Multiple drivers of leaf litter decomposition under salinity stress.

Salinity shifts driven by global change can strongly influence detritus-based stream ecosystems by altering leaf litter decomposition, a key ecological process known to depend on both the identity and diversity of leaf litter. We conducted a controlled microcosm experiment to assess the interactive effects of salinity and leaf litter identity and diversity on decomposition mediated by a detritivorous isopod (Caecidotea sp.), on microbial decomposition, and on aquatic hyphomycete sporulation rate and assemblage composition. Our design included three salinity levels (0, 5 and 10 PSU) and three leaf litter treatments: monocultures of Acer rubrum (a freshwater-associated tree species) and Spartina alterniflora (a saltwater-associated grass species) and a mixture of both species. We found a significant interaction between salinity and leaf litter type when decomposition was driven by detritivores, with higher decomposition rates of S. alterniflora at low salinity and of A. rubrum at intermediate salinity. Aquatic hyphomycete sporulation was also influenced by our treatments, increasing on S. alterniflora and decreasing on A. rubrum. Low salinity enhanced aquatic hyphomycete activity, but further increases in salinity suppressed this effect. Overall, our study highlights how simultaneous changes in environmental conditions and riparian vegetation can shape stream ecosystem functioning.