Environmental drivers of dominant phytoplankton species succession in a subtropical reservoir: a species-specific ecological analysis of Shanzai Reservoir, Fujian, China

Context Subtropical drinking-water reservoirs face increasing threats from cyanobacterial blooms driven by warming and eutrophication. Aims This study investigated environmental drivers of dominant phytoplankton succession in Shanzai Reservoir, Fujian Province, China. Methods Monthly surface-water monitoring was conducted across four sites (April 2023–April 2024, n = 52). Kruskal–Wallis tests, canonical correspondence analysis (CCA), path analysis and receiver operating characteristic (ROC) curve analysis were applied to identify and validate environmental drivers and bloom-risk thresholds. Key results Seven dominant species were identified; Microcystis spp. (44.2%) and Melosira sp. (36.5%) together accounted for over 80% of occurrences. CCA (Axis 1: r = 0.895) identified pH, temperature and chemical oxygen demand (COD) as strongest multivariate discriminators. Path analysis showed that temperature acts primarily through indirect pathways (R2 = 0.780) by a temperature → stratification → cyanobacteria biomass chain. ROC analysis validated bloom-risk thresholds as follows: temperature ≥ 25.6°C (area under the curve, AUC = 0.904), pH ≥ 9.12 (AUC = 0.941) and COD ≥ 2.70 mg L−1 (AUC = 0.956). Conclusions Distinct ecological niches were confirmed for all dominant species, with temperature and phosphorus as primary succession drivers. Implications The statistically validated thresholds provide a practical two-stage early warning framework for reservoir managers to predict and prevent harmful cyanobacterial blooms.