Ecological Safety Assessment And Research Of The Lake Of Doushui Based On F-IBI And Nutrient Criterion

Doushui Lake is located in Shangyou County and Chongyi County of Ganzhou City in Jiangxi Province.The catchment area of the lake is 2,750 km~2.It is a typical reservoir-type lake in the headwater region of Shangyou River Basin,a branch of the upper reaches of Ganjiang River.In recent years,along with the development of the socio-economy,especially the tourism industry,the aquatic environment of the lake has greatly been threatened.The water quality in some bays of the reservoir has dropped in recent 5 years and the bloom phenomenon has occurred.In order to maintain and recover the ecological safety of Lake Doushui,it is crucial to conduct an eco-safety assessment on the lake ecosystem.Based on the new methods of the nutrient criteria and fish index of biotic integrity(F-IBI),this paper evaluated the water ecological security of Lake Doushui,and mainly drawn the results as follows.(1)Through the analysis of the spatiotemporal dynamics on water quality of Lake Doushui,it was in Grade III of national standard and water quality in the different areas was different.For instance,TP in the water column of the left lake areas(e.g.Shekengzi Bay,Sheguan Bay and Guobu Bay)all exceeded the standard limits(Grade III).In the dry season,nutrients exceeded the limits more frequently than those in other seasons.Both of TN and TP were the main nutrient pollutants and out of the standard limits in the lake,followed by a gradual increase in the concentration.(2)A statistical method was used to formulate the nutrient criteria of four indicators for the lake,in which the criteria values were TN=0.67mg/L,TP=0.03mg/L,Chl-a=2.73μg/L and SD=308cm,respectively.The reference values of nutrient criteria were consistent with the fundamental characteristics of the east zones of Lake Doushui and those stood for the original states for of the lake exclusively.These values were also in line with the measured values of these nutrients concentration meeting for requirements of the function and usage in all of the water environment function areas of Lake Doushui.(3)As the aim to figure out the health state of Doushui Lake in the degradation trend of water quality,the assessment frame method of F-IBI was employed to diagnose the health state of its aquatic ecosystem by the IBI data matrix of natural fish community.After the screening of the 13 candidate indicators,discriminating ability and correlation analysis,the assessment system of F-IBI was finally built for Doushui Lake and 4 valid indicators for calculating its F-IBI values were determined(A1-species number,A5-species percentage of exotic fishes,A6-percentage of omnivorous fishes,and A9-percentage of tolerant fishes).We calculated the F-IBI values of the lake during the different periods through the score assignment method of 1,3,and 5 points.In the left and right lake area,both of the sub-lake ecosystems were all in the poor states of health,the middle lake area was in the sub-healthy state,and so the safety state of this aquatic ecosystem was not very good.(4)Based on the combined methods and data of nutrient criteria and F-IBI,the water ecological security assessment of Doushui Lake was first taken and found that the overall lake ecosystem was in the sub-healthy state.During the dry season,the water ecological safety state was worse than those of other water periods,and the water ecological security state of the lake area was better than those of the lake bays.The smaller water volume leads to insufficient dilution of pollutants,resulting in the phenomenon that the water ecological security state in the dry period was worse than those of other water periods.A large amount of sewage entering the bay via the tributaries was one of the major causes for the poor health condition of the lake bays.Therefore,the strict control of wastewater containing nitrogen and phosphorus into Doushui Lake was suggested to be the most important measures to protect the water ecological safety of the lake.