Potential impact of DOM accumulation on fCO2 and carbonate ion computations in ocean acidification experiments

The internal consistency of measurements and computations of components of the CO2-system, namely total alkalinity (A(T)), total dissolved carbon dioxide (C-T), CO2 fugacity (fCO(2)) and pH, has been confirmed repeatedly in open ocean studies when the CO2 system had been over determined. Differences between measured and computed properties, such as Delta fCO(2) (= fCO(2) (measured) - fCO(2) (computed from AT and CT)) / fCO(2) (measured) x100), are usually below 5 %. Recently, Hoppe et al. (2012) provided evidence of significantly larger Delta fCO(2) in some experimental setups. These observations are currently not well understood. Here we discuss a case from a series of phytoplankton culture experiments with Delta fCO(2) of up to about 25 %. Delta fCO(2) varied systematically during the course of these experiments and showed a clear correlation with the accumulation of dissolved organic matter (DOM). Culture and mesocosm experiments are often carried out under high initial nutrient concentrations, yielding high biomass concentrations that in turn often lead to a substantial build-up of DOM. In such experiments, DOM can reach concentrations much higher than typically observed in the open ocean. To the extent that DOM includes organic acids and bases, it will contribute to the alkalinity of the seawater contained in the experimental device. Our analysis suggests that whenever substantial amounts of DOM are produced during the experiment, standard computer programmes used to compute CO2 fugacity can underestimate true fCO(2) significantly when the computation is based on A(T) and C-T. Unless the effect of DOM-alkalinity can be accounted for, this might lead to significant errors in the interpretation of the system under consideration with respect to the experimentally applied CO2 perturbation. Errors in the inferred fCO(2) can misguide the development of parameterisations used in simulations with global carbon cycle models in future CO2-scenarios. Over determination of the CO2-system in experimental ocean acidification studies is proposed to safeguard against possibly large errors in estimated fCO(2).