长江口及邻近海域富营养化趋势分析及与环境因子关系

Analysis on the trend of eutrophication in the Changjiang(Yangtze River) Estuary and in the adjacent East China Sea and its relation to environmental factors

依据2007-2009年春、夏季在长江口及邻近海域(包括长江口、杭州湾和舟山渔场)的调查监测资料,采用富营养指数法、潜在性富营养化法和有机污染指数法对该海域的富营养化状况、时空分布特征及与环境因子的关系进行了分析评价。富营养指数法计算结果表明:富营养化覆盖比例很高,达到70%以上;在春季,该海域的富营养化程度呈现上升趋势,所占比例从2007年的77.0%上升到2009年的89.8%,在夏季最低达到89.3%,最高到100.0%。3年来,富营养化在春、夏2季基本上处于逐年增加趋势,富营养化趋势越来越明显。在春季富营养指数平面分布呈由近岸向远海逐渐递减的趋势,梯度分布明显,其中,杭州湾富营养化情况较严重;夏季其平面分布与春季类似,但长江口和杭州湾两个水域的富营养指数的等值线都比较密集,夏季的富营养化情况较为严重。潜在性富营养化法计算结果表明:该海域富营养级主要集中在III级(富营养)和V P级(磷中等限制潜在性富营养);春、夏2季时,长江口V P级所占比例均从0.0%增加到66.7%,受磷限制性富营养化程度越来越高。另外,营养盐结构显示,N/P比值从9.1到50.9,营养盐比例明显不平衡,势必影响浮游生物的生长。有机污染指数法计算结果表明:该海域有机污染指数主要处于5级(重污染),水质污染严重;长江口春、夏2季5级的比例均从50.0%分别上升到83.3%和100.0%,杭州湾分别从60.0%和80.0%均上升到100.0%;舟山渔场分别从22.2%和66.7上升到66.7%和77.7%,说明受污染程度逐年上升。另外,3年来春、夏2季长江口有机污染指数达5级的占50%以上,杭州湾60%以上,舟山渔场20%以上,杭州湾受污染情况最为严重,其次是长江口,舟山渔场最小。在春、夏2季有机污染平面分布表现为污染程度从西向东、从北向南逐渐降低。富营养指数和有机污染指数与盐度、溶解氧、pH均不呈线性相关。调查海域富营养化水平的时空变化主要是由于沿岸入海河流,浮游植物等所引起。

Eutrophication has become an overwhelming phenomenon and an increase in nutrient concentration, noxious algal blooms or hypoxia in near-bottom waters have been of more frequent occurrence in the Changjiang （Yangtze River） Estuary and in the adjacent East China Sea. In spring and summer 2007-2009, six comprehensive investigations were undertaken in three zones （including Changjiang Estuary, Hangzhou Bay and Zhoushan fishing ground） in the estuary and in the adjacent sea. The eutrophication index, potential eutrophication and organic pollution index were used to present the eutrophication status. The results indicated that more than 70.0% of the waters were in an eutrophication state. Eutrophication status of the studied area showed an increase trend over the period from 2007 to 2009. The percentage of waters in eutrophication state was 77.7% in spring of 2007, and, has increased to 89.3% in spring of 2009. The percentage of waters in eutrophication state ranges from 89.3% to 100% in summer. Spatially, eutrophication index clearly decreased from the alongshore to the open sea. The Hangzhou bay was the most serious polluted sea in spring. The isolines of eutrophication index were dense both in Changjiang estuary and Hangzhou bay in summer, thus the eutrophication status was more serious in summer than those in spring. Based on potential eutrophication analysis, the eutrophication levels of the studied area mainly centralized on level III （eutrophication） and level VP （moderate P-limited potential eutrophication）. In spring and summer, the percentage of area with level VP increased from 0.0% to 66.7%, which indicated the P-limited eutrophication level was more serious in summer. In addition, N/P ration has increased consistently from 9.1 to 50.9, which could affect the structure and growth of marine phytoplankton. In addition, the waters of the Changjiang Estuary and the adjacent East China Sea were in a serious pollution （main level five） state, according to organic pollution index. The percentage of waters in a serious pollution state was 50.0% in the Changjiang estuary in 2007, and has increased to 83.3% and 100.0% in 2009; and has increased from 60.0% and 80.0% to 100.0% in the Hangzhou bay, increased from 22.2% and 66.7% to 66.7% and 77.7% in the Zhoushan fishing ground, which indicated that the pollution level has increased gradually. Otherwise, more than 50% of the waters in the Changjiang estuary were belonging to the organic pollution index of level five, in the Hangzhou bay, 20% in the Zhoushan fishing ground. Thus, the organic pollution was most serious in the Hangzhou bay, moderate serious in Changjiang estuary, and least in the Zhoushan fishing ground. Spatially, the organic pollution index gradually decreased from west to east, from north to south. No significant correlations were observed between the eutrophication index, organic pollution index and the water salinity, dissolved oxygen, or pH. The spatio-temporal distribution of the eutrophication in the studied area was mainly influenced by the runoff and phytoplankton.