Fungal richness does not buffer the effects of streams salinization on litter decomposition

Freshwater salinization is a world-wide phenomenon threatening stream communities and ecosystem functioning. In these systems, litter decomposition is a main ecosystem-level process where fungi (aquatic hyphomycetes) play a central role linking basal resource and higher levels of food-web. The current study evaluated the impact of aquatic hyphomycete richness on leaf litter decomposition when subjected to salinization. In a microcosm study, we analysed leaf mass loss, fungal biomass, respiration and sporulation rate by fungal assemblages at three levels of species richness (1, 4, 8 species) and three levels of salinity (0, 8, 16g NaCl L-1). Mass loss and sporulation rate were depressed at 8 and 16g NaCl L-1, while fungal biomass and respiration were only negatively affected at 16gL(-1). A richness effect was only observed on sporulation rates, with the maximum values found in assemblages of 4 species. In all cases, the negative effects of high levels of salinization on the four tested variables superimposed the potential buffer capacity of fungal richness. The study suggests functional redundancy among the fungal species even at elevated salt stress conditions which may guarantee stream functioning at extreme levels of salinity. Nonetheless, it also points to the possible importance of salt induced changes on fungal diversity and identity in salinized streams able to induce bottom-up effects in the food webs. The depressive effect of salinization on detritus processing in streams overrides the potential buffer capacity of aquatic hyphomycetes higher richness, main drivers of litter decomposition. These findings, suggest that this stressor is as worldwide threat for stream ecosystem functioning independently of the fungal species abundance.