Competitive Release in HABs of TGR

The global increase in harmful algal blooms (HABs) is driven by nutrients enrichment, climate change and resource competition, though their relative impact remains unclear. This study examined phytoplankton succession during HABs in the Pengxi River, a key tributary of the Three Gorges Reservoir (TGR). Over 26 days in spring 2023, daily samples from three transects revealed two bloom events. Each began with cyanobacterial bloom, (Dolichospermum circinale and Chroococcus minor), followed by a dinoflagellate bloom dominated by Ceratium hirundinella (C. hirundinella), which had a larger biomass. Thermal stratification led to a surface density layers (SDL), creating nutrient limited conditions that restricted cyanobacterial growth. In contrast, the motile C. hirundinella exploited deeper nutrient rich waters by crossing the SDL, reaching a peak biomass of 28.6 mg L-1, while cyanobacteria accumulated only 0.86 mg L-1. At the bloom termination, hypoxia developed in the middle layers, where high C. hirundinella biomass was observed across all transects. These findings suggest that C. hirundinella’s ability to access nutrients below the SDL provided a competitive advantage, allowing it to replace cyanobacteria as the dominant species. This study highlights how competitive release from direct nutrient competition shapes algal community succession. Understanding these mechanisms offers valuable insights for managing HABs in the TGR and similar large canyon reservoirs.