On CO₂ pertubation experiments: over-determination of carbonate chemistry reveals inconsistencies

Abstract. Seawater carbonate chemistry is typically calculated from two measured parameters. Depending on the choice of these input parameters, discrepancies in calculated pCO₂ have been recognized by marine chemists, but the significance of this phenomenon for CO₂ perturbation experiments has so far not been determined. To mimic different pCO₂ scenarios, two common perturbation methods for seawater carbonate chemistry (changing either DIC or TA) were applied using state-of-the-art protocols and equipment. The carbonate system was over-constrained by measuring DIC, TA, pH, and pCO₂. Calculated pCO₂ matched measured pCO₂ if pH and TA or pH and DIC were chosen as input parameters, whereas pCO₂ calculated from TA and DIC was considerably lower than measured values. This has important implications for CO₂ perturbation experiments. First, calculated pCO₂ values may not be comparable if different input parameters were used. Second, responses of organisms to acidification may be overestimated when using TA and DIC for calculations. This is especially troublesome for experiments with calcifiers, as carbonate ion concentration and thus calcite or aragonite saturation state are overestimated. We suggest refraining from measuring TA and DIC only and rather include pH as input parameter for carbonate chemistry calculations.