中文摘要：

于2009年9～11月在贫营养型的大型深水水库（流溪河水库）中进行了围隔（体积约为85 m3）实验,分析了罗非鱼对浮游植物的影响.根据水库中现有鱼类的生物量,设置加鱼处理组（2 g/m3）和无鱼对照组两组.每组有3个重复,实验开始后每周采样一次,测定水质与采集浮游生物定性和定量样品.实验结果表明,加鱼处理组和对照组的总氮、可溶性无机氮没有明显差别,实验后期的总磷与正磷有一定差别,处理组略高于对照组.加鱼处理组浮游植物丰度及生物量、叶绿素a的含量显著高于对照组（P〈0.05）,且透明度低于对照组,个体小于30μm的浮游植物种类的丰度与生物量较明显增加,浮游动物丰度显著低于对照组（P〈0.05）,但是生物量差异不显著（P〉0.05）.总的来说,在贫营养条件下,低生物量的罗非鱼对水库浮游植物群落结构仍有着明显的影响,罗非鱼排泄产生的上行效应大于摄食产生的下行效应.

英文摘要：

To examine the effect of tilapia（Oreochromis nilotica） on phytoplankton assemblage in oligotrophic water,an enclosure experiment was performed in the Liuxihe Reservoir,Guangdong,China,a large and oligotrophic water body,lasting for six weeks from September 10 to November 14 of 2009.Six large enclosures were used,among which three were for fish treatment（2 g/m3） and three for control（no fish）.Each enclosure had a water volume of 85 m3.Water samples were collected every week for plankton and nutrient analysis.The results showed that there was no significant difference in TN and IDN and the biomass of zooplankton between the fish treatment and the control,TP and PO4 were higher in those of fish treatment only in the last three weeks than in those of control.However,the abundance and biomass of phytoplankton and chlorophyll a concentration were significantly higher in those of fish treatment than in those of control（P0.05）.The presence of tilapia resulted in the change of species and size structure of phytoplankton assemblage,and small-sized species（less than 30 μm） markedly increased and the abundance of zooplankton markedly decreased（P0.05） at the end of the experiment.In conclusion,tilapia had a significant impact on the structure of phytoplankton assemblage,and the bottom-up effect on phytoplankton from nutrient excretion of tilapia was stronger than top-down effect from its grazing.