Temperature and host diet jointly influence the outcome of infection in a Daphnia‐fungal parasite system

Climate change has the potential to shape the future of infectious diseases, both directly and indirectly. In aquatic systems, for example, elevated temperatures can modulate the infectivity of waterborne parasites and affect the immune response of zooplanktonic hosts. Moreover, lake warming causes shifts in the communities of primary producers towards cyanobacterial dominance, thus lowering the quality of zooplankton diet. This may further affect host fitness, resulting in suboptimal resources available for parasite growth. Previous experimental studies have demonstrated the respective effects of temperature and host diet on infection outcomes, using the zooplankter Daphnia and its microparasites as model systems. Although cyanobacteria blooms and heat waves are concurrent events in nature, few attempts have been made to combine both stressors in experimental settings. Here, we raised the zooplankter Daphnia (two genotypes) under a full factorial design with varying levels of temperature (the standard 19°C and elevated 23°C), food quality (Scenedesmus obliquus as high‐quality green algae, Microcystis aeruginosa and Planktothrix agardhii as low‐quality cyanobacteria) and exposed them to the parasitic yeast Metschnikowia bicuspidata. We recorded life history parameters of the host as well as parasite traits related to transmission. The combination of low‐quality cyanobacterial diets and elevated temperature resulted in additive detrimental effects on host fecundity. Low‐quality diets reduced parasite output, while temperature effects were context dependent. Overall, we argue that the combined effects of elevated water temperature and poor‐quality diets may decrease epidemics of a common fungal parasite under a climate change scenario.