Jialing River is the largest tributary of the Three Gorges Reservoir Area and an important water resource of Chongqing urban area. Diatom density in the confluence section of the Jialing River to the Yangtze River was continuously investigated from August 2005 to March 2008 in order to study the occurrence regularity of Diatom bloom, which can be regarded as a sign of water quality deterioration. The results indicated that Cyclotella asterocoststs was the dominant algae species (0.02 ＜ Y ＜0. 28 ), which annually proliferated quickly and led to water blooms accompanied by conspicuous peaks of algae density at the turn of winter into spring (from late January to early March ), while disappearing after every April. The water bloom was particularly serious in Ciqikou and Chaetianmen, which had higher contents of pollutants. The quantity of Cyclotella asterocoststs reached 269 × 104 L-1 and accounted for 92％ of all algae in the right bank of Ciqikou, especially in March 2008. Due to the short generation time (5-7 d), Cyclotella asterocoststs is able to proliferate rapidly and become the dominant species in the confluence section of Jialing River if given suitable biotope.%嘉陵江是三峡库区最大支流,是重庆主城区重要的饮用水源,其主城段硅藻水华的发生预示着水质下降.2005年8月-2008年3月连续对嘉陵江出口段水体水华硅藻密度进行监测并对数据进行分析.结果表明:水华优势藻种为星肋小环藻(原变种)(Cyclotella asterocoststs),于每年的冬、春之交(1-3月)发生,生长速度快,发生数量有明显峰值并在总藻比例中绝对占优(0.02＜优势度＜0.28),4月以后水华现象消失直至次年同期再次发生,具有明显的年规律性.水华发生时,污染物含量较高的磁器口右岸与朝天门右岸2个采样点的水华现象严重,尤其在2008年3月,磁器口右岸小环藻密度达到269×104 L-1,占同期总藻密度的92%.较短的生活世代(5～7 d)使小环藻在适宜的生境中迅速增殖并在短期内迅速成为嘉陵江出口段富营养化敏感水体水华的优势藻种.
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