Combined effects of land-use- and climate-driven stressors on stream fungi and organic matter decomposition

1. Freshwater microbial communities are essential for maintaining ecosystem functions and services, with aquatic fungi playing a particularly critical role in decomposing terrestrial organic matter entering streams and converting it into energy and nutrients that sustain higher trophic levels. However, freshwater ecosystems face growing threats from multiple stressors. The combined effects of these stressors on fungal biodiversity and functioning in streams remain poorly understood. This study examines the joint effects of land-use and climate-driven stressors on fungal biodiversity and two key functions—fungal biomass accrual and organic matter decomposition—across two stream compartments: the water column and streambed sediments. 2. We conducted an extensive two-year survey across 62 Iberian stream sites in southwestern Europe. To assess stressor impacts, we analysed responses from both the water column and streambed sediments, examining compartment-specific differences. Models were developed to evaluate six stressor types - organic matter and inorganic nutrient enrichment, oxygen depletion, thermal stress, drying stress and riparian degradation- and hydrology. Fungal responses were evaluated across multiple biodiversity dimensions, including taxonomic diversity, functional diversity, and community composition. 3. Results revealed that organic matter enrichment positively influenced fungal biodiversity and functioning, while thermal and drying stresses, along with riparian degradation, had significant negative effects. Stressor effects were predominantly additive, with limited interactions observed. Inorganic nutrient enrichment showed weak effects, likely reflecting minimal nutrient limitation in Iberian streams. Differences between stream compartments emerged, with streambed sediments buffering the negative impacts of thermal and drying stresses on fungal biodiversity and functions. 4. Our findings underscore the influence of multiple stressors on fungal biodiversity and functioning, which are expected to intensify under climate change. The predominance of additive stressor effects suggests that individual stressors can be targeted independently, simplifying management strategies. The buffering role of streambed sediments highlights their importance in providing resilience against global stressors, particularly thermal and drying stresses. 5. This study emphasises the urgent need to safeguard conditions that support fungal biodiversity and ecosystem functions in streams to ensure the continued provision of vital ecosystem services in the face of global change.