Study On Characteristic Of Phytoplankton Communities And The Effecting Factors On Backwater Area Of Xiaojiang River In Three Gorges Reservoir

The water level of Three Gorges Reservoir moved up to 156m at the end of October 2006, representing Three Gorges Project had begun its primary operation period. In order to realize the actuality of water environment and the seasonal variation of phytoplankton community of the backwater area of Xiaojiang River, the author has been observing the area regularly since the beginning of 2007. Based on the original observation on the backwater area of the branch in Three Gorges Reservoir (Xiaojiang River), the Characteristic of phytoplankton communities and the effecting factors had been studied via field monitoring of one year.From March, 2007 to March, 2008, In general, 7 phyla, 57 genera and 143 species (including varieties) of phytoplankton community on the backwaters of Xiaojiang River were identified. The proportion of the community composition was: Chlorophyta, 40.6%, includes 19genera 58species; Bacillariophyta, 36.4%, includes 19genera 52species; Cyanophyta, 14.0%, includes 11genera 20species; Euglenophyta, 3.5%, includes 3genera, 5species; Cryptophyta 2.5%, Chrysophyta, 2.1%, includes 2genera, 3species; Xanthophyta, 2.1%, 1genera, 3species; and Dinophyta, 1.4%, includes 2genera, 2species. The mostly dominant taxa of the phytoplankton community were Asterionnella Formosa, Synedra ulna and Cyclotella meneghiniana, in Bacillariophyta; Coelastrum microporum, Chlorella vulgaris, Scenedesmus bijug, Aclinastrum fluviatile and Dictyosphaerium pulchellum in Cyanophyta; Anabaena flos-aquae and Aphanizomenon flos-aquae in Cyanophyta etc. The species of Chlorophyta, Bacillariophyta and Cyanophyta achieved 85% of total number in every sampling, which constructed the principle part of phytoplankton community on the backwater area of Xiaojiang River. The variety of each site were almost the same, the general succession mode with time was: Chlorophyta - Bacillariophyta (early spring)→Cyanophyta - Chlorophyta (spring& early summer)→Chlorophyta– Cyanophyta - Bacillariophyta(summer)→Chlorophyta– Bacillariophyta (autumn)→Bacillariophyta– Chlorophyta(winter).Chla on the backwater of Xiaojiang River has obvious seasonal discrepancy. It ranged fluctuates between 0.27~48.16mg/m3. The average value of Chla was 8.15mg/m3.The phytoplankton cell densities and biomass fluctuates between 0.977×105~90.000×105cell/L and 0.126~34.813mg/L. The lowest value appeared at Hekou site in winter while the highest one appeared at Gaoyang site in spring. By Shannon- Wiener index, Margalef index,Pielou index, the estimate results showed that the trophic levels of the area were all meso-eutrophic during the most time of whole year. The water quality level of the upriver area was relative better.The simulant formula of density,biomass and water temperature on backwater of Xiaojiang River was put forward in this study: lg y1 = 0.0619T + 4.643, lg y 2= 0.0707T ? 1.4161. The relative analyse shows that the growth of phytoplankton has remarkable positive correlation with water temperature. It can be said that the water temperature is the key factor that affect the growth of phytoplankton and its community in this area. Meanwhile, the simulant formula of density,biomass and SD was also put forward: lg y1 = ? 0.2191S + 6.3226, lg y 2= ? 0.3067 S+ 0.591. It shows the growth of phytoplankton has remarkable negative correlation with SD. All the nutrition indexes, doesn't has remarkable correlation of the growth of phytoplankton It is indicated the nutrition has little control of the growth of phytoplankton in this area.Through this study, the growth and community structure of phytoplankton on backwater area of Three Gorges Reservoir after the water level moved up to 156m was realized, and the relation between phytoplankton and the key facrots was analyzed, which could provide necessary basic for finding out the eutrophic occuring condition and mechanism of typical branch river in Three Gorges Reservoir.