Seasonal evolution of deep phytoplankton assemblages in the Black Sea

The seasonal evolution of the species composition and depth of the deep maximum of phytoplankton (DPM) in the Black Sea has been studied. Vertical series of phytoplankton samples were collected at 278 stations in the deep-water basin during a 35-year period from 1985 to 2019. On average, the bulk of the total phytoplankton biomass is in the upper mixed layer (UML) throughout the year. The maximum total phytoplankton biomass is located in a narrow depth range of 10–15 m and irradiance level of 10–20% of the surface photosynthetically available radiation, regardless of the season. DPM is regularly present from May to September. However, the frequency of its occurrence is low (only 40%) compared to the UML maximum profiles (60%). In the DPM, diatoms predominate in biomass in spring, dinoflagellates in late spring, coccolithophores in early summer, dinoflagellates in summer, and nanoflagellates in autumn. Out of 41 main species, only eight form the maximum biomass in the thermocline or sub-thermocline. Among them are dinoflagellates Tripos muelleri and T. fusus, Diplopsalis lenticula, Pronoctiluca pelagica, and Gyrodinium fusus, which also produce biomass maxima in the UML. Сoccolithophore Acanthoica quattrospina and silicoflagellate Octactis speculum reach maximum biomass in winter in the UML, which indicates that these species just survive at depth during the warm seasons. Cyanobacterium, Synechococcus sp., is the only species which develops en masse exclusively in the thermocline in summer. With this exception, all species growing at depth in the Black Sea cannot be classified as a specific obligate deep flora.