浅水湖泊聚藻区沉积物-水系统多位点产甲烷过程及受控机制研究

Methane is the second important greenhouse gas, and lake has been proven as a quite important natural source of methane emission. In lakes suffering harmful algal blooms, some districts of a lake is often accumulated with a large quantity of algae cells due to the topographic feature or meteorological factor. Accompany with the successive accumulation of algal cells, some cells dies and degrades, a lot of algal particles fall onto the sediment surface, the dissolved organic carbon increases fast, and the lake water and the surficial oxic sediment become to be hypoxic or anoxic. All these provided plenty substrate and good environmental for methanogenesis, and methanogenesis is expected to occur in both sediment and water column simultaneously. In this study, the northwestern bay of Lake Chaohu, a large and shallow eutrophic lake in eastern China, is chosen to be studied as the high algal biomass accumulation occurs often in this district. Firstly, field monitoring will be conducted to examine the environmental characteristics of water, and the content and chemical structure of organic matter in both water and sediment. Methane in the air, different water and sediment depths will be measured by gas chromatography using a flame ionization detector. The ebullition and diffusion of methane across the sediment-water and water-air interface will be quantified respectively by means of gas trap funnel, drifting gas flux chamber, and diffusion calculation. The spatio-temporal distribution transportation of methane in the algae accumulated district will be gained after a whole year study. Secondly, field enclosures will be constructed to simulate algal accumulation progress. The change of O2, pH, and Eh across the sediment-water interface, acetate acid and H2 in the sediment and water will be examined successively with the progress of algae accumulation. The production of methane in the sediment and water column will be measured, and contributions of acetate acid decomposion and CO2 reduction by H2 to the total methanogenesis will be calculated. Methane transportation will also be examined, the enhancement of algae accumulation on methane ebullition will be assessed, and the contribution of diffusion and ebullition will be assessed. Thirdly, the influence of algal organic carbon increase in the sediment and water on methane production will be studied by means of 13C on the basis of enclosure system. The effects of environmental change, such as O2 depletion and pH decreasing on methanogenesis in the water and surficial sediment will be examined by means of incubation of sediment cores. The quantitative PCR, high-throughput sequencing, and stable isotope probe will be used to study the abundance, community composition, and activity of methanogens, which aims to reveal the microbial mechanisms in methane production. The study results will provide scientific evidence on the enhanced methanogenisis and methane transportation and the driving mechanisms in the shallow algal bloom lakes.

浅水湖泊中水华藻体大量聚积会造成藻源有机颗粒在沉积物表面大量沉降、藻源溶解有机碳在水中迅速增加，水体及表层沉积物呈厌氧或缺氧状态，这为沉积物-水系统多位点产甲烷提供了良好的物质基础与环境条件。本申请以浅水湖泊聚藻区这一特殊生境为研究对象，分别开展多站点原位观测与藻体聚积过程动态研究，基于倒置漏斗式气泡捕获、漂流式气体通量观测技术，明确聚藻区甲烷溶存分布特征及跨沉积物-水-气界面迁移规律；并借助13C示踪、多参数微电极探测、原位间隙水实时获取及分析技术，探明沉积物-水系统多位点产甲烷对藻体聚集过程的动态响应。开展环境变化单因子控制与藻源有机质输入实验研究，甄别影响产甲烷主导因子，并结合qPCR检测、SIP探测、高通量测序等分析产甲烷功能菌，解析沉积物-水系统多位点产甲烷的内在机制，为深入认识浅水湖泊中藻华驱动下的碳循环及甲烷排放提供科学理论依据。

湖泊是甲烷(CH4)重要的自然排放源，在气候变暖的同时，湖泊富营养化不断加剧、蓝藻水华在全球不断扩张。浅水湖泊中水华藻体大量聚积会造成藻源有机颗粒在沉积物表面大量沉降、藻源溶解有机碳在水中迅速增加，水体及表层沉积物可能呈厌氧或缺氧状态，这为沉积物-水系统多位点产甲烷提供了良好的物质基础与环境条件。本项目分别在我国两大富营养化浅水湖泊——巢湖与太湖开展了聚藻区的长周期原位观测及藻体聚积动态过程研究，分析了富营养化及蓝藻水华聚集驱动下CH4溶存分布及跨界面迁移特征；通过开展蓝藻水华聚集实验以及湖泛发生过程模拟实验，阐明了水华藻体聚积作用下沉积物-水系统多位点产甲烷过程、微生物机制及其对甲烷跨界面释放的驱动作用。结果表明，营养水平相关参数是决定浅水湖泊沉积物-水-气界面CH4扩散通量的关键，其中总磷起到最重要作用，离岸距离与水深在一定程度上改变了影响水平的影响作用；浅水湖泊聚藻区内CH4溶存水平高，且呈自岸边向湖心递减的趋势，河流外源输入在一定程度上减弱或强化这一分布特征；巢湖西北湖湾聚藻区CH4总释放通量比对照点位高出1~2个数量级，且聚藻区内约93% CH4通过冒泡释放；藻类聚集下藻体的死亡分解增加了产甲烷菌丰度、改变了产甲烷菌的群落架构，显著促进系统中CH4浓度升高与释放速率增加。湖泛易发区水与沉积物中CH4含量普遍较高，湖泛引发的缺氧降低了温度对水中CH4的直接影响作用，湖泛易发区是浅水湖泊中CH4排放的热点；湖泛发生时，水与沉积物中产甲烷菌群落结构变化显著，产甲烷菌丰度升高，沉积物产甲烷潜力大幅提升，水-气界面甲烷通量增大1~3个数量级。研究结果为深入认识浅水湖泊聚藻区碳循环及CH4产生与排放提供了科学依据，为预测全球广泛存在的蓝藻水华驱动下淡水生态系统甲烷排放提供了科学基础，完成项目预定研究目标，发表研究论文7篇。