Roles of parasitic fungi in aquatic food webs: a theoretical approach

P>1. Parasites are ubiquitous in ecosystems, but their roles in material transfer are poorly understood. Fungal parasites in freshwater ecosystems are of major importance to small heterotrophic eukaryotes and consume large phytoplankton that are resistant to zooplankton grazing. 2. To evaluate their ecosystem-level effects, we developed a simple food web model that incorporates competition between small and large phytoplankton for nutrients, zooplankton grazing on small phytoplankton, fungal parasitism on large phytoplankton and includes a newly discovered trophic link from fungal zoospores to zooplankton (F-Z link). 3. Our model demonstrates the likely occurrence of an indirect mutualism between fungi and zooplankton, in which fungal parasitism increases zooplankton production by reducing the biomass of inedible large phytoplankton. Contradicting the expectation from a previous short-term experiment that the F-Z link may benefit zooplankton, the presence of the F-Z link can reduce material transfer from phytoplankton to zooplankton because of the negative effect of the indirect mutualism. The model indicates that high growth efficiency of fungi on host tissue and their high nutrient status for zooplankton are crucial for the F-Z link to increase zooplankton production. 4. The model also indicates that the contribution of material transfer via F-Z link to zooplankton increases with nutrient availability. Our results suggest that parasitic fungi may be a key player in material transfer, especially in eutrophic ecosystems.