Sources of and processes controlling CO2 emissions change with the size of streams and rivers

Carbon dioxide (CO₂) evasion from streams and rivers to the atmosphere represents a substantial flux in the global carbon cycle. The proportions of CO₂ emitted from streams and rivers that come from terrestrially derived CO₂ or from CO₂ produced within freshwater ecosystems through aquatic metabolism are not well quantified. Here we estimated CO₂ emissions from running waters in the contiguous United States, based on freshwater chemical and physical characteristics and modelled gas transfer velocities at 1463 United States Geological Survey monitoring sites. We then assessed CO₂ production from aquatic metabolism, compiled from previously published measurements of net ecosystem production from 187 streams and rivers across the contiguous United States. We find that CO₂ produced by aquatic metabolism contributes about 28% of CO₂ evasion from streams and rivers with flows between 0.0001 and 19,000 m³ s^-1. We mathematically modelled CO₂ flux from groundwater into running waters along a stream-river continuum to evaluate the relationship between stream size and CO₂ source. Terrestrially derived CO₂ dominates emissions from small streams, and the percentage of CO₂ emissions from aquatic metabolism increases with stream size. We suggest that the relative role of rivers as conduits for terrestrial CO₂ efflux and as reactors mineralizing terrestrial organic carbon is a function of their size and connectivity with landscapes.