Phosphorus enrichment increases the prevalence of a microsporidian parasite in experimental Daphnia populations

Nutrient availability can influence the population size of primary consumers and their parasites, but identifying the mechanisms responsible for nutrient‐induced shifts in parasite population sizes is challenging. Parasites of consumers may respond to complex feedbacks between consumers and their resources via shifts in both within‐ and between‐host processes. We investigated these effects by allowing well‐studied host–parasite (Daphnia‐microsporidian) and consumer–resource (Daphnia‐algae) systems to respond to phosphorus (P) manipulation in mesocosms. We quantified consumer–resource dynamics by measuring the effects of our P additions on algal %P and algal and host density at regular intervals. After 60 d, we measured total parasite population size, infection prevalence, and individual parasite load. Because our microsporidian parasite transmits via multiple routes and achieves higher loads in vertical (from mothers to offspring) than horizontal (via ingestion of spores) infections, we could indirectly assess the effect of nutrient additions on the number of new horizontally transmitted infections, and, by proxy, qualitatively evaluate nutrient effects on within‐host processes relative to those on between‐host processes (i.e., horizontal transmission). We found positive associations of total parasite population size, prevalence, and parasite load with algal %P. In addition, parasite load was negatively associated with algal density. We could not detect a relationship between any of the parasite responses and host density. Our experiment shows how nutrient additions can enhance parasite prevalence in ways that may not be directly mediated by host density and that within‐host processes can play a critical role in shaping the responses of entire parasite populations to nutrient additions.