Alternative food sources interfere with removal of a fungal amphibian pathogen by zooplankton.
While the amphibian disease chytridiomycosis is causing ongoing population declines and biodiversity losses around the globe, efficient mitigation strategies are lacking. The free-living zoospores of the causative agents of this disease, the chytrid pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal), are a potential food source for filter-feeding micropredators as part of the aquatic food web. While consumption of zoospores can lower environmental pathogen loads, alternative food sources may interfere with pathogen removal rates. We compared the ability of three filter-feeding zooplankton species, the cladoceran Daphnia magna, the rotifer Brachionus calyciflorus and the ostracod Heterocypris incongruens, to remove Bd zoospores in water and investigated the effect of alternative food sources, the green algae Pseudokirchneriella subcapitata and Chlorella vulgaris, on zoospore ingestion by D. magna. Daphnia magna was the only micropredator candidate that effectively removed Bd zoospores from its environment, with an average removal rate of 1,012 ± 542 GE ind.-1 hr-1 within our test system. High concentrations (1 × 105 cells/ml) of large and easily ingestible P. subcapitata reduced pathogen removal rates, whereas the small and less edible C. vulgaris did not interfere with pathogen removal. Synthesis and applications. We showed that Daphnia spp., which are keystone species in all sorts of aquatic habitats worldwide, are promising target agents for biologically mitigating chytridiomycosis infections and how natural food sources may interfere with this strategy. We also suggest potential management actions for biological disease mitigation, aiming to optimize environmental conditions for these target filter feeders, thereby reducing pathogen densities and eventually infection pressure in amphibian hosts. Examples of such management actions include, but are not limited to, removal of planktivorous fish, habitat restoration, nutrient control or agrochemical regulation in the vicinity of amphibian breeding ponds. Further studies, including field trials, are needed to confirm the effects of pathogen consumption on infection dynamics in natural situations and investigate the impact of intervention actions.