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| Titel |
Technical Note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters |
| VerfasserIn |
G. Abril, S. Bouillon, F. Darchambeau, C. R. Teodoru, T. R. Marwick, F. Tamooh, F. Ochieng Omengo, N. Geeraert, L. Deirmendjian, P. Polsenaere, A. V. Borges |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 1 ; Nr. 12, no. 1 (2015-01-06), S.67-78 |
| Datensatznummer |
250117759
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| Publikation (Nr.) |
 copernicus.org/bg-12-67-2015.pdf |
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| Zusammenfassung |
| 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
(pCO2). Currently, direct measurements of water pCO2 remain
scarce in freshwaters, and most published pCO2 data are calculated
from temperature, pH and total alkalinity (TA). Here, we compare
calculated (pH and TA) and measured (equilibrator and headspace)
water pCO2 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 pCO2
(36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to
3970 μmol L−1). Calculated pCO2 were >10%
higher than measured pCO2 in 60% of the samples (with a median
overestimation of calculated pCO2 compared to measured pCO2 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 pCO2 with respect to
measured pCO2 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 pCO2 to TA in acidic and
organic-rich waters. No empirical relationship could be derived from our
data set in order to correct calculated pCO2 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 pCO2 are recommended in inland waters
in general, and in particular in acidic, poorly buffered freshwaters. |
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