Global carbon dioxide emissions from inland waters. Nature, 503(7476): 355-359http://dx

Lay Abstract The exchange of gasses between water and air is important to the budgets of carbon, nutrients, and pollutants. This exchange is driven, in part, by the turbulent energy at the air–water interface. Turbulent energy at the air–water interface scales with the gas transfer velocity (k), which can be measured in streams through various methods. We performed a metadata analysis of studies that have measured k in streams using direct gas tracer releases. We evaluated models that predict k based on stream morphology. We found that models that use slope and velocity to predict k perform reasonably well and are consistent with general theory. We also used the data set to provide new stream hydraulic equations that predict stream morphology (width, depth, velocity) based on discharge.

No abstract

Carbon dioxide (CO 2 ) evasion from streams and rivers to the atmosphere represents a substantial flux in the global carbon cycle 1-3 . The proportions of CO 2 emitted from streams and rivers that come from terrestrially derived CO 2 or from CO 2 produced within freshwater ecosystems through aquatic metabolism are not well quantified. Here we estimated CO 2 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 CO 2 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 CO 2 produced by aquatic metabolism contributes about 28% of CO 2 evasion from streams and rivers with flows between 0.0001 and 19,000 m 3 s −1 . We mathematically modelled CO 2 flux from groundwater into running waters along a stream-river continuum to evaluate the relationship between stream size and CO 2 source. Terrestrially derived CO 2 dominates emissions from small streams, and the percentage of CO 2 emissions from aquatic metabolism increases with stream size. We suggest that the relative role of rivers as conduits for terrestrial CO 2 e ux and as reactors mineralizing terrestrial organic carbon is a function of their size and connectivity with landscapes.Inland waters play a central role in the global carbon (C) cycle by transforming, outgassing and storing more than half of the C they receive from terrestrial ecosystems before delivery to oceans 1-3 . Terrestrial C inputs to freshwaters are often of similar magnitude to terrestrial net ecosystem production (NEP; refs 1,2,4). Consequently, ignoring inland waters in landscape C budgets may overestimate terrestrial CO 2 uptake and storage 1,5 . In fact, not accounting for terrestrial C exports to and emissions from freshwaters could bias terrestrial NEP and net ecosystem exchange measurements by 4-60% (refs 6-8). Despite small areal coverage, running waters are hotspots for CO 2 emissions 3,9 , with high rates of outgassing relative to lake and terrestrial ecosystems on an areal basis 3,10,11 . Given their significant role in landscape C transformations, transport and emissions, there is a fundamental need to understand rates and drivers of C cycling in running waters.A mechanistic understanding of the processes regulating CO 2 emissions from streams and rivers is necessary for sound predictions of the present and future role of freshwaters in global C cycling and the climate system. Inland waters are often supersaturated with CO 2 due to inputs of terrestrially derived CO 2 and in situ aquatic mineralization of terrestrial OC (refs 12-15) (hereafter, 'internal production') as well as abiotic CO 2 production (Supplementary Section 1). CO 2 concentrations and emissions from running waters will thus vary with changes in land cover, climate, terrestrial ecosystem processes, land-water c...