Subsurface pH and carbonate saturation state of aragonite on the Chinese side of the North Yellow Sea: seasonal variations and controls

Abstract. Based upon eight field surveys conducted between May 2011 and May 2012, we investigated seasonal variations in pH, carbonate saturation state of aragonite (Ω_arag), and ancillary data on the Chinese side of the North Yellow Sea, a western North Pacific continental margin of major economic importance. Subsurface waters were CO₂-undersaturated in May and nearly in equilibrium with atmospheric CO₂ in June. From July to October, the fugacity of CO₂ (fCO₂) of bottom water gradually increased from 438 ± 44 μatm to 630 ± 84 μatm, and pH_T decreased from 8.02 ± 0.04 to 7.88 ± 0.06 due to local aerobic remineralization of primary-production-induced biogenic particles. The subsurface community respiration rates in summer and autumn were estimated to be from 0.80 to 1.08 μmol-O₂ kg⁻¹ d⁻¹ within a relatively high salinity range of 31.63 to 32.25. From November to May in the next year, however, subsurface fCO₂ gradually decreased and pH increased due to cooling and water column ventilation. The corresponding bottom water Ω_arag was 1.85 ± 0.21 (May), 1.79 ± 0.24 (June), 1.75 ± 0.27 (July), 1.76 ± 0.29 (August), 1.45 ± 0.31 (October), 1.52 ± 0.25 (November), and 1.41 ± 0.12 (January). Extremely low Ω_arag values (from 1.13 to 1.40) were observed mainly in subsurface waters within the high salinity range of 31.63 to 32.25, which covered a major fraction of the study area in October and November. Of the China seas, the North Yellow Sea represents one of the systems most vulnerable to the potential negative effects of ocean acidification.