Potassium stimulates fungal epidemics in Daphnia by increasing host and parasite reproduction.

As natural enemies, parasites can dramatically harm host populations, and even catalyze their decline. Thus, identifying factors that promote disease spread is paramount. Environmental factors can drive epidemics by altering traits involved in disease spread. For example, nutrients (such as nitrogen and phosphorus) can stimulate reproduction of both hosts and parasites or alter rates of disease transmission by stimulating productivity and nutrition of food resources of hosts. Here, we demonstrate nutrient-trait-epidemic connections between the greatly understudied macronutrient potassium (K) and fungal disease (Metschnikowia bicuspidata) in a zooplankton host (Daphnia dentifera). In a three-year survey, epidemics grew larger in lakes with more potassium. In laboratory assays, potassium enrichment of low-K lake water enhanced both host and parasite reproduction. Parameterized with these data, a model predicted that potassium addition catalyzes disease spread. We confirmed this prediction with an experiment in large mesocosms (6000 L) in a low K-lake: potassium enrichment caused larger epidemics in replicated Daphnia populations. Consequently, the model--data combination mechanistically explained the field pattern and revealed a novel ecological role for the nutrient potassium. Furthermore, our findings highlight the need for further development of theory for nutrient limitation of epidemics. Such theory could help to explain heterogeneous eruptions of disease in space, connect these outbreaks to natural or anthropogenic enrichment of ecosystems, predict the ecological consequences of these outbreaks, and reveal novel strategies for disease management.