Hypoxia in the Pearl River Estuary, the South China Sea, in July 1999

Abstract

The Pearl River Estuary empties into the South China Sea in southeastern China, and hypoxia is one of the major environmental problems here during summer. It can be caused by coastline, topography, hydrodynamics, tidal dynamics and biological processes (mainly by stratification and oxidation reaction). Since ecological processes are closely related to dynamic processes, we revise and link together both a physical and a biological model to establish a coupled hydrodynamic-ecological numerical model to study the summertime hypoxia. The model simulates physical and biological processes and reproduces the observed hypoxia in the bottom layer in two regions located in west shallow water and around Nei Lingding Island. A low-salinity plume forms after the freshwater drifts offshore. Strong stratification exists in the water body shoreward where the vertical mixing is weak. The salinity front and stratification cause a poor exchange in the vertical direction and greatly influence biological processes for nutrients, phytoplankton, and detritus, which induce the hypoxia occurrence in the bottom layer of the Pearl River Estuary. On the other hand, in two troughs the tide diminishes the strength of stratification and prohibits the large-scale growth of the hypoxia. The physical processes related to topography, run off and tidal mixing regulate the distribution of biological factors and the occurrence of hypoxia.

Recent oxygen depletion in the Pearl River Estuary, South China: geochemical and microfaunal evidence

“…In addition, the high turbidity significantly reduces light penetration, and the contribution of photosynthesis is insignificant in bottom waters (Yin et al 2004b;Zhang et al 1999). Decomposition of the phytoplankton material deposits on the bottom, and sediment oxygen demand then causes low oxygen concentration or hypoxia in the estuary (Luo et al 2009;Zhang and Li 2010). Regarding the DO budget above the pycnocline, the oxygen depletion processes is mainly balanced by re-aeration and photosynthesis, thus, surface waters usually present relatively high concentrations of DO (Lin and Li 2002).…”

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

“…Moreover, the coastal hypoxia is forecast to increase owing to the combined effects of continued spread of coastal eutrophication and global warming largely associated with human activities (Diaz and Rosenberg ; Vaquer‐Sunyer and Duarte ). More than 400 coastal systems of hypoxia (termed “dead zone”) have been identified, with a total area of more than 245,000 km 2 (Diaz and Rosenberg ), e.g., Gulf of Mexico (Rabalais et al ; Zhang et al ), Chesapeake Bay (Hagy et al ; Murphy et al ), Lower St. Lawrence Estuary (Bourgault et al ; Lefort et al ), Yangtze River (Changjiang) Estuary (Chen et al ; Wei et al ), and Pearl River (Zhujiang) Estuary (Yin et al ; Luo et al ; Wang et al ). While it is generally accepted that this increase was the result of increased anthropogenic nutrient loads to the estuaries or coastal ocean, studies that attempt to statistically relate the inter‐annual variations in hypoxic volume to nutrient loading often fail to explain the majority of the variability (Hagy et al ; Murphy et al ; Scully , ).…”

“…Since summertime hypoxia in the Pearl River Estuary (PRE) was first reported in 1970s to 1980s, it has attracted a widespread attention of ocean scientists and public administrators (Yin et al ; Luo et al ; Zhang and Li ; Wei et al ; Wang et al ). The seasonal hypoxia occurred on the west and east shoals in the Lingdingyang Estuary, and in the Modaomen Estuary (Cai et al ).…”

“…The seasonal hypoxia occurred on the west and east shoals in the Lingdingyang Estuary, and in the Modaomen Estuary (Cai et al ). Density stratification (Luo et al ; Hong et al ) and SOD (Yin et al ; Zhang and Li ) were thought to be major processes for controlling the maintenance of hypoxia. In addition, the front of river plume and its stable stratification were also thought to cause the hypoxia occurrence and development (Luo et al ).…”

Responses of the river plume to the external forcing in Pearl River Estuary

“…From west to east, the Pearl River Estuary (PRE) comprises 4 bell-shaped estuaries, namely: Huangmaohai (HMH), Jitimen (JTM), Modaomen (MDM) and Lingdingyang (LDY) and empties through 8 channels into the South China Sea in southeast China (Figure 1a). The PRE and adjacent waters have lowered water quality due to the presence of some of the most urbanized and industrialized regions in the basin (Luo et al, 2009). Since the river plume plays an important role in the biochemical processes and sediment transport in estuary and adjacent shelf waters (Warrick et al, 2004;Geyer et al, 2000), the mechanism of the Pearl River plume and its external forcing should be explored.…”