Search

Global carbon dioxide (CO2) evasion from inland waters (rivers, lakes, and reservoirs) and carbon (C) export from land to oceans constitute critical terms in the global C budget. However, the magnitudes, spatiotemporal patterns, and underlying mechanisms of these fluxes are poorly constrained. Here, we used a coupled terrestrial–aquatic model to assess how multiple changes in climate, land use, atmospheric CO2 concentration, nitrogen (N) deposition, N fertilizer and manure applications have affected global CO2 evasion and riverine C export along the terrestrial‐aquatic continuum. We estimate that terrestrial C loadings, riverine C export, and CO2 evasion in the preindustrial period (1800s) were 1,820 ± 507 (mean ± standard deviation), 765 ± 132, and 841 ± 190 Tg C yr−1, respectively. During 1800–2019, multifactorial global changes caused an increase of 25% (461 Tg C yr−1) in terrestrial C loadings, reaching 2,281 Tg C yr−1 in the 2010s, with 23% (104 Tg C yr−1) of this increase exported to the ocean and 59% (273 Tg C yr−1) being emitted to the atmosphere. Our results showed that global inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans, highlighting the important role of inland waters in the global C balance, an amount that should be taken into account in future C budgets. Our analysis supports the view that a major feature of the global C cycle–the transfer from land to ocean–has undergone a dramatic change over the last two centuries as a result of human activities. Plain Language Summary: Despite occupying only 1% of the Earth's surface, inland waters (rivers, lakes, and reservoirs) play a critical role in global carbon (C) cycling by linking two of the Earth's largest C pools, terrestrial and marine ecosystems, as well as by exchanging CO2 with the atmosphere. Inland waters emit and bury C before it reaches the oceans, with important implications for the global C budget. Although global estimates of lateral C fluxes have been made previously, much uncertainty exists in their magnitudes, spatiotemporal patterns, and underlying controls (anthropogenic vs. natural processes). By improving a coupled terrestrial–aquatic model, we assess how climate, land use, atmospheric CO2, and nitrogen enrichment affected global CO2 evasion and riverine C export along the terrestrial–aquatic continuum since the 1800s. We estimate a 25% increase in terrestrial C loading since the 1800s, of which 59% was emitted to the atmosphere and 23% was exported to the ocean. The increased riverine C exports were primarily due to increasing atmospheric CO2 level and nitrogen inputs; additionally, climate and land conversion dominated interannual and decadal variations in CO2 evasion. Our findings indicate that anthropogenic‐induced climate change and multiple environmental stresses since the preindustrial era have resulted in significant increases in terrestrial C export to oceans and CO2 evasion. Global inland water recycles and exports nearly half of the net land C sink into the atmosphere and ocean, underscoring the importance of inland waters for closing the global carbon budget. Key Points: Terrestrial carbon loading since 1800 has increased by 25%, with 23% of this increase exported to the ocean and 59% being emitted to the atmosphereAtmospheric CO2 and N inputs dominated C export increase, while the climate and land use change dominated the decadal variations in CO2 evasionGlobal inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans in the 2010s [ABSTRACT FROM AUTHOR]

The role of streams and rivers in the global carbon (C) cycle remains unconstrained, especially in headwater streams where CO2 evasion (FCO2) to the atmosphere is high. Stream C cycling is understudied in the tropics compared to temperate streams, and tropical streams may have among the highest FCO2 due to higher temperatures, continuous organic matter inputs, and high respiration rates both in-stream and in surrounding soils. In this paper, we present paired in-stream O2 and CO2 sensor data from a headwater stream in a lowland rainforest in Costa Rica to explore temporal variability in gas concentrations and ecosystem processes. Further, we estimate groundwater CO2 inputs (GWCO2) from riparian well CO2 measurements. Paired O2–CO2 data reveal stream CO2 supersaturation driven by groundwater CO2 inputs and large in-stream production of CO2. At short time scales, CO2 was diluted during storm events, but increased at longer seasonal scales. Areal fluxes in our study reach show that FCO2 is supported by greater in-stream metabolism compared to GWCO2. Our results underscore the importance of tropical headwater streams as large contributors of carbon dioxide to the atmosphere and show evaded C can be derived from both in-stream and terrestrial sources. [ABSTRACT FROM AUTHOR]

The evasion of CO2 from inland waters, a major carbon source to the atmosphere, depends on dissolved inorganic carbon (DIC) concentrations. Our understanding of DIC dynamics across gradients of climate, geology, and vegetation conditions however have remained elusive. To understand its large‐scale patterns and drivers, we collated instantaneous and mean (multiyear average) DIC concentrations from about 100 rivers draining minimally‐impacted watersheds in the contiguous United States. Within individual sites, instantaneous concentrations (C) measured at daily to seasonal scales exhibit a near‐universal response to changes in river discharge (Q) in a negative power law form. High concentrations occur at low discharge when DIC‐enriched groundwater dominates river discharge; low concentrations occur under high flow when relatively DIC‐poor shallow soil water predominates river discharge. Such patterns echo the widely observed increase of soil CO2 and DIC with depth and the shallow‐and‐deep hypothesis that emphasizes the importance of flow paths and source water chemistry. Across sites, mean concentrations (Cm) decrease with increasing mean discharge (Qm), a long‐term climate measure, and reachs maxima at around 200 mm/yr. A parsimonious model reveals that high mean DIC arises from soil CO2 accumulation when rates of DIC‐generating reactions are relatively high compared to its export fluxes in arid climates. Although instantaneous and mean DIC concentrations similarly decrease with increasing discharge, results here highlight their distinct drivers: daily to seasonal‐scale instantaneous concentration variations (C) are controlled by subsurface CO2 distribution over depth (from below), whereas long‐term mean concentrations (Cm) are regulated by climate (from above). The results emphasize the significance of land‐river connectivity via subsurface flow paths. They also underscore the importance of characterizing subsurface CO2 distribution to illuminate belowground processes in order to project the future of water and carbon cycles in a warming climate. Key Points: Within individual sites, instantaneous dissolved inorganic carbon (DIC) (C) exhibits a near‐universal dilution pattern with lower concentrations at high discharge (Q)Across sites, DIC (Cm) similarly exhibits a dilution pattern with a lower mean concentration at high mean discharge (Qm) in humid climatesAlbeit similar patterns, results highlight distinct drivers of instantaneous and mean concentrations by subsurface CO2 profiles and climate, respectively [ABSTRACT FROM AUTHOR]

The transfer of CO2 from lakes to the atmosphere is a component of the global carbon equilibrium, while the quantification of the CO2 partial pressure (pCO2) is critical for exploring the contribution of freshwater CO2 emissions in the regional/global carbon budget. To investigate the daily variability of pCO2 and CO2 fluxes (fCO2), we conducted in situ biweekly pCO2 detection at 7:00, 10:00, 14:00, and 17:00 China Standard Time (CST) from Jan. to Sept. 2020 in the subtropical urbanizing Qinglonghu Lake in Chengdu, Sichuan, China. The pCO2 during the daytime varied greatly from 8.3 to 1,061.3 μatm, with an average of 137.7 μatm, while the average pCO2 (n = 11) clearly gradually decreased from 7:00 CST (204.9 ± 295.7 μatm) to 17:00 CST (93.5 ± 105.5 μatm). Similarly, the average fCO2 values were −19.3 (±27.5), −24.8 (±20.7), −29.2 (±9.1) and −30.4 (±10.7) mmol m2 h−1 at 7:00–17:00 CST, respectively. Further, we observed a negative correlation between pCO2 and water temperature and dissolved oxygen, but a positive correlation between pCO2 and total organic carbon and chlorophyll a. By a systematic overview of previously published data, we also discussed the differences and uncertainties in pCO2 and fCO2 estimates at regional and global scales. We therefore speculate that uncertainties may exist in the contributions of CO2 balance on lake surface in regional/global carbon budgets due to this daily pCO2 variation.

Abstract Streams in high‐elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO2) to the atmosphere by transforming terrestrial carbon to gaseous CO2. Studies of these environments are scarce, and estimates of CO2 fluxes are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO2 evasion and other biogeochemical processes. The first method, kinematic k600 (k600‐K), is derived from an empirical relationship between temperature‐adjusted k (k600) and the physical characteristics of the stream. The second method, measured k600 (k600‐M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO2 (pCO2) and CO2 evasion to the atmosphere, adjusting for temperature. Measurements were collected throughout a 5‐week period during the wet season of a peatland‐stream transition within a páramo ecosystem located above 4000 m in elevation in northeastern Ecuador. We characterized the spatial heterogeneity of the 250‐m reach on five occasions, and both methods showed a wide range of variability in k600 at small spatial scales. Values of k600‐K ranged from 7.42 to 330 m/d (mean = 116 ± 95.1 m/d), whereas values of k600‐M ranged from 23.5 to 444 m/d (mean = 121 ± 127 m/d). Temporal variability in k600 was driven by increases in stream discharge caused by rain events, whereas spatial variability was driven by channel morphology, including stream width and slope. The two methods were in good agreement (less than 16% difference) at high and medium stream discharge (above 7.0 L/s). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L/s, which represents 60% of the observations collected). Our study provides the first estimates of k600 values in a high‐elevation tropical catchment across steep environmental gradients and highlights the combined effects of hydrology and stream morphology in co‐regulating gas transfer velocities in páramo streams.

Streams in high‐elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO2) to the atmosphere by transforming terrestrial carbon to gaseous CO2. Studies of these environments are scarce, and estimates of CO2 fluxes are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO2 evasion and other biogeochemical processes. The first method, kinematic k600 (k600‐K), is derived from an empirical relationship between temperature‐adjusted k (k600) and the physical characteristics of the stream. The second method, measured k600 (k600‐M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO2 (pCO2) and CO2 evasion to the atmosphere, adjusting for temperature. Measurements were collected throughout a 5‐week period during the wet season of a peatland‐stream transition within a páramo ecosystem located above 4000 m in elevation in northeastern Ecuador. We characterized the spatial heterogeneity of the 250‐m reach on five occasions, and both methods showed a wide range of variability in k600 at small spatial scales. Values of k600‐K ranged from 7.42 to 330 m/d (mean = 116 ± 95.1 m/d), whereas values of k600‐M ranged from 23.5 to 444 m/d (mean = 121 ± 127 m/d). Temporal variability in k600 was driven by increases in stream discharge caused by rain events, whereas spatial variability was driven by channel morphology, including stream width and slope. The two methods were in good agreement (less than 16% difference) at high and medium stream discharge (above 7.0 L/s). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L/s, which represents 60% of the observations collected). Our study provides the first estimates of k600 values in a high‐elevation tropical catchment across steep environmental gradients and highlights the combined effects of hydrology and stream morphology in co‐regulating gas transfer velocities in páramo streams. [ABSTRACT FROM AUTHOR]

Robust estimates of CO2 budget, CO2 exchanged between the atmosphere and terrestrial biosphere, are necessary to better understand the role of the terrestrial biosphere in mitigating anthropogenic CO2 emissions. Over the past decade, this field of research has advanced through understanding of the differences and similarities of two fundamentally different approaches: "top‐down" atmospheric inversions and "bottom‐up" biosphere models. Since the first studies were undertaken, these approaches have shown an increasing level of agreement, but disagreements in some regions still persist, in part because they do not estimate the same quantity of atmosphere–biosphere CO2 exchange. Here, we conducted a thorough comparison of CO2 budgets at multiple scales and from multiple methods to assess the current state of the science in estimating CO2 budgets. Our set of atmospheric inversions and biosphere models, which were adjusted for a consistent flux definition, showed a high level of agreement for global and hemispheric CO2 budgets in the 2000s. Regionally, improved agreement in CO2 budgets was notable for North America and Southeast Asia. However, large gaps between the two methods remained in East Asia and South America. In other regions, Europe, boreal Asia, Africa, South Asia, and Oceania, it was difficult to determine whether those regions act as a net sink or source because of the large spread in estimates from atmospheric inversions. These results highlight two research directions to improve the robustness of CO2 budgets: (a) to increase representation of processes in biosphere models that could contribute to fill the budget gaps, such as forest regrowth and forest degradation; and (b) to reduce sink–source compensation between regions (dipoles) in atmospheric inversion so that their estimates become more comparable. Advancements on both research areas will increase the level of agreement between the top‐down and bottom‐up approaches and yield more robust knowledge of regional CO2 budgets. [ABSTRACT FROM AUTHOR]

Streams play an important role in the global carbon (C) cycle, accounting for a large portion of CO2 evaded from inland waters despite their small areal coverage. However, the relative importance of different terrestrial and aquatic processes driving CO2 production and evasion from streams remains poorly understood. In this study, we measured O2 and CO2 continuously in streams draining tundra‐dominated catchments in northern Sweden, during the summers of 2015 and 2016. From this, we estimated daily metabolic rates and CO2 evasion simultaneously and thus provide insight into the role of stream metabolism as a driver of C dynamics in Arctic streams. Our results show that aquatic biological processes regulate CO2 concentrations and evasion at multiple timescales. Photosynthesis caused CO2 concentrations to decrease by as much as 900 ppm during the day, with the magnitude of this diel variation being strongest at the low‐turbulence streams. Diel patterns in CO2 concentrations in turn influenced evasion, with up to 45% higher rates at night. Throughout the summer, CO2 evasion was sustained by aquatic ecosystem respiration, which was one order of magnitude higher than gross primary production. Furthermore, in most cases, the contribution of stream respiration exceeded CO2 evasion, suggesting that some stream reaches serve as net sources of CO2, thus creating longitudinal heterogeneity in C production and loss within this stream network. Overall, our results provide the first link between stream metabolism and CO2 evasion in the Arctic and demonstrate that stream metabolic processes are key drivers of the transformation and fate of terrestrial organic matter exported from these landscapes. [ABSTRACT FROM AUTHOR]

The article presents a study which examined the diel variations in carbon dioxide (CO2) concentrations and exchanges with the atmosphere in the flooded forests in the Amazon basin. Also cited are the estimated air-water gas exchange, gas transfer velocities, and mean daily CO2 fluxes that were recorded in the research.

The Canadian Arctic is warming at an unprecedented rate. Warming-induced permafrost thaw can lead to mobilization of aged carbon from stores in soils and rocks. Tracking the carbon pools supplied to surrounding river networks provides insight on pathways and processes of greenhouse gas release. Here, we investigated the dual-carbon isotopic characteristics of the dissolved inorganic carbon (DIC) pool in the main stem and tributaries of the Mackenzie River system. The radiocarbon ( 14 C) activity of DIC shows export of "old" carbon (2,380 ± 1,040 14 C years BP on average) occurred during summer in sampling years. The stable isotope composition of river DIC implicates degassing of aged carbon as CO 2 from riverine tributaries during transport to the delta; however, information on potential drivers and fluxes are still lacking. Accounting for stable isotope fractionation during CO 2 loss, we show that a large proportion of this aged carbon (60 ± 10%) may have been sourced from biospheric organic carbon oxidation, with other inputs from carbonate weathering pathways and atmospheric exchange. The findings highlight hydrologically connected waters as viable pathways for mobilization of aged carbon pools from Arctic permafrost soils., (© The Author(s) 2024. Published by Oxford University Press on behalf of National Academy of Sciences.)

Inland waters are major sources of CO2 to the atmosphere. The origin of this CO2 is often elusive, especially in high‐altitude streams that remain poorly studied at present. Here we study the spatial and seasonal variations in streamwater CO2, its potential sources and drivers in an Alpine stream network (Switzerland). High‐resolution sampling combined with stable isotope analysis and mixing models enabled us to capture the fine‐scale spatial heterogeneity in streamwater pCO2 as the stream network expanded and contracted during seasons. We identified soil respiration as a major source of CO2 to the stream. We also identified a major groundwater upwelling zone as an ecosystem "control point" that disproportionately influenced stream biogeochemistry. This was particularly pronounced when the stream network expanded during snowmelt, when it covered a five times larger area compared to winter (35,300 m2 compared to 7,100 m2). Downstream from this control point, CO2 evaded rapidly owing to high gas transfer velocity. The stream network was a net source of CO2 to the atmosphere with an average areal evasion flux of 30.1 (18.0–43.1) μmol · m‐2 · s‐1 and a total flux at network scale ranging from 237 (141–339) kg C/day in winter to 1793 (1069–2565) kg C/day during spring snowmelt. Our study highlights the role of stream network dynamics and control points for the CO2 dynamics in high‐altitude streams. Plain Language Summary: Streams are significant sources of CO2 to the atmosphere. However, the origin of this CO2 and its spatial and temporal dynamics are often not clear. Here we study the small‐scale spatial variation of streamwater CO2 and its evasion from an Alpine stream network in Switzerland. We found that groundwater upwelling, as constrained by local geomorphology, delivered large quantities of respiratory CO2 from adjacent soils to the stream. The relevance of these groundwater deliveries changes seasonally as the stream network expanded and contracted. Our spatially resolved sampling design allowed us to identify hot spots of CO2 evasion, based on which we were able to assess CO2 evasion at the scale of the entire stream network. Our findings emphasize the relevance of such approaches to properly constrain CO2 evasion fluxes beyond the reach scale. Key Points: We identified a geomorphological "control point" to deliver respiratory CO2 from soils to an Alpine stream via groundwaterWe show how streamwater CO2 dynamics and evasion varies with stream network expansion and contractionCO2 evasion from mountain streams might be underestimated due to high spatiotemporal variability in pCO2 [ABSTRACT FROM AUTHOR]

The river–floodplain network plays an important role in the carbon (C) cycle of the Amazon basin, as it transports and processes a significant fraction of the C fixed by terrestrial vegetation, most of which evades as CO2 from rivers and floodplains back to the atmosphere. There is empirical evidence that exceptionally dry or wet years have an impact on the net C balance in the Amazon. While seasonal and interannual variations in hydrology have a direct impact on the amounts of C transferred through the river–floodplain system, it is not known how far the variation of these fluxes affects the overall Amazon C balance. Here, we introduce a new wetland forcing file for the ORCHILEAK model, which improves the representation of floodplain dynamics and allows us to closely reproduce data‐driven estimates of net C exports through the river–floodplain network. Based on this new wetland forcing and two climate forcing datasets, we show that across the Amazon, the percentage of net primary productivity lost to the river–floodplain system is highly variable at the interannual timescale, and wet years fuel aquatic CO2 evasion. However, at the same time overall net ecosystem productivity (NEP) and C sequestration are highest during wet years, partly due to reduced decomposition rates in water‐logged floodplain soils. It is years with the lowest discharge and floodplain inundation, often associated with El Nino events, that have the lowest NEP and the highest total (terrestrial plus aquatic) CO2 emissions back to atmosphere. Furthermore, we find that aquatic C fluxes display greater variation than terrestrial C fluxes, and that this variation significantly dampens the interannual variability in NEP of the Amazon basin. These results call for a more integrative view of the C fluxes through the vegetation‐soil‐river‐floodplain continuum, which directly places aquatic C fluxes into the overall C budget of the Amazon basin. [ABSTRACT FROM AUTHOR]

Highlights • Strong declining trends of p CO 2 and CO 2 flux were found in lake surface water. • The lake was a CO 2 source in early morning but a sink in late afternoon. • p CO 2 dynamics were mainly driven by aquatic metabolism in the daytime. • Findings suggest uncertainties in the current carbon evasion estimates from lakes. Abstract Evasion of carbon dioxide (CO 2) from lakes is a significant component in the continental carbon balance, but most current CO 2 evasion estimates ignore daily CO 2 fluctuations. To test the hypothesis that partial pressure of CO 2 (p CO 2) and CO 2 evasion vary throughout a day due to biological processes driven by solar radiation, we conducted in-situ p CO 2 and ambient water measurements over eleven 10-h periods in a subtropical, eutrophic shallow lake from November 2017 to May 2018. In-situ measurements were performed at 7:00, 10:00, 14:00, and 17:00 Central Standard Time of the United States (CST), and CO 2 evasion rates were estimated based on the field p CO 2 records. Strong daily declining trends of p CO 2 and CO 2 flux were found throughout the seasons except for one winter day with unusually low temperatures. At 7:00, 10:00, 14:00, and 17:00 CST of a day, average p CO 2 were 1131, 839, 345 and 205 µatm, respectively, while average CO 2 fluxes were 80, 67, −10, and −34 mmol m2 h−1. Significant differences were found in average p CO 2 between any two measured time points in a day, while significant reductions in CO 2 flux were observed between 10:00 and 14:00 CST and between 14:00 and 17:00 CST. p CO 2 and CO 2 flux dynamics were most likely driven by the air-water exchanges during nighttime hours and mainly driven by aquatic metabolism in the daytime. These findings suggest possible large uncertainties in the estimation of carbon emitted from trophic lakes, highlighting the need for further research on diurnal p CO 2 fluctuation from different aquatic ecosystems to improve CO 2 evasion estimation. [ABSTRACT FROM AUTHOR]

Abstract: Lakes and reservoirs transform, emit, and bury carbon that is exported from land and are thus significant components of terrestrial carbon budgets. Their significance is often assessed by integrating these water bodies into terrestrial primary production. However, the transfer of inorganic carbon (IC) is likely a sticking point for these integrations because IC is not part of net ecosystem production. Here we integrated carbon evasion and organic carbon (OC) burial in a lake in the context of inorganic and OC cycling in a karst catchment from a system perspective. The lake emitted carbon dioxide (CO2) and buried OC at rates of 1.0 ± 0.2 and 0.9 ± 0.2 g C m−2 a−1, respectively, approximately equaling 13% and 11% of catchment net ecosystem production, respectively. These proportions represent significant influences on terrestrial carbon budgets, given an organic origin. However, catchment carbon export is dominated by IC that is derived from carbonates dissolved by soil CO2. Lake CO2 evasion accounts for less than 0.1% of soil CO2 efflux, suggesting little potential in significantly altering terrestrial carbon budgets. This comparison indicates the significance of aquatic CO2 evasion, requiring an adjustment of terrestrial carbon budgets to recognize their dependence on carbon origins. The significance may be overstated if inorganic origin is ignored. Our study suggests that a careful reassessment of the significance of CO2 evasion and OC burial in freshwater ecosystems to local and global carbon budgets, with full consideration of their sources, is necessary and pressing. [ABSTRACT FROM AUTHOR]

[Display omitted] • A eutrophic subtropical lake functioned as a CO 2 sink throughout diurnal hours in winter. • The lake functioned primarily as a CO 2 source in hot summer, but a sink in later afternoon hours. • The source-sink shifting indicates possible overestimation of CO 2 emission from global eutrophic lakes. • Large diurnal CO 2 fluctuation suggests a need for standardizing field measurement around 10o'clock local time. Carbon dioxide (CO 2) outgassing from inland eutrophic waters is a small but critical component of the global carbon cycle that is currently not well constrained. Eutrophic lake systems have been reported to be supersaturated with dissolved CO 2 (p CO 2), therefore considered a carbon source to the atmosphere. In this study, we aimed to test two hypotheses: 1) there is a substantial diurnal fluctuation in p CO 2 in eutrophic lakes, making them both a source and a sink of carbon; and 2) the diurnal fluctuation is seasonally dependent, with a pronounced variability during the summer. As a case study, we used a hypereutrophic lake in a highly urbanized area in subtropical southern USA. We conducted in-situ measurements on p CO 2 , photosynthetically active radiation (PAR), and other ambient parameters at 3-hour intervals for two days each in the winter and summer of 2022. The study shows that there was a clear diurnal fluctuation of p CO 2 with the peak in the early morning and the lowest in the later afternoon in both seasons. In the winter, the lake water was mainly CO 2 -unsaturated in reference to the atmospheric CO 2 of 420 ppm, ranging from a low of 128 ppm in the mid-afternoon to a high of 557 ppm in the early morning hours. In the summer, the lake water was mostly CO 2 -oversaturated, with p CO 2 increasing drastically from 309 ppm during the early afternoon to 3895 ppm in the early morning hours. In both winter and summer, CO 2 outgassing was nearly three times greater at night than in the daytime, increasing from −9.98 to –3.75 mg C m-2h−1 in the winter and from 13.17 to 45.34 mg C m-2h−1 in the summer. Consequently, a clear diurnal trend of CO 2 evasion rate (FCO 2) was found, showing that the lake often acted as a carbon sink during the daylight hours but as a carbon source at night, strongly affected by PAR availability and colored dissolved organic matter (cDOM). These findings validate our initial hypotheses and suggest that the diurnal and seasonal fluctuations of CO 2 evasion in eutrophic lakes should be considered in the global carbon accounting of lake systems to constrain uncertainties. Furthermore, we found that p CO 2 measurements at 10:00o'clock in the morning have the least deviation to the diurnal average and, therefore, propose standardizing this time for future studies in field p CO 2 measurements in order to make data comparable over seasons, geographical regions, and lake trophic levels. [ABSTRACT FROM AUTHOR]

Inland water systems are generally supersaturated in carbon dioxide (CO2) and are increasingly recognized as playing an important role in the global carbon cycle. The Arctic may be particularly important in this respect, given the abundance of inland waters and carbon contained in Arctic soils; however, a lack of trace gas measurements from small streams in the Arctic currently limits this understanding. We investigated the spatial variability of CO2 evasion during the summer low-flow period from streams and rivers in the northern portion of the Kolyma River basin in north-eastern Siberia. To this end, partial pressure of carbon dioxide (pCO2) and gas exchange velocities (k) were measured at a diverse set of streams and rivers to calculate CO2 evasion fluxes. We combined these CO2 evasion estimates with satellite remote sensing and geographic information system techniques to calculate total areal CO2 emissions. Our results show that small streams are substantial sources of atmospheric CO2 owing to high pCO2 and k, despite being a small portion of total inland water surface area. In contrast, large rivers were generally near equilibrium with atmospheric CO2. Extrapolating our findings across the Panteleikha–Ambolikha sub-watersheds demonstrated that small streams play a major role in CO2 evasion, accounting for 86% of the total summer CO2 emissions from inland waters within these two sub-watersheds. Further expansion of these regional CO2 emission estimates across time and space will be critical to accurately quantify and understand the role of Arctic streams and rivers in the global carbon budget.

• In-situ mass spectrometry allows to map the gas exchanges along headwater streams. • Gas exchange rate coefficients are highly heterogeneous along headwater streams. • Empirical equations systematically underestimate gas exchanges in small cascades. • Small cascades are hot spots for both stream oxygenation and CO 2 emission. • Overlooking small cascades leads to underestimate CO 2 emissions from headwaters. Using continuous injections of helium coupled to in-situ continuous flow membrane inlet mass spectrometry (CF-MIMS), we mapped the gas exchanges along two low-slope headwater streams having discharges of 25 L s−1 and 90 L s−1. Mean reaeration rate coefficients (k 2) were estimated at 130 d−1 and 60 d−1, respectively. Our study revealed that gas exchanges along headwater streams are highly heterogeneous. The variable morphology of the streambed causes gas exchanges to be focused into small areas, namely small cascades made up of stones or wood, with reaeration rate coefficients up to 40 times higher than in low-turbulent zones. As such, cascades appear to be hot spots for both oxygenation and greenhouse gases emissions. Additional O 2 and CO 2 measurements effectively showed fast exchanges between the stream and the atmosphere in the cascades, following the partial pressure gradients. These cascades allow a fast oxygenation of the eutrophic streams depleted in O 2 , which sustains respiration. Simultaneously, cascades release the oversaturated CO 2 originating from groundwater inputs to the atmosphere. By comparing measured reaeration rate coefficients to ten predictive equations from literature, we showed that all equations systematically underestimate reaeration rate coefficients, with significantly higher discrepancies in cascades than in low-turbulent zones. The inadequate characterization of the processes occurring in cascades causes empirical equations to have poor predictive capabilities, leading to a global underestimation of CO 2 emission from headwater streams. [ABSTRACT FROM AUTHOR]