The species composition of microscopic algae of 14 estuaries of the North-Western Black Sea (NWBS) coast has been studied. During the period of 1982–2017, 1817 samples were collected and processed at 103 sampling stations; 1200 permanent slides were prepared and examined. A total of 544 species of microalgae were revealed in benthos of the estuaries located in the NWBS Region. They belonged to 187 genera, 89 families, 49 orders, 11 classes, and 8 divisions of algae. Bacillariophyta (372 species), Cyanoprokaryota (79), and Chlorophyta (54) led in species diversity. The richest genera are Nitzschia Hassall (39 species), Navicula Bory (31), Amphora Ehrenb. (20), Tryblionella W. Sm. (12), Gyrosigma Hassall (11), Diploneis Ehrenb. (11), Phormidium Kütz. (10), Cocconeis Ehrenb. (9), Surirella Turpin (10), and Cymbella C. Agardh (9). Among estuaries, the Dniester Estuary led in diversity of microphytobenthos (296 species); the Dofinovsky Estuary was the poorest in terms of species number (30). Revealed species included 26 species first cited for the study area; among them, 15 species are new for the flora of Ukraine, four species are new records for the Black Sea, and three species are new for science. According to the ratio of ecological groups of algae, algal flora of the NWBS estuaries is freshwater-brackish, alkaliphilic, and β-mesosaprobic. In terms of salinity tolerance, revealed species belong to groups of freshwater (318 species), brackish (107), and marine (117) forms. Substrates found in microphytobenthos communities included 400 species on the silty bottom, and 277 on the sandy bottom. There were 398 periphytic species found on macrophytes, 166 species on stones, 136 on shells of molluscs, 125 species inhabited concrete constructions, 27 species of algae occurred on barnacles, 78 species were found in foam, and 14 – in ice. In terms of biogeography, algal flora of the NWBS estuaries were represented by species with cosmopolitan (340) and boreal (150) distribution; the last group includes boreal-natal (22), north-alpine (11), boreal-tropical (13), arctic-boreal (5), and tropical (1) elements.
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