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Outgassing of carbon dioxide (CO2) from rivers and streams to the atmosphere is amajor loss term in the coupled terrestrial-aquatic carbon cycle of major low]gradient riversystems (the term griver system h encompasses the rivers and streams of all sizes thatcompose the drainage network in a river basin). However, the magnitude and controls onthis 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 theAmazon 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 mwide) than in large rivers (channels >100 m wide). Small rivers and streams also hadsubstantially higher variability in k values than large rivers. Observed FCO2 and k valuessuggest that previous estimates of basinwide CO2 evasion from tropical rivers andwetlands have been conservative and are likely to be revised upward substantially in thefuture. Data from the present study combined with data compiled from the literaturecollectively suggest that the physical control of gas exchange velocities and fluxes in lowgradientriver systems makes a transition from the dominance of wind control at the largestspatial scales (in estuaries and river mainstems) toward increasing importance of watercurrent velocity and depth at progressively smaller channel dimensions upstream. Theseresults highlight the importance of incorporating scale-appropriate k values into basinwidemodels of whole ecosystem carbon balance.

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