Physical Oceanography Physical Oceanography 1573-160X 20230205 Experimental and field research Research Article Inorganic Forms of Nitrogen in the Deep Part of the Black Sea Based on the Expeditionary Data, 2016–2019 https://orcid.org/0000-0002-2049-7750 Kondratev S. I.
Russian Federation
skondratt@mail.ru https://orcid.org/0000-0001-5033-4576 H-1880-2014 56960448000 3277-7914 Varenik A. V.
Russian Federation
alla.varenik@mhi-ras.ru https://orcid.org/0000-0002-1387-970X I-1755-2017 35784884700 Orekhova N. A.
Russian Federation
natalia.orekhova@mhi-ras.ru Marine Hydrophysical Institute of RAS 2 Kapitanskaya Str. Sevastopol 299011 Russian Federation 30 04 2023 30 2 186 201 06 05 2022 12 01 2023 16 01 2023 Copyright ©; 2023, S. I. Kondratev, A. V. Varenik, N. A. Orekhova 2023 S. I. Kondratev, A. V. Varenik, N. A. Orekhova https://creativecommons.org/licenses/by-nc/4.0/ https://physical-oceanography.ru/repository/issues/2023/02/05/

Purpose. The purpose of the study is to describe quantitatively the structure of vertical distribution of the nitrogen inorganic forms, namely nitrites, nitrates and ammonium ions, at the present post-eutrophication period of the deep-sea ecosystem evolution in the Black Sea based on the field data obtained in 2016–2019.

Methods and Results. The data obtained in the Black Sea within the economic zone of Russia in 2016–2019 by the scientists of Marine Hydrophysical Institute were used. At more than 200 deep-sea stations, a cassette of 12 bathometers (Seabird-Electronics CTD-instrument) was applied for taking hydrochemical samples at certain isopycnic surfaces, usually at σt = 16.30; 16.25; 16.20; 16.15; 16.10; 16.05; 16.00; 15.95; 15.90; 15.80; 15.50, 14.0 kg/m3. Such a scheme permits to determine the suboxic zone locations (including its upper boundary), the depth of hydrogen sulfide formation, and also to study in detail the changes in the nitrogen forms during transition from the oxic conditions to the anoxic ones.

Conclusions. At the present post-eutrophication period of the deep-sea ecosystem evolution in the Black Sea, qualitative characteristics of the vertical distribution of inorganic nitrogen forms correspond to the already known features. The quantitative ones are characterized by the following values: from the surface to the oxycline onset near the isopycnic surface σt = 14.5 kg/m3, the contents of nitrites and nitrates were at the level 0.06 μM and 2–3 μM, respectively, and the ammonium concentration did not exceed 0.6 μM. As for the nitrite vertical profiles, two maximums are noted at the isopycnic surfaces σt = 14.0 kg/m3 and σt = 15.9 kg/m3; the value of the second peak does not exceed 0.07 μM. The nitrate vertical profiles are characterized by a maximum within the range of isopycnic surfaces σt = 15.2–15.5 kg/m3; its value reached 4–5 μM. The ammonium considerable concentrations were recorded at the isopicn σt = 16.0 kg/m3, after which the ammonium content monotonously increased with depth reaching the value 96 ± 5 μM at the 2000-meter depth. The molar ratio NH4+/H2S near the upper boundary of the hydrogen sulfide zone was 0.58 that indicated a significant equivalent excess of ammonium as compared to the theoretical stoichiometric value 0.30. At greater depths where the H2S concentrations are high (≥ 380 μM), it decreased to 0.25 and approached the theoretical value.

 Black Sea hydrogen sulfide nutrients inorganic forms of nitrogen vertical profile field data The investigation was carried out within the framework of the state assignments No. 0555-2021-0004 "Fundamental studies of the oceanological processes which determine state and evolution of the marine environment influenced by natural and anthropogenic factors, based on the observation and modeling methods" and FNNN 0555-2021-0005 "Complex interdisciplinary studies of oceanological processes that determine functioning and evolution of the ecosystems of coastal zones of the Black and Azov seas", as well as with support of the RFFI project No. 21-55-52001.

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