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. The variations of dissolved N2O concentrations were modest 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). Surface waters were very strongly over-saturated in CO2 and CH4 with respect to atmospheric equilibrium. Spatial variations were overwhelmingly more important than seasonal variations and than diurnal (day-night) variations. 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 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).