Coastal hypoxia diminished by intrusion of open ocean water after long El Nino Events: Case study of Hong Kong waters

Coastal regions suffer from increasing terrestrial inputs of nutrients and organic matter. Consequently, hypoxia (dissolved oxygen (DO) < 30% or 2 mg/L) in the coastal regions has become more serious. In the study of coastal eutrophication and hypoxia, incoming offshore seawater has rarely been addressed. With references to the time-series data in the coast of Hong Kong and at the South East Asia Time Series Study (SEATS) station located in the northern South China Sea (SCS), this study demonstrates that coastal waters of Hong Kong have suffered hypoxia for over a decade. The hypoxia condition, however, diminished between 2002 and 2004, most likely owning to a large scale intrusion of the West Philippine Sea (WPS) seawater. For instance, at station SM18 located south of Hong Kong, the summer DO minimum has generally decreased from a saturation state of about 60% to as low as 5% from 1990 to 2013. The almost anoxic condition occurred in 2011 after a La Nina event. On the other hand, the summer DO minimum reached a high value of 79% in 2004 after a long El Nino event. Meanwhile, seawater at the SEATS site also contained the highest proportion of the WPS water, reflecting the large intrusion of the WPS seawater into the SCS. Such a result illustrates a situation that coastal eutrophication and hypoxia could be worsened when the intrusion of open ocean water decreases, and vice versa.