| The Pengxi River is the largest sub-tributary of Yangzi River in the north shore of the Three Gorges reservoir areas. Since the Three Gorges reservoir began to store water in 2003, "algae bloom" occurs in the Pengxi River for several times. Besides,2009 is the first year after the Three Gorges reservoir areas carried on pilot water storage (store to 172.3m). In order to explore the influence of water level's rising on the Pengxi River's water environment and the structure of plankton populations, water samples were taken in spring (March), summer (July) respectively, the two "algae bloom" high-incidence seasons in 2009. Sampling sites included in five sampling locations from upstream to mouth of the Pengxi River, namely, YangLu (YL) (E108°32'58", N31°5'55"), GaoYang (GY)(E108°40'33",N31°5'29"), HuangShi (HS)(E108°42'30'',N31°0'31"), ShuangJiang (SJ) (E108°39'44",N31°57'11"), and HeKou (HK) (E108°38'52") N30°57'11"). Water quality, E. coli, bacterial, the buildup and distribution of phytoplankton and the buildup and distribution of zooplankton were studied by Margalef (d), Shannon-Wiener(H), and Pielou(J) indexes to evaluate the eutrophication in order to figure out causes of eutrophication in the backwater area of the Pengxi River. The consequences are following:(1)In spring of 2009 in the backwater area of the Pengxi River, the concentration of Total Nitrogen (TN) was 1.8-2.7 mg.L-1 and the concentration of Total Phosphorus (TP) 0.02-0.09 mg.L" The ratio of Nitrogen and Phosphorus (N/P) was 28.7-98.9 and P is limiting factor of Algae breeding. Both of them are over the threshold of water eutrophication and water was in a moderate-severe eutrophication. But in summer, TN is 0.38-0.70 mg.L-1 and TP is 0.007-0.060 mg.L-1. N/P was 11.5-56.4 and P is limiting factor of Algae breeding. The figures show water quality was better in summer than that in spring. The concentration of Total Nitrogen (TP) increased from upstream to downstream in spring and summer in the backwater area of the Pengxi River.(2) E. coli test results of 2009 in the backwater area of the Pengxi River:the mean of E. coli of among sampling sites was in 110-16000 cfu/L in spring while 20-16000 cfu/L in summer but decreased in most sampling sites comparing with spring data. The states of being polluted were serious in spring and moderate in summer.(3) Bacteria number of sampling sites had a range of 300-15850 cfu/mL in spring, which was 1/9-1/16 folders the data in summer, which was 380-980 cfu/mL. The number of germ in each section in spring is lager than the number in the same section in summer. The data shows again that the polluted degree of water body in spring is higher than that of summer.(4) 7 phylum 57 genus 100 species phytoplankton were found, with Bacillariophyta cover 17 genera including 33 species, Chlorophyta 24 genera 40 species, Cyanophyta 7 genera 10 species, Dinoflagellate 2genera 3 species, Euglenophyta 4 genera 10 species, Xanthophyta 2 genera 2 species and Cryptophyta 1 genus 2 species. The results show Bacillariophyta and Chlorophyta were the dominant populations in the region. In spring, phytoplankton community was dominated by diatom algae-green algae, with the former was more than the latter in biomass. In summer, phytoplankton shows type of diatoms-blue-green algae, with more green algae. In spring of 2009, rust red "spray" occurred in Gaoyang site, the slowest water velocity part in Pengxi, and algal density was as high as 103.6×105 cells/L. The values of the monitoring sections calculated by d, H, and J were 0.68-1.70, 0.99-1.44, and 0.228-0.345 respectively in spring and 1.0-1.91,1.24-2.10, and 0.30-0.49 in summer. In conclusion, water environment in backwater area of Pengxi River was in moderate-severe eutrophication in spring and in moderate degree of eutrophication in summer of 2009. Water quality in summer was better than that in spring.(5) 4 groups including 40 genera and 78 species zooplankton were found in the survey duration in the backwater area of the Pengxi River, with Protozoa covering 7 genera and totally having 7 species, Rotifera 21 genera 47 species, Cladocera 7 genera 17 genera and Coepoda 3genera 7 species. Rotifera was the species with absolute advantage in each section both in spring and summer. In spring, the species with absolute advantage were few but the degrees of dominance were very high, which is a important features of eutrophication. Asplachna priodonta Gosse, Keratella quadrata, Brachionus calyciflorus, Amoeba radiosa, Keratella cochlearis, Bosmina longirostris and Polyarthra vulgaris, etc were regarded as the direction species of eutrophication. In summer, the species with absolute advantage were more with lower degrees of dominance, showing a better environment and less pollution comparing with situation in spring. Among the species of advantage there were also Brachionus calyciflorus, Ascomorpha ecaudis, Polyarthra vulgaris, Epiphanes senta.In spring in the Pengxi River, the slow water velocity, the suitable luminous intensity and temperature, and the enrichment of nutritive salts were the direct causes of the frequently occurred "algae bloom". The living creatures in each site can make use of the temporary accumulation of phosphorus and then likely cause "algae bloom". Correspondingly, in summer some factors helpful for accumulating nitrogen, namely, the increasing of nitrogen fixation due to enough illumination and the nitrogen poured from the farmland runoff are raising the possibility of the occurring of "algae bloom" in great degree.
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