Abstract:

 Quarterly investigation of the zooplankton community in TianEZhou Oxbow in the middle reach of the Yangtze River was conducted during April 2011 to January 2012 A total of 87 zooplankton species were observed，among which，Protozoa （23 genera 38 species），Rotifer （25 genera 36 species），Cladocera （8 genera 9 species），Copepoda （4 species） accounted for 437%，414%，103%，46% of the total species，respectively The annual average density of zooplankton was 25 7119 ind./L with protozoa and rotifers accounting for 993% of total density The annual average biomass of zooplankton was 36mg/L，with protozoa and cladocera accounting for 60% of total biomass Among the 15 dominant species （7 species of protozoa and 8 species of rotifer），9 species were indicator species of eutrophic status Four species of protozoa，Strombidium viride，Urotricha furcated，Cyclidium oblongum， Tintinnidium fluviatile and one species of rotifer，Anuraeopsis fissa were predominant all year around Three species of rotifer，Polyarthra trigla，Keratella ticinensis，Brachionus angularis were dominant in spring，summer and autumn，and one species of rotifer，Keratella cochlearis was dominant only in spring and autumn Higher density of dominant species Strombidium viride，Urotricha furcated，Cyclidium oblongum occurred in the sampling sites of 6 and 10，with a large number of Polyarthra trigla in sampling site of 10 Analysis on the zooplankton community in Tianezhou Oxbow demonstrated dominance of microzooplankton because protozoa and rotifer，the smallsized zooplankton，accounted for 993% in density，while cladocera and copepods，the largesized zooplankton，accounted for only 07% in density Intensive feeding by stocked filterfeeding fish，silver carp and bighead carp and the selective predation on macrozooplankton by abundant smallsized fish such as Taihu icefish，sharpbelly and shortjaw tapertail anchovy were the possible reasons for the changes in zooplankton community According to the standing crop and the dominant species，the trophic state of Tianezhou Oxbow was classified into eutrophic level which were in agreement with the assessment results based on Shannons index and evenness of Pielou.