Perennial rivers and streams make a disproportionate contribution to global carbon (C)cycling. However, the contribution of intermittent rivers and ephemeral streams, which

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Many inland waters exhibit complete or partial desiccation, or have vanished due to global change, exposing sediments to the atmosphere. Yet, data on carbon dioxide (CO 2) emissions from these sediments are too scarce to upscale emissions for global estimates or to understand their fundamental drivers. Here, we present the results of a global survey covering 196 dry inland waters across diverse ecosystem types and climate zones. We show that their CO 2 emissions share fundamental drivers and constitute a substantial fraction of the carbon cycled by inland waters. CO 2 emissions were consistent across ecosystem types and climate zones, with local characteristics explaining much of the variability. Accounting for such emissions increases global estimates of carbon emissions from inland waters by 6% (~0.12 Pg C y −1). Our results indicate that emissions from dry inland waters represent a significant and likely increasing component of the inland waters carbon cycle.

The potential for rivers to alter the flux of dissolved organic matter (DOM) from land to ocean is widely accepted. Yet anticipating when and where rivers behave as active reactors vs. passive pipes of DOM stands as a major knowledge gap in river biogeochemistry, resulting in uncertainties for global carbon models. Here, we investigate the controls on in-stream DOM dynamics by evaluating changes in DOM concentration and composition along several reaches of a medium-sized river network over one full hydrological year. Roughly half of the observations over time and space showed active reactor conditions and, among these, similar proportion of gains and losses was measured. High water residence times promoted the active over passive behavior of the reaches, while DOM properties and nitrate availability determined whether they supplied or removed DOM from the river. Among different DOM fractions, protein-like DOM both of terrestrial and aquatic origin seemed to drive bulk DOM patterns. Our study emphasizes the role of water residence time as a physical constraint for in-stream processes, and provides new insights into the key factors governing the net balance between in-stream gains and losses of DOM in rivers.

Temporary watercourses that naturally cease to flow and run dry comprise a notable fraction of the world's river networks, yet estimates of global carbon dioxide (CO 2 ) emissions from watercourses do not consider emissions from these systems when they are dry. Using data from a sampling campaign in a Mediterranean river during the summer drought period, we demonstrate that the CO 2 efflux from dry watercourses can be substantial, comparable to that from adjacent terrestrial soils and higher than from running or stagnant waters. With an up-scaling approach, we show that including emissions from dry watercourses could increase the estimate of CO 2 emissions from watercourses in our study region by 0.6-15%. Moreover, our results tentatively illustrate that emissions from dry watercourses could be especially important in arid regions, increasing the estimate of global CO 2 emissions from watercourses by 0.4-9%. Albeit relatively small, the contribution of dry watercourses could help to constrain the highly uncertain magnitude of the land carbon sink. We foresee that in many areas of the world, the expected increase in the extent of temporary watercourses associated with future global change will increase the relevance of CO 2 emissions from dry watercourses.

Most fluvial networks worldwide include watercourses that recurrently cease to flow and run dry. The spatial and temporal extent of the dry phase of these temporary watercourses is increasing as a result of global change. Yet, current estimates of carbon emissions from fluvial networks do not consider temporary watercourses when they are dry. We characterized the magnitude and variability of carbon emissions from dry watercourses by measuring the carbon dioxide (CO2) flux from 10 dry streambeds of a fluvial network during the dry period and comparing it to the CO2 flux from the same streambeds during the flowing period and to the CO2 flux from their adjacent upland soils. We also looked for potential drivers regulating the CO2 emissions by examining the main physical and chemical properties of dry streambed sediments and adjacent upland soils. The CO2 efflux from dry streambeds (mean ± SD = 781.4 ± 390.2 mmol m−2 day−1) doubled the CO2 efflux from flowing streambeds (305.6 ± 206.1 mmol m−2 day−1) and was comparable to the CO2 efflux from upland soils (896.1 ± 263.2 mmol m−2 day−1). However, dry streambed sediments and upland soils were physicochemically distinct and differed in the variables regulating their CO2 efflux. Overall, our results indicate that dry streambeds constitute a unique and biogeochemically active habitat that can emit significant amounts of CO2 to the atmosphere. Thus, omitting CO2 emissions from temporary streams when they are dry may overlook the role of a key component of the carbon balance of fluvial networksThis research was funded by the Spanish Ministry of Economy and Competitiveness through the Projects CGL2011-30474-C02-01 and CGL2014-58760-C3-1-R

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 ...