Greenhouse gases (CO2, CH4, N2O) in the Congo River are driven by fluvial-wetland connectivity

We carried out ten field expeditions between 2010 and 2015, in the lowland part of the Congo River network, to describe the spatial variations of fluvial dissolved carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) concentrations. We investigated the drivers of the spatial variations of dissolved CO2, CH4 and N2O concentrations by analysing co-variations with several other biogeochemical variables, aquatic metabolic processes (primary production and respiration), catchment characteristics (land cover) and wetland spatial distribution. 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). The integrated CO2 emission from the Congo River network (251±46 TgC yr-1) was more than three times higher than terrestrial net ecosystem exchange (NEE) on the whole catchment. It is unlikely that the fluvial CO2 emissions were sustained by the hydrological carbon export from terra firme soils (typically very small compared to terrestrial NEE), but most likely, they were sustained by wetlands (with a much higher hydrological connectivity with rivers and streams).