Abstract:

Since the first impoundment of Three Gorges Reservoir,trophic state,algae biomass and the structure of community has changed significantly.Series of investigating results during different impoundment stages showed that eutrophication of tributary river deteriorated and differed on time and space.The eutrophication in spring and summer were more serious than that in autumn and winter,making the general trophic state of ecosystem shift from oligotrophicmesoerophic status to mesoerophiceutrophic status.Nutrient was not limited factor for algae growth because of high and relatively stable nutrient level during different impoundment stages in tributary river.Therefore,the reason for eutrophication was the reduced velocity of flow,making transparency rise，light transmission increase and growing condition improve.Then it brought algae biomass increased largely and water bloom occured frequently as well as the dominant algae diversified. llariophyta,Pyrrophyta,Crytophyta,Cayanophyta and Chlorophya could be dominant in algae population.Generally speaking,the percentage taken by Bacillariophyta and Pyrrophyta decreased,as well as Crytophyta,Cayanophyta and Chlorophya increased.Dominant algae succession trend was from rivertyped to laketyped.However,with the evaluation of the water level,the general occuring rate of bloom decreased