Hydrodynamic control on the interannual variability of algal bloom in the Pearl River Estuary from 2021 to 2024: evidences from buoy data and hydrodynamic modeling.

Algal blooms have increased in frequency in the Pearl River Estuary (PRE) due to intensive human activities. While seasonal effects of hydrodynamics and water quality on blooms are well-known, influences at intramonthly and interannual timescales remain unclear due to limited high-frequency long-term observations. Using daily buoy data and a coastal ocean model, this study investigates hydrodynamic controls on interannual bloom variability. Results showed 55 bloom events (Chl-a > 10 μg/L) occurred between 2021 and 2024, with 35 events occurred in the dry season (October-April) when residence time (RT) was long and nutrients were low. Most blooms formed along salinity fronts, where convergence accumulates nutrients and biomass. The strongest event (Chl-a > 35 μg/L) occurred in December 2024. Interannual variability in dry season blooms was regulated by wind: winter storms in 2022 and 2023 enhanced offshore flushing, shortening RT and suppressing blooms, whereas stronger easterlies in 2021 and 2024 increased onshore Ekman transport, extending RT and favor blooms. The December 2024 bloom was particularly intense due to weaker winds and a more stable water column compared with 2021. RT primarily controlled blooms in the western PRE, while vertical stability was more important in the eastern PRE. These findings emphasize the combined roles of RT, vertical stability, and frontal intensity in regulating blooms, suggesting they may serve as key indicators for bloom forecasting in the PRE.