Significant changes in pH and saturation state of calcium carbonate in coastal ocean waters in Korea

Coastal ocean is more vulnerable to ocean acidification (OA) than open ocean due to high inputs of nutrients from land, large biological production, and various human activities. We started coastal acidification monitoring at three coastal (Busan, Jeju, and Ulleung) and one offshore (3km away from the Busan site) sites in Korea to examine long-term trends of the OA and their effects on coastal ocean environment. Discrete surface seawater samples for measurement of total dissolved inorganic carbon and total alkalinity were collected at the Busan, the Jeju and the Ulleung, and the offshore sites on a weekly, biweekly, and monthly basis, respectively. Here, we report changes in pH and saturation state of seawater with respect to aragonite (Ω) at the four sites for the period of 2019–2022. At the Busan and the Ulleung sites, both pH and Ω showed significant decreases, but there were no trends at the other two sites. The change rates of deseasonalized pH and Ω (-0.011 ± 0.005 yr^-1 for pH and -0.049 ± 0.024 yr^-1 for Ω) found at the Busan site were similar to those (-0.010 ± 0.005 yr^-1 for pH and -0.058 ± 0.025 yr^-1 for Ω) of the Ulleung site. These rates are about six times greater than the global long-term mean rates (-0.016 per decade for pH and -0.07 per decade for Ω). Sea surface temperature and salinity did not show any significant trends for the same period. Continuous monitoring of carbonate parameters at these sites is necessary to get robust long-term OA trends and understand coastal OA processes by finding their drivers.