Nutrients Dynamics in the Surface Waters of the Black Sea

Purpose. One of the key characteristics of water quality and marine ecosystems’ sustainability is nutrients supply, which are the main factors of phytoplankton bloom. Since the mid-20 th century, in the northwestern Black Sea and on its western shelf significant changes in nutrients concentrations took place, which were manifested in increase of the inorganic nitrogen and phosphorus concentrations and decrease of the dissolved silicon content, that in its turn, led to a strong anthropogenic eutrophication of the above-mentioned regions. The aim of the work is to assess the current state of the Black Sea based on the data on the nutrients distribution and dynamics in the surface water layer. Methods and Results. The data obtained in the cruises of R/V Professor Vodyanitsky in 2016–2019 were analyzed. The main regions under study included the deep part of the Black Sea, as well as the Crimean and Caucasian sectors of the economic zone of Russia. The data obtained showed significant variability in the nutrients concentration both in the coastal and open areas. Conclusions. A decrease of nutrients concentration in 2016–2019 as compared to that in 2009–2014 was noted, that can indicate a decrease in the eutrophication degree and an improvement of the Black Sea waters quality. Intra-annual variability of the nutrients concentration in the sea surface layer is observed. No pronounced features in spatial variability of the nutrients concentration in this layer were revealed. It is assumed that in the central deep part of the sea, their concentrations are governed mainly by the physical processes of water transfer. In addition to physical transport, the increased nutrients concentrations in the coastal areas are, probably, also related to influence of the anthropogenic factor, namely inflow of nutrients from the coastal sources or their formation resulting from the biogeochemical processes with organic matter. 0555-2021-0004, and the RFBR projects No. 18-05-80028 “Studies and scaling of water dynamics and biogeochemical processes in development of oxygen deficit and sulfidic conditions in coastal areas of the Crimea and the Kerch Strait” (analysis of the data obtained in the coastal regions of the Black Sea) and No. 21-55-52001 “Comparing structure, functions and niche differentiation in marine picophytoplankton communities with and without Prochlorococcus cyanobacteria in the East China Sea and the Black Sea”.