Physical dynamics structures and oxygen budget of summer hypoxia in the Pearl River Estuary

A summertime hypoxia sporadically occurred in the lower Pearl River Estuary (PRE) for more than three decades. Although its mechanism has already been extensively studied, the topic on why seasonal hypoxia is persistent in patchy waters is still an open question. Here, we presented the investigation of physical dynamics structures and dissolved oxygen (DO) processes for controlling the spatial distribution and maintenance of coastal hypoxia. Field observations were conducted in the 2015 summer in the PRE and adjacent shelf sea. High river discharge forms intense haloclines in the river plume, while salinity intrusion of shelf benthic waters results in a notable pycnocline at the top of salt wedge. A mid-depth transitional layer with the weakest mixing over water column functions as a barrier for DO vertical exchange between river plume and shelf salt wedge. A benthic hypoxia in the 2015 summer appears at the overlapping zone between river plume and shelf salt wedge. Based on physical and biological processes, a DO budget for the hypoxic system was established. The DO advection by gravitational circulation from shelf benthic waters is roughly balanced by bacterial respiration in water column. The DO diffusion from river plume to benthic hypoxia is completely inhibited by the barrier layer. The patchy distribution of benthic hypoxia for the 30-yr period in the PRE can be satisfactorily predicted by the numerical simulations of the overlapping zones between river plume and shelf salt wedge. These findings will have an important implication for predicting and mitigating coastal hypoxia. Physical structures and processes of DO dynamics were investigated to understand the spatial distribution and maintenance of coastal hypoxia. Summertime hypoxia appear near the head of shelf salinity intrusion, where a mid-depth barrier layer inhibits the vertical exchange between river plume and shelf salt wedge. DO advection by gravitational circulation from DO-rich shelf benthic waters is roughly balanced by bacterial respiration in water column. The spatial distribution of coastal hypoxia can be well predicted by the overlapping zone between river plume and shelf salt wedge.