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Biogeosciences Discuss., 9, 63-108, 2012
www.biogeosciences-discuss.net/9/63/2012/ doi:10.5194/bgd-9-63-2012 © Author(s) 2012. This work is distributed under the Creative Commons Attribution 3.0 License. |
Organic matter sources, fluxes and greenhouse gas exchange in the Oubangui River (Congo River basin)
1Department of Earth and Environmental Sciences, K.U. Leuven, Leuven, Belgium
2Service de l'Agrométéorologie et de Climatologie, Direction de la Météorologie Nationale, Bangui, Central African Republic
3Woods Hole Research Center, 149 Woods Hole Road, Falmouth, Massachusetts, 02540, USA
4Department of Geology, Union College, Schenectady, NY, USA
5Department of Land, Air, and Water Resources, University of California, One Shields Avenue, Davis, CA 95616, USA
6Department of Plant Sciences, University of California, Davis, One Shields Ave, California, 95616, USA
7University of Liège, Chemical Oceanography Unit, Liège, Belgium
Abstract. 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. We estimated the total annual flux of TSM, POC, PN, DOC and DIC to be 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. 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 pCO2 (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 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. Water-atmosphere CO2 fluxes were estimated to average ~ 105 g C m−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 pCO2 values to near equilibrium values during low discharge conditions.
Citation: Bouillon, S., Yambélé, A., Spencer, R. G. M., Gillikin, D. P., Hernes, P. J., Six, J., Merckx, R., and Borges, A. V.: Organic matter sources, fluxes and greenhouse gas exchange in the Oubangui River (Congo River basin), Biogeosciences Discuss., 9, 63-108, doi:10.5194/bgd-9-63-2012, 2012.