| Summary: | In this work, the species diversity of epilithon diatoms of Donuzlav Gulf (Crimean coastal waters of the Black Sea) is presented for the first time. A total of 60 taxa of Bacillariophyta belonging to 3 classes, 18 families and 34 genera were found. The largest number of species (45) of diatoms was observed in summer, and the smallest number of species (12) in winter. A total of 25 saprobity indicator species of diatoms was found on the epilithon, 12 taxa of which belong to a group of betamesosaprobionts—indicators of moderate organic pollution of water. Marine species comprise 55% and brackish-marine ones make up 37% of them, but no freshwater species were identified. The diatom community was dominated by cosmopolites (32%) and arctic–boreal–tropical species (27%). The maximal abundance (186.2 × 103 cells cm−2) and biomass (0.083 mg cm−2) of diatoms was noted in June 2018 at salinity 16.58‰ and T = 24.4 °C with the dominant cosmopolites species of Licmophora abbreviata and Cocconeis scutellum. The minimum values, 14.8 × 103 cells cm−2 and 0.005 mg cm−2, were observed in January 2019 at salinity 16.86‰ and T = 9.0 °C with the dominant Berkeleya rutilans and Nitzschia hybrida f. hyalina. The high values of BOD5, oxidizability, silicon, Norg and Porg were noted, indicating a certain degree of water pollution by domestic sewage. It has been shown that the pollution in the southwestern part of the Donuzlav Gulf is greatest since this part of the coast was populated most densely in comparison with the northern coast of the Gulf where there are no coastal settlements except for two mariculture farms.
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