Seasonal Variability and Future Projection of Ocean Acidification on the East China Sea Shelf off the Changjiang Estuary

Ocean acidification (OA) occurs universally in the world’s oceans. Marginal seas are facing more serious OA than the open ocean due to strong anthropogenic and natural impacts. This study investigates carbonate dynamics on the East China Sea (ECS) shelf off the Changjiang Estuary using field observations made from 2015 to 2019 that cover all four seasons. In the low productivity cold seasons, the water was well-mixed vertically. The coastal area and the northern ECS were occupied by water characterized by high dissolved inorganic carbon (DIC), low pH25 (pH at 25°C), and low ΩAr (saturation state index of aragonite), and influenced by the coastal water from the Yellow Sea (YS). However, during highly productive warm seasons, pH25 and ΩAr increased in the surface water but decreased in the bottom water as a result of strong biological DIC uptake in the surface water and CO2 production by strong organic matter remineralization in the bottom water. Strong remineralization decreased pH25 and ΩAr by 0.18 ± 0.08 and 0.73 ± 0.35 in the hypoxic bottom water in summer, even though the bottom water remained oversaturated with respect to aragonite (ΩAr > 1.0) during the surveys. Under the context of global OA and the strong seasonal acidification, the projected bottom water on the ECS shelf will be corrosive for aragonite by mid-century.