Contrasting local responses but similar regional turnover in phytoplankton and rotifer assemblages in saline–alkaline wetlands, Northeast China

Freshwater salinization and alkalization are emerging threats to global biodiversity, particularly in arid and semi-arid regions. Although ecological impacts of freshwater salinization and alkalization on local biodiversity have received increasing attention, it remains unclear how diversity patterns of organisms in different trophic levels respond at a regional scale. Here, we investigated rotifer and phytoplankton assemblages across 61 wetlands spanning a broad saline–alkaline gradient in the Songnen Plain, Northeast China. We quantified local (alpha) diversity, partitioned beta diversity into turnover and nestedness components, and evaluated how saline–alkaline conditions are associated with assemblage and diversity. Our results showed salinization in wetlands was often associated with high pH, nutrient enrichment, shallower surface water and lower macrophyte coverage, association with a significant decline in local phytoplankton diversity but no significant effect on rotifer diversity. Rotifer assemblages were mainly shaped by water depth, and macrophyte coverage, while phytoplankton assemblages were more influenced by TOC, NH 4 + -N, water temperature, and macrophyte coverage. Phytoplankton diversity in local wetlands was negatively associated with pH and inorganic nitrogen concentrations, whereas rotifer diversity was associated with water temperature, water depth, and nutrient concentrations. Despite these contrasting local responses, both groups exhibited high spatial compositional dissimilarity among investigated wetlands. Mean total beta diversity reached 0.89 for rotifers and 0.91 for phytoplankton, with turnover accounting for 74.16% and 67.03% of the assemblage dissimilarities, respectively. These findings reveal the profound impacts of freshwater salinization and alkalization on wetland organisms. We highlight the importance of integrating both alpha and beta diversity dimensions in monitoring biodiversity response to these emerging threats in freshwater ecosystems under global environmental change.