Global carbon dioxide efflux from rivers enhanced by high nocturnal emissions

Abstract

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 ...

Carbon dioxide fluxes increase from day to night across European streams

“…2). Altogether, these results point towards a high relevance of night-time CO 2 fluxes as reported earlier for single pre-alpine streams 12 , stream networks 13,17 or rivers 18 , and in a recent compilation of diel CO 2 data from 66 streams worldwide 24 . A rough annual extrapolation of fluxes from our study sites (Supplementary Methods) shows that the inclusion of night-time fluxes increases annual estimates of site-specific stream CO 2 emissions by 16% [6%; 25%] (Supplementary Table S4).…”

“…Although diel variabilities of gas exchange velocities have been reported for CO 2 and other gases 31,32 , the majority of the investigated streams in this study did not show those changes. The pCO 2 as a major driver of diel CO 2 flux variability was also identified by a global compilation of high-frequency CO 2 measurements 24 . Consequently, if no major changes in physical drivers of gas exchanges occur that strongly affect the turbulence, such as heavy rain events, it is sufficient to focus on pCO 2 for assessing diel flux changes at the water-air interface.…”

Spatial and temporal variability of &lt;i&gt;p&lt;/i&gt;CO&lt;sub&gt;2&lt;/sub&gt; and CO&lt;sub&gt;2&lt;/sub&gt; emissions from the Dong River in south China

“…induced precipitation during the wet season (Guérin et al, 2007;Alin et al, 2011;Ho et al, 2018). Exceptionally high k 600 values were observed in the surveyed rivers (Fig.…”

“…However, in our studied rivers, no significant correlation (Pearson correlation p>0.05) was found between wind speed and k 600 regardless of stream size. This could be explained by the lower wind speed (0.68 ± 0.66 and 1.09 ± 1.06 m s −1 for small and large rivers, respectively; Table 2) (Guérin et al, 2007). As the wind speed decreases, the impact of flow velocity on k 600 becomes increasingly predominant (Borges et al, 2004).…”

“…Environmental factors, including wind speed and hydrological variables, could affect the gas exchange at the water-air interface and are typically used to explain the variance in k 600 (Alin et al, 2011;Raymond et al, 2012). Flow velocity generally determines the k 600 in small rivers, while wind speed becomes a more important factor in controlling the k 600 in large rivers, reservoirs, and estuaries (Guérin et al, 2007;Rasera et al, 2013;Amaral et al, 2020). In our surveyed rivers, k 600 displayed a significant linear correlation (Pearson correlation, p<0.001) with the flow velocity.…”