A cyanobacterial bloom(dominated by Anabaena mucosa) in Gaozhou Reservoir was firstly reported from January to April in 2009, occurred again in 2010.Its maximum cell density reached 3.4×10⁷ cells/L.To understand the characteristics of the cya-nobacterial blooms,the community structure and dynamics of cyanobacteria were investigated monthly from August 2009 to July 2010.There was a significant difference in species composition of cyanobacteria between dry season and flood season in the reser-voir.Cell density in dry season was higher than that in flood season when Anabaena mucosa formed cyanobacterial bloom.This phe-nomenon may attribute to the beneficial conditions to algal growth and accumulation,e.g.low precipitation,low water level and low storage capacity,but hydraulic retention time prolonged and thermal stability increased.Moreover,the decreased water trans-parency and lowering temperature would contribute to Anabaena mucosa turning into the dominant species and to form cyanobacterial blooms.Strong negative correlation between water temperature and cyanobacterial biomass suggested that Anabaena mucosa was a-daptable to low water temperature.In the flood season,increasing of precipitation resulted in high water level,and thermal stratifi-cation occurred in vertical column of the water.Microcystis flos-aquae was dominated in the algal community gradually instead of An-abaena mucosa because of the water stratification,but the bloom disappeared with a dramatic decrease of total cyanobacteral bio-mass.In conclusion,the cyanobacterial bloom in Gaozhou Reservoir was closely related to water temperature and precipitation.