Abstract
Abstract. In summer, the Pearl River estuary (PRE) experiences hypoxia, largely driven by the high input of fresh water with low dissolved oxygen (DO), abundant nutrients and particulate organic carbon from the Pearl River network. In this study, we used a well-validated physical–biogeochemical model together with a DO species-tracing method to study the responses of hypoxia and oxygen dynamics to the anthropogenic perturbations of riverine inputs (i.e. DO, nutrients, and particulate organic carbon) in July–August 2006. Model results showed that hypoxia in the PRE was most sensitive to riverine inputs of particulate organic carbon, followed by DO concentrations and nutrients. Specifically, a 50 % decrease (increase) in riverine input of particulate organic carbon led to a 47 % decrease (64 % increase) in hypoxic area, with the sediment oxygen demand and water column production being the two most important processes contributing to changes in DO concentration. Changes in the riverine inputs of DO and nutrients had little impact on the simulated hypoxia because of the buffering effects of re-aeration (DO fluxes across the air–sea interface); i.e. the re-aeration responded to the changes in surface apparent oxygen utilization (AOU) associated with river-induced variations of oxygen source and sink processes. The PRE features shallow waters (with averaged depth of 10 m) in which oxygen provided by the re-aeration could penetrate to bottom waters via vertical diffusion and largely offset the changes in DO contributed by other oxygen source and sink processes. This study highlights the importance of re-aeration in reducing hypoxia variability in shallow estuaries.
Highlights
Recent decades have seen a decline in dissolved oxygen (DO) concentrations in most of the coastal oceans because of intensifying anthropogenic disturbances, leading to an increase in the occurrence and intensity of hypoxic conditions (Diaz and Rosenberg, 2008)
Unlike the large spatial extent of hypoxia observed in the Changjiang Estuary (Wang, 2009; Wang et al, 2012) and the northern Gulf of Mexico (NGOM) (Rabouille et al, 2008), hypoxia in the Pearl River estuary (PRE) is confined to a small area as a result of the sediment oxygen demand (SOD) and the re-aeration (Wang et al, 2017)
Model results based on a simulation in 2006 show that hypoxia in the PRE starts in April, peaks in August, and disappears in October
Summary
Recent decades have seen a decline in dissolved oxygen (DO) concentrations in most of the coastal oceans because of intensifying anthropogenic disturbances, leading to an increase in the occurrence and intensity of hypoxic conditions (Diaz and Rosenberg, 2008). Relations between the riverine nutrient loading and the hypoxic conditions (DO < 2 mg L−1) have been well documented in many coastal hypoxic systems, such as the Changjiang Estuary (Li et al, 2011; Ning et al, 2011), the Chesapeake Bay (Du and Shen, 2015; Hagy et al, 2004), and the northern Gulf of Mexico (NGOM) (Forrest et al, 2011; Justicet al., 2003).
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