A five-year study of the interannual changes observed in May–June during the mass reproduction of coccolithophorids was carried out in the northeastern part of the Black Sea. The long-term dynamics were analyzed using the data on the phytoplankton collected during the last 40 years. The fraction of coccolithophorids represents either 20 or 60% of the total biomass of the algae and does not depend on either the previous winter conditions or the nitrogen content and the N: P ratio in the surface water layer. Our studies have revealed the dependency between the phosphate content and the size of the coccolythophorids’ fraction in the total phytoplankton biomass. The long-term population dynamics includes three periods. Until the mideighties, the coccolythophorid fraction in the Black Sea phytoplankton was insignificant (3%). The average biomass was equal to 8 μg/l. In the late eighties, the average biomass increased up to 106 μg/l. Since the midnineties, coccolythophorids often prevail in the number and mass among the other phytoplankton species. The general tendency for the growth of the coccolythophorid fraction in the phytoplankton cenoses coincides with the increase in the phosphate concentration in the near-surface water observed within the last 40 years. This fact corresponds to the experimental results, which demonstrate that the coccolythophorid development during May–June is phosphate-limited.

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.