Outgassing of carbon dioxide (CO2) from rivers and streams to the atmosphere is a major loss term in the coupled terrestrial‐aquatic carbon cycle of major low‐gradient river systems (the term “river system” encompasses the rivers and streams of all sizes that compose the drainage network in a river basin). However, the magnitude and controls on this important carbon flux are not well quantified. We measured carbon dioxide flux rates (FCO2), gas transfer velocity (k), and partial pressures (pCO2) in rivers and streams of the Amazon and Mekong river systems in South America and Southeast Asia, respectively. FCO2 and k values were significantly higher in small rivers and streams (channels <100 m wide) than in large rivers (channels >100 m wide). Small rivers and streams also had substantially higher variability in k values than large rivers. Observed FCO2 and k values suggest that previous estimates of basinwide CO2 evasion from tropical rivers and wetlands have been conservative and are likely to be revised upward substantially in the future. Data from the present study combined with data compiled from the literature collectively suggest that the physical control of gas exchange velocities and fluxes in low‐gradient river systems makes a transition from the dominance of wind control at the largest spatial scales (in estuaries and river mainstems) toward increasing importance of water current velocity and depth at progressively smaller channel dimensions upstream. These results highlight the importance of incorporating scale‐appropriate k values into basinwide models of whole ecosystem carbon balance.
A recent estimate of CO 2 outgassing from Amazonian wetlands suggests that an order of magnitude more CO 2 leaves rivers through gas exchange with the atmosphere than is exported to the ocean as organic plus inorganic carbon. However, the contribution of smaller rivers is still poorly understood, mainly because of limitations in mapping their spatial extent. Considering that the largest extension of the Amazon River network is composed of small rivers, the authors' objective was to elucidate their role in air-water CO 2 exchange by developing a geographic information system (GIS)-based model to calculate the surface area covered by rivers with channels less than 100 m wide, combined with estimated CO 2 outgassing rates at the Ji-Paraná River basin, in the western Amazon. Estimated CO 2 outgassing was the main carbon export pathway for this river basin, totaling 289 Gg C yr −1 , about 2.4 times the amount of carbon exported as dissolved inorganic carbon (121 Gg C yr −1 ) and 1.6 times the dissolved organic carbon export (185 Gg C yr −1 ). The relationships established here between drainage area and channel width provide a new model for determining small river surface area, allowing regional extrapolations of air-water gas exchange. Applying this model to the entire Amazon River network of channels less than 100 m wide (third to fifth order), the authors calculate that the surface area of small rivers is 0.3 ± 0.05 million km 2 , and it is potentially evading to the atmosphere 170 ± 42 Tg C yr −1 as CO 2 . Therefore, these ecosystems play an important role in the regional carbon balance.
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