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

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 δ¹³C_POC), particulate nitrogen (PN and δ¹⁵N_PN), dissolved organic carbon (DOC and δ¹³C_DOC), dissolved inorganic carbon (DIC and δ¹³C_DIC), 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 δ¹³C_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 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.