Technical Note: Large overestimation of <i>p</i>CO₂ 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 (<i>p</i>CO₂). Currently, direct measurements of water <i>p</i>CO₂ remain scarce in freshwaters, and most published <i>p</i>CO₂ data are calculated from temperature, pH and total alkalinity (TA). Here, we compare calculated (pH and TA) and measured (equilibrator and headspace) water <i>p</i>CO₂ 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 μmol L⁻¹), pH (3.94 to 9.17), measured <i>p</i>CO₂ (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 μmol L⁻¹). Calculated <i>p</i>CO₂ were >10% higher than measured <i>p</i>CO₂ in 60% of the samples (with a median overestimation of calculated <i>p</i>CO₂ compared to measured <i>p</i>CO₂ 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 <i>p</i>CO₂ with respect to measured <i>p</i>CO₂ 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 <i>p</i>CO₂ to TA in acidic and organic-rich waters. No empirical relationship could be derived from our data set in order to correct calculated <i>p</i>CO₂ 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 <i>p</i>CO₂ are recommended in inland waters in general, and in particular in acidic, poorly buffered freshwaters