Interactive comment on “On CO2 pertubation experiments: over-determination of carbonate chemistry reveals inconsistencies” by C. J. M. Hoppe et al.

Hoppe et al. report very large discrepancies between measured pCO2 and pCO2 calculated from other carbonate system parameters, in natural seawater treated to adjust pCO2. In particular, pCO2 calculated from the DIC (dissolved inorganic carbon) and TA (total alkalinity) pair is approximately 300 µatm lower than measured pCO2 and pCO2 calculated from other pairs, in the high pCO2 manipulations.

We suspect that these important discrepancies can probably be largely resolved by considering DOM (dissolved organic matter)-related alkalinity (TA-DOM), which was suggested to be important in a recent paper (Kim & Lee, 2009). Kim and Lee found large impacts on TA in phytoplankton culture experiments in which large amounts of DOM were generated, including a ratio of ~1:1 between amount of DOC (dissolved organic carbon) produced and increase in TA-DOM (their figure 3).

This could be highly relevant to explaining Hoppe et al’s results, if the source water for Hoppe et al.’s experiments (collected from the North Sea) was preconditioned by biological activity leading to high levels of TA-DOM. High levels of DOC occur in the North Sea, with levels in the central North Sea observed to vary seasonally between an average of about 100 µM C in autumn and an average of about 200 µM C in spring (figure 4 of Suratman et al., 2009).

Tyrrell, T., Interactive comment on “On CO2 pertubation experiments: over-determination of carbonate chemistry reveals inconsistencies” by C. J. M. Hoppe et al. Biogeosciences Discussions 7(2): C257-C260. Interactive comment.

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