Climate change and anthropogenic impacts drastically affect the biogeochemical regime of the Black Sea, which contains the largest volume of sulphidic water in the world. The Sea’s oxygen inventory depends on vertical mixing that transports dissolved oxygen (DO) from the upper euphotic layer to deeper layers and on dissolved oxygen consumption for the oxidation of organic matter (OM) and reduced species of S, Fe, and Mn. Here we use a vertical one-dimensional transport model, 2DBP, forced by Copernicus data, that was coupled with the FABM-family N-P-Si-C-O-S-Mn-Fe Bottom RedOx Model BROM. The research objective of this study was to analyze the oxygen budget in the upper 350 m of the Sea and demonstrate the role of the parameterization of the acceleration of the sinking of particles covered by precipitated Mn(IV). The analysis of the oxygen budget revealed distinct patterns in oxygen consumption within different depths. In the oxic zone, the primary sink for DO is the mineralization of organic matter, whereas in the suboxic zone, dissolved Mn(II) oxidation becomes the predominant sink. The produced Mn(IV) sinks down and reacts with hydrogen sulphide several meters below, making possible the existence of the suboxic layer without detectable concentrations of DO and H₂S.

Among other marine environmental problems, the issue of marine litter accumulation in the World Oceans is of increased interest. It is relevant not only in areas with direct intense anthropogenic pressure, but also in remote and presumably pristine areas, such as the Arctic Sea. As the concentration of plastic waste in the marine environment increases, it can have impacts on various components of the marine ecosystem, at sea, on the seafloor, on the coasts and in particular in accumulation areas, while it also can negatively affect human social and economic activities. To reduce the release of plastic debris into the marine environment, litter occurrence and pathways need to be studied in order to identify litter sources, requiring monitoring studies that provide comparable results. Here we present the results of studies of the level of pollution by floating marine debris carried out using ship-based visual observations through a harmonised methodology, developed to obtain comparable data. The observations were carried out in the White Sea, Barents Sea and the Kara Seas during a research cruise of the Floating University project by the Shirshov Institute of Oceanology Russian Academy of Sciences in 2021. The studies included training and involvement of a large number of students as observers. Floating macro litter was observed in all study areas along the vessel’s route. In total 2500 km route and 19,8 km2 were covered with observations. The concentration of litter varied on observed transects from 0 to 226 items/km2, with a mean value of 10.1 items/km². 95% of registered items were made of plastic, the most common were unidentified, weathered plastic objects that could indicate the long presence of litter items in the environment. 

Climate change and anthropogenic impact drastically affect the biogeochemical regime of the Black Sea, the contains the largest in the world volume of sulfidic water. The volume of the oxic layer of the sea depends on vertical mixing, that transports dissolved oxygen (DO) from the upper euphotic layer in the deeper layers and dissolved oxygen consumption for oxidation organic matter (OM). Changes in the Sea hydrodynamic properties due to warmer winters restricts renovation of the Black Sea Cold Intermediate Layer and therefore the DO flux to the deeper layers. The main goal of this study was to model upper 350 m with emphasis on redox layer.In the study we use the benthic-pelagic biogeochemical BROM model combined with 2DBP model for vertical and horizontal transport via FABM. BROM combines a relatively simple ecosystem model with a detailed biogeochemical model considering interconnected transformations of chemical species (N, P, Si, C, O, S, Mn, Fe). OM dynamics include parameterizations of production (via photosynthesis and chemosynthesis) and decay via oxic mineralization, denitrification, metal reduction, sulfate reduction and methanogenesis.Hydrophysical forcing (temperature, salinity and northward and eastward sea water velocity) was hourly data for year 2010 for a point with coordinates 43.5 °N. 37.75 °E from the E.U. Copernicus Marine Service Information “Copernicus”. The data is the result of a reanalysis calculation based on the NEMO hydrodynamic numerical model.The 2DBP/BROM model was applied for the 350 m water column with bottom boundary positioned in sulfidic layer. A steady-state solution was reached after 50 calculation years. The results of calculations were compared with the data of field observations of the expedition on the RV "Knorr" in March 2003. The obtained vertical distributions of hydrochemical characteristics are consistent with the existing understanding of the hydrochemical structure of the Black Sea. The dissolved oxygen has the similar structure in the model and observations occupying the upper 70 m layer, its onset was positioned shallower than the appearance of hydrogen sulfide at appr. 80. In the limits of the redox layer there were reproduced maxima of nitrite, Mn(IV), Mn(III), Fe(III), elemental sulfur and phosphate minimum. Below the redox layer the model reproduced maxima of Mn(II) and Fe(II).Calculated seasonal variability in the upper layer shows seasonality in development of phytoplankton and corresponding changes dissolved oxygen and nutrients. Organic matter distributions changes in accordance with seasonality of its production and destruction. At the same time, the redox layer remains practically unchanged during the year.At the present the BROM model is applied for the Black Sea and satisfactory validated against the data of observations. The calculated seasonal dynamics of the biogeochemical properties of the Black Sea will be used as an initial condition for studying of effects of changing in mixing (considering modeling interannual changes with difference in hydrodynamical scenarios) and allochthonous OM delivery on the biogeochemical structure of the Sea.This study was funded by the Ministry of Science and Higher Education of the Russian Federation, theme 13.2251.21.0008.

The Black Sea contains the largest volume of sulphidic water in the world. The position of the upper boundary of hydrogen sulfide is associated with the supply of oxygen, which is used for the oxidation of organic matter and reduced compounds of several chemical elements, such as sulfur, manganese, iron, nitrogen, etc. Climate change and anthropogenic impact dramatically affect the biogeochemical regime of the Black Sea. The depth of the oxygen layer of the sea depends on vertical mixing, which transfers dissolved oxygen from the upper euphotic layer to deeper layers, and the consumption of dissolved oxygen for the oxidation of autochthonous organic matter (OM) produced in the sea and allochthonous OM delivered with coastal runoff, and for the reduced forms of the listed elements.The study uses the BROM biogeochemical model, which describes biogeochemical processes in the water column. The BROM benthic-pelagic biogeochemical model combines a relatively simple ecosystem model with a detailed biogeochemical model for the water column, bottom boundary layer, and upper sediment, with a focus on changes in oxygen regime and redox conditions. BROM considers the interrelated transformations of chemical elements (N, P, Si, C, O, S, Mn, Fe) and organic matter (OM) in terms of nitrogen according to the Redfield ratio between the main nutrients.In this work, we used the previously combined BROM-2DBP model within the FABM framework. In order to more accurately reproduce the fine structure of the redox layer by the model, the parameterization of the change in the vertical velocity of the suspended matter due to aggregation with Mn(IV) oxide was introduced:WCi=WCi0+WMe∗Mn(IV)/(Mn(IV)+0.1) ,where WCi is the total vertical velocity, WCi0 is the own vertical velocity of the suspended matter, Mn(IV) is the concentration of Mn(IV) oxide.The hydrophysical forcing was hourly temperature, salinity, and orthogonal components of currents data for 2010 on a point with coordinates 43.5 °N. 37.75 °E from “Copernicus” portal. To take into account stationary recovery conditions, the upper 350 m were used in the calculations. The obtained data were compared with the field observations of the expedition aboard the R/V Knorr in March 2003. The obtained vertical distributions of hydrochemical parameters (a) are consistent with the existing hydrochemical structure of the Black Sea. Dissolved oxygen has a similar distribution in the model and observations, occupying the upper 70 m layer, its origin was located higher than the appearance of hydrogen sulfide at about 150 m. Within the redox layer, nitrate, Mn(IV), Mn(III ), Fe(III), elemental sulfur and a minimum of phosphate. Below the redox layer, the model reproduced the maxima of Mn(II) and Fe(II). Hydrogen sulfide appears at a horizon of about 80 m.The introduced parametrization of the additional vertical velocity of suspended matter makes it possible to more accurately reproduce the processes of a relatively thin redox layer by taking into account its aggregation with Mn(IV) oxide, which, in turn, can shift the redox layer lower or higher, depending on the availability of Mn(IV) oxide.