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.
Acidification and hypoxia are universal environmental issues in coastal seas, especially in large river estuaries such as the Pearl River estuary. In July and August of 2015, two legs of a field survey were conducted in the Pearl River plume. Leg 1 was sampled during the influence of upwelling favorable winds, while Leg 2 was during downwelling favorable winds. During both legs, instead of the typically observed dissolved inorganic carbon (DIC) consumption and dissolved oxygen (DO) over‐saturation, upwelling‐induced high DIC (>2000 μmol kg‐1), low pH (7.7‐7.8) and low DO (140‐150 μmol kg‐1) values were observed in surface waters at the estuary mouth and the area off Hong Kong. In the bottom waters, hypoxia, acidification (pH 7.6‐7.8) and DIC accumulation (DIC addition of ∼ 100‐180 μmol kg‐1) were observed. Hypoxia was less severe during Leg 2 compared to Leg 1. The stoichiometry of oxygen depletion to DIC addition was 0.89 for bottom water, suggesting remineralization was dominated by marine sourced organic matter. However, a comparison of data from the two legs showed that the stoichiometry of oxygen consumption to DIC accumulation was significantly higher during Leg 2 (0.73±0.03 for Leg 1 vs. 0.80±0.05 for Leg 2), although N/P ratios were the same (13.54±1.93 for Leg 1 vs. 13.51±2.04 for Leg 2). This phenomenon was attributed mainly to enhanced ventilation (re‐oxygenation) under the influence of the downwelling favorable winds during Leg 2. Although ventilation relieves some hypoxia, it might enhance acidification in bottom waters after a short‐term ventilation event. The enhanced acidification after short‐term ventilation is worthy of further study considering that most hypoxia and acidification are found in shallow coastal seas.
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