Distributions, Influencing Factors and Sea-Air Fluxes of Dissolved Methane in the Northern of North Yellow Sea in August 2022

Scientific assessment of the distribution and sea-air fluxes of dissolved methane (CH₄) in the marginal sea is important for understanding the regional contribution of CH₄ emission to the atmosphere. The distribution, influencing factors and sea-air fluxes of CH₄ in the northern of the North Yellow Sea were studied based on a survey in August 2022. The results showed that the dissolved CH₄ concentrations range were from 4.0 nmol/L to 63.3 nmol/L with saturations from 168% to 2 360% in the study area. High CH₄ concentrations were observed in Yalu River estuary, and the dissolved CH₄ concentrations in surface seawater decreased rapidly as the estuary extends to the sea, while the input of diluted water from terrestrial sources such as the Yalu River coupled with the degradation of seawater and sediment organic matter leaded to an increase in CH₄ in inshore area. The oxidative consumption of CH₄ in seawater mixing with the terrestrial diluted water was also an important process that could not be ignored in controlling CH₄ concentration and distribution. The action of North Yellow Sea Cold Water Mass coupled with anaerobic degradation of sediment organic matter resulted in relatively high dissolved CH₄ concentrations in bottom seawater than that in surface seawater in the offshore area. In the study area the sea-air CH₄ fluxes were from 18.4 μmol/(m²·d) to 578.8 μmol/(m²·d), being the source of atmospheric CH₄ and it was significantly higher in the inshore area than that of the offshore area. Therefore, the dissolved CH₄ concentrations and sea-air fluxes in this area had significant differences due to the inputs of diluted water, and the variation amplitude was higher than that of other sea area in the world. It is crucial to strengthen the investigation and research in coastal areas such as estuary to accurately assess the CH₄ emissions and emission source composition to formulate effective CH₄ reduction measures.