Community structure and toxin-producing traits of bloom-forming algae in Danjiangkou Reservoir

The water quality and aquatic ecological status of the Danjiangkou Reservoir are directly related to the safe and stable operation of the water transfer system of the Middle Route of the South-to-North Water Diversion Project. This study investigated the community structure and toxin-producing characteristics of bloom-forming algae in the Danjiangkou Reservoir to provide a scientific basis for bloom risk management. In July 2025, water samples were collected from 15 sites across the reservoir for analysis of water quality parameters and algal community composition, while concentrations of microcystins and cylindrospermopsins and the proportion of toxin-producing strains were measured using ELISA, quantitative PCR, and mass spectrometry. The results showed that the reservoir exhibited phosphorus-limited conditions (TN/TP ratio: 32~105), with higher TN (1.20~1.37 mg/L) and TP (0.031~0.043 mg/L) concentrations in the inflow areas and reservoir bays compared to the main reservoir (TN, 0.72~0.77 mg/L; TP, < 0.01~0.015 mg/L). Cyanobacteria dominated the phytoplankton community, accounting for over 80% of the total abundance, with dominant species including Cylindrospermopsis, Pseudanabaena, Aphanocapsa, Leptolyngbya, and Microcystis. Spatially, Cylindrospermopsis and Pseudanabaena were prevalent in reservoir bays and tributary inflows, whereas the main reservoir and downstream areas were dominated by simpler communities, primarily Cylindrospermopsis or Aphanocapsa. Toxin analysis revealed that cylindrospermopsin was only detected at the Madeng site at a low concentration, while the proportion of toxin-producing strains in this area was extremely low (0.59%~1.69%), and microcystin concentrations remained low overall, indicating a generally low toxicity risk.