[Spatiotemporal variation of epilithic algae in Xiangxi River system].

Xiangxi River system is the greatest branch in the Hubei reservoir area of the Three Gorges reservoir. In this paper, the epilithic algae in the River and its three major tributaries were investigated from July 2005 to June 2006. A total of 218 taxa were identified, including 183 species of Bacillariophyta, 24 species of Chlorophyta, 10 species of Cyanophyta, and one species of Xanthophyta. The diatom Achnanthes linearis was the most predominant species. The richness and Shannon-Wiener diversity indices varied significantly (or almost significantly) over time and space, and the total average values were 32 and 1.54, respectively. The total averages of annual epilithic algal density and chlorophyll a concentration were 8.75 x 10(9) cells x m(-2) and 14.62 mg x m(-2), respectively. There were significant differences in the algal density and chlorophyll a concentration among different sampling sites, and the maximum values were observed in Gufu River tributary, which were one order of magnitude higher than the minimum ones in Jiuchong River tributary. The algal density and chlorophyll a concentration tended to be higher in winter and spring than in summer and autumn, but no significant differences were observed in various seasons. Epilithic algal density and chlorophyll a concentration were significantly negatively correlated with elevation and water current, but positively correlated with the total nitrogen concentration in water.