Overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters
This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Inland waters have been recognized as a significant source of carbon dioxide (CO 2) to the atmosphere at the global scale. Fluxes of CO 2 between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water-air gradient of the partial pressure of CO 2 (pCO 2). Nowadays, direct measurements of water 5 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 14.2 mmol L −1), pH (3.94 to 9.17), measured pCO 2 (36 to 23 000 ppmv), and 10 dissolved organic carbon (DOC) (29 to 3970 µmol 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 overestima-tion of 9080 ppmv). We suggest these large overestimations of calculated pCO 2 with 15 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 car-bonate alkalinity and high DOC concentrations; (2) a lower buffering capacity of the carbonate system at low pH, that increases the sensitivity of calculated pCO 2 to TA in acidic and organic-rich waters. We recommend that regional studies on pCO 2 should 20 not be based on pH and TA data only, and that direct measurements of pCO 2 should become the primary method in inland waters in general, and in particular in acidic, poorly buffered, freshwaters.