Abstract: BASED ON CONTINUOUS MONITORING DATA OF PHYTOPLANKTON, WATER QUALITY, AND HYDROLOGY IN SONGHUA DAM RESERVOIR FROM 2014 TO 2023, THIS STUDY CONDUCTED AN IN-DEPTH INVESTIGATION INTO THE DYNAMIC CHANGES OF THE PHYTOPLANKTON COMMUNITY AND ITS RELATIONSHIP WITH ENVIRONMENTAL FACTORS USING MANN-KENDALL TREND TESTS, DIFFERENCE TESTS, AND REDUNDANCY ANALYSIS (RDA). THE RESULTS INDICATE THAT: ① OVER THE DECADE, SONGHUA DAM RESERVOIR REMAINED IN A MESOTROPHIC STATE. A TOTAL OF 68 PHYTOPLANKTON SPECIES FROM 8 PHYLA WERE IDENTIFIED, INCLUDING 24 DOMINANT SPECIES. THE ANNUAL AVERAGE ALGAL DENSITY RANGED FROM 1.437 MILLION TO 30.187 MILLION CELLS/L. THE SHANNON-WIENER INDEX VARIED BETWEEN 0.38 AND 3.65, AND THE EVENNESS INDEX RANGED FROM 0.10 TO 0.90. THE NUMBER OF GENERA AND SPECIES AND THE ALGAL CELL DENSITY WERE SIGNIFICANTLY HIGHER IN AUTUMN THAN IN WINTER (P < 0.05).② THE TROPHIC STATE INDEX (TSI) AND TOTAL ALGAL DENSITY SHOWED A HIGHLY SIGNIFICANT UPWARD TREND, WITH AN ABRUPT CHANGE POINT DETECTED IN 2021. THE COMMUNITY STRUCTURE OF DOMINANT SPECIES AND ALGAL DENSITY EXHIBITED CHARACTERISTICS OF SPECIES SIMPLIFICATION, CYANOBACTERIAL DOMINANCE, AND STRUCTURAL SIMPLIFICATION, INDICATING A DECLINE IN THE ECOSYSTEM'S DIVERSITY AND STABILITY AFTER 2021.③ PROLONGED HYDRAULIC RETENTION TIME DUE TO DROUGHT WAS IDENTIFIED AS THE PRIMARY ENVIRONMENTAL DRIVER BEHIND THE SIGNIFICANT ECOLOGICAL SHIFT IN THE COMMUNITY STRUCTURE. IT COLLECTIVELY EXPLAINED 55.1% OF THE VARIATION IN COMMUNITY STRUCTURE TOGETHER WITH WATER TEMPERATURE, PH, AND DISSOLVED OXYGEN.THIS STUDY ELUCIDATES THE DECADAL DYNAMICS OF THE PHYTOPLANKTON COMMUNITY IN SONGHUA DAM RESERVOIR, REVEALS THE ENVIRONMENTAL DRIVERS OF ITS ECOLOGICAL TRANSITION, AND PROVIDES A LONG-TERM CASE STUDY FOR RESEARCH ON PHYTOPLANKTON SUCCESSION IN WATER SOURCES.