Variations in aragonite saturation state and its controlling factors in the South Yellow Sea in spring and autumn.

To assess the progression of ocean acidification in the South Yellow Sea (SYS), the aragonite saturation state (Ω) was determined from dissolved inorganic carbon (DIC) and total alkalinity (TA) in the surface and bottom waters of the SYS in spring and autumn. The Ω exhibited large spatiotemporal variations in the SYS; DIC was a major factor controlling the Ω variations, whereas temperature, salinity, and TA were minor factors. Surface DIC concentrations were mainly influenced by the lateral transport of the DIC-enriched Yellow River waters and DIC-depleted East China Sea Surface Water; bottom DIC concentrations were affected by aerobic remineralization in spring and autumn. Ocean acidification is now seriously progressing in the SYS, particularly in the Yellow Sea Bottom Cold Water (YSBCW) where the mean value of Ω substantially decreased from 1.55 in spring to 1.22 in autumn. All Ω values measured in the YSBCW in autumn were lower than the critical threshold value of 1.5 necessary for the survival of calcareous organisms.