Dongting Lake Ecological Health And Safety Assessment Study

Dongting Lake, the second-largest freshwater lake in China, is located in the north of Hunan province and on the south bank of Yangtze River. Its ecological functions include adjusting the accumulation of water as well as a flood barrier, breeding the biological diversity, and serving as a pool of purification in the major stream of Yangtze River or as an ecological foregoing-reservoir, and so on. However, under the double functions of nature and human activity, Dongting Lake came forth many eco-environmental problems such as severe atrophy, shrink of the water body, destruction of biological diversity, and increasing deterioration of eco-environment. It has become the limiting factor to the development of society and economy.Based on the sufficient investigation, this thesis evaluated the water quality of the Dongting Lake using a single-factor appraisal, comprehensive pollution index and comprehensive nutrition index. Then an appraisal system to estimate the aquatic health of the lake was established by using entropy weight comprehensive health index method combined with index systems of physics, chemistry and ecology. The results of single-factor appraisal showed that the whole water quality was excellent in case that the appraisal of TN and TP was excluded in the recent 5 years. However, the pollutions of TN and TP were relatively serious. The concentrations of TP from all the monitoring section were inferior to the water quality requirements ofⅢclass and there were only 8% sections in which the TN concentration reached the water quality requirements ofⅢclass. The appraisal of comprehensive nutrition index showed that the quality of the lake was in the level of moderate nutrition and light eutrophication. The appraisal of comprehensive pollution index further showed the water quality in the lake body was inferior to the water in the entry, which was also inferior to the water in the outlet. The pollution of the water quality of the lake were mainly contributed to farming of large-scale livestock, aquaculture, and residential pollutions according to the investigation. The results of entropy weight comprehensive health index demonstrated that during the period of 2004 to 2008 the comprehensive health index of the ecological system of Dongting Lake revealed a declined trend, and index was only 0.2881 in 2008. Generally, the healthy situation of the ecological system in Dongting lake declined.The comprehensive appraisal method of ecological security about the Dongting Lake was further investigated.14 indices were chosen for establishment of the ecological security index (ESI) system, according to the driving force-pressure-state-impact-risk model. The weights of the indices were determined by analytical hierarchy process (AHP), and the ecological security status classification was evaluated by the ESI model. The results showed that the ecological security ranked the safe grade in 2000 and the general grade in 2008, indicating the decrease of ecological security. In 2008, the status score was only 43.2% of the benchmark. The total nitrogen and total phosphorus were the main contributors for the water deterioration of the Lake Dongting.The strategies to guarantee the ecological security were proposed according to the situation of ecological security of the Dongting Lake. For instance, the strategies included reinforcing protection and restoration of the marsh, developing comprehensive prevention and treatment referring to the pollution of the lake and intensifying the ecological secure management of drainage area.