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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.

The results of original studies (1987–2016) of the floristic composition of diatoms of natural and anthropogenic substrates in different regions of the Black Sea at depths of 0–53 m are summarized. 350 taxa of Bacillariophyta were identified, 77% from them are benthic, 57% – marine, 29% – boreal and 25% – boreal-tropical. In the epilithon, 140 taxa were found, in the epiphyton of macrophytes – 254, the epizoon of the mussel Mytilus galloprovincialis Lam. – 154, on the skin of dolphins – 10 and in the periphyton of anthropogenic substrates – 194. The Сzekonowsky-Sörensen coefficient of species similarity depending on the ecotope varied from 54 to 64%. In both ecotopes of epizoon and periphyton 28 species were common. The maximum values of abundance (N), biomass (B) and the Shannon index of species diversity (H) of diatoms in all ecotopes were observed in the winter-spring season: epilithon – N =416•103 cells•cm-2, B=0.4 mg•cm-2,Н = 1,89; epiphyton – N=366.3•103 cells•cm-2,B=0.82 mg•cm-2, Н=3.03; epizoon – N=179.3•103 cells•cm-2, B=0,54 mg•cm-2, Н=3.81; periphyton at exposure of experimental plates from 1 to 13 months – N =2180.8•103 cells•cm-2, B=0.543 mg•cm-2, H=0.97–3.67 and with monthly exposure – N=1229•103 cells•cm-2, B=0.35 mg•cm-2, H=1.1–3.5. The questions of similarity and dissimilarity of qualitative and quantitative characteristics of microphytobenthos diatoms in different ecotopes and areas of the Black Sea are discussed.

In hypersaline water bodies, the microphytobenthos plays a very important ecosystem role and demonstrates variability along with a salinity change. Due to anthropogenic activity, the sharp salinity increase in Bay Sivash occurred after 2014. To assess the changes in the microalgae community during the bay ecosystem transformation, the study was conducted four times in 2018 and 2019. At every sampling period, the samples were taken in a salinity gradient (from 7 to 10 sites). A total of 40 species of microalgae were identified during all research, including Cyanobacteria (Cyanophyceae, 2 species), Ochrophyta (Bacillariophyceae, 35 species), Haptophyta (Prymnesiophyceae, 2 species), and Miozoa (Dinophyceae, 1 species). According to the calculated similarity indices of Jaccard and Czekanowski–Sørensen–Dice, the species composition significantly differed during sampling periods. A total of 15 species were recorded at salinities of 80–90 psu, and 10 species at higher salinities, which contribute 64% of all species found in this study. The microalgae abundance was two times more in the floating green algae mat than on the bottom. There was no significant correlation between the number of species and salinity in all sampling periods. In November 2018, a significant positive correlation between the number of species in the sample and total suspended solids (TSS) and dissolved organic matter (DOM) was revealed. A significant correlation between the cell length in different species and salinity and DOM concentration was noted. Before the onset of the salinity increase, 61 species of microalgae were found in Eastern Sivash, of which only 12 have now been recorded, 31% of the currently found species. The characteristics of the total microphytobenthos abundance also significantly changed during all studies. Many characteristics have changed in the bay: the concentration of total suspended matter and dissolved organic matter, the temperature regime, composition of zoobenthos and plankton, and oxygen concentration. Due to this, it is unlikely that only the salinity increase caused the microphytobenthos changes in the lagoon.

The article summarizes original and literary data on different aspects of studying Cylindrotheca closterium (Ehrenberg) Reimann et Lewin 1964 in two biotopes – phytoplankton and microphytobenthos – of the Black Sea, the Sea of Azov, and the Sea of Japan for the period from 1976 to 2016. The aim of the work is to present the results of the study mainly of own data on the morphology, systematics and ecology of C. closterium in different seas and under cultivation in the laboratory. Information on the history of the species origin and its nomenclature changes is given. C. closterium belongs to the phylum Bacillariophyta, class Bacillariophyceae, order Bacillariales Hendey 1937, family Bacillariaceae Ehrenb. 1831, genus Cylindrotheca Rabenhorst 1859 emend. Reim. et Lewin 1964. This benthoplanktonic species occurs in the plankton, in littoral and sublittoral zones of the seas. The species is marine and brackish-water; it is a cosmopolite common in different geographical zones of the World Ocean. The results of studying alga by various methods under natural and experimental conditions in light and transmission electron microscopes of C. Zeiss LIBRA-120 are presented. The quantitative data of C. closterium were determined by direct counting of the cells in the Goryaev’ camera (V = 0.9 mm³) in light microscopes BIOLAM L-212, C. Zeiss Axioskop 40 with the program AxioVision Rel. 4.6 at 10×40, 10×100, and Olympus BX41 (Tokyo, Japan) with lenses UPLanF140× and 100×1/30 oil immersion. Cultivation of C. closterium was carried out in the cumulative mode on the nutrient medium F, volume of 1 L under light intensity of 13.7 klx and temperature of +20…+21 °C. Morphology data of this species from different seas were obtained. The average cell sizes of C. closterium are: 25–260 µm length, 1.5–8 µm width; 12–25 fibulae in 10 µm. The results of cultivation in the laboratory conditions showed that the average cell sizes reached 148.17 µm (length) and 8 µm (width) at the temperature of +19…+20 °C and light intensity of 13 klx; length of cells reached 162.12 µm in the exponential phase of growth and 172.07 µm – in the stationary phase. C. closterium has an important practical significance as a source of fucoxanthin, since this alga is intensively cultivated for production of biologically active substances. Our experimental data showed that during laboratory cultivation the fucoxanthin concentration in a diatom biomass can reach 11 mg·g-1 of dry mass. The new data obtained are relevant and important; they can be used in different fields of science and medicine. The seasonal dynamics of population abundance of C. closterium in different ecotopes (epizoon of invertebrates and their food spectra, epiphyton of bottom vegetation, periphyton of the experimental and anthropogenic substrates of the different seas) is presented for the first time. The maximum abundance of the species population (65.6·10³ cells·cm-2) was registered in the epizoon of the mussel Mytilus galloprovincialis Lam. in March at the water temperature of +7.7 °C at a depth of 2.5 m in the Black Sea. The maximum abundance was registered in the epiphyton of green algae (896·10³ cells·cm-2) and in the periphyton of asbestos plates (728·10³ cells·cm-2) in August at the water temperature of +24.5 °C in the Sea of Japan. The abundance dynamics of C. closterium natural populations in the local habitats changed depending on the season, the depth, and the type of substrate. The similarities and differences in the distribution of C. closterium in the sea microphytobenthos are discussed.