Consistent diversity loss but divergent functional responses of freshwater biofilms to salinity and nutrient enrichment across climatic regions.

Freshwater salinization and nutrient enrichment are intensifying globally. Despite their widespread co-occurrence, the generality of their combined effects across regions remains unclear, limiting identification of appropriate policy scales. Using outdoor freshwater ponds established >10 years ago, replicated across inland, coastal and mountain regions, we examined independent and interactive effects of elevated salinity (0, +5 and +15 g NaCl l⁻¹) and nutrient enrichment (10 : 1 N : P additions) on benthic biofilms. Salinity was the dominant driver of community structure and function. Nutrient enrichment increased green algae abundances after 30 days but did not affect biofilm function or bacterial structure. Across all regions, bacterial richness and Shannon's diversity index declined consistently with increasing salinity after 30 days and bacterial community structures differed among all salinity levels by 60 days. Responses of phototrophic groups and biofilm functions, however, were region-specific. In the inland region, high salinity increased biofilm biomass, gross primary productivity (GPP), net primary productivity (NPP) and abundances of diatoms and cyanobacteria; in the coastal region, GPP and NPP declined under high salinity with no change to phototroph abundances; while neither was affected in the mountain region. In sum, taxonomic changes in biofilm bacterial communities under salinization were consistent across regions, while functional changes varied.