Potential impact of DOM accumulation on fCO 2 and carbonate ion computations in ocean acidification experiments
The internal consistency of measurements and computations of components of the CO
2-system, namely total alkalinity (
AT, total dissolved carbon dioxide (
2 fugacity (
2) and pH, has been confirmed repeatedly in open ocean studies when the CO
2 system had been over determined. Differences between measured and computed properties, such as Δ
2 (measured) –
2 (computed from
2 (measured) × 100), are usually below 5%. Recently, Hoppe et al. (2012) provided evidence of significantly larger Δ
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 Δ
2 of up to about 25%. Δ
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 CO 2 fugacity can underestimate true fCO 2 significantly when the computation is based on AT and CT. 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 CO 2 perturbation. Errors in the inferred fCO 2 can misguide the development of parameterisations used in simulations with global carbon cycle models in future CO 2-scenarios. Over determination of the CO 2-system in experimental ocean acidification studies is proposed to safeguard against possibly large errors in estimated fCO 2.