Can induced drying modulate the response of leaf litter decomposition and fungal diversity to wastewater effluents in permanent streams?

Watercourses are among the most threatened ecosystems globally and face multiple anthropogenic stressors of different origins and intensities that reduce biodiversity and disrupt ecosystem functioning. Among these stressors, both stream drying and wastewater treatment plant (WWTP) effluents are expected to become more intense with global change, reducing water availability and consequently the dilution capacity of nutrients and pollutants. Leaf litter decomposition is a key ecosystem function in which aquatic hyphomycete communities play a crucial role, yet the interactive impacts of drying and consecutive WWTP exposure on both remain poorly understood. We conducted a field experiment in three permanent Mediterranean streams to assess the effects of induced drying and WWTP effluents on leaf litter decomposition and its associated fungal communities. After microbial colonization, leaf litter bags were subjected to two drying intensities by moving the bags into and out of the streambed: pulse (short cycles of drying and rewetting) and press (prolonged drying). The bags were then exposed to WWTP effluents downstream. We analysed the individual and joint effects of both stressors on leaf litter mass loss, fungal biomass, leaf C:N and fungal community structure via DNA sequencing. We found that drying increased fungal species richness and changed community composition through the colonization of terrestrial species but reduced leaf litter decomposition. The differences between drying intensities were minimal. In contrast, the WWTP effluent had no individual effects on leaf litter decomposition, but previously dried leaves exhibited legacy effects. The WWTP effluent also altered the fungal community composition by reducing the number of terrestrial species and promoting their replacement with more tolerant aquatic species. Our results highlight the long-lasting legacy effects of drying on leaf litter decomposition and its associated fungal community when leaves are exposed to WWTP effluent. The increase in the number of dying events limits the dilution capacity of streams and likely increases the interaction between the two stressors, leading to more pronounced changes in fungal assemblages and their decomposition capacity, which is an important threat to ecosystem functioning.