Effect Of Seasonal Stratification On The Kinetics Of Dissolved Methane In The Central Bohai Sea

Methane(CH4)is the second largest greenhouse gas after carbon dioxide.It can directly or indirectly affect the global climate.Ocean is an important source of atmospheric CH4,and coastal seas account for 75%of global marine CH4 emissions.The Bohai Sea is a marginal sea of the western Pacific Ocean.At present,CH4 researches in the Bohai Sea mainly focuse on spatiotemporal distribution,air-sea flux,and their controls.However,the effect of seasonal stratification in the central Bohai Sea on CH4 kineticis is still unclear.To address this question,we re-analized the data collected in twice summertime field investigations(August 2018 and July-August 2019)and one wintertime investigation(December 2019).Based on our own data and literature results,we quantified strengths of sources and sinks of dissolved CH4 in these seasons.Also,we set up steady-state box model and then clarified the effect of seasonal stratification on kinetics of dissolved CH4 in the central Bohai Sea.The main results are shown as follows:Water column in the central Bohai Sea was separated into two layers by summertime thermocline and CH4 concentration was higher in the bottom layer than in the surface layer.Whereas,water column was well-mixed in winter and CH4 concentration was vertically consistent.A steady-state box model was set up to mimic the water column in the central Bohai Sea,double-layer in summer and single-layer in winter.In summer,model results indicated that there was net production of dissolved CH4 in the surface layer.The total contribution of Yellow River input and upward diffusion across thermocline to the stock of dissolved CH4 in surface layer was only 2.5-10.0%.Moreover,model result indicated that there was net consumption of dissolved CH4 in the bottom layer with a rate of 2.6-14.5 μmol m-2d-1.In winter,there was net production of dissolved CH4 in the whole water column.Contribution of Yellow River input to the stock of dissolved CH4 was only 0.1%.Seasonal stratification significantly affected the the kinetics of dissolved CH4 in the central Bohai Sea.Summertime stratification resulted in a completely different biogeochemical cycling senario of dissolved CH4 to that in winter.Thermocline hindered upward transportation of dissolved CH4 from bottom layer to surface layer,leading to 99.7-99.9%of sedimetaryreleased CH4 accumulated in bottom layer,and only about 0.1-0.3%diffused upward to surface layer through the thermocline.Turnover time of dissolved CH4 was 29.1-59.2 days in bottom water.In surface layer,net production contributed 90.0-97.5%to sea-air exchange,and turnover time of dissolved CH4 was only 7.9-24.3 days.In winter,turnover time of dissolved CH4 was 33.2 days,a time longer than that in surface layer in summer but shorter than that in bottom layer in summer.