Freshwater cyanobacterial blooms have drawn public attention because they threaten the safety of water resources and human health worldwide.Heavy cyanobacterial blooms out-break in Lake Taihu in summer annually and vanish in other months.To find out the factors impacting the cyanobacterial blooms,the present study measured the physicochemical pa-rameters of water and investigated the composition of microbial community using the 16S rRNA gene and internal transcribed spacer amplicon sequencing in the months with or without bloom.The most interesting finding is that two major cyanobacteria,Planktothrix and Microcystis,dramatically alternated during a cyanobacterial bloom in 2016,which is less mentioned in previous studies.When the temperature of the water began increasing in July,Planktothrix appeared first and showed as a superior competitor for M.aeruginosa in NO3--rich conditions.Microcystis became the dominant genus when the water temperature increased further in August.Laboratory experiments confirmed the influence of temper-ature and the total dissolved nitrogen(TDN)form on the growth of Planktothrix and Mi-crocystis in a co-culture system.Besides,species interactions between cyanobacteria and non-cyanobacterial microorganisms,especially the prokaryotes,also played a key role in the alteration of Planktothrix and Microcystis.The present study exhibited the alteration of two dominant cyanobacteria in the different bloom periods caused by the temperature,TDN forms as well as the species interactions.These results helped the better understanding of cyanobacterial blooms and the factors which contribute to them.