苏南地区湖泊群的富营养化状态比较及指标阈值判定分析

Abstract

借助综合营养状态指数法、箱须图法以及拐点探测分析法，分析了苏南地区11个主要湖泊的富营养化特征以及各湖之间的差异性，并探讨湖泊由轻度富营养化向中度富营养化状态转换的相关指标阈值。结果显示苏南地区各湖泊的营养物水平存在明显的阶梯状特征，不同湖泊所处的富营养化阶段有所不同。典型封闭型湖泊水源地尚湖与傀儡湖水质最好，总体为Ⅱ-Ⅲ类，评价为中营养状态。典型过水性湖泊淀山湖与澄湖水质最差，TN和TP均劣于Ⅴ类，评价为中度富营养状态，其他湖泊属于轻度富营养化状态。基于拐点分析，提出苏南地区湖泊群富营养化状态由轻度向中度转变的指标阈值：COD_Mn为4.7mg/L，NH₃-N为1.0mg/L，TN为2.3mg/L，TP为0.125mg/L，Chl-a为25μg/L，透明度为50cm；尚湖和傀儡湖处于富营养化最轻的下游区，元荡和阳澄湖目前处于敏感的拐点区域，而淀山湖、澄湖、长荡湖等其他湖泊处于富营养化程度较重的上游区。苏南湖泊群的富营养化状态差异性主要是由不同人为干扰强度和水资源管理模式引起的，实现管理目标由资源利用向资源保护合理转变是解决富营养化问题的关键。;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 10km² to 2,428 km² and 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 (NH₃-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 (<em>TLI</em>_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, NH₃-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 moderately-eutrophicated 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 <em>TLI</em>_c. As the typical water-carrying 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 NH₃-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.