三峡水库“水华”成因初探

2004年3月、5月、7月、8月、10月和2005年5月对长江干流29个站点进行了5次调查。2004年3月（旱季）在三峡库区坝前（秭归）发现藻类“水华”，藻类密度达2.73×106 cells/L，优势种类为拟多甲藻。2004年8月（雨季）和2005年4月（旱季）沿香溪河下游及河口区以及香溪河口到三峡大坝干流江段进行了2次6个断面分层调查，两次调查中在香溪河下游以及香溪河口区发现了严重的藻类“水华”，藻类密度高达1.87×107 cells/L 和1.67×107 cells/L，优势种分别为蓝隐藻（1.84×107 cells/L）和美丽星杆藻 (1.34×107 cells/L)。相关分析结果表明：三峡库区干流藻类数量和生物量与水库的出水流量有着显著的负相关(Spearman，r=-1.000, r=-0.900, p<0.05)，而与可溶性营养盐（NO3 N, PO4 P, SiO3 Si）的浓度无显著的相关性；在2004年7~8月（雨季）香溪河下游及河口区浮游植物生物量与主要营养盐（NO3 N, PO4 P,SiO3 Si）的浓度呈显著负相关（Spearman,p<0.01, p<0.05,p<0.01, n=21）；在2005年4月（旱季）该河段藻类密度与主要营养盐（NO3 N N,SiO3 Si）呈显著负相关 (Spearman, p<0.05，p<0.01,n=28)，但与PO4 P无显著的相关性。香溪河口到秭归的坝前库区河段藻类数量与主要营养盐（NO3 N, PO4 P,SiO3 Si）没有显著相关性(Spearman, │r│<0.2, n=20)。然而香溪河下游及河口区主要营养盐（NO3 N, PO4 P, SiO3 Si）浓度却低于长江干流。可以推断三峡库区蓄水后干流和支流发生“水华”的最主要原因是筑坝后库区内水动力条件的改变而非营养盐浓度较高。随着三峡工程的全面完工，库区内水体滞留时间的进一步延长，三峡库区水体富营养化趋势将会进一步加剧。

PRELIMINARY STUDY ON ALGAL BLOOMS WITHIN THE THREE GORGES RESERVOIR

Surveys during March, May, July, October 2004 and May 2005 at 29 stations along the Yangtze River were processed. In March 2004 (dry season) bloom occurred in Zigui, just before the Three Gorge Dam (TGD); algal abundance reached 2.73×106 cells/L, and the dominant species was Peridiniopsis sp. (9.16×105 cells/L). During August 2004 ( rainy season) and April 2005 ( dry season) blooms also happened in the downstream of the Xiangxi River and the river mouth, and the algal abundance reached 1.87×107 cells/L and 1.67×107 cells/L respectively. The dominant species were Chroomonas acuta（1.84×107 cells/L）and Asterionella formosa (1.34×107cells/L). Correlation analysis indicated that algal density in the mainstream has significant negative correlation with the main current discharge (Spearman, r=-0.900, p<0.05), but not with the main resolvable nutrients（NO3 N, PO4 P, SiO3 Si）(Spearman, r=-0.031; r=-0.116; r＝0.262; n=14). However, in the downstream of the Xiangxi River and the river mouth, significant negative correlations were observed between phytoplankton abundance and nitrate (Spearman, p<0.01, n=21), phosphate (Spearman, p<0.05, n=21) and silicate (Spearman, p<0.01, n=21) in the rainy season. In the dry season, the similar correlations were also observed with nitrate (Spearman, p<0.05, n=28) and silicate (Spearman, p<0.01, n=28), but not with phosphate. In the mainstream of the Three Gorges Reservoir (T〖JP2〗GR), there were no significant correlations between phytoplankton abundance and the main nutrients (Spearman, │r│<0.2, n=20).Whereas the concentrations of the main resolvable nutrients in the mainstream of the Yangtze River are higher than those in the Xiangxi River and its river mouth, and the occurrence of the blooms within TGR could be attributed to the variations of the hydrological condition after damming rather by the nutrient concentrations. With the completion of the Three gorge Project (TGP), the residence time of the water body will be prolonged, and the eutrophication within TGR will be aggravated.