Using field measurements and a process-oriented three-dimensional coupled physical-biogeochemical numerical model, we investigated the physical and biogeochemical processes governing the bottom hypoxic zone west off the Pearl River estuary. The intensity and area of the hypoxia grew with increasing total nutrient input from the Pearl River that has increased continuously in recent decades. The hypoxic zone was formed and maintained largely associated with the stable water column where the stability was provided simultaneously by wind stress and freshwater discharge, favorable local hydrodynamics for flow convergence, and westward organic matter transport. Wind stress altered the stratification, while freshwater discharge changed the stratification and baroclinic velocity shear simultaneously. Two-layered flow with a cyclonically rotating current around a coastal salient edge of the western shelf off the estuary hydrodynamically enhanced the local convergence, allowing sufficient residence time in the bottom for the remineralization of organic matter produced in the hypoxic zone and organic matter transported into the region. Our results suggest that a combination of unique local hydrodynamic feature and decomposition of organic matter in water column (and possibly in the sediment) are the cause of the formation and maintenance of the bottom hypoxia on the western shelf of the estuary during summer.Plain Language Summary Hypoxia results in dead zones in the ocean. It often occurs in the bottom waters below surface eutrophication due to decomposition of organic matter in the water column and also possibly in sediment. Both physical and biogeochemical processes in the ocean control the formation of the eutrophication and hypoxia. This study investigates these processes for an observed strong hypoxia zone off Pearl River estuary using a numerical model and field measurement data. Through comprehensive analyses, we found that the intensity and area of the hypoxia grew with increasing total nutrient input from Pearl River. The hydrodynamic conditions of ocean flow field such as stability of the water column associated with wind forcing and freshwater buoyancy discharge, local flow convergence, and external organic matter input provide favorable conditions for the formation of hypoxia.Many studies indicate that the remineralization of organic matter in the hypoxic zone is associated with autochthonous organic carbon production fueled by excessive terrestrial nutrient input (Rabalais et al.

No abstract

Abstract. The East China Sea (ECS) and the South China Sea (SCS) are two major marginal seas of the North Pacific with distinct seasonal variations of primary productivity. Based upon field observations covering both the ECS and the northern SCS (NSCS) during December 2008-January 2009, we examined southward long-range transport of nutrients from the ECS to the northeastern SCS (NESCS) carried by the China Coastal Current (CCC) driven by the prevailing northeast monsoon in wintertime. These escaped nutrients from the ECS shelf, where primary production (PP) was limited in winter, might however refuel the PP on the NESCS shelf at lower latitude, where the water temperature remained favorable, but river-sourced nutrients were limited. By combining the field observation of nitrate+nitrite (NO 3 +NO 2 , DIN) with our best estimate of volume transport of the CCC, we derived a first-order estimate for DIN flux of 1430 ± 1024 mol s −1 . Under the assumption that DIN was the limiting nutrient, such southward DIN transport would have stimulated 8.84 ± 6.33 × 10 11 gC of new production (NP), accounting for 33-74 % of the NP or 14-22 % of PP in winter on the NESCS shelf shallower than 100 m.