<p>A study of physicochemical parameters variations and their influence on spatial and temporal variability of the dominance, biomass, and composition of phytoplankton assemblages, as well as the determination of the trophic state, was conducted at Beni Amrane Dam Lake, located in Northern Algeria. Five sites were sampled during a monthly sampling campaign, from June 2023 to May 2025. In all, 157 phytoplankton species belonging to 6 phyla were identified, including 48 taxa of Chlorophyta, 43 taxa of Cyanobacteria, 29 taxa of Euglenophyta, 21 taxa of Bacillariophyta, 13 taxa of Dinoflagellata, and 3 taxa of Cryptophyta. The phytoplankton assemblage was dominated by Cyanobacteria, with <i>Oscillatoria tenuis</i> as the dominant species. Fifteen physical and chemical factors were documented, and their relationships with the dominance of phytoplankton species were determined by redundancy analysis and Spearman correlation. The results of redundancy analysis showed that magnesium was the most significant environmental factor that influenced phytoplankton dominance, explaining 80% of the variation in species composition (<i>r</i>^<i>2</i> = 0.8, <i>p</i> &lt; 0.01). Spearman correlation analysis confirmed significant species-specific associations with magnesium and also identified additional relationships with other environmental variables, including nitrates, phosphorus, salinity, and conductivity (<i>p</i> &lt; 0.05). Our study area was classified as hypertrophic according to the Organisation for Economic Co-operation and Development criteria. This study advances understanding of phytoplankton dynamics and underscores the role of environmental variables in shaping phytoplankton assemblages and influencing water quality in Beni Amrane Dam Lake.</p>