Abstract
Acidification and hypoxia present significant ecological and environmental challenges for coastal oceans, particularly estuarine systems with high nutrient inputs such as the Changjiang Estuary (CJE). We conducted three replicate cruises from July to September in 2020 along the same transects off the CJE to investigate dynamic changes of coastal acidification and hypoxia. Bottom hypoxia expanded and intensified from July to August and alleviated in September. Changes in pHT generally followed those of dissolved oxygen (DO), although the spatial and temporal patterns did not entirely align. In August, the tight connection between hypoxia and acidification differed between the northern and southern regions. The northern region experienced widespread hypoxia and acidification with DO and pHT as low as 39 μmol kg−1 and 7.66, whereas the southern region maintained similar pHT values despite higher DO (>94 μmol kg−1). By September, hypoxia was alleviated, and pHT showed a more pronounced rise, with pHT increasing by ∼0.02 at the same DO level. Aerobic remineralization emerged as the primary driver of bottom acidification and hypoxia off the CJE during summer, peaking in August and diminishing by September. In August, the northern region experienced synchronous and severe hypoxia and acidification, whereas intermittent localized mixing in the south alleviated hypoxia. By September, enhanced mixing in the CJE increased the buffering capacity of subsurface waters, mitigating pH decline despite ongoing hypoxia. These findings improve our understanding of short-term dynamics in estuarine carbon cycling, acidification and hypoxia.
Plain Language Summary
Coastal areas, such as the Changjiang Estuary, are facing significant environmental challenges due to acidification and low oxygen levels. In 2020, we conducted three cruises from July to September to understand these changes better. We found that oxygen conditions worsened from July to August but improved by September. Interestingly, changes in acidity (pH levels) did not always match changes in oxygen levels. In August, the northern part of the estuary experienced both low oxygen and acidity, whereas the southern part had stable acidity despite higher oxygen levels. By September, enhanced water mixing by wind in the region helped improve oxygen levels and increased the resistance to acidification. Our research shows that remineralization of organic matter and wind stress are the main factors controlling pH and oxygen during summer. Understanding the drivers of acidification and deoxygenation is essential to predict future environmental changes and their potential consequences for coastal ecosystems.
Key Points
- Aerobic remineralization is the dominant driver of bottom hypoxia and acidification of the CJE, with peak intensity observed in August
- During August, intermittent localized mixing replenished bottom water oxygen, reducing hypoxia yet leaving acidification unchanged
- By September, enhanced mixing in the CJE increased buffering capacity, limiting pH decline where hypoxia persisted
Miao Y., Wang B., Carstensen J., Li D., Ma X., Sun Q., Xu Z., Jin H., Xu J., Zeng J., Zhou F. & Chen J., 2025. Diverging relationships between acidification and hypoxia off the Changjiang Estuary. Journal of Geophysical Research: Oceans 130(11): e2025JC022675. doi: 10.1029/2025JC022675. Article (restricted access).
