The East Sea (Japan Sea) in Change: A Story of Dissolved Oxygen

Dissolved oxygen (DO) is one of the most important oceanographic parameters measured for understanding various physicochemical processes in the ocean. This situation has been particularly true for the East Sea study ever since the first extensive investigation in the area during the 1930s (<xref ref-type="bibr" rid="bib23">Uda, 1934</xref>). Uda found very high and uniform concentrations of DO, around 250 µM (5.6 ml/l), for waters below a few hundred meters over entire basins, and assumed that a very fast ventilation system was operating in the East Sea. The Circulation Research of the East Asian Marginal Seas (CREAMS), Japan-Korea-Russia international cooperative studies on the East Sea have provided a unique opportunity to investigate the entire East Sea for the first time since Uda’s study. A spectrophotometrically modified Winkler method (<xref ref-type="bibr" rid="bib16">Pai et al., 1993</xref>) and a DO sensor (Sea Bird Model SBE 13) were tested successfully during the CREAMS studies for improving the precision and accuracy of DO measurement. The study further confirmed an earlier observation by <xref ref-type="bibr" rid="bib7">Gamo et al. (1986)</xref> that DO structures in the East Sea have been changing drastically in such a way that the DO minimum depths have deepened by more than 1000 meters during the last 30 years. While the causes for these changes are not known at the present time, the analysis of DO profiles strongly suggests that the mode of deep water ventilation system in the East Sea has shifted from bottom water formation in the past to intermediate water formation at the present time (<xref ref-type="bibr" rid="bib8">Kim and Kim, 1996</xref>). Studies of precise and accurate DO monitoring, along with other chemical tracers, deserve the highest priority for future research in the East Sea.