New insights into the taxonomic and functional response of phytoplankton to the effect of salinity in semiarid reservoirs

The salinization of freshwater systems, intensified by climate change, is a growing threat to aquatic ecosystems, particularly in semiarid regions. While salinity acts as an ecological filter, its effects on phytoplankton diversity remain poorly understood. Here, we assessed how salinity influences phytoplankton community structure in semiarid reservoirs using a combined taxonomic and functional trait-based approach. The study was conducted in 11 reservoirs in northeastern Brazil. Salinity was associated with reduced water volume. Although climate and other environmental variables were the main drivers of phytoplankton diversity, salinity had a relatively greater, albeit modest, influence on functional diversity. Moderate salinity increased taxonomic and functional diversity, including species richness and functional redundancy, while reducing functional evenness, indicating a transitional phase with coexistence of tolerant and sensitive species. High salinity favored filamentous cyanobacteria taxa with heterocytes, aerotopes, and akinetes, as well as chlorophyceans and dinoflagellates, whereas low salinity selected for species with silicified cell walls and Zygnematophyceae. Despite cyanobacterial dominance under high salinity, species-specific responses were observed across taxa. These findings indicate salinity as a relatively more important environmental filter for functional diversity, although multiple environmental factors shape phytoplankton structure.