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Since half a century ago, the number and area of dead zones (dissolved oxygen (DO) < 2 mg L–1 or 30% saturation) in the coastal oceans has increased dramatically. As widely recognized, the increased terrestrial nutrient and organic matter inputs are the two main factors causing the eutrophication of many coastal oceans. Here we show with decadal observed time series data from stations off the Pearl River Estuary and in the northern South China Sea (nSCS) that a strong intrusion into the nSCS of the West Philippine Sea (WPS) seawater in the form of Kuroshio branch occurred during the warm phase of the Pacific Decadal Oscillation (PDO) around 2003–2004 and 2015–2016 (also a strong El Niño event). Consequently, the DO concentration increased but NO3– and PO43– concentrations decreased in the subsurface layers of the nSCS. The WPS seawater was observed to reach the hypoxic area off the Pearl River Estuary in 2003–2004. Likely, due to the oxygen supply carried by the Kuroshio, little hypoxia developed. Yet, anoxic condition developed in the cold phase of PDO or strong La Niña years with weak Kuroshio intrusions.

The Kuroshio is the most significant current in the western North Pacific Ocean and affects a wide area. This work shows that the intrusion of the oligotrophic upper-layer West Philippine Sea seawater into the South China Sea (SCS) as the branch of Kuroshio reduced the productivity and hence the fluxes of sinking particles in the SCS between 2013 and 2017. Conversely, the productivity in the SCS increased during a large scale Kuroshio intrusion in 1998–2006, indicating that other factors also affected the productivity. Further, the western North Pacific Ocean is acidifying, with the surface seawaters to the west having lower acidification rates. This phenomenon is likely a consequence of enhanced productivity owing to more anthropogenic nutrient inputs from the continent in the west, but needs further investigation. In the East China Sea, the Kuroshio Intermediate Water has increased nutrient concentrations, but decreased in both dissolved oxygen (DO) concentration and pH, most likely owing to reduced ventilation in the North Pacific Intermediate Water. Further warming of the surface oceans would strengthen the stratification of the surface ocean, weakening ventilation. Consequently, DO and pH would continue to decline while nutrients level increases.

[1] Export of dissolved inorganic carbon (DIC) to adjoining oceans enhances the potential of CO2 sequestration in marginal seas. By using a series of measured DIC depth profiles and reported flow transports, we estimated that the intermediate outflow (100–600 m) from the South China Sea is capable of transporting 6.5 ± 4.1 Tg (1 Tg = 1012g) of biologically mediated carbon (DICbio) annually to the East China Sea (ECS) via the northwardly flowing Kuroshio current. The mixing and transport of these DIC-rich waters would raise 3% and 16% of DIC/TA ratio and the Revelle factor of the adjoining seawaters, respectively. Upon upwelling onto the ECS shelf, these DIC-rich waters would counteract the potential of CO2 uptake of shelf waters that might have been enhanced by the accompanying increase in nutrient inputs, thus complicating assessment of the ECS as a net CO2 source or sink.