淀山湖富营养化过程的统计学特征

湖泊营养物输入及响应指标的统计学规律正在受到越来越广泛的关注。对淀山湖在不同富营养化阶段和近期总磷TP、总氮TN和叶绿素Chl a的频率分布以及TP-Chl a关系的经验方程进行了分析,结果表明:(1)淀山湖TP、TN和Chl a的平均浓度和离散程度随着湖泊富营养化程度的加剧而增加,其中以Chl a的增幅最大;(2)在富营养化条件下,即使营养物TP得到一定程度的控制,Chl a大于15μg/L的概率继续增加了20%以上。仅仅削减营养物的峰值,对降低湖泊初级生产力水平的贡献有限;(3)TP-Chl a对数回归方程的斜率随湖泊富营养化程度的升高而增加,由20世纪80年代的0.54增加到目前的2.46。淀山湖营养物输入及响应指标的统计学特征,可以用来表征水体富营养化程度,评价湖泊生态恢复的进程和效果,为湖泊营养物基准和标准的制定提供最为实际的统计学支持。

Statistical characteristics of eutrophication process in Dianshan Lake

The statistical characteristics of total nitrogen(TN) and total phosphorus(TP) nutrient input and their responses during the lake eutrophication process are of increasing concern.An increase in the mean(μ) and/or variance of TN and TP input may increase the probability of exceeding the critical threshold at which algal blooms proliferate. Dianshan Lake,a shallow temperate lake in the Taihu Plain,is an important source of drinking water for Shanghai and is subject to a subtropical monsoon climate.Its trophic state transitioned from eutrophic to hypereutrophic in 1999,causing the lake to suffer from bluegreen algal blooms ever since.Beginning in 2004,some restoration measures have been taken to reduce nutrient inputs into the lake.However,the immediate reduction of nutrient inputs did not instantly reduce the frequency of algal bloom events. In order to understand nutrient input and their responses in a lake ecosystem from a statistical point of view,the frequency distributions of TP,TN and chlorophyll a(Chl a),and the TP-Chl a relationships during two different trophic stages(eutrophic stage: 1986—1999;hypereutrophic stage: 1999—2009) of the lake were analyzed based on long-term data from 1985 to 2009.The means and variances of these parameters were compared with those after restoration measures were undertaken on the lake in 2004. The means and variances of TN and TP increased with the eutrophication process.The probabilities of TN and TP values exceeding the Surface Water Standard for Class V Waters(TN = 2.0mg/L;TP = 0.2mg/L) increased from 62.8% and 12.1% during the eutrophic stage to 91.4% and 51.9% during the hypereutrophic stage,respectively.The increase in Chl a concentration was even greater,with the probability of exceeding the limit of 15 μg/L increasing from 0.5% during the eutrophic stage to 67.5% during the hypertrophic stage. The log-log relationship between TP and Chl a was calculated as per Organization for Economic Co-operation and Development(OECD) methods using three data sets: 1985—1986,2000—2004 and 2004—2008.All of the observed patterns fit the TP control paradigm well,which indicated that TP had been the primary limiting factor.Over the 24-year eutrophication period examined,the slope of the TP-Chl a log-log relationship increased from 0.54 during the 1980s to 0.83 in the early 2000s and then further to 2.46 by 2008,which might indicate an increasing of proportion of bluegreen algae and the frequency of their blooms. After restoration measures were undertaken in 2004,mean TP stayed constant and the discrete degree(variance decreased to 50%,whereas the probability of Chl a exceeding 15 μg/L continued to increase by an additional 20%.Under the hypereutrophic condition,a reduction in peak nutrient values or variance only had a limited contribution to primary algal production.Further study on the effects of TN and TP reductions in eutrophication control is necessary. Here,we showed that statistical parameters such as mean,variance,slope and frequency distribution of nutrients and their responses were able to describe the eutrophication process of Dianshan Lake.These characteristics also offer statistical support for lake restoration assessments,risk assessments for Harmful Algal Bloom and nutrient criteria and standard development.