Environment-Forming Effect of Bubble Gas Emissions in the Golubaya Bay, Black Sea: Oxygen Regime and Bacterial Mats

Abstract

Gas seep and fluid flows from the seabed are an environment-forming factor of the aquatic environment, mainly due to their influence on the dissolved gases in the water, including dissolved oxygen. During the summer seasons from 2019 to 2021 in the area of shallow water gas emission site off the southern coast of the Heracles Peninsula, series of vertical probing profiles were carried out to determine hydrological parameters of the water: dissolved oxygen concentration (O2), temperature (T), salinity (S), and flow velocity (U). The study area is an underwater ledge with faults in the form of three canyons composed of dense limestones, two of which contained bubble gas emissions. Significant variability in O2 was identified in canyons where gas emissions are observed: from 1 to 80% saturation in the bottom layer, in contrast to normoxia at the background sites. Hypoxia was observed in the bottom layer above the emission sites in the absence of turbulence at temperature stratification. The values S decreased with depth, and the maximum difference reached 0.4‰. The bubble gas was dominated by methane (68.5–75.5%), and the carbon isotope composition of the bubble methane gas varied from –67.9 to –59.8‰ VPDB in 2019 and 2020, respectively. This generally indicates that the CH4 is of predominantly microbial genesis, was formed under different conditions, and matured in various periods of research during the monitoring period. Bacterial mats (mostly sulfur-oxidizing bacteria) were found in the areas of gas emissions.