Ocean acidification in the surface water of subarctic western North Pacific Ocean

Chair: James Orr

Masahide Wakita(1), Akira Nagano (2), Tetsuichi Fujiki(2), Shuichi Watanabe(1)

1 JAMSTEC MIO, Mutsu, Aomori, 035-0022, Japan
2 JAMSTEC RCGR, Yokosuka, Kanagawa, 237-0061, Japan

Background
The subarctic western North Pacific Ocean is characterized by the high primary productivity and abundant marine resources. In the winter mixed layer of this region, pH decreased from 1997 to 2011 because of reduction of CO2 emission in winter by an increase in TA (Wakita et al., 2010, 2013). However, this detection of the pH decrease only in the winter is not enough to clarify the impact of acidification on biological production and ecosystems.

Methods
We investigate progression of acidification in the surface water by examining the temporal changes of pH using the CO2 system data sets previously observed at stations KNOT and K2 from 1997 to 2014. In order to estimate mean rate of temporal change by using observations during various seasons in different year, we eliminated the seasonal bias according to Takahashi et al. (2006). By comparison, we estimate the winter mixed layer of parameters from the temperature minimum layer following Wakita et al. (2013).

Findings
The pH decrease (-0.0018 ± 0.0005 yr-1) and xCO2 (1.6 ± 0.4 ppm yr-1) increase from deseasonalized anomaly were similar to those in the winter (-0.0013 ± 0.0003 yr-1; 1.3 ± 0.3 ppm yr-1). Deseasonalized TA increase in the surface water (0.6 µmol kg-1 yr-1) agreed with that in the winter (0.5 µmol kg-1 yr-1), while the increases of deseasonalized DIC (1.6 µmol kg-1 yr-1) and phosphate (0.018 µmol kg-1 yr-1) were significantly higher than those in the winter(1.0 µmol kg-1 yr-1; 0.007 µmol kg-1 yr-1).

Conclusions
The TA increase in the surface water suppresses oceanic acidification and CO2 uptake by anthropogenic CO2 increase in this region (CaCO3 Counter Pump). The higher increases of deseasonalized DIC and nutrients indicate that seasonal variations didn’t remain unchanged, and DIC and nutrient accumulated over time. It suggests the reduction of net community production.