Research on the difference in eutrophication state and indicator threshold value determination among lakes in the Southern Jiangsu Province,China

Cultural eutrophication is the Earth's most widespread water quality problem,causing harmful algal blooms,fish deaths,underwater plant deaths,decreasing biodiversity,reduction of water clarity,and many other related problems. Southern Jiangsu Province( SJP),which is by far the most densely populated and most developed area of the east of the People's Republic of China,has in total 11 shallow lakes with a surface area ranging from 10km2to 2,428 km2and an average water depth of less than 2. 5m. Unfortunately,accompanying the fast economic development in SJP is the serious deterioration and eutrophication of the water environment,with high levels of algal biomass,frequent algal blooms and low water clarity,which are threatening both the water supply to these regions and their economic development. In this article, in order to investigate the eutrophication features and differences in state of 11 lakes within the same geographic area,we obtained 1090 field monitoring data sets of potassium permanganate index( COD Mn),ammonia nitrogen( NH 3-N),total phosphorus( TP),total nitrogen( TN),algal Chlorophyll-a( Chl-a) and Secchi depth( SD) of lakes from the environmental monitoring agencies attached to the EPA of Southern Jiangsu Province. All water samples were collected monthly or quarterly from 62 field sites across the 11 lakes and measured in the laboratory during the period from 1993 to 2012. We applied the method of trophic level indices( TLI c) to assess the trophic level of all lakes,using Box-Whisker plots to compare the levels with current standards and provide spatial discrimination of eutrophication indicators( e. g., COD Mn,NH 3-N,TP,TN,Chl-a,SD) among all lakes. Furthermore,change-point analysis was performed to determine the threshold values of indicators showing transition among different eutrophication levels in SJP. It was revealed that different eutrophication levels among all 11 lakes can be divided into mesotrophic,lightly-eutrophicated and moderatelyeutrophicated states. Both Lake Shanghu and Lake Kuilei,which are under strict management as semi-enclosed drinking water resource areas,had the best water quality,achieving category II to III standards according to the state standards for surface water quality( GB 18918—2002) and belong to mesotrophic lakes category based on TLI c. As the typical watercarrying lakes,Lake Dianshan and Lake Chenghu had the heaviest eutrophication level,with TN and TP concentrations recorded as an inferior Category V,classified as moderately-eutrophicated lakes. The other 7 lakes,including Lake Taihu were classified as lightly-eutrophicated. Based on change-point analysis, the threshold values for all indicators as transitional marks,from lightly-eutrophicated to moderately-eutrophicated states,were recommended as: 4. 7mg / L of COD Mn,1. 0mg/L of NH 3-N,2. 3mg / L of TN,0. 125mg / L of TP,25μg / L of Chl-a,0. 5m of SD. It is notable that Lake Yangchenghu and Lake Yuandang were lingering near the turning point between the lightly-eutrophicated state and moderately-eutrophicated state; thereby,more efforts should be made to prevent these two lakes sliding into more heavily eutrophicated states. According to the main results,there is further discussion to be carried out on the advantages and disadvantages of lake-group analytical methods,as well as the reasons that caused differences in eutrophication level among all lakes within one geographic area( e. g. SJP),such as the natural characteristics of lakes,intensity of human disturbance and water resource management strategies. It can be suggested that setting the primary management goal as water-resource-protection rather than water-resource-utilization is the key to control lake eutrophication in SJP.