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The functional groups approach is an efficient way to analyze seasonal changes in phytoplankton biomass as it is based on the physiological, morphological, and ecological attributes of the species. In this study, we identified the functional groups and driving factors behind short-term succession in phytoplankton communities. We analyzed physical, chemical, and biological factors of the Maixi River in Baihua Reservoir (BHR) from August to September, 2013 (summer, phase I) and March to May, 2014 (late spring and early summer, phase II). The 226 samples collected were divided into 23 functional groups. In phase I, phytoplankton biomass ranged from 4.88 to 30.59 mg·L-1, and the group S1 (Pseudanabaena limnetica) had the greatest biomass. In phase II, phytoplankton biomass ranged from 2.22 to 50.61 mg·L-1, and groups Y (Cryptomonas sp.) and S1 (P. limnetica) had the greatest biomass. Dominant functional groups in the Maixi River changed from S1 + D + Y + Lo in phase I to Y + S1 in summer. Changes in the phytoplankton community varied between 0 and 0.144 day-1 in phase I and between 0.008 and 0.389 day-1 in later spring and early summer. This showed a steady-state phytoplankton community during the two phases, in which the functional groups S1 (P. limnetica) and Y (Cryptomonas sp.) were dominant. Pseudanabaena limnetica, Synedra sp., Peridinium sp., and Cryptomonas sp. were dominant during summer, contributing to 70% of the total biomass in the steady-state community, and P. limnetica, Synedra sp., Cryptomonas sp., and Chlamydomonas sp. were dominant during later spring and early summer, contributing to 60% of the total biomass in the community. Groups S1, D, and Y formed easily in the Maixi River, but P. limnetica was the dominant species in the eutrophic conditions of the Maixi River. According to biotic and abiotic factors, we concluded that the Maixi River is hypertrophic, and water resource management should take blooms of P. limnetica occurring in May into account. Temperature and dissolved oxygen were the critical factors affecting the steady-state of the phytoplankton community in late spring and early summer and summer, respectively. Because the Maixi River is an important source in the BHR, its phytoplankton functional groups directly affect the ecological balance of the water environment.