Ocean acidification in the Tsugaru warm current

The Tsugaru Warm Current (TWC) flows into the North Pacific from the Tsugaru Strait, between Honshu (main Japanese island) and Hokkaido, and is the branch of Tsushima warm current which is dominant warm current in the Sea of Japan. At the strait, fishing activities are particularly high but ocean acidification had little been monitored so far, as in other Japanese coastal regions. In February 2014, we initiated time-series measurements of temperature, salinity, nutrients, dissolved inorganic/organic carbon (DIC/DOC), total alkalinity (TA), total dissolved nitrogen and ¹⁸O-H₂O by weekly bucket sampling in the breakwater and floating pier of Sekine-hama port of MIO, JAMSTEC, located at the southern edge of the eastern strait. In addition, ship-based observations in the eastern area of this strait have been carried out every season since the summer of 2012. Using the data, we revealed the progression of acidification in the TWC. The annual mean pH calculated from DIC and TA of the surface mixed layer water (SML) is found to significantly decrease from 2012 to 2018 at a rate of −0.0024 ± 0.0006 /yr, which is faster than at other time-series sites in open ocean (~ −0.0016 /yr). This fast acidification is attributed to an enhanced increase rate of annual mean DIC (~2.8 mmol/kg/yr) due not only to oceanic uptake of anthropogenic CO₂ but also to other physical and/or biogeochemical processes. Moreover, below the surface mixed layer (20−200m depth) by ship-based observations, pH decline and DIC increase was significantly faster (−0.0068 to 0.0080 /yr and 3.3 to 7.0 mmol/kg/yr) than the SML, being associated with significant increases in phosphate. These results suggest that the fast mixed-layer pH decline is caused by the vertical mixing of DIC from subsurface water enhanced at the western strait. Accordingly, the coastal acidification in the Tsugaru Strait has progressed rapidly all over the depth, and its effect on marine calcifying organism must be paid close attention.