| 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 in different time section and the effecting factors had been studied via field monitoring for one year.According to the changes of the water level in XiaoJiang, in order to explore changes in phytoplankton community scheduling to the water level,the annual will be divided into March-May June-September and October to February the three study periods. This study explored the phytoplankton species composition, biomass, process of changes and characteristics in the composition of each time section; Based on characteristics of the population in each time section,comparative study was done to find the of the reasons for the that formation; The main factors impacting the growth of algal species was also explored.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 19 genera 58 species; Bacillariophyta,36.4%, includes 19genera 52species; Cyanophyta,14.0%,includes 11 genera 20 species; Euglenophyta, 3.5%,includes 3 genera, 5 species; Cryptophyta 2.5%, Chrysophyta, 2.1%, includes 2genera, 3species; Xanthophyta,2.1%, 1 genera, 3 species; and Dinophyta,1.4%,includes 2 genera, 2 species. 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.Between March and May,the phytoplankton cell densities and biomass fluctuated between 2.972~97.116×105cell/L and 0.701~28.121mg/L,and significant changes appeared both in the composition of cell density and biomass during this period, that is, Cyanophyta becomes dominant phyla instead of Chlorophyta. Between June and September,the cell densities and biomass of phytoplankton fluctuated between 3.000~72.069×105cell/L and 0.544~34.813mg/L,and Cyanophyta and Chlorophyta were dominant phyla during this period. Because environmental conditions conducive to the diversification of phytoplankton populations, so the composition of cell density did not show obvious regularity during this period. Between October and February,the cell densities and biomass of phytoplankton fluctuated between 1.512~55.419×105cell/L and 0.076~5.546mg/L,and Chlorophyta and Bacillariophyta became dominant phyla alternately.The number of diatom species was dominant between March and May, 6-September and October to the number of Chlorophyta species was dominant between June and February of next year;The species number of Cyanobacteria was fewer between March and May,but the number increased slightly between June and February next year. Chlorophyta had the highest frequency of occurrence and the the dominant species of Chlorophyta appearing in high frequency mostly appeared in the June to September or October to February; The dominant species of Cyanobacteria appeared in the March-May with high frequency; Frequency of occurrence of Diatoms in March-May and October-February was significantly higher than the frequency of June to September. Dinophya became dominant species in the low frequency. During March-May , June-September , October-February , spots with the highest mean Shannon-Wiener index were Gaoyang , Quma, Hekou ; During March-May ,June-September,October-February,spots with the highest mean Margalef index were Quma,Shuangjiang,Quma;During March-May,June-September,October-February,spots with the highest mean Pielou index were Gaoyang,Quma,Hekou.The relative analyse shows that Cyanophyta,Chlorophyta and Bacillariophyta have remarkable positive correlation with water temperature. But, the adaptability for water temperature is different,Chlorophyta adapts to the widest range of water temperature, followed by Bacillariophyta, Cyanobacteria grows better in the range of 20 ~ 30℃.Cyanobacteria had better correlation with DSi/DIN, DSi/DTP than TN/TP, DIN/ TP;Chlorophyta has better adaptability on the TN/TP, DIN/DTP than Cyanobacteria. Study has shown Cyanobacteria has a strong power of phosphorus uptaking than Chlorophyta; Bacillariophyta has better adaptability on the TN/TP, DIN/DTP than less than Cyanobacteria but less than Chlorophyta. As a result of better DSi uptaking ability, when silicon uptaked by Bacillariophyta, and the silicon source of supply lagged behind, so resulting in a strong negative correlation with DSi/DIN. Cyanobacteria and the surface light intensity was positively related, light intensity distribution of the annual impact study found Cyanobacteria has a good adaptability to high light intensity. Chlorophyta and surface light green algae was positively correlated with better competitiveness comparing with other phyta. Bacillariophyta shows a strong competitive advantage after Chlorophyta, especially in low light intensity.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 factors was analyzed, which could provide necessary basic for finding out the eutrophic occurring condition and mechanism of typical branch river in Three Gorges Reservoir.
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