Abstract: According to the aquatic ecosystem investigations in October of 2003 and April of 2004, the composition and quantity of phytoplanktons after impounding of the Three Gorges Reservoir were studied. We discussed the influence of water dynamics condition, water nutrient level on the phytoplankton components and quantity. The results indicated that dominated species of phytoplankton in investigated area were diatoms, and the quantity of phytoplankton investigated on October of 2003 was not obviously different from that before impounding, and was between 2.02×104 individuals/L and 31.6×104 individuals/L. The quantity of phytoplankton increased obviously between 3.18×104 individuals/L and 16 288×104 individuals/L on April of 2004. According to the change of hydraulic condition, the reservoir is plotted to three types of waterbody; the upstream of Wushan County of Yangtze River belongs to rivertype, and section from Wushan County to the Three Gorges Reservior is transitiontype, and the estuaries and backwaters of tributary located in section from Wushan County to the dam are laketype. The phytoplankton growing in rivertype water region has not increased obviously after the impounding, and those in laketype and transitiontype water region have increased notably. Therefore, the transition type and lake type are the eutrophication sensitive regions in the Three Gorges Reservoir.

Key words: Three Gorges Reservoir, impound, phytoplankton, quantity, structure