Intrusion of Kuroshio Helps to Diminish Coastal Hypoxia in the Coast of Northern South China Sea

Hon-Kit Lui,Yi-Chia Hsin,You-Lin Wang,Chen-Tung Arthur Chen,Jian-Ming Liau,Jay Lee,Wen-Chen Chou,Yan-Yu Choi,Chau-Ron Wu,Wei-Ping Hou

doi:10.3389/fmars.2020.565952

Abstract

Since half century ago, the number and area of dead zones (dissolved oxygen (DO) < 2 mg L-1 or 30% saturation) in the coastal oceans have increased dramatically. As is 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 Nino 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 Nina years with weak Kuroshio intrusions.

Highlights

Coastal oceans are important habitats and spawning grounds for many marine organisms, which need dissolved oxygen (DO) to live
We look at the data from the South East Asia Time-Series Study (SEATS, 18◦N, 116◦E, with bottom depth > 3700 m) station located in the northern South China Sea (nSCS) and from the station 21 located in the West Philippine Sea (WPS) (Figure 1)
To examine whether the changes in salinity were due to the changes in the mixing proportion between the South China Sea (SCS) and the WPS seawaters, we firstly modeled the amount of Kuroshio intrusion using the Princeton Ocean Model along 120.75◦E in the Luzon Strait, which connects the nSCS and the WPS (Figure 4C)

Summary

Introduction

Coastal oceans are important habitats and spawning grounds for many marine organisms, which need dissolved oxygen (DO) to live. Eutrophication due to the increasing terrestrial nutrient flux is one of the main factors that causes hypoxia, hypoxia can form naturally, for instance by the intrusion of upwelled low DO water into coastal oceans (Feely et al, 2008; Rabalais et al, 2010; Howarth et al, 2011; Breitburg et al, 2018). Dead zones mostly exist off river mouths. This is because the excess nutrients from riverine inputs enhance biological productivity in the surface ocean. Dead phytoplankton sink to the bottom of the coastal zone, where they decompose and consume oxygen. The stratification of seawater in coastal oceans in summer reduces bottom

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Conclusion