The Poyang Lake Basin serves as a critical ecological barrier in the middle and lower reaches of the Yangtze River. Investigating the response mechanisms of phytoplankton communities to the dual stressors of extreme climate events and anthropogenic activities is essential for regional ecological conservation. Our study focused on the dynamic characteristics of phytoplankton dominant species in key aquatic conservation zones of Jiujiang City during the extreme drought of 2022 and the subsequent hydrological recovery period in 2023. Through systematic surveys at 18 sampling sites across dry and wet seasons， we integrated niche breadth， niche overlap index （O_ik）， variance ratio （VR） method， and interspecific association analyses （χ² test， association coefficient AC， and Spearman's rank correlation） to elucidate the mechanisms underlying phytoplankton community stability under hydrological fluctuations. Results showed that： （1） A total of 161 phytoplankton species from seven phyla were identified， with 13 dominant species from three phyla. Bacillariophyta （Aulacoseira granulata and Ulnaria acus） dominated both extreme drought and recovery phases. （2） During the 2022 extreme drought， niche overlap values significantly increased. Generalist species with broad niches formed stable symbiotic networks through complementary resource utilization， resulting in overall positive community associations （VR>1， 51.31% of species pairs positively linked）. （3） The persistent dominance of A. granulata and U. acus was attributed to their morphological adaptability and niche differentiation， which alleviated competitive pressures from high-overlap species pairs （e.g.， O_ik=0.93 with Navicula sp.）. （4） Post-drought recovery reduced mean niche breadth （7.54） while increasing specialist species proportions， indicating hydrological stabilization drove succession toward functionally resilient community structures. Our study pioneers in exploring niche differentiation and interspecific synergy mechanisms of phytoplankton in mid-Yangtze waters under extreme drought， providing baseline data for ecological monitoring in Poyang Lake and the Yangtze Basin， as well as theoretical insights for freshwater ecosystem management under climate extremes.