Zooplankton Community Composition And Abundance Is Structured By Aquatic Fungi In A Large Estuary

Fungi are a little known but important part of pelagic aquatic ecosystems. A recent hypothesis – the “mycoloop hypothesis” – proposes that fungal parasites of algae improve energy transfer to the zooplankton and higher trophic levels by converting the biomass of inedible algae to highly nutritious fungal biomass and by separating phytoplankton aggregates into smaller and more ingestible fragments. This interaction can form an important resource for zooplankton during blooms of inedible or graze-resistant algae. Previous work in freshwater lakes and marine environments suggests that the mycoloop is most influential in highly productive systems such as estuaries, though no previous study has focused on this pathway in estuaries. To address this research gap we conducted a survey of the aquatic fungi within the third largest estuary and one of the most productive bodies of water in the world, Chesapeake Bay, USA. We compared the efficacy of two widely used fungal metabarcoding markers, the ITS and LSU regions, and examined correlations between the algal parasites and zooplankton abundance and community composition. Sampling was conducted over 3 months (June, July, August) in 2023 at 24 sites throughout the estuary, totaling 70 sampling events. We collected samples of zooplankton using traditional methods and characterized fungal communities in the water and the gut contents of zooplankton by DNA metabarcoding of the fungal LSU (28S) region. We found that zooplankton and aquatic fungal communities both had spatial and temporal variation in diversity and community structure that correlated with environmental variables. Congruent with our predictions based on the mycoloop hypothesis, the density of zooplankton was positively correlated with the diversity of algal parasites. A trophic connection between algal parasites and zooplankton was confirmed by zooplankton gut DNA contents. This study improves our understanding of the functional roles of fungi in estuarine systems and supports the importance of the aquatic fungi in the lower food web of a major estuary.