Benthic Diatom Distribution Along a Salinity Gradient in Southeast Caspian Sea Watersheds, Northeast Iran

In coastal areas, salinity is an important variable structuring diatom composition. The Caspian Sea is the world's largest enclosed water body. Variations in benthic diatom composition along salinity gradients were investigated in two important rivers flowing into the southeast Caspian Sea, Gorganroud and Atrak. Epipelic diatom samples were collected from 11 sites from November 2020 to April 2021, along with water samples. One hundred three taxa belonging to 34 genera were identified, of which 47 taxa (≥ 1% relative abundance in ≥ 5 samples were used to model optima and tolerance). The results revealed a shift from freshwater to brackish taxa in dominant and subdominant diatoms with increasing salinity (from 0.02 to 6.64 g.L −1 ). Taxa such as Amphora inariensis and Cymbella excisa dominated low‐salinity sites, while Navicula salinarum, Nitzschia cf. thermaloides , and Haslea spicula increased downstream. Salinity significantly affected the diatom distribution and separated brackish taxa from freshwater ones. However, ammonium and temperature were equally important in Atrak, and phosphate and dissolved oxygen also contributed. However, salinity was not the only factor determining diatom distribution, as nutrients and thermal regime modulated assemblages where salinization was less extreme. Shannon diversity and evenness declined at intermediate salinities (5–7 g.L −1 ), reflecting Remane's species‐poor zone. These findings underscore diatom's value for monitoring salinization and nutrient impacts in Caspian tributaries and support their use in catchment management and long‐term ecological assessments.