Implications of observed inconsistencies in carbonate chemistry measurements for ocean acidification studies

Abstract. The growing field of ocean acidification research is concerned with the investigation of organism responses to increasing pCO₂ values. One important approach in this context is culture work using seawater with adjusted CO₂ levels. As aqueous pCO₂ is difficult to measure directly in small-scale experiments, it is generally calculated from two other measured parameters of the carbonate system (often A_T, C_T or pH). Unfortunately, the overall uncertainties of measured and subsequently calculated values are often unknown. Especially under high pCO₂, this can become a severe problem with respect to the interpretation of physiological and ecological data. In the few datasets from ocean acidification research where all three of these parameters were measured, pCO₂ values calculated from A_T and C_T are typically about 30% lower (i.e. ~300 μatm at a target pCO₂ of 1000 μatm) than those calculated from A_T and pH or C_T and pH. This study presents and discusses these discrepancies as well as likely consequences for the ocean acidification community. Until this problem is solved, one has to consider that calculated parameters of the carbonate system (e.g. pCO₂, calcite saturation state) may not be comparable between studies, and that this may have important implications for the interpretation of CO₂ perturbation experiments.