Divergent biophysical controls of aquatic CO2 and CH4 in the World’s two largest rivers

Carbon emissions to the atmosphere from inland waters are globally significant and mainly occur at tropical latitudes. However, processes controlling the intensity of CO 2 and CH 4 emissions from tropical inland waters remain poorly understood. Here, we report a data-set of concurrent measurements of the partial pressure of CO 2 (pCO 2 ) and dissolved CH 4 concentrations in the Amazon (n = 136) and the Congo (n = 280) Rivers. The pCO 2 values in the Amazon mainstem were significantly higher than in the Congo, contrasting with CH 4 concentrations that were higher in the Congo than in the Amazon. Large-scale patterns in pCO 2 across different lowland tropical basins can be apprehended with a relatively simple statistical model related to the extent of wetlands within the basin, showing that, in addition to non-flooded vegetation, wetlands also contribute to CO 2 in river channels. On the other hand, dynamics of dissolved CH 4 in river channels are less straightforward to predict and are related to the way hydrology modulates the connectivity between wetlands and river channels.