Rotifer species are cosmopolitan and the endemism is very low.They are highly susceptible to physical and chemical changes in their environment due to their small size and permeable integument,and they are known to be an important link between the microbial loop and higher trophic levels,so rotifer is an excellent indicator of the water status of a lake.Yangcheng Lake is one of the most important freshwater lakes in China.The total area of the lake is 119.04 km²,aquatic plants are dense providing a food source for crabs and the lake yields high numbers of the Chinese mitten crab(Eriocheir sinensis).Currently,although the ecological effects of the pen culture on the environment have received extensive attention,expert mainly focus on aquatic plants,physical and chemical factors,environmental effects and benthic animals,but rarely reported on the rotifer community structures.To date,no studies have been done on rotifers in Yangcheng Lake in detail.Thus,the aim of this study was to analyze the spatial and temporal variation of rotifer community structure and species diversity in net-pen closing and lake areas,in order to provide the data and theoretical basis for aquatic ecosystem of Yangcheng Lake,biodiversity conservation and sustainable development of ecological-pen-culture.Rotifer community structure and several feature parameters of water quality were investigated in 3 controlled sites in net-pen closing and lake areas of Yangcheng Lake,during June 2008 to May 2009.The relationship of rotifer community structure and environmental parameters was analyzed,various parameters were used to assess the water quality.The totals of 83 rotifer species were collected,belonging to 30 genera and 12 families.The study showed that the annual average density and biomass of 3 sites in netpen closing were lower than those in lake areas,density exhibited significant positive correlations with temperature.The CCA biplot of samples and environmental variables showed that rotifer assemblages followed a water temperature gradient,suggesting that water temperature was the most important factor in determining seasonality,but the pH was the lowest.Different months were affected by different environmental parameters,there was no differentia in sites except April.The CCA biplot of species and environmental variables shows that the warmstenotherm species like Trichocerca sp.,Anuraeopsis fissa,Hexarthra mira in the downleft quadrant have strong positive correlations; the cold-stenotherm species like Keratella quadrata,Synchaeta sp.in the top-right have strong negative correlations; the eurytherm species like Polyarthra trigla and Keratella cochlearis are in the middle of the biplot,which support these two species are dominant ones in a year.According to the Jaccard similarity index(S_J),the high similarity in dominant species occurred between net-pen closing and adjacent lake areas.Water quality showed that 6 sites were all eutrophication and β-α mesosaprobic,although there was a slight degree in sites C₁ and A₁,no significant differences in 6 sites,which was similar to the results by TSI.