Joint Effects of Extrinsic Biophysical Fluxes and Intrinsic Hydrodynamics on the Formation of Hypoxia West off the Pearl River Estuary

Abstract

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.

Production and Transformation of Dissolved and Particulate Organic Matter as Indicated by Amino Acids in the Pearl River Estuary, China

“…Temperature and salinity profiles were obtained using a SBE 25 conductivity‐temperature‐depth/pressure unit (Sea‐Bird Co.). Temperature, salinity, and nutrient data were reported in the previous study (Lu et al, ). Samples for DOC and TDAA analysis were obtained by filtering water samples through precombusted 0.7‐μm GF/F filters.…”

Genomic adaptation to eutrophication of ammonia‐oxidizing archaea in the Pearl River estuary

“…The rapid growth of industries and urbanization along the Pearl River since the 1990s has led to a massive terrigenous release of waste into the estuary, which resulted in serious eutrophication and contamination of the surrounding environment (Huang et al ., ). The concentrations of inorganic nitrogen in the estuarine water reaches ~100 μM and chlorophyll a > 10 mg m −3 during summer wet season (Yin, ; Chen et al ., ), resulting to high production of organic matter and temporal hypoxic condition in the deep layer in the far‐field of the estuary (Lu et al ., ). Microorganisms in the estuary play an irreplaceable role in the degradation of complex chemicals and organic matter, and they are therefore essential for sustaining a high level of productivity in estuarine waters (Herbert and Nedwell, ; Herbert, ).…”

Large benthic fluxes of dissolved iron in China coastal seas revealed by 224Ra/228Th disequilibria

“…In previous studies, benthic Fe fluxes were often assumed to increase with decreasing dissolved oxygen (DO) content in the bottom water because hypoxia conditions tend to shoal the oxygen penetration depth into the Table 1 are replaced by molecular diffusive fluxes, which are used to represent a lower estimate of true benthic Fe fluxes. The data of St. F401 are excluded in the diagrams because this site is located in an uncommon convergence zone of regional summer flows (Lu et al, 2018). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)…”