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Vertical entrainment of nutrients and phytoplankton from the subsurface layers in autumn -winter is a cause of the surface phytoplankton bloom in the Black Sea in winter. Relationship of the winter heat flux and the sea surface temperature (SST) with the integral and surface phytoplankton biomass is assessed based on a series of simulations performed due to a 1D-biogeochemical model. The results show that during severe winters, intensity of phytoplankton bloom is 50% higher than that in warm winters. Winter entrainment of nutrients influences the phytoplankton biomass in the subsurface layer in summer, namely, after cold winters its maximum value exceeds the analogous one after warm winters by ≈30%. In-situ data is used to estimate the relation between the upper mixed layer depth and density, and the integral concentration of nitrates and phosphates in various regions of the basin. It is revealed that growth of the upper mixed layer density from 1014.0 to 1014.2 kg/m 3 results in increase of the integral concentration of nutrients in the upper layer by 2-2.5 times in the center and on the periphery of the basin; and when the density value achieves 1014.5 kg/m 3 the integral concentration becomes higher by 4-5 times. Thus the upper mixed layer density serves a good indicator of intensity of the nutrients inflow to the sea upper layers. Impact of winter cooling upon the upper mixed layer density is investigated using the model and in-situ data. It is shown that density equal to 1014.2 kg/m 3 is achieved in the basin center at SST 7.5-8° C, and on the sea periphery -at 6.5° C. The maximum density value 1014.8 kg/m 3 is recorded in the center of the sea (depth exceeds 2000 m) at SST < 5.5° C. During the same atmospheric conditions, the vertical nutrient transport is different in various regions of the Black Sea, which can affect the spatial features of bloom intensity in the basin. Acknowledgements:The data were obtained and processed within the framework of the State Order No. 0827-2014-0011 "The research of regularities of marine environment condition changes on the basis of operational observations and the data of marine area condition nowcast, forecast and reanalysis system" ("Operative oceanography" code); model calculations were carried out with the financial support of RFBR

One of the main characteristics of the marine ecosystem is the vertical distribution of chlorophyll-а (Chla) concentration, which is a proxy of phytoplankton abundance reflecting the level of primary production in