| Water eutrophication is one of the important environmental problems which human beings face. The three gorges project is a large-scale water conservancy project which integrates various functions including flood control, generate electricity and ship. Thus, the water environment security of this artificial reservoir has very important significance in improving social economic development of the reservoir area and middle and lower of Yangtze River. The Jialing River is the biggest anabranch in Three Gorges Reservoir (TGR); also, it is one of the most important drinking water resources of Chongqing. Water quality condition of local water body of outlet section of the Jialing River had become deterioration. Water bloom which was mainly caused by Cyclotella had broken out frequently during recent years. With the operation of TGR, the flow velocity slowed down, nutrient dilution effect was weakened. As a result, both algous activity space and occurrence degree had an increasing trend. The study on Cyanobacteria bloom which broke out following a regular basis in lake or reservoir was common at home and abroad, but the study on diatom bloom which broke out in river basin was obviously lag. This phenomenon not only foreshowed water quality deterioration of outlet section of the Jialing River, but also put forward strict demand for water pollution control of TGR.In order to study the occurrence regularity and biological toxicity of Cyclotella bloom which broke out in outlet section of the Jialing River, field investigation and indoor test were carried out aimed at related water of ChaoTianMen, HuaLongQiao and CiQiKou from January 2007 to March 2008.Used correlation analysis to find out the relation between these environmental factors and algae, then, cultivated Cyclotella which caused diatom bloom for toxicity test. The principal conclusions of this paper were obtained as follows.①The appraisal result showed that the Cyclotella asterocoststs which belongs to Bacillariophyta, Centricae, Coscinodiscales, Coscinodiscaceae, Cyclotella was the dominant algae which caused the water bloom in outlet section of the JiaLing River in early spring.②The occurrence time of Cyclotella bloom showed a clear yearly periodicity regularity. The rapid proliferation of Cyclotella would cause water bloom in outlet section of the Jialing River in early spring (from January to March) each year and its amount had a statistically evident peak value. The proportion of Cyclotella density to total algae density reached to 60%~80%.③Physical and chemical indexes and the amount of Cyclotella were treated and statistically analyzed with SPSS16.0 software. Statistical analysis showed that Cyclotella bloom is result from the interaction of physical and chemical factors. That is to say, it might occur under these preconditions including lower velocity of flow (0.05-0.20 m/s), adequate nutrients (total nitrogen concentration is 2.17-2.24 mg/L and total phosphorus concentration is 0.7-0.14 mg/L), enough illumination time (11.24 hours), lower water temperature (8-11℃), and so on. A short generation time (between two and three days) could made itself proliferated rapidly in suitable living environment and then became the dominant algae species in outlet section of the JiaLing River in short order. It could be concluded that maybe water temperature was the dominant factor for the occurrence of Cyclotella bloom under certain conditions.④The crushing effect of silica shell of Cyclotella was the guarantee of toxicity test and it had a close bearing on the quality monitoring of drinking water. Ultrasonic cell-break was considered to the best crushing method for Cyclotella in terms of analyzing and comparing the crushing effect of grinding method, freeze-thaw method and ultrasonication. The result of orthogonal test showed that the frequency, power, total action time of ultrasonic, unit of action time and time interval of ultrasonic cell-break method under the optimum condition were 22 kilohertz, 200 wattages, 20 minutes, 15 seconds, 5 seconds, respectively.⑤The result of toxicity test showed that the food intake, vital movement and growth status of white mice were in order in the observation time (48 hours) after injected with cytolymph of Cyclotella, and there were no poisoning symptom or death phenomenon. This result could meet the requirement of State Pharmacopoeia (2005 edition). Therefore, it was considered that the Cyclotella which caused water bloom in outlet of the Jailing River does not have biological toxicity.
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