Freshwater ecosystems rely heavily on microbial communities, particularly aquatic fungi (aquatic hyphomycetes), to decompose organic matter and sustain aquatic food webs. Despite their ecological importance, large-scale studies on how fungal communities respond to environmental variation are scarce. This project investigated how substrate identity, land use, river position, and environmental conditions shape aquatic fungal diversity across the Deschutes River watershed in western Washington. The study pursued four main objectives: (1) determining how leaf substrate type affects fungal diversity and composition; (2) assessing whether stream position structures fungal assemblages; (3) evaluating land use influences; and (4) examining relationships between fungal diversity and environmental variables like nutrients and trace metals. Together, these aimed to identify key drivers of fungal community assembly and explore the potential of aquatic fungi as freshwater health indicators.
Leaf litter from three riparian tree species — red alder, Oregon white oak, and willow — was deployed across twelve sites representing a range of land uses, including urban, agricultural, timber, and natural areas. After a standardized incubation period, fungal communities were identified via high-throughput DNA sequencing, yielding over 3,100 operational taxonomic units. Data were analyzed using diversity metrics, ordination, and multivariate statistics. Leaf substrate identity emerged as the strongest driver of fungal community structure, with significant differences in diversity and composition among leaf types. Land use, water body type, and river position had comparatively weaker effects on overall diversity, though they did influence community composition. Environmental variables showed limited relationships with fungal diversity.
These results carry meaningful implications for freshwater conservation. The strong effect of substrate identity underscores the importance of maintaining diverse riparian vegetation, as plant species composition directly shapes the microbial communities that drive nutrient cycling and ecosystem function. Homogenization of riparian vegetation — through land use change or invasive species — could therefore impair aquatic ecosystem resilience. The study also reinforces the value of aquatic fungi as bioindicators. Because fungal communities respond rapidly to disturbance, they can signal ecosystem stress before impacts reach higher trophic levels, making them useful tools for early detection in monitoring programs. Overall, this project links microbial ecology to practical watershed management, supporting the case for riparian plant diversity as a cornerstone of freshwater ecosystem health. Findings may inform restoration planning, bioassessment frameworks, and regional monitoring programs in the Pacific Northwest and beyond. ($13,000)