Variations of dissolved greenhouse gases (CO₂, CH₄, N₂O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity

We report the spatial variations of dissolved carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) concentrations in the lowland part of the Congo River network obtained during ten field expeditions carried out between 2010 and 2015, in the eastern part of the basin (Democratic Republic of Congo). Two transects of 1,650 km were carried out from the city of Kisangani to the city of Kinshasa, along the longest possible navigable section of the river, and corresponding to 41 % of the total length of the mainstem. Additionally, three time series of CH4 and N2O were obtained at fixed points in the mainstem of the middle Congo (2013–2018, biweekly sampling), in the mainstem of the lower Kasaï (2015–2017, monthly sampling), and in the mainstem of the middle Oubangui (2010–2012, biweekly sampling). The variations of dissolved N2O concentrations were modest, with values oscillating around the concentration corresponding to saturation with the atmosphere, with N2O saturation level (%N2O) ranging between 0 % and 561 % (average 142 %). The relatively narrow range of %N2O variations was consistent with low NH4+ (2.3±1.3 µmol L−1) and NO3− (5.6±5.1 µmol L−1) levels in these near pristine rivers and streams with low agriculture pressure on the catchment (croplands correspond to 0.1 % of catchment land cover of sampled rivers), dominated by forests (~ 70 % of land cover). The co-variations of %N2O, NH4+, NO3−, and dissolved oxygen saturation level (%O2) indicate N2O removal by sedimentary denitrification in low O2, high NH4+ and low NO3− environments (typically small and organic matter rich streams) and N2O production by nitrification in high O2, low NH4+ and high NO3− (typical of larger rivers that are poor in organic matter). Surface waters were very strongly over-saturated in CO2 and CH4 with respect to atmospheric equilibrium, with values of the partial pressure of CO2 (pCO2) ranging between 1,087 and 22,899 ppm (equilibrium ~ 400 ppm), and dissolved CH4 concentrations ranging between 22 and 71,428 nmol L−1 (equilibrium ~ 2 nmol L−1). Spatial variations were overwhelmingly more important than seasonal variations for pCO2, CH4 and %N2O, and than diurnal (day-night) variations for pCO2. The wide range of pCO2 and CH4 variations was consistent with the equally wide range of %O2 (0.3–122.8 %) and of dissolved organic carbon (DOC) (1.8–67.8 mg L−1), indicative of intense processing of organic matter that generated these two greenhouse gases. However, the emission rate of CO2 to the atmosphere from riverine surface waters was on average about 10 times higher than the flux of CO2 produced by aquatic net heterotrophy (as evaluated from measurements of pelagic respiration and primary production). This indicates that the CO2 emissions from the river network were sustained by lateral inputs of CO2 (either from terra firme or from wetlands). The pCO2 and CH4 values decreased and %O2 increased with increasing Strahler order, showing that stream size explains part of the spatial variability of these quantities. In addition, several lines of evidence indicate that lateral inputs of carbon from wetlands (flooded forest and aquatic macrophytes) were of paramount importance in sustaining high CO2 and CH4 concentrations in the Congo river network, as well as driving spatial variations: the rivers draining the Cuvette Centrale Congolaise (CCC) (a giant wetland of flooded forest in the core of the Congo basin) were characterized by significantly higher pCO2 and CH4 and significantly lower %O2 and %N2O values than those not draining the CCC; pCO2 and %O2 values were correlated to the coverage of flooded forest on the catchment. The flux of GHGs between rivers and the atmosphere averaged 2,469 mmol m−2 d−1 for CO2 (range 86 and 7,110 mmol m−2 d−1), 12,553 µmol m−2 d−1 for CH4 (range 65 and 597,260 µmol m−2 d−1), 22 µmol m−2 d−1 for N2O (range −52 and 319 µmol m−2 d−1). The estimate of integrated CO2 emission from the Congo River network (251 TgC (1012 gC) yr−1) corresponded to nearly half the CO2 emissions from tropical oceans globally (565 TgC yr−1) and was nearly two times the CO2 emissions from the tropical Atlantic Ocean (137 TgC yr−1). Moreover, the integrated CO2 emission from the Congo River network is more than three times higher than the estimate of terrestrial net ecosystem exchange (NEE) on the whole catchment (77 TgC yr−1). This shows that it is unlikely that the CO2 emissions from the river network were sustained by the hydrological carbon export from terra firme soils (typically very small compared to terrestrial NEE), but most likely, to a large extent, they were sustained by wetlands (with a much higher hydrological connectivity with rivers and streams).