Long-term trends of oxygen concentration in the waters in bank and shelves of the Southern Japan Sea

Tsuneo Ono

doi:10.1007/s10872-021-00599-1

Abstract

While multiple studies have investigated oxygen decrease in Japan Sea Proper Water (JSPW; > 300 m in depth), oxygen variation in continental slope and shelf waters (< 300 m) must also be investigated in order to assess its socioecological impacts. In this study, historical oxygen data in the waters of three continental shelves and a bank of Japan Sea, off-Awashima area (AW), Wakasa Bay (WB), East of Tsushima Straight (ETS), and Yamato Bank (YB), were collected and analyzed to assess temporal variation of oxygen in each region from 1960 to 2000s. Significant decreasing trends of oxygen were detected in the waters below 150 m depth in WB and YB, and below 300 m in AW, in the summer season. In winter, a decreasing trend of oxygen was detected throughout the water column from 300 m to the sea surface in WB and YB. In ETS, a deoxygenation trend was detected throughout the water column from the bottom to the sea surface in the summer season, while no trend was detected in winter. The results suggested that oxygen decreases in AW, WB, and YB were the consequence of the upward propagation of the deoxygenation signal from JSPW, while that of ETS was caused by horizontal propagation of deoxygenation signal from the East China Sea. Assuming that the observed trend will continue in future, it is predicted that part of the water in Tsushima Strait area will reach the general sublethal threshold of oxygen (134 μmol kg−1) by the end of this century.

Highlights

Watanabe et al (2003) estimated that the analytical uncertainty of historical oxygen data in Japan Sea was less than 5 μmol kg−1, but the typical standard variation of oxygen obtained from a single-year time slice of Fig. 2 was 10 μmol kg−1
The results revealed that the oxygen concentrations in these waters have decreased, similar to those observed in Japan Sea Proper Water (JSPW)
The mechanism of oxygen decreases differed between the Tsushima Strait area and other coastal areas

Summary

Introduction

There are growing concerns about oxygen declines that are being detected in oceans worldwide, oceanic subsurface, and coastal regimes (e.g., Helm et al 2011; IPCC 2013, 2019; Ito et al 2017; Koslow et al 2011, 2015; Oschlies et al 2018; Sasano et al 2015, 2018; Schmidtko 2017; Stramma et al 2010, 2011, 2012, 2020; Rabalais et al 2010; Whitney et al 2013; Zhang et al 2010). The Japan Sea is known as a one of key spot of ocean deoxygenation, where oxygen concentration has dramatically decreased in the waters below a depth of 300 m (i.e., Japan Sea Proper Water, hereafter JSPW) since no later than 1950s (e.g., Chen, 1999; Gamo 1999, 2011; Gamo et al 1986, 2001, 2014; Minami et al 1999; Watanabe et al 2003). These phenomena are attributed to the result of diminished deep-water. The temporal change in oxygen content in shallow waters, especially those in continental-shelf areas,

Results

Discussion

Conclusion