Long-term trend of CO ₂ and ocean acidification in the surface water of the Ulleung Basin, the East/Japan Sea inferred from the underway observational data

Anthropogenic carbon is responsible for both global warming and ocean acidification. Efforts are underway to understand the role of ocean in a high CO ₂ world on a global context. However, marginal seas received little attention despite their significant contribution to biogeochemical cycles. Here we report the CO ₂ increase and ocean acidification in the surface waters of the Ulleung Basin (UB) of the East/Japan Sea, and possible causes are discussed. Fourteen observations of surface fCO ₂ were made in the period from 1995 to 2009. The contribution of temperature variation to the seasonality of fCO ₂ was almost equivalent to the non-thermal effect in the UB. However, the difference of relative contribution with the season makes two seasonal peaks of fCO ₂ in the surface water of the UB. Non-thermal effect contributed to the surface fCO ₂ drawdown in summer, whereas the surface fCO ₂ elevation in winter. The decadal trend of fCO ₂ increment was estimated by harmonic analysis. The estimated rates of increase of fCO ₂ were 1.8 ± 0.4 μatm yr ⁻¹ for the atmosphere and 2.7 ± 1.1 μatm yr ⁻¹ for the surface water. The ocean acidification trend, calculated from total alkalinity and fCO ₂, was estimated to be −0.03 ± 0.02 pH units decade ⁻¹. These rates seem to be higher than observations at most other ocean time-series sites during the same period of time. Sustained observations are required to understand more accurate trend in this area.