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

[摘要] The internal consistency of measurements and computations of components ofthe CO₂-system, namely total alkalinity (A_T, total dissolvedcarbon dioxide (C_T), CO₂ fugacity (fCO₂) and pH, hasbeen confirmed repeatedly in open ocean studies when the CO₂ system hadbeen over determined. Differences between measured and computed properties,such as ΔfCO₂ (= fCO₂ (measured) –fCO₂ (computed from A_T and C_T)) / fCO₂ (measured) × 100), are usually below 5%.Recently, Hoppe et al. (2012) provided evidence of significantly largerΔfCO₂ in some experimental setups. These observations arecurrently not well understood. Here we discuss a case from a series ofphytoplankton culture experiments with ΔfCO₂ of up to about25%. ΔfCO₂ varied systematically during the course of theseexperiments and showed a clear correlation with the accumulation of dissolvedorganic matter (DOM).

Culture and mesocosm experiments are often carried out under high initialnutrient concentrations, yielding high biomass concentrations that in turnoften lead to a substantial build-up of DOM. In such experiments, DOM canreach concentrations much higher than typically observed in the open ocean.To the extent that DOM includes organic acids and bases, it will contributeto the alkalinity of the seawater contained in the experimental device. Ouranalysis suggests that whenever substantial amounts of DOM are producedduring the experiment, standard computer programmes used to compute CO₂fugacity can underestimate true fCO₂ significantly when thecomputation is based on A_T and C_T. Unless the effect ofDOM-alkalinity can be accounted for, this might lead to significant errors inthe interpretation of the system under consideration with respect to theexperimentally applied CO₂ perturbation. Errors in the inferredfCO₂ can misguide the development of parameterisations used insimulations with global carbon cycle models in future CO₂-scenarios.Over determination of the CO₂-system in experimental ocean acidificationstudies is proposed to safeguard against possibly large errors in estimatedfCO₂.

[学科分类] 地球化学与岩石