Modelling organic alkalinity in the Baltic Sea using a Humic-Pitzer approach

• A Pitzer and WHAM combination is used to model fulvic acid in the Baltic Sea.
• Measured organic alkalinity is consistent with DOC comprising 100% fulvic acid.
• The effect of fulvic acid on alkalinity titrations is assessed.

Significant excess alkalinity, of the order of 30 μmol kg− 1 and attributed to dissolved organic matter, has recently been measured in the Baltic Sea. Chemical speciation modelling shows that the measured excess alkalinity is consistent with an organic alkalinity derived from dissolved organic carbon, assuming that this dissolved organic carbon consists entirely of terrestrial humic substances. The contribution of polydisperse material such as humic substances to titration alkalinity invalidates the assumptions on which the current definition of titration alkalinity is based. It is therefore concluded that alkalinity should currently not be one of the parameters used to characterise the CO2 system in organic-rich waters. The use of a simple relationship to estimate organic alkalinity from the dissolved organic carbon concentration is assessed for the limited Baltic Sea data set currently available.