Dynamic variability of methane and controlling mechanisms in the northern North Yellow Sea

The North Yellow Sea (NYS) is considered a typical continental shelf marginal sea, characterized by a complex driving mechanism among land-source runoff, thermocline, and monsoon, that change the transport and exchange of nearshore water masses, indirectly influencing the distribution of methane (CH₄) in the NYS. During cruises conducted in May, August, and October 2022 and February, June, and September 2023, the surface and bottom dissolved CH₄ concentrations ranged from 4.0 to 26.9 nmol/L and 6.8 to 27.8 nmol/L in the northern of NYS, respectively. The results explain reasons that the response of surface and bottom dissolved CH₄ to changed these parameters (temperature, salinity, DO, chl-a). By integrating the CH₄ concentrations with temperature, salinity, DO%, and depth, the random forest machine-learning models was employed to predict CH₄ concentrations and assess the importance of various variables in the northern NYS. The surface seawater CH₄ were significantly supersaturated, with the saturation (CH₄%) ranging from 103 % to 1211 %, and the sea-to-air CH₄ fluxes ranging from 0.5 to 190.2 μmol/(m²·day), respectively. Overall, the study area had an annual CH₄ emission of 7.8 × 10⁷ mol/yr. Consequently, the northern NYS is identified as a net source of atmospheric CH₄.