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| Titel |
Organic matter sources, fluxes and greenhouse gas exchange in the Oubangui River (Congo River basin) |
| VerfasserIn |
S. Bouillon, A. Yambélé, R. G. M. Spencer, D. P. Gillikin, P. J. Hernes, J. Six, R. Merckx, 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 ; 9, no. 6 ; Nr. 9, no. 6 (2012-06-08), S.2045-2062 |
| Datensatznummer |
250007117
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| Publikation (Nr.) |
 copernicus.org/bg-9-2045-2012.pdf |
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| Zusammenfassung |
| The Oubangui is a major tributary of the Congo River, draining an area of
~500 000 km2 mainly consisting of wooded
savannahs. Here, we report results of a one year long, 2-weekly sampling
campaign in Bangui (Central African Republic) since March 2010 for a suite
of physico-chemical and biogeochemical characteristics, including total
suspended matter (TSM), bulk concentration and stable isotope composition of
particulate organic carbon (POC and δ13CPOC), particulate
nitrogen (PN and δ15NPN), dissolved organic carbon (DOC
and δ13CDOC), dissolved inorganic carbon (DIC and δ13CDIC), dissolved greenhouse gases (CO2, CH4 and
N2O), and dissolved lignin composition. δ13C signatures of
both POC and DOC showed strong seasonal variations (−30.6 to −25.8‰, and −31.8 to −27.1‰,
respectively), but their different timing indicates that the origins of POC
and DOC may vary strongly over the hydrograph and are largely uncoupled,
differing up to 6‰ in δ13C signatures.
Dissolved lignin characteristics (carbon-normalised yields,
cinnamyl:vanillyl phenol ratios, and vanillic acid to vanillin ratios)
showed marked differences between high and low discharge conditions,
consistent with major seasonal variations in the sources of dissolved
organic matter. We observed a strong seasonality in pCO2, ranging
between 470 ± 203 ppm for Q < 1000 m3 s−1
(n=10) to a maximum of 3750 ppm during the first stage of the rising
discharge. The low POC/PN ratios, high %POC and low and variable δ13CPOC
signatures during low flow conditions suggest that the
majority of the POC pool during this period consists of in situ produced
phytoplankton, consistent with concurrent pCO2 (partial pressure of
CO2) values only slightly above and, occasionally, below atmospheric
equilibrium. Water-atmosphere CO2 fluxes estimated using two
independent approaches averaged 105 and 204 g C m−2 yr−1, i.e. more
than an order of magnitude lower than current estimates for large tropical
rivers globally. Although tropical rivers are often assumed to show much
higher CO2 effluxes compared to temperate systems, we show that in situ
production may be high enough to dominate the particulate organic carbon
pool, and lower pCO2 values to near equilibrium values during low
discharge conditions. The total annual flux of TSM, POC, PN, DOC and DIC are
2.33 Tg yr−1, 0.14 Tg C yr−1, 0.014 Tg N yr−1, 0.70 Tg C yr−1, and 0.49 Tg C yr−1, respectively. While our TSM and POC fluxes
are similar to previous estimates for the Oubangui, DOC fluxes were
~30% higher and bicarbonate fluxes were ~35% lower than previous reports. DIC represented 58% of the total
annual C flux, and under the assumptions that carbonate weathering
represents 25% of the DIC flux and that CO2 from respiration drives
chemical weathering, this flux is equivalent to ~50% of
terrestrial-derived riverine C transport. |
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