Appropriate pCO2 treatments in ocean acidification experiments

Experiments in which organisms are reared in treatments simulating current and future pCO(2) concentrations are critical for ocean acidification (OA) research. The majority of OA exposure experiments use average atmospheric pCO(2) levels as a baseline treatment. We conducted an ecoregion-scale analysis of global carbon chemistry datasets. For many locales, atmospheric pCO(2) levels are not an appropriate characterization of marine carbon chemistry. We argue that atmospheric pCO(2) should be disregarded when setting baseline treatment conditions and experimental design should rely on measurements of carbon chemistry in a study subject's habitat. As carbon chemistry conditions vary with space and time, we suggest using a range of pCO(2) values as a control rather than a single value. We illustrate this issue with data on the habitat of Euphausia pacifica, which currently lives in waters with a pCO(2) around 900 mu atm, a concentration much higher than the current global atmospheric mean.