Destruction and reinstatement of coastal hypoxia in the South China Sea off the Pearl River Estuary

Abstract. We examined the evolution of intermittent hypoxia off the Pearl River Estuary during three cruise legs conducted in July 2018: one during severe hypoxic conditions before the passage of a typhoon and two post-typhoon legs showing destruction of the hypoxia and its reinstatement. The lowest ever regional dissolved oxygen (DO) concentration of 3.5 μmol kg⁻¹ (~ 0.1 mg L⁻¹) was observed in bottom waters during Leg 1, with a ~ 660 km² area experiencing hypoxic conditions (DO < 63 μmol kg⁻¹). Hypoxia was completely destroyed by the typhoon passage but was quickly restored ~ 6 days later, resulting primarily from high biochemical oxygen consumption in bottom waters that averaged −14.6 ± 4.8 μmol O₂ kg⁻¹ d⁻¹. The shoreward intrusion of subsurface shelf waters contributed to an additional 8.6 ± 1.7 % of oxygen loss during the reinstatement of hypoxia. Freshwater input-induced stratification, suppressing turbulent mixing induced by wind stress and/or tidal forcing, stabilized the water column and facilitated the hypoxia formation. The rapid reinstatement of summer hypoxia has a comparable timescale with water residence time and that of its initial disturbance from frequent tropical cyclones or high-wind events throughout the season. This has important implications towards better understanding the intermittent nature of hypoxia, and predicting coastal hypoxia in a changing climate.