Rising levels of carbon dioxide in the surface ocean (matching those in the atmosphere) are turning the oceans ever more acidic. Addition of nutrients to the ocean will reduce the carbon dioxide, which will lower the acidity. However, the actual effects are more complex than this simple picture. Modelling studies have indicated that there may also be a movement of acidity from the surface ocean to the deep ocean. Since the deep ocean has a much greater volume than the surface ocean, a small increase in acidity in the deep ocean will correspond to a considerably larger decrease in acidity through the surface ocean. This important issue is seriously under investigated. Work is needed to clarify the processes, to gain a better understanding of what is happening, and to utilise global models to predict the mitigation effects of Ocean Nourishment on surface ocean pH.