The combined effects of anthropogenic and biological CO₂ inputs may lead to more rapid acidification in coastal waters compared to the open ocean. It is less clear, however, how redox reactions would contribute to acidification. Here we report estuarine acidification dynamics based on oxygen, hydrogen sulfide (H₂S), pH, dissolved inorganic carbon and total alkalinity data from the Chesapeake Bay, where anthropogenic nutrient inputs have led to eutrophication, hypoxia and anoxia, and low pH. We show that a pH minimum occurs in mid-depths where acids are generated as a result of H₂S oxidation in waters mixed upward from the anoxic depths. Our analyses also suggest a large synergistic effect from river–ocean mixing, global and local atmospheric CO₂ uptake, and CO₂ and acid production from respiration and other redox reactions. Together they lead to a poor acid buffering capacity, severe acidification and increased carbonate mineral dissolution in the USA’s largest estuary.

The Changjiang River supplies huge amounts of fresh water and dissolved and particulate substances to the East China Sea, thereby exerting a great influence on the coastal ecosystem. Meanwhile, the construction of the Three Gorges Reservoir (TGR) has reallocated the annual discharge, likely affecting the transportation of carbon in its various forms. The transport and transformation of carbon in Changjiang River and the effect of the TGR were discussed based on three field campaigns, a 1 year time series investigation, and historical data. Our results indicated the following: (1) Dissolved inorganic carbon (DIC) was derived from the upper stream and was significantly diluted downstream by the low‐DIC waters from two large lakes. Dissolved organic carbon (DOC) was a product of anthropogenic input and showed no clear relationship with discharge. particulate organic carbon (POC) within total suspended matter (POC%) was below the global average. (2) The TGR has not measurably affected the transport of DOC downstream of the reservoir dam. However, downstream grain size has decreased and autochthonous processes have increased, resulting in a sharp increase in POC% since reservoir construction. (3) For the period 1997–2010, estimated annual DIC flux was 16.9 Tg yr⁻¹. The regulation of river flow by the TGR has decreased the river DIC flux to the East China Sea in the autumn and increased it in the spring. Furthermore, the South‐North Water Diversion will reduce the high‐DIC water from the upper reach, thus affecting the biogeochemistry of the Changjiang estuary and the ecosystem of the nearby coastal ocean.