Relative Water Column Stability Drives Harmful Algal Blooms in a Tributary of The Three Gorges Reservoir

Harmful algal blooms (HABs) in reservoir tributaries are a global concern, yet their regulating mechanisms remain unclear. We hypothesized that the variation in relative water column stability (RWCS) is a critical driver of when and where HABs develop in reservoir tributaries. We tested this by intensive field sampling and CE-QUAL-W2 hydrodynamic modeling in three transects (S1, S2, S3) of Pengxi River, a tributary of the Three Gorges Reservoir (TGR), from April 17 to May 20, 2021. Each transect had unique hydrological conditions affecting RWCS. Transect S3 (downstream), most affected by backwater intrusion and thermal stratification, exhibited prolonged surface density layer (SDL) gradients and the highest chlorophyll-a (Chl-a) levels, with HAB development dominated by the Ceratium hirundinella. Surface-layer Chl-a at S3 peaked at 262.6 µg L−1, ∼86.4% higher than upstream (S1). Water mixing influenced dissolved total phosphorus (DTP) at S1; SDL formation corresponded with surface-water DTP reduction at midstream (S2) and downstream (S3), while backwater intrusion enriched bottom-water DTP at S3. This study identifies RWCS, as reflected by the presence and persistence of SDLs, as a key driver of HAB development in reservoir tributaries. These findings support early warning and targeted management of HABs in high-risk downstream backwater-influenced tributary reaches.