Comparing Subsurface Seasonal Deoxygenation and Acidification in the Yellow Sea and Northern East China Sea Along the North-to-South Latitude Gradient

Qin-Sheng Wei,Yi-Xing Zhang,Cheng-Long Li,Tian-Qi Xiong,Song-Yin Wang,Si-Qing Yu,Shuo-Jiang Liu,Wei-Dong Zhai

doi:10.3389/fmars.2020.00686

Abstract

To better understand the relationship between subsurface seasonal deoxygenation and acidification in the Yellow Sea and northern East China Sea (ECS), we examined carbonate system parameters and dissolved oxygen (DO) of seven field surveys conducted in 2017–2018, spanning all four seasons. Low pHT values of 7.71‒7.80 and critically low aragonite saturation state (Ωarag) values of 1.07‒1.40 along with undersaturated DO of mostly higher than 150 μmol O2 kg–1 occurred in the Yellow Sea Cold Water Mass area in summer and autumn, while hypoxic DO values of 49‒63 μmol O2 kg–1 and extremely low pHT values of 7.68‒7.74 as well as critically low Ωarag values of 1.21‒1.39 were observed in the northern ECS in July 2018. At the beginning of warm-season stratification formation, the cold Yellow Sea waters had much higher DO but lower Ωarag values than those in relatively warmer ECS waters, while yearly initial pHT values rarely exhibited differences between the two coastal seas. During warm seasons, the central Yellow Sea accumulated respiration products beneath the thermocline in summer and autumn, while the northern ECS bottom waters preserved them only in summer. This study highlights fundamental roles of wintertime carbon dioxide (CO2) solubility along a north-to-south latitude gradient in the coastal acidification development. In comparison with the relatively low-latitude northern ECS subject to seasonal hypoxia, relatively high-latitude Yellow Sea exhibits higher CO2 solubility in winter and longer respiration-product accumulations in warm seasons, leading to lower Ωarag in the central Yellow Sea than those in the northern ECS. However, the present-day central Yellow Sea is free from hypoxia.

Highlights

The oceanic absorption of anthropogenic carbon dioxide (CO2) has lowered sea surface pH and calcium carbonate (CaCO3) mineral saturation state ( ) as compared with the preindustrial era, known as ocean acidification (Caldeira and Wickett, 2003; Orr et al, 2005; Doney et al, 2009)
Seasonal variations and the controls of bottom-water carbonate system parameters and DO in the Yellow Sea and northern ECS were examined in this study
At the beginning of warm-season stratification formation, the colder Yellow Sea waters had higher DO values but lower arag values than those in the relatively warmer northern ECS waters, while yearly initial pHT values were similar in the two coastal seas

Summary

Introduction

The oceanic absorption of anthropogenic carbon dioxide (CO2) has lowered sea surface pH and calcium carbonate (CaCO3) mineral saturation state ( ) as compared with the preindustrial era, known as ocean acidification (Caldeira and Wickett, 2003; Orr et al, 2005; Doney et al, 2009). PH is the negative logarithm of the sum of the concentrations of hydrogen (H+) and bisulfate (HSO4−) ions, i.e., total hydrogen ion concentration scale, pHT = -log10[H+]T, where [H+]T = [H+] + [HSO4−]. It affects chemical/biochemical properties of seawater, including chemical reactions, equilibrium conditions, and biological toxicity. In the Pacific Ocean, the present surface arag values are 3–4.5 in low-latitude regions while only 1–2 in high-latitude regions (Feely et al, 2012) This latitude gradient of arag is largely attributed to higher solubility of CO2 in colder seawaters of high-latitude regions. Seasonal aragonite undersaturation (i.e., arag < 1) has already been observed in surface and shallow subsurface waters of some northern polar seas (Bates et al, 2009; Fabry et al, 2009; Qi et al, 2017)

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Conclusion