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- Title
Organic matter sources, fluxes and greenhouse gas exchange in the Oubangui River (Congo River basin).
- Authors
Bouillon, S.; Yambélé, A.; Spencer, R. G. M.; Gillikin, D. P.; Hernes, P. J.; Six, J.; Merckx, R.; Borges, A. V.
- Abstract
The Oubangui is a major tributary of the Congo River, draining an area of ~500 000km² 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. We estimated the total annual flux of TSM, POC, PN, DOC and DIC to be 2.33 Tg yr-1, 0.14 TgCyr-1, 0.014 TgNyr-1, 0.70 TgCyr-1, and 0.49 TgCyr-1, respectively. Most elements showed clear hysteresis over the hydrograph. δ13C signatures of both POC and DOC showed strong seasonal variations (-30.6 to -25.8 ‰, and -31.8 to -27.1 ‰, respectively) but with contrasting patterns. Our data indicate that the origins of POC and DOC may vary strongly over the hydrograph and are largely uncoupled, differing up to 6‰ in δ13C signatures. The low POC/PN ratios, high %POC and low and variable δ13CPOC signatures during low flow conditions suggest that during this period, the majority of the POC pool consists of in situ produced phytoplankton, consistent with concurrent Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed (partial pressure of CO2) values only slightly above and occasionally, below, atmospheric equilibrium. 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. We observed a strong seasonality in Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed , ranging between 470±203 ppm for Q<1000m³ s-1 (n=10) to a maximum of 3750 ppm during the first stage of the rising discharge. Water-atmosphere CO2 fluxes were estimated to average ~105 gCm-2 yr-1, i.e. more than an order of magnitude lower than current estimates for large tropical rivers globally. 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. 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 Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed values to near equilibrium values during low discharge conditions.
- Subjects
UBANGI River; CONGO River; GREENHOUSE gas mitigation; CARBON content of water; SAVANNAS; BIOGEOCHEMISTRY; PARTICULATE matter; STABLE isotopes
- Publication
Biogeosciences Discussions, 2012, Vol 9, Issue 1, p63
- ISSN
1810-6277
- Publication type
Article
- DOI
10.5194/bgd-9-63-2012