Program

The nutrient discharge from Pearl River is continuously increasing in recent decades, eutrophication and hypoxia have more and more frequently been observed off the Pearl River Estuary (PRE). Based on both field measurement and a three dimensional coupled physical and biogeochemical model, we investigated the process and formation mechanism of the bottom hypoxia in this region during wet season. We found that low oxygen water from upper reach of the Pearl River has only slight effect on the hypoxia off the PRE because of the oxygen replenishment through air-sea flux/ventilation and the exhaustion of total organic carbon inside the shallow PRE. Besides the nutrient discharged from PRE, the area of the hypoxic region to the west off PRE is significantly affected by the lateral advection of organic matters via the eastward coast current, while the intensity of hypoxia is mainly regulated by local physical/biogeochemical processes. Higher freshwater discharge increases water column buoyancy frequency and spreads nutrient farther off the PRE, both of these conditions are favorable to the enhancement of phytoplankton bloom in surface layer and intensification of hypoxia in bottom layer. Weaker wind stress provides lower vertical velocity shear which strengthens hypoxia by insulating ventilation between bottom and upper layer, but forms a more disordered shelf circulation and makes the hypoxia less stable. The center of hypoxic region was mainly confined at the western coast off the PRE as a result of the combined effect of local water column stability and convergence of bottom flow field. Sensitivity experiment shows that the hypoxia index (HI, represented by the area and intensity of hypoxic region) increases exponentially in response to enhancement of river nutrient discharge. Given the fact that the increasing trend of nutrient inputs from river, more severe hypoxia in terms of both intensity and coverage can be expected in the PRE and adjacent waters.