Abstract:Human inputs of nutrients to coastal waters can lead to the excessive production of algae, a process known as eutrophication. Microbial consumption of this organic matter lowers oxygen levels in the water1, 2, 3. In addition, the carbon dioxide produced during microbial respiration increases acidity. The dissolution of atmospheric carbon dioxide in ocean waters also raises acidity, a process known as ocean acidification. Here, we assess the combined impact of eutrophication and ocean acidification on acidity in the coastal ocean, using data collected in the northern Gulf of Mexico and the East China Sea—two regions heavily influenced by nutrient–laden rivers. We show that eutrophication in these waters is associated with the development of hypoxia and the acidification of subsurface waters, as expected. Model simulations, using data collected from the northern Gulf of Mexico, however, suggest that the drop in pH since pre-industrial times is greater than that expected from eutrophication and ocean acidification alone. We attribute the additional drop in pH—of 0.05 units—to a reduction in the ability of these carbon dioxide-rich waters to buffer changes in pH. We suggest that eutrophication could increase the susceptibility of coastal waters to ocean acidification.