Photosynthetic Production Determines Bottom Water Oxygen Variations in the Upwelling Coastal South China Sea Over Recent Decades

Xiaowei Zhu,Yuhang Tian,Wen Yan,Li Miao,Guodong Jia,Shendong Xu,Aibin Mo,Weihai Xu

doi:10.3389/feart.2021.759317

Abstract

Dissolved oxygen (DO) in seawater is fundamental to marine ecosystem health. How DO in coastal upwelling areas responds to upwelling intensity under climate change is of particular interest and vital importance, because these productive regions account for a large fraction of global fishery production and marine biodiversity. The Yuedong upwelling (YDU) in the coastal northern South China Sea can be served as a study case to explore long-term responses of DO to upwelling and climate due to minor influence of riverine input. Here, bottom water DO conditions were recovered by sedimentary C28Δ22/Δ5,22 ratios of steroids in three short cores, with lower ratio value indicating higher DO concentration. The ratio records showed oscillations in varying degrees and exhibited no clear trends before ∼1980s, after which, however, there occurred a persistent decreasing trend or basically remained at lower values. Thus, inferred DO variations by the C28Δ22/Δ5,22 ratio records are not compatible with regional YDU-involved physical processes under climate change, such as southwesterly wind-induced onshore advection of reduced-oxygenated source waters from outer shelf and oceanic warming that would rather lead to less oxygenation in bottom waters in recent decades. Intriguingly, the alcohol records of n-C20:1/C28Δ5,22 and br-C15/C28Δ5,22 ratios, indicative of the relative strengths between biogeochemical oxygen consumption (i.e., by zooplankton and microbes) and photosynthetic oxygen production (i.e., by phytoplankton), changed almost in parallel with the C28Δ22/Δ5,22 records in three cores. Accordingly, we propose that net photosynthetic oxygen production outweighs source water– and warming-induced increasing deoxygenation in the study area. This study may suggest an important biogeochemical mechanism in determining bottom water DO dynamics in shallow coastal upwelling regions with minor contribution of riverine input.

Highlights

IntroductionThe fate of dissolved oxygen (DO) in seawater has attracted increasing attentions from multi-disciplines, owing to its vital role in ecological, biological, and geochemical dynamics in marine environments (Diaz and Rosenberg, 2008; Breitburg et al, 2009; Breitburg et al, 2018; Levin and Breitburg, 2015; Watson, 2016; Schmidtko et al, 2017; Fennel and Testa, 2019)
We suggest that net photosynthetic oxygen production outweighs upwelling-involved source water– and oceanic warming–induced deoxygenation in bottom waters in the Yuedong upwelling (YDU) area
A series of sterols and stanols were identified, and the 5αstanol/Δ5-sterol ratios were examined for their applicability for redox reconstruction in sediment cores in the YDU area in the coastal northern South China Sea (SCS)

Summary

Introduction

The fate of dissolved oxygen (DO) in seawater has attracted increasing attentions from multi-disciplines, owing to its vital role in ecological, biological, and geochemical dynamics in marine environments (Diaz and Rosenberg, 2008; Breitburg et al, 2009; Breitburg et al, 2018; Levin and Breitburg, 2015; Watson, 2016; Schmidtko et al, 2017; Fennel and Testa, 2019). Close links between upwelling-favorable wind and climate change proposed previously (Bakun, 1990) have been recently evidenced by observations, models, and records in many upwelling regions (Gutierrez et al, 2011; Sydeman et al, 2014; Wang et al, 2015), showing strengthened wind-induced upwelling in response to climatic warming. This scenario may further complicate long-term responses of DO condition to upwelling variation and climate change; this issue has not been investigated in upwelling regions in the coastal northern South China Sea (SCS) (Hu and Wang, 2016), including the Yuedong upwelling (YDU)

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Discussion

Conclusion