Daphnia can protect diatoms from fungal parasitism

Many phytoplankton species are susceptible to chytrid fungal parasitism. Much attention has been paid to abiotic factors that determine whether fungal infections become epidemic. It is still unknown, however, how biotic factors, such as interactions with zooplankton, affect the fungal infection process. Because the size of fungal zoospores is well within the preferred range of food particle size for Daphnia, we hypothesize that Daphnia will affect the fungal infection process by reducing the abundance of fungal zoospores. To examine the effects of zooplankton on the fungal parasitism of phytoplankton, we performed grazing experiments. These experiments revealed that Daphnia significantly decreased fungal parasitism by grazing on the fungal zoospores. Daphnia also had a small positive effect on fungal infection, probably by increasing the encounter rate between fungi and host phytoplankton cells. These results suggest that Daphnia can affect the seasonal succession of chytrids and their host phytoplankton species. In addition, these results imply that zoospore-producing fungi may play an ecological role as food sources for Daphnia in aquatic food webs.