CO₂ evasion from freshwater lakes is an important component of the carbon cycle. However, the relative contribution from different lake sizes may vary, since several parameters underlying CO₂ flux are size dependent. Here we estimated the annual lake CO₂ evasion from a catchment in northern Sweden encompassing about 30,000 differently sized lakes. We show that areal CO₂ fluxes decreased rapidly with lake size, but this was counteracted by the greater overall coverage of larger lakes. As a result, total efflux increased with lake size and the single largest lake in the catchment dominated the CO₂ evasion (53% of all CO₂ evaded). By contrast, the contribution from the smallest ponds (about 27,000) was minor (<6%). Our results emphasize the importance of accounting for both CO₂ flux rates and areal contribution of various sized lakes in assessments of CO₂ evasion at the landscape scale.

Streams are important emitters of CO₂ but extreme spatial variability in their physical properties can make upscaling very uncertain. Here, we determined critical drivers of stream CO₂ evasion at scales from 30 to 400 m across a 52.5 km² catchment in northern Sweden. We found that turbulent reaches never have elevated CO₂ concentrations, while less turbulent locations can potentially support a broad range of CO₂ concentrations, consistent with global observations. The predictability of stream pCO₂ is greatly improved when we include a proxy for soil‐stream connectivity. Catchment topography shapes network patterns of evasion by creating hydrologically linked “domains” characterized by high water‐atmosphere exchange and/or strong soil‐stream connection. This template generates spatial variability in the drivers of CO₂ evasion that can strongly bias regional and global estimates. To overcome this complexity, we provide the foundations of a mechanistic framework of CO₂ evasion by considering how landscape process domains regulate transfer and supply.

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 O-2 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.

Global warming is enhancing the mobilization of organic carbon (C) from Arctic soils into streams, where it can be mineralized to CO₂ and released to the atmosphere. Abiotic photo‐oxidation might drive C mineralization, but this process has not been quantitatively integrated with biological processes that also influence CO₂ dynamics in aquatic ecosystems. We measured CO₂ concentrations and the isotopic composition of dissolved inorganic C (δ¹³C_DIC) at diel resolution in two Arctic streams, and coupled this with whole‐system metabolism estimates to assess the effect of biotic and abiotic processes on stream C dynamics. CO₂ concentrations consistently decreased from night to day, a pattern counter to the hypothesis that photo‐oxidation is the dominant source of CO₂. Instead, the observed decrease in CO₂ during daytime was explained by photosynthetic rates, which were strongly correlated with diurnal changes in δ¹³C_DIC values. However, on days when modeled photosynthetic rates were near zero, there was still a significant diel change in δ¹³C_DIC values, suggesting that metabolic estimates are partly masked by O₂ consumption from photo‐oxidation. Our results suggest that 6–12 mmol CO₂‐C m⁻² d⁻¹ may be generated from photo‐oxidation, a range that corresponds well to previous laboratory measurements. Moreover, ecosystem respiration rates were 10 times greater than published photo‐oxidation rates for these Arctic streams, and accounted for 33–80% of total CO₂ evasion. Our results suggest that metabolic activity is the dominant process for CO₂ production in Arctic streams. Thus, future aquatic CO₂ emissions may depend on how biotic processes respond to the ongoing environmental change.