Distribution, Emission And Production Of Methane In The Yellow Sea, East China Sea And Sanggou Bay

Methane (CH4) is an important greenhouse gas, which influences global warmingand atmospheric chemistry. In recent years, CH4concentration has risen continuouslydue to the imbalance of its sources and sinks. Ocean is one of the natural sources ofatmospheric CH4, especially the coastal area, where CH4emissions are significantlyactive as a result of terrestrial input and human activities and accounts for a relativelylarge part of oceanic CH4emissions. It is essential and valuable to comprehensivelystudy the biogeochemistry processes of dissolved CH4in different oceanicenvironments, to quantify various sources and sinks, and to explore the vital paths ofoceanic CH4production.Distribution, emission, seasonal variation and production mechanism of dissolvedCH4were studied in this thesis based on eight cruises in three different marineenvironments, which include the continental shelf sea (the Yellow Sea, the East ChinaSea), estuarine area (Changjiang Estuary) and the representative aquaculture area(Sanggou Bay). The main results are as follows:1. Two cruises were conducted in the Yellow Sea and the East China Sea in Octoberand December2011, respectively. CH4concentrations in both surface and bottomwaters gradually declined from the Changjiang Estuary towards the open sea inautumn due to the impact of Changjiang diluted water. The average CH4concentration in autumn was5.00±1.68nmol·L-1in surface waters and7.51±2.93nmol·L-1in bottom waters. Bottom CH4concentrations are slightly higher thanthose of the surface due to CH4emissions from the sediments. CH4distributions inthe coastal area of the Yellow Sea and the East China Sea were influenced lesssignificantly by Changjiang diluted water in winter, however, high CH4concentrations were still observed in the Changjiang Estuary and its adjacent area.Average CH4concentrations in surface and bottom waters were4.74±1.69nmol·L-1and5.32±2.25nmol·L-1respectively in winter, which were relatively lower than those in autumn. Dissolved CH4in surface waters were oversaturatedin both autumn and winter, and the saturations in autumn were slightly higher thanthose in winter. In general, the Yellow Sea and the East China Sea were netsources of atmospheric CH4. It was estimated that CH4emission from the YellowSea and the East China Sea in2011was (3.34-5.83)×109mol (about0.053-0.093Tg), which accounts for0.52%of the annual global oceanic CH4fluxes. Thisvalue is higher than its corresponding area proportion of0.32%, indicating thatthey are active areas for CH4production and emission. CH4emission from thesediment in the Yellow Sea and the East China Sea was estimated to be about6.09×108mol/yr, indicating that sediment is an important source of CH4in bottomwaters.2. Distribution and air-sea fluxes of methane in the Changjiang Estuary and itsadjacent area were investigated during two cruises in March and July2012,respectively. Surface CH4concentration in Spring and Summer were28.33±38.33nmol·L-1and19.92±19.18nmol·L-1, respectively, and showed a decrease from theinner estuary to the open sea. CH4concentration and saturation in surface andbottom waters showed obvious seasonal variation. In the inner estuary, higher CH4concentration and saturation occurred in spring, however, outside the estuaryhigher concentration and saturation occurred in summer. This may attribute to theseasonal variation of temperature and CH4concentration in the Changjiang dilutedwater. Distribution of dissolved CH4was obviously influenced by terrestrial input.In summer, the impact area of Changjiang diluted water was much wider and CH4release from the sediments was more obvious than in Spring in the outer Estuaryand its adjacent area. Surface CH4were all oversaturated during our surveys,hence the Changjiang estuary and its adjacent area are net sources of atmosphericmethane. Annual CH4emission from the Changjiang estuary and its adjacent areawas preliminarily estimated to be1.08×109mol·yr-1, which accounted for about0.10%of the global annual oceanic CH4fluxes, and the value was5-fold of itscorresponding area proportion（about0.02%）, indicating that the Changjiangestuary and its adjacent area were active sites for CH4production and emission. 3. Four cruises were carried out in Sanggou Bay in June and September2012, Apriland July2013, to investigate distributions, air-sea fluxes, sediment-water fluxesand production of dissolved CH4in the bay. CH4concentrations in Sanggou Bayshowed substantially seasonal variation with CH4concentrations in summer andautumn obviously higher than those in spring. The aquaculture of shellfish andkelp in the bay was vitally responsible for the seasonal differences of CH4.Dissolved CH4in surface waters of the Sanggou Bay were oversaturated in spring,summer and autumn, indicating that Sanggou Bay is a net source of atmosphericCH4. CH4concentrations in rivers and underground waters around the bay showedobviously seasonal variation, and CH4concentrations in rivers were significantlyhigher than those in underground waters. Water incubation experiment indicatedthat seawater CH4production rates in the bay was much higher than the adjacentYellow Sea due to the aquacultural activities, while CH4production in seawater inthe bay in summer was more noticeable than that in spring. CH4budget inSanggou Bay was estimated with a box-model according to our results andliterature data, which suggests that in situ CH4production in seawater was thedominant source in Sanggou Bay, accounting for above90%of CH4sources,while CH4from river runoff, underground water input and sediment emission onlycontribute to a small proportion. CH4was mainly output through air-sea exchange.