Study On The Generation, Release And Driving Mechanism Of Methane In Taihu And Xuanwu Lake

Wetland systems including lake are one of the important natural sources that release methane into atmosphere. Abundant nutrient release causes the eutrophycation in lakes due to human activities. Eutrophic water column not only induce the imbalance of nutrient cycle, the burst of algae and the accumulation of organic matters in the sediments but also induce the formation of anaerobic environment and enhance the release of methane from lakes to atmosphere. In China, abundant shallow lakes are currently being subject to the varying degrees of eutrophication. Many processes including reduction the loads of exterior nutrient, cyanobacteria salvage and ecological dredging project were considered to improve the quality of water. However, factors about the production and release of methane from sediments to the air remain unclear, particularly in the lake, where the ecological dredging projects are performing.The methane current at water-gas interface and methane concentrations in water column, air and sediments were determined in the Taihu Lake and Xuanwu Lake (located in Nanjing) and the effects of environmental engineering, physico-chemical parameters of water (sediments) and biological parameters on the fate of methane were also discussed.Methane concentration was determined in the atmosphere above the surface water (0.5m) of Taihu Lake and Xuanwu Lake, and it varied more significant among sampling site in that of Taihu Lake than in that of XuanWu Lake and was higher in XuanWu Lake than that in Taihu Lake. The methane fluxes at water-air interface associated with the season and it was lower in winter and spring than that in summer and autumn. The distribution and thickness of sediment and depth of overlying water also affected the vertical distribution of methane concentration in water column, and the methane concentration was higher at regions with thick sediments that those with little sediments.Principal component analysis were performed to analysis the relationships among methane content and many physical and chemical parameters in water column of Xuanwu Lake and Taihu Lake. The results indicated that methane and nitrogen parameters in Taihu Lake showed positive relationship while methane and phosphorus showed positive relationship in Xuanwu Lake, indicating there are different mechanisms affecting the fate of methane in the two lakes. In addition, the pH, DO, TOC and algae parameters were grouped in the same class and can be explained, suggesting the principal component analysis method is reliable to analyze these data and the methane content in water can be affected by many factors as it was revealed that there are difference among the methane content and other parameters.Methane concentration and many physico-chemical parameters were determined in the surface sediments and found that no significant alterations were found among the parameters like partical size, pH, total nitrogen and water content; principal component analysis revealed that there were some differences about the relationship between methane content and other indicators in the surface sediments. Principal component analysis on the level of total nitrogen, and ammonia nitrogen, total phosphorus, phosphate and metal ions like Mn, and Zn, and Fe, and Ni, Cu and Mg in porewater of surface sediment from Xuanwu Lake showed that three main components were generated. Ammonia, phosphate and nickel and methane main weighed on the second components; iron and methane fluxes at water-gas interface weighted on the third components. For Taihu Lake, Mn, Mg and methane weighted mainly on the second components while methane flux at water-air interface and Ni mainly weighted on the third component.Results about the methane content and physic-chemical parameters on the vertical sediments showed that the methane were lower in surface sediment (0-6cm) than that in deeper sediment (6-21cm). The pH values, total phosphorus, total nitrogen and organic matters varied with the increase of depth of sediments. PCA results showed that the water content, pH values, total nitrogen and organic matters weighted mainly on the first component and methane and total phosphorus weighted on the second component for Xuanwu Lake and Taihu Lake, all these parameters weighted on the same component. Catalase activity was lower in the deeper layer of sidiments than that in the surface layer in Xuanwu Lake; while it increased with the depth of sediments and then decreasd thereafter in Taihu Lake. Cellulose activity showed similar trend that decreased with the depth of sediments in both lakes. Sucrase activity reached peak values at3-6cm below surface sediments in Xuanwu Lake and no significant alterations were observed among other layers, while it decreased with the depth of sediments in Taihu Lake. Urease activity detected in the vertical sediments of Xuanwu Lake and increased normally with the increase of depth of sediment in Taihu Lake.Copy number of bacterial16S rRNA gene were2.44-15.86×109g-1fresh weight in sediments of Xuanwu Lake, while it varied significantly in the vertical sediments.16S rRNA gene copy number of archaebacteria was4.84-17.71×108and0.11×108-17.44×108g-1fresh weight in vertical sediments of Xuanwu Lake and Taihu Lake, respectively.Methanogens belonging to MBT (Methanobacteriales), MMB (Methanomicrobiales) and MSL (Methanosarcinales) genus and the Methanosarcinaceae and Methanosaetaceae family were detected in the sediments of Xuanwu Lake and Taihu Lake, and abundance of MMB and MSL are usually higher than that of MBT. MrcA gene was detected in Xuanwu Lake and Taihu Lake, however, it was absent in the vertical sediment of Xuanwu Lake at0-3cm and Taihu lake at0-6cm. Two sequence of MrcA gene were cloned by using mrcA gene library construction technology, and showed a high similarity to the MrcA gene deposited in Genebank.According to fitting evaluation, Taihu methane emissions was22800tons per year, the methane emissions of the Taihu Lake Basin was26400tons per year, and emissions of primary lakes in Jiangsu province methane was55200tons per year. Methane emission of freshwater hydrological basin was163000tons per year. The percentage of the annual methane release volume of Taihu, primary lakes and freshwater hydrological basin are1.8%,4.3%and12.75%, respectively.The influence to methane content in air of the desilting region caused by Taihu ecological dredging project is significantly. Methane content in air of the desilting region is decreased23.3%compared with the content in the air of un-desilting area. In past five years, methane content of Taihu lake basin was cut of about1573600tons from lake, river water by the ecological dredging project, which accounting for about at123%of the total methane release content of Jiangsu province in2010. The co reduction amount caused by ecological dredging project in Taihu lake body is about522000tons, to reduce water endogenous methane content, reduce greenhouse gas emissions in Jiangsu Province outstanding contribution.The results obtained in this study are forward-looking significance for bringing the Jiangsu Province Lake methane release into IPCC greenhouse gas emissions list, for accounting contribution of ecological dredging project to endogenous methane content in water and introduction of greenhouse gas emission trading capital.