Numerical Modelling of RedOx Condition Dynamics at the Water-Sediment Interface in Sevastopol Bay

Yu. S. Gurova1,*, E. V. Yakushev2, 3, A. V. Berezina2, 3, M. O. Novikov2, K. I. Gurov1, N. A. Orekhova1

1 Marine Hydrophysical Institute of RAS, Sevastopol, Russia

2 Shirshov Institute of Oceanology RAS, Moscow, Russia

3 Norwegian Institute for Water Research, Oslo, Norway

* e-mail: kurinnaya-jul@yandex.ru

Abstract

The paper aims at assessing the variability of characteristics of redox conditions in the water column and the surface layer of sediments under changing anthropogenic load using in situ observational data and results of numerical modelling (the case of Sevastopol Bay). A comprehensive analysis is carried out of the chemical characteristics of the water column and pore water as well as geochemical characteristics of the bottom sediments. It is confirmed that there is the previously determined violation of the natural hydrochemical regime due to phytoplankton blooms in summer and the location of a large amount of stormwater and municipal wastewater in the bay. Despite the saturation of waters with oxygen in the bottom layer (94–113 % sat.), suboxic conditions are registered in the surface layer of bottom sediments. This is explained by predominance of the fine-grained fraction and high content of organic carbon. Mathematical calculations were performed using the one-dimensional benthic-pelagic Bottom RedOx Model (BROM). The numerical modelling results were validated using in situ observational data. The results showed that the model reproduces the natural seasonal variations of hydrochemical parameters associated with phytoplankton blooms, the occurrence of high concentrations of organic matter and its oxidation by the dissolved oxygen. Two numerical experiments with decreased and increased concentrations of organic matter were conducted to assess the effects of varying amounts of the organic matter entering the bay. It was found that the increased load on the bay results in a decrease in the oxygen concentration (up to 12 µM) and the development of anaerobic conditions in the bottom layer of water. Reduced organic matter input promotes aerobic conditions in the water column and in the bottom water layer. However, for bottom sediments, such a reduction in the load is not sufficient given the level of excess organic matter accumulated in them. The pore waters still consume oxygen and nitrates heavily and produce reduced forms of iron and manganese.

Keywords

bottom sediments, pore waters, oxygen, organic carbon, modelling, Black Sea, Sevastopol Bay, BROM model

Acknowledgments

The work was carried out under state assignment no. FNNN-2021-0005 “Coastal research” of Marine Hydrophysical Institute and state assignment no. FMWE-2021-0001 of Shirshov Institute of Oceanology RAS; as well as funded by grants no. 20-35-90103 of the RFBR and no. 21-17-00191 of the RSF. The authors are grateful to A. I. Kubryakov, Dr.Sci. (Phys.-Math.), for the provision of calculation results obtained using the POM model.

For citation

Gurova, Yu.S., Yakushev, E.V., Berezina, A.V., Novikov, M.O., Gurov, K.I. and Orekhova, N.A., 2023. Numerical Modelling of RedOx Condition Dynamics at the Water-Sediment Interface in Sevastopol Bay. Ecological Safety of Coastal and Shelf Zones of Sea, (2), pp. 71–90. doi:10.29039/2413-5577-2023-2-71-90

DOI

10.29039/2413-5577-2023-2-71-90

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