Dissolved Methane in Coastal Waters of the Northeastern Black Sea

Abstract

As the largest methane reservoir in the world, the Black Sea is characterized by significant variability in its dissolved methane distribution patterns. Aerobic waters in the coastal regions are influenced by various factors governing the biogeochemical processes in the water column; however, their impact on the distribution pattern of dissolved methane is not always well studied, especially in the shallow areas of the northeastern Black Sea, which have hitherto not been thoroughly covered by observations. Here, we consider the measurements of methane content in several regions of the northeastern Black Sea carried out from 2013–2017 with large and detailed spatial coverage in order to identify the key factors determining the dissolved methane pattern in each region of the study area. The CH4 pattern in the region of Sochi is dominated by the impact of river flow. The increased methane saturation in surface waters of the southeastern Crimean shelf (up to 40 nM) is caused by the influence of the Azov Sea outflow. The waters of the Feodosia Bay and to the south of it are mainly characterized by an increase in methane content towards the bottom (up to 100 nM), which is associated with water–sediment interactions and gas emission from the bottom.

Seasonal Variability of Dissolved Methane in the Shallow Coastal Zone: The Case Study of the Golubaya Bay, Northeastern Part of the Black Sea

“…These areas include the surface layer-the river plume located in the northern part of the bay near the mouth of the Ashamba River, along with plumes in the northwest and east areas of the bay near inflowing streams. It is known that fresh watercourses are often characterized by an increased methane content in the estuarine zones [23,[46][47][48]. Continental waters carry a significant amount of suspended solids that accumulate in these zones.…”

“…In a short time, this led to the discharge of solid and suspended matter to the bay in volumes comparable to amounts worth several years under usual conditions. The salinity of the coastal water dropped to 2‰ [41]. Overall, during the catastrophic flood conditions, the Ashamba river carried out to the bay at least 130,000 tons (of 65 thousand m 3 ) of suspended matter [40].…”

“…According to obtained estimation, the average scatter on parallel measurements reached a value of about 4%. The method has been already tested and implemented before for the Black Sea conditions [23] and has been adapted for a wide range of salinity conditions from brackish to hypersaline environments [44]. Atmospheric equilibrium concentrations of methane in water were calculated from the equation suggested by [45] using the temperature and salinity data.…”

“…In addition, the observed anthropogenic impacts result in the pollution of coastal waters with organic matter, which also contains methane. Thus, the formation of dissolved methane maxima in the surface layer of the water column caused by the continental runoff was observed on the shallow Black Sea shelf in the region of Sochi and the Kerch Strait [23]. Vertical gradients of dissolved methane content towards the bottom were observed in the regions adjoining the Feodosia Bay and were associated with water-sediment interactions and gas emissions from the seafloor [23].…”

“…Thus, huge volumes of dissolved methane and hydrogen sulfide are concentrated in the anaerobic zone of the Black Sea beneath the cold intermediate layer [21]. At the same time, many studies of the methane content in the upper aerobic zone of the Black Sea indicate an increased gas content above the equilibrium value with the atmospheric air [22,23]. The main reason for the methane paradox in the Black Sea is the decomposition of methane in the anaerobic microenvironments mentioned above.…”