Significant efflux of carbon dioxide from streams and rivers in the United States

The evasion of carbon dioxide from inland waters was only recently included in assessments of the global carbon budget(1-3). Present estimates of carbon dioxide release from global freshwater systems, including lakes and wetlands, range from 0.7 to 3 : 3 Pg C yr(-1) (refs 1,4-7). However, these estimates are based on incomplete spatial coverage of carbon dioxide evasion, and an inadequate understanding of the factors controlling the efflux of carbon dioxide across large drainage networks(6). Here, we estimate the amount of carbon degassed from streams and rivers in the United States using measurements of temperature, alkalinity and pH, together with high-resolution data on the morphology and surface area of these waterways. We show that streams and rivers in the US are supersaturated with carbon dioxide when compared with the atmosphere, emitting 97 +/- 32 Tg carbon each year. We further show that regionally, carbon dioxide evasion from streams and rivers is positively correlated with annual precipitation, which we attribute to climatic regulation of stream surface area, and the flushing of carbon dioxide from soils. Scaling our analysis from the US to temperate rivers between 25 degrees N and 50 degrees N, we estimate a release of around 0.5 Pg carbon to the atmosphere each year.