Technical Note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters

Inland waters have been recognized as a significant source of carbon dioxide (CO2) to the atmosphere at the global scale. Fluxes of CO2 between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water-air gradient of the partial pressure of CO2 (pCO(2)). Currently, direct measurements of water pCO(2) remain scarce in freshwaters, and most published pCO(2) data are calculated from temperature, pH and total alkalinity (TA). Here, we compare calculated (pH and TA) and measured (equilibrator and headspace) water pCO(2) in a large array of temperate and tropical freshwaters. The 761 data points cover a wide range of values for TA (0 to 14 200 mu mol L-1), pH (3.94 to 9.17), measured pCO(2) (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 mu mol L-1). Calculated pCO(2) were > 10% higher than measured pCO(2) in 60% of the samples (with a median overestimation of calculated pCO(2) compared to measured pCO(2) of 2560 ppmv) and were > 100% higher in the 25% most organic-rich and acidic samples (with a median overestimation of 9080 ppmv). We suggest these large overestimations of calculated pCO(2) with respect to measured pCO(2) are due to the combination of two cumulative effects: (1) a more significant contribution of organic acids anions to TA in waters with low carbonate alkalinity and high DOC concentrations; (2) a lower buffering capacity of the carbonate system at low pH, which increases the sensitivity of calculated pCO(2) to TA in acidic and organic-rich waters. No empirical relationship could be derived from our data set in order to correct calculated pCO(2) for this bias. Owing to the widespread distribution of acidic, organic-rich freshwaters, we conclude that regional and global estimates of CO2 outgassing from freshwaters based on pH and TA data only are most likely overestimated, although the magnitude of the overestimation needs further quantitative analysis. Direct measurements of pCO(2) are recommended in inland waters in general, and in particular in acidic, poorly buffered freshwaters.