Features of Methane Distribution in the Euphotic Layer of the Northern Black Sea in Summer, 2018 (Based on the Data of the 102nd Cruise of R/V "Professor Vodyanitsky")

Abstract

The main goal of the work was to assess the horizontal and vertical structure of methane (CH4) distribution, its emission to the atmosphere in the euphotic zone of the northern part of the Black Sea in summer, 2018 as well as to reveal its relationship with the chlorophyll a concentration. Methods and Results. The CH4 concentration in the surface layer was determined by the gas chromatographic method at 104 stations, including 45 stations at which the vertical profiles of the CH4 content in the upper 0-50 m layer were done. The chlorophyll a concentration was defined by the fluorometric method. The CH4 distribution in this region showed distinct spatial heterogeneity. The concentration of the dissolved in the surface water CH4 varied within 0-39.2 nmol/l at all the stations. The average calculated value of the CH4 seawater-air flux was 2.3 nmol/m 2 /day, and the average CH4 saturation of surface water -460 %. The major part of the sea area under study represented a source of methane emission to the atmosphere near-water layers, except for 15 stations where the CH4 concentration in the surface layer was smaller than its equilibrium values. The calculated integral value of the CH4 flux from the whole region under study (its area is equal to 88•10 3 km 2 ) constituted 3.2 tons of CH4 per day. Vertical distribution of CH4 in the upper 50 m layer was different in the coastal and deep-sea areas. At the deep-sea stations, the maximum values were revealed in the sub-surface layers, whereas at the coastal stations (not deeper than 100 m), the highest CH4 concentrations, up to 86 nmol /l, were observed mainly at the near-bottom horizons. Conclusions. The average concentration of CH4 in the deep-sea profiles was 2 times lower than that in the coastal ones. Sub-surface maximums of the CH4 concentration coincided in general with the maximums of the chlorophyll a concentration. It was also found that the CH4 highest concentration corresponded to the increased chlorophyll a content (0.58 mg/m 3 ) in the Feodosiya Gulf surface water. The abnormally high concentration of CH4 (269 nmol /l) at the bottom horizon, by an order of magnitude exceeding the average methane content in the adjacent areas, was revealed at the station near the Dnieper paleo-channel. Such an increased concentration is assumed to be caused by the methane emission from the gas seeps densely located in this region.