Oxygen Dynamics during the Period of Dystrophic Processes in the Black Sea

A. V. Masevich✉, S. K. Konovalov

Marine Hydrophysical Institute of RAS, Sevastopol, Russian Federation

✉ e-mail: anna_vidnichuk@mhi-ras.ru

Abstract

Purpose. The paper is aimed to analyze (i) peculiarities of the oxygen content and distribution in the Black Sea waters and (ii) the ratio of the observed changes in the oxygen distribution, the total primary production level and changes in the cold intermediate layer ventilation.

Methods and Results. The expedition data (2015–2019) as well as the data array (1980–2013) from the Oceanographic Data Bank of Marine Hydrophysical Institute, RAS, were used to analyze the oxygen content. The data for the deep part of the Black Sea (exceeding the 200 m depth) were selected. To analyze the primary production values, the shipboard fluorometric measurements of the chlorophyll a concentrations (1980–2001), and also the surface chlorophyll a concentrations derived from the SeaWiFS and MODIS-Aqua color scanners remote sensing (1998–2019) were utilized. The primary production was calculated using the regression equations in the form y = a + bx that bound up the primary production in the water column with the surface chlorophyll concentration. The calculated data reveal a significant increase in the annual primary production level (to 400 g C/m2·year) in the first half of the 1980s, then it declined on the average to ~ 140 g C/m2·year from 1985 to 1995, and it remains at the level of ~ 100 g C/m2·year from 1998 up to the present time. These variations in the primary production values correspond to the observed changes in the vertical distribution of nitrate and an increase in the temperature of the cold intermediate layer core, resulting in a decrease of the oxygen concentration in the deep oxic layers.

Conclusions. An increase in temperature in the upper layers of the sea and a reduction of the winter convective mixing intensity resulted in a decrease in the oxygen supply to all layers of the aerobic zone of the Black Sea. The lowest oxygen content recorded for the whole period of 1980–2019 was revealed in 2010. At the same time, dystrophication process drove the Black Sea system to its natural state, when the oxygen content dynamics depended mainly on the intensity of physical ventilation.

Keywords

oxygen, oxygen concentration, eutrophication, primary production, cold intermediate layer, Black Sea

Acknowledgements

The investigation was carried out within the framework of the state task on theme No. 0555-2021-0004 “Fundamental studies of oceanological processes which determine the state and evolution of the marine environment influenced by natural and anthropogenic factors, based on observation and modeling methods”, and at the RFBR financial support within the framework of research project No. 19-35-90062.

Original russian text

Original Russian Text © A. V. Masevich, S. K. Konovalov, 2022, published in MORSKOY GIDROFIZICHESKIY ZHURNAL, Vol. 38, Iss. 1, pp. 89-104 (2022)

For citation

Masevich, A.V. and Konovalov, S.K., 2022. Oxygen Dynamics during the Period of Dystrophic Processes in the Black Sea. Physical Oceanography, 29(1), pp. 83-97. doi: 10.22449/1573-160X-2022-1-83-97

DOI

10.22449/1573-160X-2022-1-83-97

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