcystis aeruginosa and the parasite that causes White Bacterial Disease in D. magna. Increased M. aeruginosa concentrations reduced the percentage of infected individuals and as such protected D. magna against parasitism. Interactions between M. aeruginosa and the parasite were antagonistic in terms of percentage of surviving Daphnia, total offspring per female and clutch size. Additional plating experiments showed a direct negative effect of Microcystis on bacterial growth.
3. The results suggest that changes in phytoplankton affect host-parasite interactions in zooplankton. Contrary to the prevailing paradigm that multiple stressors often induce additive or synergistic effects, we report an antagonistic effect of the presence of cyanobacterial stress on parasites in Daphnia. Thus, assessment of the outcome of host-parasite interactions needs to incorporate the environmental context.