on a desk study to identify likely sources of error in the measurements of carbonate system parameters and related calculations , particularly with respect to coastal waters and ocean acidification experiments

Investigating the future of eco-systems in a high CO2 world requires experiments that use seawater that has been modified beyond the limits of our knowledge of the carbonate system. The partial pressure of carbon dioxide (pCO2) is difficult to measure directly in experimental systems and tends to be calculated from measurements of two of the other three components of the carbonate system (pH, total alkalinity TA, total dissolved inorganic carbon DIC). This desk study was commissioned by DEFRA following recent concerns about the accuracy with which the pCO2 of experimental systems can be determined by calculation from measurements of TA and DIC. The evidence reported by (Hoppe et al., 2010 [1]) suggests errors as large as 25 % may occur at high concentrations of pCO2 (~ 1000 μatm). This study has looked at the literature on the methodologies involved. It concludes that for work in open ocean waters the evidence is that measurements of TA and DIC can usually be used to calculate pCO2 within recognised limits that are tighter than the errors suggested by Hoppe et al. We provide maps to illustrate how the calculated values of pCO2 and carbonate ion vary with and are sensitive to changes in the values of the input pair of variables. These help to show graphically that accurate measurements of pH in conjunction with either TA or DIC can be used to accurately calculate both pCO2(aq) and the concentration of carbonate ion. Conversely, the maps also show that inaccurate measurements of pH severely compromise any subsequent calculations of concentrations of pCO2(aq) and carbonate ion. Calculation of the carbonate system is supported by three main software packages CO2SYS, SEACARB and SWCO2. These give closely similar results; and the scale of discrepancy, when different carbonic acid dissociation constants are used, is shown to be small relative to the errors reported by Hoppe et al. However it is not clear that the problem described by Hoppe et al., is due to inaccuracies either in the constants and equations describing the seawater chemistry or in the sampling and analytical measurements used by them. The ocean acidification community needs to be aware that seawater carbon measurements are not straightforward, and that it is as difficult to be confident in the accuracy of a measurement that is made without following best practice. A weakness with present procedures is in the measurement of carbonate alkalinity. Carbonate alkalinity is calculated from measurements of total alkalinity. The definition of total alkalinity is left open ended to allow for the inclusion of unknown protolytes. While the evidence from work in ocean waters is that carbonate alkalinity can generally be successfully calculated from total alkalinity, there is evidence that significant amounts of organic alkalinity may be present in enriched experimental systems and coastal waters. This is in the form of dissolved organic compounds and bacterial cells which can act as titrateable bases over the pH range of the alkalinity titration. We recommend that work be carried out: (i) to see if the finding of Hoppe et al. (2010) are reproduced when measurements are done following best practice (Dickson et al., 2007 [2]); (ii) to determine in which water types and systems the presence of organic alkalinity is significant; (iii) to provide a needed improvement in the accuracy and precision of measurements of pH. Where possible at least three of the carbonate system variables should be measured so that the system is over determined and any differences between calculated (and where possible observed) values of pCO2 can be determined. Certified reference buffers are now available to aid improvement in the accuracy of measurements of pH. Careful use of appropriate seawater based buffer solutions may enable a range of laboratories to achieve measurements of pH to better than 0.01 units equivalent to a pCO2 variation of 10 μatm if best practice is followed scrupulously. [1] Hoppe, C. J. M. , G. Langer, S. D. Rokitta, D. A. Wolf-­‐Gladrow, and B. Rost, 2010. On CO2 pertubation experiments: over-­‐determination of carbonate chemistry reveals inconsistencies. Biogeosciences Discuss. , 7, 1707–1726. [2] Dickson, A. G. , Sabine, C. L. , and Christian, J. R. : 2007. Guide to best practices for ocean CO2 measurements, PICES Special Publication, 3, Sidney, Canada, 2007. ACKNOWLEDGEMENTS Preparation of this report was funded (djh and sl) by DEFRA (contract ME4133) as part of DEFRA’s contribution to the UK Ocean Acidification programme of research. Toby Tyrrell’s contribution was supported by the EU FP7 Integrated Project EPOCA (European Project on OCean Acidification). The authors gratefully acknowledge the great effort that has already gone into supporting work in this field particularly documents such as Dickson et al., 2007 and Dickson, 2010 and the work of Frank Millero and co-workers. We also thank Andrew Dickson and Cynthia Dumousseaud, for useful exchanges in the early stage of preparation of this report, and Andrew Dickson, Jean Pierre Gattuso, Francois Muller, Zongpei Jiang, Clara Hoppe and Rachel Cave for comments on the final draft.