Reoxygenation of the Hypoxia in the East China Sea: A Ventilation Opening for Marine Life

Chung-Chi Chen,Chun-Chi Lien,Hung-Jen Lee,Fuh-Kwo Shiah,Yu-Sin Wita Huang,Wen-Chen Chou,Gwo-Ching Gong,Dong S Ko

doi:10.3389/fmars.2021.787808

Abstract

Hypoxia and upwelling co-occur in the summer, and well-mixed water typically reaches the subsurface in the East China Sea (ECS), especially off the Changjiang River estuary. The impact of upwelling on hypoxia and, therefore, on the ecosystem in the ECS is not known. This study demonstrates both positive and negative effects of upwelling on hypoxia and its impact on the ecosystem. With upwelling, the spatial extent of hypoxic water increases with a lower pH but waters with high regenerated nutrients and fugacity of CO2 (fCO2), which are normally confined to the near-bottom, are found just up to 5–10 m below the surface. This upwelled high nutrient water can enhance phytoplankton growth in this region. On one occasion in August 2014, upwelling reached to the surface and lasted for 3 weeks, with the area of coverage ranging from 326.8 to 24,368.0 km2. During this event, the water was mixed thoroughly throughout the water column, with high concentrations of nutrients, chlorophyll a, and slightly undersaturated dissolved oxygen but saturated fCO2, alongside a normal pH. This event may have served as an important pathway from the ocean to the atmosphere for the regenerated CO2. It also provided a productive and suitable environment for marine life and ventilation to alleviate low-oxygen stress in this hypoxic but upwelling region in the ECS.

Highlights

In coastal oceans, the hypoxic zone has been expanding globally over the last half-century (Diaz and Rosenberg, 2008; Breitburg et al, 2018)
Hypoxia was observed in July 2014, and it covered a wider area in the East China Sea (ECS) in August of 2003 and 2014, especially off the CRE and along the Zhejiang coast (Figures 2b–d)
Well-mixed water often reaches 5–10 m below the surface in this upwelling during summer

Summary

Introduction

The hypoxic zone (commonly known as a “dead zone”) has been expanding globally over the last half-century (Diaz and Rosenberg, 2008; Breitburg et al, 2018). In the inner shelf of the East China Sea (ECS), especially outside the Changjiang River estuary (CRE, or Yangtze River; Figure 1), hypoxia has been frequently observed since the 1950s using evidence from sediment microfossils (Li et al, 2002, 2011). This and the nearby area form an important fishing ground (Zhang et al, 2016). Acidification and high nutrient regeneration have been observed in the hypoxic regions in the ECS (Cai et al, 2011; Wang et al, 2016; Zhu et al, 2017; Chen et al, 2020)

Methods

Results

Conclusion