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Carbon dioxide (CO2) emissions to the atmosphere from running waters are estimated to be four times larger than the total carbon (C) flux to the oceans. However, these fluxes remain poorly constrained because of substantial temporal variability in dissolved CO2 concentrations. Using a global compilation of high frequency CO2 measurements, we demonstrate that nocturnal CO2 emissions are consistently larger, by an average of 27% (0.9 g C m -2 d -1 ), than those estimated from diurnal concentrations alone. Canopy shading is the principal control on observed diel (24 hr) variation, suggesting this nocturnal increase arises from daytime fixation of dissolved inorganic C by photosynthesis. Because contemporary global estimates of CO2 emissions to the atmosphere from running waters (0.65 -1.8 Pg C yr -1 ) rely primarily on discrete measurements of dissolved CO2 obtained during the day, they substantially underpredict the magnitude of this important flux. Accounting for night-time CO2 elevates global estimates of emissions from running waters to the atmosphere by 0.20-0.55 Pg C yr -1 .Carbon dioxide (CO2) emission from inland waters to the atmosphere is a major flux in the global carbon (C) cycle, and four-fold larger than the lateral C export to oceans 1 . Streams and rivers are hotspots for this flux, accounting for ~85% of inland water CO2 emissions despite covering <20% of the freshwater surface area 2 . Despite this importance, the magnitude of global CO2 emissions from streams and rivers remains highly uncertain with estimates revised upwards over the past decade from 0.6 to 3.48 Pg C yr -1 (3,4) . Changes to this estimate follow improvements in the spatial resolution for upscaling emissions 2,5 , as well as new studies from previously underrepresented areas such as the Congo 6 , Amazon 7 , and global mountains 8 . Further refinements have emerged from considering temporal variability in CO2 emission rates 9 . However, despite recent studies showing dramatic day-night changes in stream and river water CO2 concentrations 10-14 the significance of systematic sub-daily variation on overall CO2 emissions remains unexplored.Diurnal cycles in solar radiation impose a well-known periodicity on stream biogeochemical processes, creating diel (i.e., 24-hr period lengths) patterns for many solutes and gases, including nutrients, dissolved organic matter, and dissolved oxygen (O2) 15 . Indeed, diel variation in O2 arising from photosynthetic activity is the signal from which whole-system metabolic fluxes are estimated 16 . Photosynthetic production of O2 is stoichiometrically linked to the day-time assimilation of dissolved inorganic carbon (principally bicarbonate and dissolved CO2), lowering CO2 concentrations during the day. The resulting diel variation, with higher night-time CO2 concentrations when respiration reactions dominate, implies increased emissions at night. Despite the obvious connection between photosynthesis and CO2 consumption, the implications for total aquatic CO2 emissions has been neglected, most likely ...

Evasion of carbon dioxide (CO 2 ) from headwater streams is a dominant process controlling the fate of terrestrially derived carbon in inland waters. However, limitations of sampling techniques inhibit efforts to accurately characterize CO 2 evasion from streams, and particularly headwater streams with steep gradients, complex morphologies, and challenging terrain. CO 2 source dynamics coupled with turbulence conditions control gas transfer velocities of CO 2 (k CO2 ) and therefore drive CO 2 evasion. We present estimates of k CO2 and CO 2 evasion from a steep, turbulent headwater stream in southwestern British Columbia, Canada, collected using an automated in situ CO 2 tracer technique. Gas transfer velocities scaled positively with discharge, with a median k CO2 of 36.8 m/day and a range of 13.5 to 169 m/day. Gas transfer velocities were highest during high-flow events, with 84% of all CO 2 emissions occurring when discharge was higher than Q 50 , the median discharge (92.6 L/s). Widely used models overestimated gas transfer velocities with a mean relative error of 24% but underestimated k 600 values above 165 m/day. Our determinations of gas transfer velocities for a range of streamflow suggest that CO 2 evasion may be higher than previously estimated from direct measurements or models, particularly during high-flow events. These findings illustrate the need for direct, frequent, in situ determinations of k CO2 to accurately characterize CO 2 evasion dynamics in steep headwater streams.Plain Language Summary Characterizing the global carbon cycle is crucial for many aspects of earth science. Inland waters, such as streams, rivers, lakes, and estuaries, play a major role in the carbon cycle by carrying carbon from the land into water bodies and processing it, such that much of the carbon that enters an inland water body is either stored in sediments or evades into the atmosphere as carbon dioxide (CO 2 ), a potent greenhouse gas. Headwater streams are particularly active sites of CO 2 evasion, but a lack of data and limited sampling techniques inhibit our ability to accurately estimate and characterize this important system. This study provides new data on CO 2 evasion from a headwater stream and suggests that high streamflow events, such as storms, drive CO 2 evasion. This study also outlines a new technique for measuring CO 2 evasion from streams, which will allow researchers to collect high-quality data more frequently.

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