Increasing atmospheric carbon dioxide (CO(2)) is raising seawater CO(2) concentrations and thereby acidifying ocean water. But a second environmental problem, eutrophication, is also causing large CO(2) inputs into coastal waters. This occurs because anthropogenic inputs of nutrients have fueled massive algal blooms, which deplete bottom waters of oxygen (O(2)) and release CO(2) when the organic matter from these blooms is respired by bacteria. On the basis of a biogeochemical model, these CO(2) inputs are predicted to decrease current pH values by 0.25 to 1.1 units, effects that increased with decreasing temperature and salinity. Our model predictions agreed well with pH data from hypoxic zones in the northern Gulf of Mexico and Baltic Sea, two eutrophic coastal systems with large temperature and salinity differences. The modeled and measured decreases in pH are well within the range shown to adversely impact marine fauna. Model calculations show that the acidification from respiratory CO(2) inputs interacts in a complex fashion with that from increasing atmospheric CO(2) and that these pH effects can be more than additive in seawater at intermediate to higher temperatures. These interactions have important biological implications in a future world with increasing atmospheric CO(2), increasing anthropogenic inputs of nutrients, and rising temperatures from CO(2)-linked global warming.