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.

In the coastal waters of the Black Sea near Sevastopol, a gradual temperature elevation and an increase in anthropogenic pressure since the early 2000s have caused significant structural and functional changes in phytoplankton. Currently, there is a significant decrease in the contribution of small diatom species (Skeletonema sp. and Chaetoceros socialis H.S.Lauder as well as coccolithophorids Emiliania huxleyi (Lohmann) W.W.Hay and H.P.Mohler) to the total phytoplankton biomass in these waters. Previously these species caused regular weak blooms. In the warm periods (from May to October), during which the main phytoplankton biomass is formed, large diatom species Pseudosolenia calcar-avis (Schultze) B.G.Sundström, 1986, Proboscia alata (Brightwell) Sundström and dinoflagellates predominate. Therefore, the maximum values of the phytoplankton community’s specific growth rate are about two times lower than in the preceding periods and do not exceed 1.10–1.40 day⁻¹. There was also a decrease observed in the microzooplankton grazing rate, which, during the year, was no higher than 0.70–1.20 day⁻¹. This is primarily conditioned by the increased role of large algae in phytoplankton, which means a decline in nutrition quality for microzooplankton. As a result of the joint influence of nutrition quality and water pollution, the relative share of net primary production consumed by microzooplankton in the warm periods of the year averaged only 32%, which is two times lower than the average values generally accepted for marine ecosystems. This means that the transfer of matter and energy from phytoplankton to higher trophic levels is significantly decreased.

The structural and functional characteristics of the large-celled diatom alga Pseudosolenia calcar-avis isolated from the Black Sea plankton have been studied. The main reasons for its intensive development and dominance in the Black Sea phytoplankton under conditions of different nutrient content in water have been determined. The high values of the photosystem II efficiency (0.67–0.69), relative chlorophyll a content (~1% of the organic carbon), and the ratio of the cell surface area to cytoplasm volume (~0.80–0.90 μm^–1), as well as the presence of a large intracellular vacuole, allowed this species to grow at a high rate (1.40 day^–1) under optimal environmental conditions. The transfer of P. calcar-avis cells having the maximum intracellular pool of nutrients to seawater depleted in nutrients causes a rapid decrease in photosystem II efficiency, relative content of chlorophyll a, and specific growth rate. Due to the intracellular pool of nutrients, this algal species provides 4.5 cell divisions within 4 days. After the depletion of intracellular nutrient reserves, the cell abundance and the culture viability remains almost unchanged for the next 5–7 days. The complete restoration of the functional activity and the further growth of this species require access to the external source of nutrients able to maintain cell functioning at a high level. The possible mechanisms of the replenishment of P. calcar-avis cells with nutrients under conditions of their deficiency in the seawater are discussed.

Stelmakh L.V., Gorbunova T.I., 2018: Emiliania huxleyi blooms in the Black Sea: influence of abiotic and biotic factors. - Botanica, 24(2): 172-184. The studies were conducted in the Black Sea in deep and shallow water areas in October 2010 and May 2013. The main abiotic and biotic factors, which control spring and autumn coccolithophorid Emiliania huxleyi blooms, were identified. During late May this phenomenon was observed under optimal light and temperature conditions, and also optimal ratio between mineral forms of nitrogen and phosphorus (N/P) in the water. Biotic variables (the net growth rate of phytoplankton and relative dinoflagellates share in its total biomass) determined the uneven distribution of E. huxleyi within a bloom. In October, water temperature was almost 4ºС lower compared to that in May, and solar radiation intensity decreased approximately by 2-3 times. However, as a result of seasonal adaptation to light and temperature, E. huxleyi abundance reached blooming level. In that period, the variability of N/P ratio in the water and the relative share of diatoms in total biomass of phytoplankton played a major role in the uneven distribution of this coccolithophorid within the studied area. In the areas with low water salinity, a bloom was not developing. Within the main part of the studied water area, the major source of nitrogen was ammonium, which was favourable for the growth of E. huxleyi and dinoflagellates, but limited the growth of diatoms.

Experimental studies have been performed on Tetraselmis viridis (Rouchijajnen) R.E. Norris, Hori & Chihara (Chlorophyta) and Phaeodactylum tricornutum Bohlin (Bacillariophyta) cultures of Black Sea microalgae using TsV-S1 and PtV-S18 algal virus strains isolated from the Black Sea ecosystem. The study assesses the effects of light intensity and the initial abundance of microalgae cells on the onset of their infection by viruses and the decline in their abundance. As early as on the second or third day, the cells changed their shape and increased in volume in the cultures infected by viruses. At this time, a decrease was observed in chlorophyll a red autofluorescence, variable fluorescence per cell, and photochemical efficiency of photosystem 2. In the studied cultures, a reduction in cell abundance due to viral lysis was noted 1 to 2 days later. The threshold cell abundance at which this process was observed was independent of light conditions and amounted to 3 × 10⁵ cells/mL for T. viridis and 18 × 10⁵ cells/mL for Ph. tricornutum. The complete lysis of algae cells was detected by the end of the fourth to sixth day. In the dark, the effect of viruses on microalgae was insignificant.

Using the electron microscopy method, the species identity of the diatom Cerataulina pelagica, isolated in pure culture from the coastal waters of the Black Sea in September 2021, has been confirmed. The range of optimal temperature values for the development of this species is identified. The impact of light and biogenic substances on its main structural and functional characteristics has been studied. The acclimation of C. pelagica to various light intensities is carried out by changing the efficiency of photosystem II, the C/Chl a ratio, and the specific growth rate. The morphometric parameters of cells (volume, surface area, and specific surface area) change slightly in the studied light range: 8.5–510 μE/(m² s). The transfer of C. pelagica cells, which have the maximum intracellular pool of nutrients, to seawater depleted in nutrients causes a rapid increase in the C/Chl a ratio, as well as a decrease in the efficiency of photosystem II, the relative rate of electron transport, and the specific growth rate. A high degree of toxicity with copper ion in low concentrations in relation to the studied species is shown.