Hydrologic controls on pCO2 and CO2 efflux in US streams and rivers

Streams and rivers emit petagrams of CO2, yet there is little known about how discharge variability impacts stream CO2 at broad scales. This article analyzed changes of pCO(2) and CO2 efflux with discharge in 813 conterminous US streams and rivers. Half of the streams and rivers (49%) showed positive pCO(2)-discharge responses, while significantly more (80%) showed positive CO2 efflux-discharge responses. More CO2 efflux therefore occurred at higher discharge. While CO2 efflux tended to increase with discharge at all stream orders, pCO(2) tended to decrease with discharge in small rivers (stream order 1-6) and increase in large rivers (stream order 7-10). Small streams and rivers tended to release more CO2 at higher discharge due to highly responsive gas transfer velocity. While high CO2 efflux in small streams (stream order 1-3) pointed to strong terrestrial input, high CO2 responsiveness in large rivers was likely due to increased riverine metabolism and river-corridor exchanges.