In mollusk cultivation areas large amount of biomass and metabolites is accumulated. For this reason, biological monitoring in the farming areas, which includes study of microalgae as environmental quality indicators, is of considerable importance. Samples of mussels harvested from collectors at 6 m depth over the period February 2015 – March 2016 have been utilized for studying epizoon microalgae residing on mollusk shells. At the same time, sea water at depths of 0 and 6 m was sampled for determining phytoplankton and hydrochemical parameters of environment in the mussel-and-oyster farm area. Dissolved oxygen, biological oxygen demand after five days of incubation in the dark (BOD5), alkaline permanganate oxidizability, silicates, organic and inorganic forms of nitrogen and phosphorus have been quantified in the water samples using conventional methods. In the epizoon of the mussel shells, 108 taxa of microalgae of four phyla have been identified: 3 species of Сyanoprokaryota, 6 of Dinophyta, 6 of Haptophyta and 93 of Bacillariophyta. The maximum values of the species richness (26) and abundance of microalgae were observed in February (74,78·103 cells·cm-2, t = 9,7 °C) and April 2015 (62,0·103 cells·cm-2, t = 10,3 °C), as well as in January 2016 (65,1·103 cells·cm-2, t = 9,5 °C). The highest biomass was registered in August (0,272 mg·cm-2, t = 25,5 °C). The main contribution to the total abundance was made by the diatoms Tabularia fasciculata while Navicula ramosissima, and cyanobacteria were prevalent in the total biomass. In phytoplankton at the depths of 0 and 6 m, 135 taxa belonging to eight phyla have been found: 2 species of Cyanoprokaryota, 47 of Acillariophyta, 57 of Inophyta, 17 of Haptophyta, 5 of Chlorophyta, 2 of Euglenophyta, 3 of Cryptophyta and 2 of Chrysophyta. The genus Chaetoceros dominated by the number of diatoms species (18). In terms of abundance and biomass, the dinoflagellate Prorocentrum micans and haptophyte Emiliania huxleyi were dominant. The maximum abundance (370·107 cells·m-3) and biomass (7560 mg·m-3) of the phytoplankton were observed in spring and autumn. In total, 213 of microalgae taxa have been identified in the phytoplankton and mussel shell epizoon, with 30 ones being common for both. Furthermore, 26 potentially toxic species and 24 indicator species have been determined, among which 26 ones are betamesosaprobionts, the indicators of moderate level of water pollution. Thermohaline characteristics of water in the mollusk farm area did not exceed those of the long-term observations. At all horizons, the oxygen content was at the level of 93–125 % of saturation. The sea water oxidizability did not exceed the maximum permissible level established by fishery standards. The concentration of nutrients was high with a large fluctuation range, which indicates anthropogenic impact on the water area. The values of the total inorganic nitrogen-to-phosphorus and silicon-to-phosphorus ratios suggested nitrogen and silicon limitations for the microalgae community development from July to December. The mussel epizoon microalgae abundance strongly correlated with water temperature and dissolved oxygen, and a strong correlation of the biomass with inorganic phosphorus was observed, too. Moderate correlations were also found with inorganic phosphorus and organic nitrogen. For the phytoplankton, moderate correlations of abundance with hydrological and hydrochemical characteristics were identified: with nitrates in the surface layer and with temperature, dissolved oxygen, and organic nitrogen in the subsurface water layer. The phytoplankton biomass moderately correlated with the silicate concentration. The hydrological and hydrochemical structure of sea water, especially in the mollusk farming areas, affected species composition and quantitative characteristics of planktonic and benthic microalgae communities.

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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The studies of seasonal phytoplankton dynamics, its growth rate, and microzooplankton grazing were conducted on two stations in the western Black Sea coastal waters near Sevastopol from January 2021 to December 2022. The phytoplankton species composition has remained relatively the same during recent years compared to the end of the last century and the beginning of the 2000s. However, significant changes have occurred in the ratio between different species of diatoms, and the proportion of dinoflagellates was increased, especially in the autumn. Large diatoms and dinoflagellates play a crucial role in forming the phytoplankton biomass seasonal peaks. The first central maximum was observed in July, and the second smaller one was in September–November. Whereas two decades ago, the small diatoms generated three peaks annually: in February, May, and September–October. The maximum values of the phytoplankton growth rate and the rate of its consumption by microzooplankton decreased 2–3 times compared to the beginning of the 2000s. The relative share of primary production consumed by microzooplankton annually averages 35%, two times lower than before.