The global development of cyanophyte blooms has been related to many factors such as increased nutrients, climate change and resource competition but there is little information on the relative importance of these drivers. This study examined the spatiotemporal dynamics of phytoplanktonic species successions during algal blooms, particularly focusing on Ceratium hirundinella (C. hirundinella) and cyanobacteria, in the Pengxi River, a key 1st order tributary of the Three Gorges Reservoir (TGR). Over a 26-day period in spring 2023, daily samples were collected from three transects (S1 to S3, following the water flow) in the middle reach of Pengxi River, 4 km apart from each other. The objective was to resolve the influence of water quality and hydrological regimes on phytoplankton community dynamics during algal blooms. The results indicated that 2 bloom events developed during the sampling period. Each bloom showed similar algal species succession, i.e. cyanobacterial bloom, mainly composed of Anabaena circinalis (A. circinalis) and Chroococcus minor (C. minor) dominated initially, followed by a dinoflagellate bloom, dominated by C. hirundinella with a larger biomass. The thermal stratification and formation of a surface density layer (SDL) resulted in nutrient-limited conditions in the surface waters that limited the cyanobacterial bloom in the surface waters. This provided the opportunity for more mobile C. hirundinella to dominate the algal community with its ability to move into and out of the SDL. At the end of the bloom, hypoxic conditions developed in the middle layers of water columns. The occurrence of hypoxia was significantly associated with high biomass of C. hirundinella at middle layers at all transects, suggesting that C. hirundinella was able to take advantage of nutrient availability below the SDL and thus replace cyanobacteria in dominance. During the study period，C. hirundinella reached a maximum biomass 28.6 mg L-1, while cyanobacteria displayed an accumulation of 0.72 mg L-1. This study highlights how competitive release from direct competition for nutrients can modify the algal community succession. C. hirundinealla’s ability to quickly move between nutrient rich deeper waters below the SDL and the photic zone provided a significant competitive advantage over the cyanobacteria. This research fills a critical gap in understanding how environmental factors drive the dominance and succession of phytoplankton species in blooms in the TGR, providing insights into managing and mitigating algal blooms in large and canyon reservoirs.