Abstract:The oceans represent a natural source of atmospheric methane (CH4), with estuaries, shelf areas, and near-shore seas collectively accounting for 16% of the global ocean area and contributing approximately 75% of the total annual CH4 release. The release of CH4 from estuaries and near-shore areas is influenced by human activity such as sewage discharge and aquaculture. Hence, it is a scientific priority to study the production and release of CH4 in near-shore aquaculture areas and understand the factors influencing its distribution. Three cruises were conducted in the Yangma Island aquaculture area of the North Yellow Sea at the end of June, and in early and late August 2023. Surface and bottom seawater samples were collected to understand regional dissolved CH4 distribution characteristics, and sea-air fluxes were estimated based on the CH4 concentrations in surface water and wind speeds. The dissolved CH4 concentrations in surface waters obtained during the three cruises during summer were (8.88±4.99), (11.30±4.81), and (9.10±3.03) nmol/L (Mean±SD), and the dissolved CH4 concentrations in bottom seawaters were (14.25±7.99), (16.15±5.93), and (10.88±4.08) nmol/L, respectively. The CH4 concentrations in the bottom water were significantly higher than those at the surface, because of CH4 release from the sediments. The bottom CH4 concentrations were 2~6 nmol/L higher than those in the surface water at most stations at the end of June and beginning of August, owing to the presence of water column stratification, which effectively impeded the transportation of CH4 produced by the sediment, to the upper seawater. By the end of August, the water column stratification had dissipated, resulting in a notable reduction in the discrepancy between the surface and bottom CH4 concentrations. The distribution of dissolved CH4 concentrations in the Yangma Island aquaculture area was predominantly influenced by river inputs, aquaculture activity, and algal and microbial processes. High CH4 concentrations were observed in the nearshore estuaries (Xin'an, Yuniao and Qinshui Rivers) throughout summer due to river input. Dissolved CH4 content in the water body was strongly influenced by aquaculture activity (primarily bivalve shellfish), which provided favorable conditions for CH4 production in the water column, thereby affecting the suspended particulate and organic matter contents in the water column and the sediments. The anaerobic microenvironments of the intestinal tract and excreta of shellfish also represent an optimal setting for anaerobic CH4 production. Consequently, the dissolved CH4 concentration in seawater within the aquaculture zone was markedly elevated compared to that in the non-farming regions. Phytoplankton abundance was high in this area during summer, with Bacillariophyta and Pyrrophyta being the primary groups. Previous studies have demonstrated that the dominant algal species in this area, Leptocylindrus danicus, can directly produce CH4 at a rate of 0.0136 μmol·CH4/(g·dry weight∙h). Moreover, in this study, another dominant algal species, Pseudo-nitzschia, was subjected to laboratory-controlled culture experiments, which demonstrated that it can also produce CH4 at a rate of 46.59 ag/(cell∙d). In addition, some Bacillariophyta and Pyrrophyta indirectly produce CH4 through the degradation of dimethyl sulfoniopropionate (DMSP) released from the algae. Simultaneously, this area showed phosphorus limitation at the end of June and beginning of August, and the high DOC content in the aquaculture area provided rich C-P bonds, allowing microorganisms to degrade organic phosphorus compounds to produce CH4. In early August, the ∆CH4 concentration in the surface layer had a significantly positive correlation with Chl-a, verifying that aerobic processes such as phytoplankton production and methyl compound degradation can provide ΔCH4 sources in aerobic surface waters during summer. Spatiotemporal variations in CH4 saturation and air-sea fluxes in the surface seawater showed trends of aquaculture area > non-aquaculture area and bay area > coastal shelf area. The CH4 saturation in the surface water obtained during the three cruises in the coastal waters adjacent to Yangma Island aquaculture area during summer were (377±209)%, (527±224)% and (391±130)%, respectively, and were all oversaturated with respect to atmospheric CH4. The air-sea exchange flux (estimated using the W2014 relationship) of CH4 in the surface water during summer was (18.87±28.82) μmol/(m2∙d). Overall, we estimated the annual CH4 emissions from the coastal waters adjacent to Yangma Island aquaculture area to be approximately 5.87×10-5 Tg/yr, indicating that this region is a net source of atmospheric CH4.
