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

Abstract

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.

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

“…Rabouille et al (2008) demonstrated that tidal mixing could disrupt stratification and break hypoxia. However, the DO concentration outside of the Lingdingyang sub-estuary exhibited a tidal fluctuation and spring-to-neap oscillation, with a maximum neighboring oxygen range only of 0.2 and 0.5 mg L -1 , respectively (Cui et al 2019). Our observations showed the maintenance of hypoxia during Leg 1 and the reinstatement of hypoxic or oxygen-deficient conditions from Leg 2 to Leg 3, both over the transformation from a neap tide to a spring tide (Fig.…”

“…4a, b). It therefore acted as a barrier layer, with weak dissipation of oxygen into the bottom layer (Cui et al 2019). The inherent pycnocline between the offshore surface water and shelf benthic waters mainly driven by steep temperature gradients (Qu et al 2007), such as at Station A14, was observable as a second pycnocline in the plume region (e.g., stations A8 and F304, Fig.…”

“…For revisited stations, DO is the difference in DO values measured between the two sampling periods. The physical-mixing-induced DO variations were derived using a three-endmember mixing model, which construct the conservative mixing scheme among different water masses: Brackish Plume Water (PW), Offshore Surface Water (SW) and Upwelled Subsurface Water (SUB) (Su et al 2017, Cui et al 2019. The model is constrained by salinity (S) and potential temperature () according to the following equations:…”

“…A stable water column is a key prerequisite for the formation and maintenance of hypoxia in coastal oceans (Wang and Justić 2009, Obenour et al 2012, Testa and Kemp 2014, which restricts the oxygen supply by suppressing advective and diffusive mixing with oxygen-rich waters (Murphy et al 2011, Cui et al 2019. Many studies have demonstrated that density stratification becomes enhanced and stabilizes subsurface waters when freshwater flows over seawater , MacCready et al 2009, Bianchi et al 2010, allowing oxygen depletion over a longer timescale (Fennel and Testa 2019).…”

Wind-driven stratification patterns and dissolved oxygen depletion in the area off the Changjiang (Yangtze) Estuary

“…Besides eutrophication (Diaz and Rosenberg, 2008), climate change (Altieri and Gedan, 2015) intensifies DO depletion. Estuaries and other regions of fresh water influence are typically very productive regions and often experience hypoxia (Conley et al, 2009;Cui et al, 2019;Obenour et al, 2013;Testa and Kemp, 2014). Natural and anthropogenic nutrient 50 load in these areas leads to intensive sedimentation of organic matter.…”