The oxygen balance analysis in the water column of the Black Sea based on a mathematical model
Authors: Novikov Matvey , Berezina Anfisa , Pakhomova Svetlana , Yakushev Evgeniy 
Published: February 2023

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&#8727;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 &#176;N. 37.75 &#176;E from &#8220;Copernicus&#8221; portal.&#160;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.