Environmental Drivers for Freshwater Fungal Communities Across a Gradient of Land Uses in Agriculturally Dominated Watersheds

Abstract

Freshwater fungi are vital to the aquatic food web, essential for nutrient cycling, energy flow, and ecosystem regulation. Their distribution is particularly contingent upon agricultural runoff, which can carry agrochemicals capable of influencing the freshwater mycobiota and potentially impacting the ecosystem services which they provide. While such impacts are well documented for freshwater bacterial communities, fungal communities are critically understudied. Here, we address this research gap by assessing the impact of anthropological and environmental perturbations on the freshwater mycobiota in the agriculturally dominated South Nation River basin in Eastern Ontario, Canada. We undertook biweekly water sampling from 2016-2021, complemented by rich ancillary data including water properties, hydrology, weather conditions, and fungal ITS2 metabarcoding. Our study yielded 6,571 Operational Taxonomic Units from 503 water samples, spanning 15 fungal phyla, dominated by Ascomycota, Basidiomycota, and Chytridiomycota. Agricultural land use decreased the mycobiota alpha diversity and distinct fungal communities were observed at agricultural ditch and mixed-use sites compared to the forested site. Notably, river discharge emerged as a predominant influencer of both alpha and beta diversity, likely transporting fungi via precipitation, especially from plant-rich catchment basins. Intriguingly, environmental data only explained a fraction of fungal community variation, underscoring the significance of unmeasured factors such as fungicide application, alongside stochastic community assembly processes. This work highlights the complex interplay of factors influencing the freshwater fungal community in agriculturally impacted watersheds and shows the need for further investigation for a more comprehensive understanding of the freshwater fungal ecology.