Environmental filtering structures phytoplankton communities in a tropical river–estuarine transition: evidence consistent with species sorting

Abstract Phytoplankton communities provide sensitive models for testing how environmental filters organize plankton assemblages across river–estuarine transitions. We evaluated whether salinity, hydrological seasonality, and nutrient-related gradients structure phytoplankton biomass, diversity, composition, indicator taxa, and turnover in the lower Itapecuru River, on the Brazilian equatorial margin. Five sites, including one mesohaline sector and four predominantly limnetic reaches, were sampled during rainy and dry seasons. The community comprised 93 taxa distributed mainly among Bacillariophyta, Chlorophyta, Cyanobacteria, Euglenophyta, Charophyta, and Dinophyta. The mesohaline sector showed higher salinity, electrical conductivity, and total dissolved solids, separating it from upstream limnetic reaches. Phytoplankton density increased during the dry season, whereas salinity reduced density and richness. Bacillariophyta dominated most sites and seasons, and IndVal identified taxa associated with mesohaline, limnetic, and seasonal conditions. Beta diversity was dominated by turnover, indicating taxonomic replacement along the gradient. Redundancy analysis and variance partitioning showed that salinity, pH, nitrate, and silicate explained a significant fraction of compositional variation, exceeding pure spatial effects. These findings indicate that local environmental filters, especially salinity and hydrological seasonality, structure phytoplankton assemblages and provide evidence consistent with species sorting in tropical river–estuarine systems under hydrological connectivity and salinization pressure.