Overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters

Abstract

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.

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Showing 1-10 of 32 references

Carbon dioxide concentration and atmospheric flux in the Hudson River, Estuaries

  • P A Raymond, N F Caraco, Cole
  • 1997
Highly Influential
15 Excerpts

Carbon dioxide and methane emissions and the carbon budget of a 10-year old tropical reservoir

  • G Abril, F Guérin, +7 authors M A Matvienko
  • 2005
Highly Influential
5 Excerpts

Temperature independence of carbon dioxide supersaturation in global lakes

  • S Sobek, L J Tranvik, Cole
  • 2005
Highly Influential
3 Excerpts

Direct and indirect pCO 2 measurements in a wide range of pCO 2 and salinity values

  • M Frankignoulle, A V Borges
  • 2001
Highly Influential
4 Excerpts

Acid-base properties of dissolved organic matter in the estuarine waters of Georgia

  • W.-J Cai, Y Wang, R E Hodson
  • 1998
Highly Influential
2 Excerpts

Limnological features of lakes on the Sepik-Ramu floodplain, Papua New Guinea

  • W Vyverman
  • 1994
Highly Influential
1 Excerpt

Global chemical weathering of surficial rocks estimated from river dissolved 20 loads

  • M Meybeck
  • 1987
Highly Influential
3 Excerpts
Showing 1-9 of 9 extracted citations