From fresh to hypersaline: diatom communities of sodium chloride springs in the taiga zone of the Middle Urals (Russia)

Intracontinental sodium chloride springs, as distinctive hydrochemical habitats, make a substantial contribution to regional biodiversity and influence the formation of aquatic landscapes in the taiga zone. This work presents the first comprehensive description of the species composition and variability of structural parameters in diatom assemblages from salt springs of the Middle Urals Piedmont region (Russia), examined across a broad salinity range (1.2–54.9 g l‒1). A total of 147 taxa were identified, of which 44 were new records for the region. The greatest species diversity is noted in the slightly saline waters of the Usolka and Solyanka rivers. The ecological preferences of numerous diatom species were analysed, and proposed changes and additions to the halobicity characteristics of several species are presented. Water salinity is a key limiting factor, causing differences in species composition within communities based on their halobicity characteristics. For Diatoma tenuis, Halamphora holsatica, Navicula lanceolata, Navicula salinarum and Nitzschia palea, the mineralization and specific ions are the most important factors. A number of species (Navicula cincta, Navicula viridula, Nitzschia scalpelliformis, Prestauroneis protracta, Staurosira subsalina, Tabularia fasciculata) thrive in high NaCl concentrations, indicating mesohalobicity rather than halophilicity. Synedra famelica should be considered a halophilic species, and Achnanthidium saprophilum is indifferent. Typical marine algae species were found in the studied springs, the routes and time of penetration of which have not yet been clarified. These findings expand current knowledge of poorly studied diatom communities associated with ancient inland brines and provide a basis for using diatom assemblages in environmental and geochemical monitoring of inland waters with complex salinity regimes.