东平湖菹草腐烂的沉积物响应机制及温室气体排放

Dongping Lake as the important regulating reservoir in the eastern route of China's south-to-north water diversion project is of great importance to guarantee the transferred water quality. Potamogeton crispus, the dominant species in the aquatic vascular plants in Dongping Lake, begins to decay and decompose in late May and early June, which causes the deterioration of the aquatic ecological environment and the death of fishes. Currently, researches on the effect of decomposition of submerged macrophyte on water quality have been carried out; however, the overall environmental effect of decomposition of submerged macrophyte has still to be further studied systematically across the macrophyte-sediment-water-air interface. By using a combination of the field observation and laboratory simulation and culture methods, the project is to study the decomposition law and nutrient element release of Potamogeton crispus, and to explore the effect of the decomposition of different biomass of Potamogeton crispus on the basic physiochemical properties and denitrification rates of sediment. Meanwhile, the fluxes of greenhouse gases, i.e. carbon dioxide, methane, and nitrous oxide across water-air interface and the fluxes of nutrient across sediment-water interface will also be determined. Based on the above analysis, the response process and mechanism of sediment to the decomposition of Potamogeton crispus are to be revealed. Moreover, the release/accumulation of nutrient and the sedimentary flux caused by the decomposition of Potamogeton crispus will be estimated. The research results are expected to provide important scientific basis for the prevention of eutrophication of Dongping Lake and the safety guarantee of water quality of south-to-north water transfer project, and simultaneously enrich and develop theoretical research contents about the geochemical cycle of nutrient elements in shallow lake ecosystem.

东平湖是国家南水北调东线工程的重要调蓄站，其水质状况对输水水质具有重要影响。菹草是东平湖水生植物的优势种群，每年5月下旬6月初开始衰亡、腐烂分解，导致水生态环境急剧恶化，鱼类大量死亡。当前对沉水植物腐烂分解对水质影响的研究已有开展，但还有待于系统的从植物-沉积物-水-气界面开展沉水植物腐烂分解环境效应的研究。本项目拟通过野外现场观测和实验室模拟培养技术的综合运用，研究菹草集中衰亡后的遗体分解和营养元素的释放规律，探究不同生物量菹草腐烂分解对沉积物基本理化性质和反硝化速率的影响，并结合水-气界面温室气体（CH4、CO2和N2O）排放通量和沉积物-水界面营养盐交换通量变化，揭示菹草衰亡分解过程中沉积物的响应过程和机理，估算菹草腐烂对东平湖营养物质循环和沉降通量的影响。以期为东平湖水体富营养化的防治和南水北调供水水质安全保障提供科学依据，同时丰富和发展湖泊水生态系统物质地球化学循环理论研究内容。

菹草已经成为当前北方浅水湖泊中的优势物种，其在生长过程中会吸收上覆水和沉积物中的营养物质和其他污染物质，但其集中的衰亡会改变水体的理化性质，导致水质的恶化。本项目采用野外现场观测和实验室模拟培养的方法，研究了东平湖菹草生长对水质的影响，探究了菹草腐烂分解的物质衰减规律和营养元素释放规律，阐明了菹草腐烂分解对水质和沉积物理化性质的影响，通过菹草腐烂期水体温室气体排放的变化，初步解析了沉积物的响应机制。结果表明：（1）东平湖菹草的腐烂分解过程符合二次指数衰减模型，在分解过程中释放C、N、S和重金属元素（Cd、Cr、Cu、Fe、Mn、Pb和Zn），且C和S的释放量大于N；（2）东平湖菹草生长期有利提高湖水中DO含量，对P有一定的净化作用，而在菹草腐烂分解期，湖水DO浓度急剧降低，TP浓度显著上升，沉积物中硝态氮含量有显著增加，水体中CO2浓度和排放通量有显著升高，而N2O和CH4浓度和排放通量在腐烂后期有显著增加，水温是影响温室气体排放的主要因子，而N2O则主要来源于沉积物的反硝化作用；（3）模拟培养实验的结果表明，菹草腐烂分解期间，高生物量组上覆水中pH和DO浓度显著降低，而N、P浓度显著增加，沉积物有机质和硝态氮含量增加，但氨氮含量降低，高生物量组温室气体的浓度、饱和度和排放通量均显著高于低生物量组和空白组，表现为大气中CO2、CH4的源，N2O的汇，孔隙水中温室气体浓度整体表现为随深度增加而降低。以上研究结果较为系统的从植物-沉积物-水-气界面阐明了菹草腐烂分解的环境效应，基本揭示了沉积物的响应机制，为东平湖水体富营养化防治和输水水质安全保障提供了重要的科学依据。