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

[摘要] The Oubangui is a major tributary of the Congo River, draining an area of~500 000 km² mainly consisting of woodedsavannahs. Here, we report results of a one year long, 2-weekly samplingcampaign in Bangui (Central African Republic) since March 2010 for a suiteof physico-chemical and biogeochemical characteristics, including totalsuspended matter (TSM), bulk concentration and stable isotope composition ofparticulate organic carbon (POC and δ¹³C_POC), particulatenitrogen (PN and δ¹⁵N_PN), dissolved organic carbon (DOCand δ¹³C_DOC), dissolved inorganic carbon (DIC and δ¹³C_DIC), dissolved greenhouse gases (CO₂, CH₄ andN₂O), and dissolved lignin composition. δ¹³C signatures ofboth 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 POCand 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 dissolvedorganic matter. We observed a strong seasonality in pCO₂, rangingbetween 470 ± 203 ppm for Q < 1000 m³ s⁻¹(n=10) to a maximum of 3750 ppm during the first stage of the risingdischarge. The low POC/PN ratios, high %POC and low and variable δ¹³C_POC signatures during low flow conditions suggest that themajority of the POC pool during this period consists of in situ producedphytoplankton, consistent with concurrent pCO₂ (partial pressure ofCO₂) values only slightly above and, occasionally, below atmosphericequilibrium. Water-atmosphere CO₂ fluxes estimated using twoindependent approaches averaged 105 and 204 g C m⁻² yr⁻¹, i.e. morethan an order of magnitude lower than current estimates for large tropicalrivers globally. Although tropical rivers are often assumed to show muchhigher CO₂ effluxes compared to temperate systems, we show that in situproduction may be high enough to dominate the particulate organic carbonpool, and lower pCO₂ values to near equilibrium values during lowdischarge conditions. The total annual flux of TSM, POC, PN, DOC and DIC are2.33 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 fluxesare 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 totalannual C flux, and under the assumptions that carbonate weatheringrepresents 25% of the DIC flux and that CO₂ from respiration driveschemical weathering, this flux is equivalent to ~50% ofterrestrial-derived riverine C transport.

[学科分类] 地球化学与岩石