基于拐点分析的汉江水华暴发突变与归因研究

The Evolution and Mutation Attribution of Water Bloom in Hanjiang River Based on Mutation Test Method

科学认知河流生态退化的突变过程一直是水生态学科的研究难点。该文面向汉江河流水华的演变规律和成因,建立基于拐点分析(Mutation Test)的河流水华诊断与归因分析方法。综合采用STL趋势分解法(Seasonal Trend Decomposition using LOSS)、MK突变检验法(Mann-Kendall)以及冗余分析(Redundancy Analysis,RDA)等多源统计诊断与归因分析方法,重点分析了2004—2017年2—4月汉江武汉段藻类密度的时间变化趋势和突变点,识别了水华年和非水华年期间藻类密度主要影响因子。结果表明,(1)2004—2017年2—4月汉江武汉段宗关、白鹤嘴与琴断口3个观测站藻类密度均值为0.37×10^(7) cell·L^(−1),峰值浓度为5.3×10^(7) cell·L^(−1),其中2004—2008年呈显著上升趋势,2008—2014年呈显著下降趋势,2008年前后为汉江藻类密度突变点。(2)汉江武汉段水华年主要发生在2008—2011年和2016年,2月中旬至3月上旬藻类密度最大,2008年水华事件为近20年最严重。(3)汉江武汉段藻类密度2008年前后出现峰值及拐点的主要原因为总氮、总磷、流量以及水位差的突变;其中,与非水华年相比,水华年流量下降15%,总氮、总磷均偏高2%左右。(4)较高的总磷含量、适宜的水位差及较小的流量综合导致2004—2017年春季汉江水华现象发生。总磷含量是水华年与非水华年汉江藻类生长的主要限制营养元素,水华年水位差对藻类生长影响显著。建议下一步重点开展汉江总磷源解析,监控汉江武汉段的流量,预防“顶托”作用的形成。

Scientific understanding of the mutation process of river ecological degradation is a difficulty of environmental science in recent years.This paper faces to the evolvement mechanism and cause of river algal bloom,by utilizing multiple statistical diagnosis and attribution analysis approach,including the Seasonal Trend Decomposition using LOSS(STDL),Mann-Kendall(MK)and Redundancy Analysis(RDA),comprehensively analyzed the temporal variations and abrupt change points of algal density in Wuhan section of Hanjiang River(HR)from February to April in 2004-2017.Moreover,the main driving factors of algal density were identified during the blooms years and non-bloom years.Results showed that:(1)The average algal density was 0.37×10^(7)cell·L^(−1)and the peak value was 5.3×10^(7)cell·L^(−1)at the observation stations of Zongguan,Baihezui and Qinduankou in the downstream of HR.From 2004 to 2008,algal density showed a significant increase trend,and it became a significant decrease trend after 2008 until to 2014.The abrupt transition point of algal density was happened in year of 2008.(2)The algal bloom years in Wuhan section of HR mainly occurred from 2008 to 2011 and 2016 and the bloom event in 2008 were the most serious in recent 20 years and its algal density was reached to the peck value from mid-February to early March.(3)The variation of total nitrogen,total phosphorus,flow rate and water level differences were the main reason lead to the appearance of a point of mutation.Compared with non-bloom years,the annual flow rate decreased by 15%,and the total nitrogen and phosphorus were both increased by 2%.And(4)final result indicated that the higher total phosphorus content,the suitable water level difference and the smaller discharge comprehensively led to the occurrence of water bloom in the spring of 2004-2017.TP was the major limiting nutrient factor caused algal growth during the bloom and non-bloom years and the difference of water level in bloom years has a significant influence on the growth of algae.In the next step,we should focus on the source apportionment of total phosphorus in Hanjiang River,monitor the flow in Wuhan section of Hanjiang River,and prevent the formation of“jacking”effect.