Local Seasonal Hypoxia and Hydrogen Sulphide Formation in the Bottom Waters of the Sevastopol Bay in 2009–2019

S. I. Kondratev*, A. V. Vidnichuk

Marine Hydrophysical Institute of RAS, Sevastopol, Russia

* e-mail: skondratt@mail.ru

Abstract

The purpose of this work is to identify the causes of hypoxia conditions in one of the Sevastopol Bay areas. The article used the results of a quarterly environmental monitoring of the Sevastopol Bay performed by the Marine Biogeochemistry Department of the Marine Hydrophysical Institute. During the monitoring samples of dissolved oxygen, elements of the carbonate system and main biogenic cycle were taken from 36 stations at surface and bottom horizons. The GAP-16 STD-probe equipped with an oxygen sensor was used to obtain vertical temperature, salinity and oxygen profiles. During environmental monitoring in summer of 2009, hydrogen sulfide at a concentration of 37 μM was detected in the bottom waters of Station 8. Analysis of oxygen content over the past 10 years showed a regular occurrence of hypoxia conditions (oxygen content less than 62.5 μM) in the bottom waters of this station. This is due to artificial deepening of the bottom in the area of this station made for immersion of a floating dock. Warming of the surface water in May – June leads to emergence of a thermocline, which gradually descends 10–12 m deep. The difference in water temperatures can reach 8 °C. The resulting thermocline, together with a layer of less salty water at the horizon of 10–14 m (the origin of this layer is not quite clear) weaken the vertical exchange, thus extraction of dissolved oxygen begins for the process of mineralization of particulate organic matter. This process ended with formation of hydrogen sulfide in September 2009 and 2014.

Keywords

Sevastopol Bay, bottom water hypoxia, hydrogen sulfide formation, biogenic elements, anthropogenic factors

Acknowledgments

The research is performed under state order on topics No. 0827-2019-0003 “Oceanological processes” and 0827-2019-0004 “Coastal studies” and supported by the Russian Foundation for Basic Research as part of scientific projects No. 18-45-920018 “Studies and evaluation of anthropogenic and natural influences on the distribution of oxygen in the Sevastopol Bay based on monitoring data and numerical simulations” and No. 18-05-80028 “Studies and assessment of the role of water dynamics and biogeochemical processes in development of oxygen deficit and sulfidic conditions in coastal areas of Crimea and the Kerch Strait”.

For citation

Kondratev, S.I. and Vidnichuk, A.V., 2020. Local Seasonal Hypoxia and Hydrogen Sulphide Formation in the Bottom Waters of the Sevastopol Bay in 2009–2019. Ecological Safety of Coastal and Shelf Zones of Sea, (2), pp. 107–121. doi:10.22449/2413-5577-2020-2-107-121 (in Russian).

DOI

10.22449/2413-5577-2020-2-107-121

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