Controls on the interannual variability of hypoxia in a subtropical embayment and its adjacent waters in the Guangdong coastal upwelling system, northern South China Sea
Coastal embayments located downwind of large rivers under an upwelling-favorable wind are prone to develop low-oxygen or hypoxic conditions in their bottom water. One such embayment is Mirs Bay, off the Guangdong coast, which is affected by upwelling and the Pearl River Estuary (PRE) plume during summer. The relative importance of physical and biochemical processes on the interannual variability of hypoxia in Mirs Bay and its adjacent waters was investigated using statistical analyses of monthly hydrographic and water quality monitoring data from 2001 to 2015. The results reveal that the southwesterly wind duration and the PRE river discharge together explain 49% of the interannual variability in the size of the hypoxic area, whereas inclusion of the nutrient concentrations inside Mirs Bay and phytoplankton on the shelf explains 75% of the interannual variability in the size of the hypoxic area. This finding suggests that the interannual variability of hypoxia in Mirs Bay is regulated by coupled physical and biochemical processes. Increase of the hypoxic area under a longer-lasting southwesterly wind is caused by increased stratification, extended bottom water residence time, and onshore transport of a low-oxygen water mass induced by stable upwelling. In contrast, a reduction in the size of the hypoxic area may be attributed to a decrease in the surface water residence time of the particulate organic matter outside Mirs Bay due to increased discharge from the PRE. The results also show that the effects of allochthonous particulate organic matter outside Mirs Bay on bottom hypoxia cannot be neglected.
KeywordsMirs Bay Pearl River estuary Hypoxia Upwelling Plume Southwesterly wind duration
This research was primarily supported by the National Natural Science Foundation of China (NSFC) through grant #41506102 and #41576089, and also supported by Joint Research Projects NSFC-NOW (Netherlands Organization for Scientific Research)-RCUK (UK Research Council) through grant #5161101656 and the Fundamental Research Funds for the Central Universities (15lgjc39). We thank the Environmental Protection Department of the Hong Kong Government (HKEPD) for providing the observation data and allowing us to use them. The authors thank anonymous reviewers for their constructive suggestions.
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