Abstract:

Ecological risk assessment is an emerging area. It is developed with the change of environmental management goal and conception. In addition to the general meaning of risk, it is characterized by uncertainty, intrinsic value, objectivity and dynamic nature. Dongting Lake, the second largest freshwater lake in China, is an important storage lake in the middle reaches of the Yangtze River, as well as a worldfamous scenic spot. It plays important roles in controlling flood, keeping balance between flow and sediment and maintaining species diversity. In recent years, the industry and agriculture development around the lake brought about a series of problems, such as water quality deterioration, aggravated eutrophication, sediment accumulation and increasing shortage of water resources. Ecological safety has been a horrible threat to the sustainable development of lake area. Therefore, it is considerably necessary to carry out ecological risk assessment of Dongting Lake. An ecosystembased ecological risk assessment method, which is different from traditional methods, was developed depending on the concepts of system ecology and complex system theory. The Ecosystem Services were selected as assessment endpoints and the “PressureResponse” relationship between external stress on ecosystem and Ecosystem Services was quantified. In this method, the whole ecosystem was taken as the object to analyze. It took account of not only the elements that make up the ecosystem, but also the complex relationship among them. For the ecological risk assessment based on single pressure source and risk receptor, the connection between the ecosystem elements followed a onetoone relationship. However, the ecological risk assessment based on whole ecosystem is the typical of multiple pressure sources, mutipaths, mutireceptors and mutiendpoints. Eventually, it was applied to the ecological risk assessment of Dongting Lake, and the assessment results were analyzed. The results showed that the degree of ecological risk is highest in East Dongting Lake while lowest in South Dongting Lake. The major pressure sources were urbanization, human life and climate change. Organic pollutant, nutritive material and water volume change of the upstream turned out to be the critical stress factors. The ecosystem indicators which were greatly affected were the biodiversity, riverlake connectivity and carbon cycle. The risk degree of nutrient cycle, water quality improvement and biodiversity were comparatively higher. The ecological risk assessment method proposed in this study considered risk resources comprehensively and objectively. Quantitative evaluation facilitated identification of the risk level. It provided basis for wetland management and ecological protection. Management of Dongting Lake should not only focus on the regular indices, like water quality and water quantity, but also biological indicators, such as species diversity. Since the three parts of Dongting Lake are different in landscape, hydrology and economic activity, different measures should be adopt to suit local conditions according ecological risk level