The superimposed hydrological regulation of cascade reservoirs significantly alters the structure and function of river ecosystems. Identifying the spatiotemporal variations in phytoplankton communities and their key environmental drivers in the lower reaches of the Jinsha River can provide scientific basis for ecological restoration and management of cascade reservoirs in this region. Phytoplankton surveys were conducted in March 2025 (low-water period) and July 2025 (high-water period), during which taxonomic composition, density and biomass, diversity, dominant taxa, and niche attributes were quantified; Pearson correlation and redundancy analysis (RDA) were subsequently applied to examine relationships between dominant taxa and environmental variables. In total, 104 phytoplankton species (83 genera, 9 phyla) were recorded across the cascade reservoirs, with Chlorophyta, Bacillariophyta, and Cyanobacteria as the predominant groups. Species richness was significantly higher in the high-water period than in the low-water period (P<0.01) but remained relatively uniform among reservoirs. By contrast, phytoplankton density ranged from 0.14×105 to 362.37×105 cells/L and biomass from 0.01 to 19.05 mg/L, and neither exhibited significant spatiotemporal variation. Diversity metrics showed no significant differences among reservoirs; however, the richness, Shannon–Wiener, and Simpson indices were significantly higher in the high-water period than in the low-water period, whereas evenness did not vary significantly between seasons. The diversity-based assessment further indicated that the study area overall corresponded to an α–β mesosaprobic (moderately polluted) level. During the low-water period, diatoms predominated and exhibited relatively narrow? niche breadth, whereas the high-water period was characterized by co-dominance of multiple taxonomic groups and an overall increase in niche breadth. Notably, Cyclotella remained consistently dominant in both seasons, while Cryptomonas exhibited concurrent increases in dominance and niche breadth in the high-water period. At the community level, niche overlap was generally higher in the high-water period than in the low-water period: overlap patterns were relatively even across dominance classes in the low-water period, whereas the high-water period showed an increased proportion of dominant-taxon pairs with low overlap. Permanganate index (CODMn), pH, and dissolved oxygen (DO) were consistently identified by Pearson correlation and RDA as the primary environmental drivers shaping the spatiotemporal patterns of dominant phytoplankton taxa.