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

We carried out 10 field expeditions between 2010 and 2015 in the lowland part of the Congo River network in the eastern part of the basin (Democratic Republic of the Congo), to describe the spatial variations in fluvial dissolved carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) concentrations.We investigate the possible drivers of the spatial variations in dissolved CO₂, CH₄ and N₂O concentrations by analyzing covariations with several other biogeochemical variables, aquatic metabolic processes (primary production and respiration), catchment characteristics (land cover) and wetland spatial distributions. We test the hypothesis that spatial patterns of CO₂, CH₄ and N₂O are partly due to the connectivity with wetlands, in particular with a giant wetland of flooded forest in the core of the Congo basin, the "Cuvette Centrale Congolaise" (CCC). Two transects of 1650 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 main stem. Additionally, three time series of CH₄ and N₂O were obtained at fixed points in the main stem of the middle Congo (2013-2018, biweekly sampling), in the main stem of the lower Kasai (2015-2017, monthly sampling) and in the main stem of the middle Oubangui (2010-2012, biweekly sampling). The variations in dissolved N₂O concentrations were modest, with values oscillating around the concentration corresponding to saturation with the atmosphere, with N₂O saturation level (%N₂O, where atmospheric equilibrium corresponds to 100 %) ranging between 0% and 561% (average 142 %). The relatively narrow range of %N₂O variations was consistent with low NH⁺₄ (2.3±1.3 μmol L^-1) and NO^-₃ (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 covariations in %N₂O, NH⁺₄, NO^-₃ and dissolved oxygen saturation level (%O₂) indicate N₂O removal by soil or sedimentary denitrification in low O₂, high NH⁺₄ and low NO^-₃ environments (typically small and organic matter rich streams) and N₂O production by nitrification in high O₂, low NH⁺₄ and high NO^-₃ (typical of larger rivers that are poor in organic matter). Surface waters were very strongly oversaturated in CO₂ and CH₄ with respect to atmospheric equilibrium, with values of the partial pressure of CO₂ (pCO₂) ranging between 1087 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 as well as 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 generation of these two greenhouse gases from intense processing of organic matter either in "terra firme" soils, wetlands or in-stream. 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 CCC 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 greenhouse gases (GHGs) between rivers and the atmosphere averaged 2469 mmolm^-2 d^-1 for CO₂ (range 86 and 7110 mmolm^-2 d^-1), 12 553 μmolm^-2 d^-1 for CH₄ (range 65 and 597 260 μmolm^-2 d^-1) and 22 μmolm^-2 d^-1 for N₂O (range-52 and 319 μmolm^-2 d^-1). The estimate of integrated CO₂ emission from the Congo River network (251±46 TgC (10¹²gC) yr^-1), corresponding to nearly half the CO₂ emissions from tropical oceans globally (565 TgC yr^-1) and was nearly 2 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 3 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).