Abstract:

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.

Key words: Three Gorge Reservoir, hydrological conditions, algal blooms