Studies of the annual dynamics of phytoplankton in the NE Black Sea at two stations on the shelf and the continental slope were conducted in 2016, 2017, and 2019. The species composition of phytoplankton has not undergone significant changes compared to previous decades. The coccolithophore Emiliania huxleyi, small flagellates, and diatoms determined the abundance of phytoplankton; and diatoms, coccolithophores, and dinoflagellates determined the total biomass. The annual dynamics of the satellite-derived chlorophyll-a showed peaks in spring and autumn, and sometimes in summer. During the stratified water column period, strong winds in most cases led to a detectible increase in chlorophyll-a. The annual dynamics of phytoplankton followed the pattern: small diatoms (spring) → coccolithophores (late spring, early summer) → large diatoms (summer, autumn). Such a pattern was typical for the previous decades. Coccolithophores dominated in weak SE winds, diatoms in NE winds. The combined effect of sustained offshore wind and strong current can cause diatom blooms during stratified water, even if the wind velocity is moderate.

Field studies, experiments, and mathematical modeling showed that seasonal cycle of the Black Sea phytoplankton is regulated by three major physiological mechanisms. The first mechanism is a high specific growth rate, high values of the uptake rate and the half-saturation constant of the uptake of the limiting nutrient. It operates in spring when small-cell diatoms Pseudo-nitzschia pseudodelicatissima and Chaetoceros curvisetus grow actively. The second mechanism is notable for relatively low rates of growth, the uptake rate, and half-saturation constant; it operates mainly at the end of spring and in the beginning of summer when coccolithophore Emiliania huxleyi is in abundance. The third mechanism prevails in summer and autumn when the mass of water is regularly agitated by wind and waves with domination of macrocellular diatoms Proboscia alata and Pseudosolenia calcar-avis. The cells of these algae can accumulate nutrient ensuring numerous divisions.

In August 2020, during a dramatical summer retreat of sea ice in the Nansen Basin, a study of phytoplankton was conducted on the transect from two northern stations in the marginal ice zone (MIZ) (north of 83° N m and east of 38° E) through the open water to the southern station located in the Franz Victoria Trench. The presence of melted polar surface waters (mPSW), polar surface waters (PSW), and Atlantic waters (AW) were characteristic of the MIZ. There are only two water masses in open water, namely PSW and AW, at the southernmost station; the contribution of AW was minimal. In the MIZ, first-year and multiyear ice species and Atlantic species were noted; Atlantic species and first-year ice species were in open water, and only ice flora was at the southernmost station. The maximum phytoplankton biomass (30 g · m⁻³) was recorded at the northernmost station of the MIZ, and 99% of the phytoplankton consisted of a large diatom Porosira glacialis. Intensive growth of this species occurred on the subsurface halocline separating mPSW from PSW. A thermocline was formed in open water south of the MIZ towards the Franz Victoria Trench. A strong stratification decreases vertical nutrient fluxes, so phytoplankton biomass decreases significantly. Phytoplankton formed the maximum biomass in the thermocline. When moving south, biomass decreased and its minimum values were observed at the southernmost station where the influence of AW is minimal or completely absent. A transition from the silicon-limited state of phytoplankton (MIZ area) to nitrogen-limited (open water) was noted.

The biogeochemistry of waters is an essential regulator of phytoplankton dynamics, determining the level of species bloom and the change in dominants. This paper investigated the seasonal dynamics of phytoplankton and the nutrient concentrations and their ratios in the northeastern Black Sea in 2017–2021. Two taxonomic groups, diatoms and coccolithophores, determine the seasonal dynamics and significantly contribute to the total phytoplankton biomass. Coccolithophores formed blooms in early June annually, except in 2020. Large diatoms dominated in summer with a biomass exceeding 1000 mg m⁻³ annually, except in 2019. During the blooms of these taxonomic groups, their contribution to the total phytoplankton biomass exceeded 90%. Each group has characteristic biogeochemical niches in the nitrogen and phosphorus concentration coordinates. The position of the seasonal thermocline regulates the biogeochemistry of the water. With a high-lying and sharp gradient thermocline (the average for five years is 6.87 m), low nitrogen concentrations and a nitrogen-to-phosphorus ratio below the Redfield ratio are created in the upper mixed layer. These conditions are optimal for the dominance of coccolithophores. When the thermocline is deepened (the average for five years is 17.96 m), the phosphorus concentration decreases significantly and the ratio of nitrogen to phosphorus is significantly higher than the Redfield ratio, and these conditions lead to the dominance of large diatoms. The results of experimental studies with nitrogen and phosphorus additives in the natural phytoplankton population confirm the above statements. The addition of phosphorus leads to the increased role of coccolithophores in the total phytoplankton biomass, the addition of nitrogen alone contributes to the growth of large diatoms, and the combined addition of phosphorus and nitrogen in a ratio close to the Redfield ratio leads to the dominance of small diatoms.