Organic matter sources, fluxes and greenhouse gas exchange in the Oubangui River (Congo River basin)

The Oubangui is a major tributary of the Congo River, draining an area of similar to 500 000 km(2) 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 physicochemical and biogeochemical characteristics, including total suspended matter (TSM), bulk concentration and stable isotope composition of particulate organic carbon (POC and delta C-13(POC)), particulate nitrogen (PN and delta N-15(PN)), dissolved organic carbon (DOC and delta C-13(DOC)), dissolved inorganic carbon (DIC and delta C-13(DIC)), dissolved greenhouse gases (CO2, CH4 and N2O), and dissolved lignin composition. delta C-13 signatures of both POC and DOC showed strong seasonal variations (-30.6 to -25.8 parts per thousand, and -31.8 to -27.1 parts per thousand, 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 parts per thousand in delta C-13 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 pCO(2), ranging between 470 +/- 203 ppm for Q < 1000 m(3) 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 delta C-13(POC) 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 pCO(2) (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 pCO(2) 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 TgC yr(-1), 0.014 Tg N yr(-1), 0.70 TgC yr(-1), and 0.49 Tg Cyr(-1), respectively. While our TSM and POC fluxes are similar to previous estimates for the Oubangui, DOC fluxes were similar to 30% higher and bicarbonate fluxes were similar to 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 similar to 50% of terrestrial-derived riverine C transport.