Seasonal Variations And Regulatory Mechanisms Of Dissolved Methane Concentration And Its Sea-to-air Fluxes In The Seasonal Oxygen Deficient Zones In Bohai Sea

The Bohai Sea is a shallow semi-closed region in the northern China waters,covering an area of 77,000 km² with a mean depth of 18 meters.It is surrounded by continent,and only connected to the northern Yellow Sea through the narrow Bohai Strait.The preliminary survey showed that the Bohai Sea was the source of atmospheric CH₄.But until now,few studies about the source-sink processes of dissolved CH₄,and the regulatory mechanisms of dissolved CH₄ concentration distribution and its sea-to air flux were reported.In recent years,for the coincidence effects of human activities and nature processes,the oxygen depletion phenomenon was detected in the western Bohai Sea,where the dissolved oxygen concentration declined year by year.The area of the new oxygen deficient zone was second only to that in the Yangtze River Estuary.To research the source-sink processes,spatiotemporal distribution dissolved CH₄concentration and its sea-to-air flux in the oxygen deficient zones in the Bohai Sea,a detailed study plan was designed and conducted.In addition,due to the potential bubble ebullition phenomenon,a study of bubble-mediated CH₄ and its regulatory mechanisms was performed in the polluted Dalian Bay.This study was valuable for deepening understanding the biogeochemical cycle of CH₄ in coast seas,the effects of human activities on the source-sink processes of CH₄ in shelf shallow seas,and useful for improving the marine environment by formulating efficient policies,through which the anthropogenic source strength of CH₄ would be reduced and beneficial for the global climate change more effeciently.Based on self-design and optimization tests,the underway ship-based cavity ring-down spectroscopy system and method was established.The system is used for the continuous measurement of CH₄ mole fraction in marine boundary air,with a max drift less than 0.10%,and with precision and accuracy less than 0.03%and 0.08%,respectively,better than the requirements of the World Meteorological Organization-Global Atmosphere Watch associated to atmospheric CH₄.And the headspace equilibrium gas chromatograph system equipped with flame ionization detector was established,which used for the measurement of dissolved CH₄ in discrete seawater samples,with precision and accuracy less than 0.80%and 1.50%,respectively.Moreover,the typical rigid plastic inverted funnel-style bubble trap was set up and used to collect bubbles in polluted coastal seas.An incubation method and pore-water sampling method were also established,and used to measurements of dissolved CH₄concentration in sediment and its net release rate from sediment to seawater.The CH₄ in boundary air,seawater column and sediment in Bohai Sea and related hydrographic&meteorological data were observed.The atmospheric CH₄ and dissolved CH₄were simultaneous measured in spring,summer and winter cruises.Based on the in-situ observation and related experiments data,we found that the sediment was the main source of dissolved CH₄.And the bottom seawater was the weak sink of dissolved CH₄ in seawater of the oxygen deficient zones.The highest and lowest concentration of dissolved CH₄ in the oxygen deficient zones was observed in early autumn and witer,respectivley.But the vertical gradient concentration distribution of dissolved CH₄ was largest in summer.The hydrodynamic condition which derived by seasonal stratification and submarine topography was the most important and direct regulate factor.The oxygen depletion and the high dissolved CH₄ concentration in bottom seawater was not a direct cause-effect relationship.And both of them were results of sedimenting organic materials from phytoplankton blooms which related to human activities and natural processes.But the oxygen depletion would induce more CH₄ free to be oxidized in bottom seawater in summer.So,it was reasonable to say that an effect of source-strengthen and sink weakened could occur in the oxygen deficient zones in summer.And the accumulation effect of dissolved CH₄ in bottom seawater in oxygen deficient zones may be enhanced if the oxygen depletion worse year by year in future.The dissolved CH₄ saturation and its sea-to-air flux which calculated according to the global mean atmospheric CH₄ mole fraction were 14%and 15%higher than that calculated according to the in-situ observed atmospheric CH₄ mole fraction,respectively.Obviously,the saturation and flux were more accuracy that calculated by using the in-situ observed data.So,the observation and study methods about sea-to-air flux of CH₄ were optimized,and the CH₄in seawater and boundary air was observed simultaneously.The oxygen deficient zones in Bohai Sea were net source of atmospheric CH₄ because the dissolved methane was always oversaturated against to the marine boundary air.The sea-to-air flux of CH₄ varied dramatically in different season,which dominated by seasonal hydrodynamic conditions,wind,sea surface temperature and salinity,etc.The seasonal hydrodynamic conditions which derived by seasonal stratification and submarine topography were the most important and direct regulate factor of sea-to-air flux of CH₄.The sea-to-air flux of CH₄ reduced from spring to summer due to the form of stratification and the accumulation of dissolved CH₄ in bottom seawater in summer.And it would increase from summer to autumn due to the terminated stratification with a highest observed value of 37.9?mol/m²/d observed in mid-September,2017.The oxygen deficient zones were or may be an area with bubble ebullition as it happened in the Black Sea and the Baltic Sea,etc.The pre-study about the bubble-mediated CH₄ was conducted in coastal area of the polluted Dalian Bay in summer of 2016 when the bubble ebullition was the most intensive,by using the self-established invert funny trap and watertight camera.The observed results showed that the mole fraction of CH₄ in bubble sample reached to 0.378 mol/mol.The components of bubble released from sediment of the polluted Dalian Bay included CO₂,CH₄,N₂O,etc.The regulate factors of bubble ebullition were tide movement,temperature and organic matters in sediment,etc.the tide movement regulate the bubble release rate through periodic changed pressure on sediment and mixing of water bodies.And temperature influenced the saturation and the productivity of CH₄ in sediment.Sea-to-air flux caused by bubble-mediated CH₄ was 14.8 times of that caused by dissolved CH₄ in the same area,and would influenced the vertical and horizontal distribution of atmospheric CH₄ in local scale efficiently.So,the bubble ebullition may be a hot-spot of CH₄ emission in oceans which could not be ignored.