From June 2006 to December 2007,monthly samplings of 3 sites were carried out for measuring chlorophyll a concentration,composition and density of phytoplankton.Phytoplankton sampling yielded 65 taxonomic units(mostly to genus levels) in 53 genera.Microcystis aeruginosa was predominant with 92.23% of the total taxa.The average algal density for all the sites was 1.23×10⁸ ind·L^-1 and the maximum was 1.39×10⁹ ind·L^-1,while the minimum was 3.21×10⁵ ind·L^-1.With the increased distance between Dianchi Lake and the sampling sites(from site 3 to 1),total density of phytoplankton and Microcystis aeruginosa decreased dramatically,while the Shannon-Wiener,Simpson(1/Dominance) diversity and evenness went up significantly.Besides,Microcystis aeruginosa of site 2 and 3 was similar with that of Dianchi Lake,while site 1 was different from site 2 and 3 due to its long distance away from Dianchi Lake,indicating significant impacts of Dianchi Lake water on taxonomic composition of site 2 and 3.From the percentage similarity index of community(PSc) among 3 sites,we found that PSc index between site 1 and 3 was significantly lower than that of site 1&2 and site 2&3.Except for April,May and July 2007,the PSc index between site 2 and 3 was higher than 60%.After the project operation,Microcystis aeruginosa of site 1 and 2 showed decreasing trend.Nevertheless,Microcystis aeruginosa of site 3 increased.At the same time,species richness of the 3 sites increased dramatically after the project.Changes in algal community structure(i.e.,the species present and their relative abundances) following project operation were evaluated by non-metric multi-dimensional scaling(NMS) ordinations.The results showed that overall algal assemblage structure of the 3 sites remained similar throughout the study period(MRPP,p>0.05),indicating no significant impacts on phytoplankton community 1-year after project operation.