Appropriate pCO2 treatments in ocean acidification experiments

Experiments in which organisms are reared in treatments simulating current and future pCO₂ concentrations are critical for ocean acidification (OA) research. The majority of OA exposure experiments use average atmospheric pCO₂ levels as a baseline treatment. We conducted an ecoregion-scale analysis of global carbon chemistry datasets. For many locales, atmospheric pCO₂ levels are not an appropriate characterization of marine carbon chemistry. We argue that atmospheric pCO₂ 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 pCO₂ 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 pCO₂ around 900 μatm, a concentration much higher than the current global atmospheric mean.