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.

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.

The seasonal dynamics of the NE Black Sea phytoplankton follow the following pattern: small diatoms (spring) → coccolithophorid Emiliania huxleyi (late spring–early summer) → large diatoms (summer). Our hypothesis states that nitrogen and phosphorus concentrations regulate the seasonal phytoplankton dynamics. A minimum number of parameters is enough to understand the mechanisms of dominant species change. Based on the concept of intracellular regulation, the following parameters were evaluated: the minimum nitrogen and phosphorus quotas; half-saturation constants for nitrogen and phosphorus uptake; the maximum specific growth rate of the dominant phytoplankton species. Computational experiments on the model show the following: (1) in spring, a species with a high maximum specific growth rate becomes dominant; (2) in late spring and early summer, a species with a low minimum nitrogen quota and a low half-saturation constant for nitrogen uptake is observed; (3) in summer, a low minimum phosphorus quota and a low half-saturation constant for phosphorus uptake allow the species to become dominant.

The basis of the theory of evolution of transgressive the Akchagyl Sea is based on the hypothesis that the transgression was created by water coming from an as yet unidentified ocean. This causes some doubt due to the fact that in the Akchagyl time the ocean level was about 100 m below sea level. The article calculates changes in salinity in the sea. A comparison of the results of calculations with geological data shows that in reality the salinity of the water entering the sea was 2–3 orders of magnitude lower than the salinity of ocean water. Theoretically, only in 1000 years, the salinity of seawater was supposed to increase to 120‰, and geological studies showed that during the whole time of transgression the salinity of the water was not higher than 25‰. This indicates that during the transgression the water came not from the ocean, but from melting glaciers, i.e. water theoretically could not flow upwards, and it never really flowed upwards.