Effects of Pearl River estuarine-front-induced convergence on formation of bottom hypoxia in summer

The estuarine plume front is a typical dynamic interface in estuaries, where plume and oceanic waters converge at the surface, subducting ocean water beneath the plume and forming a barrier layer between the surface and bottom water. Phytoplankton blooms, fueled by nutrient-rich freshwater in the plume, generate substantial amounts of sinking organic matter, enhancing bacterial consumption of dissolved oxygen (DO) in the bottom water, but the effects of the frontal convergence on DO remain insufficiently understood. In this study, field surveys were conducted in the Pearl River estuary (PRE) during August 27 and September 7 in 2016, including a 24-h time series at an anchored station (ED) near the front. The results showed that the occurrence of bottom hypoxia at ED coincided with surface phytoplankton blooms (high DO and chlorophyll fluorescence). Three distinct water masses were identified: (1) the Pearl River estuarine water (PRW), where nutrients stimulated blooms, (2) the hypoxic bottom water (BW), receiving sinking organic matter, and (3) offshore surface water (OSW), subducted beneath the plume as a middle layer, preventing PRW–BW mixing. The hypoxic layer average DO correlated negatively with equivalent thickness (ET) of PRW (p < 0.05), which negatively correlated with tidal level (p < 0.01) and OSW ET (p < 0.01), indicating that tidal-dynamics-induced front modulated the OSW barrier effect, exacerbating bottom hypoxia. These findings highlight the critical role of frontal processes in regulating oxygen depletion in stratified estuaries.