Purpose. Monitoring of coccolithophore "bloom" is becoming increasingly important due to their influence on the biogeochemical cycles. The purpose of the study is to analyze the frequency, intensity and area of Gephyrocapsa huxleyi "bloom" in different sub-regions of the Black Sea, as well as to assess the effect of environmental conditions on the intensity and interannual variability of "bloom".

Methods and Results. Based on the satellite data and using the regional algorithms, the values of chlorophyll a concentration, the particulate backscattering coefficient and the coefficient of light absorption by colored detrital matter were retrieved with a two-week averaging for different Black Sea sub-regions for the period 1998–2023. Application of the normalized anomalies made it possible to reveal the annual cycles of variability of these parameters, as well as to identify their common patterns and variability features in particular sub-regions of the sea.

Conclusions. It has been established that in early June in all the regions of the sea, the "bloom" of Gephyrocapsa huxleyi was regularly observed; at that it was characterized by a year-to-year variability in its intensity and area. High light intensity in the sea upper mixed layer is a key factor for a shift in the phytoplankton species structure and for the transition to Gephyrocapsa huxleyi dominance and "bloom" due to the physiologically determined ability of these algae to grow at an extremely high light intensity with no inhibition, whereas at similar light conditions the growth of other plankton microalgae is suppressed. In most sub-regions of the Black Sea, the "bloom" of Gephyrocapsa huxleyi is not followed by an increase in the phytoplankton biomass, only the structure of phytoplankton species changes. The exception is the coastal waters affected by the river runoffs: the "bloom" there is observed when the phytoplankton biomass grows. In the coastal waters during a cold period (December, February), the Gephyrocapsa huxleyi abundance sometimes increases due to a decrease of nutrient supply that results from weakening of the river runoffs and/or increasing stability of the water column. Decrease in phytoplankton supply with nutrients contributes to the competitive growth of coccolithophores as compared to other types of phytoplankton. The development of Gephyrocapsa huxleyi "bloom" in different seasons can reflect its genetic and physiological plasticity.