The winter period is seldom included in the estimates of aquatic‐atmospheric carbon exchange. In this study we quantified the flux of carbon dioxide (CO₂) and methane (CH₄) over 3 years from 12 small subarctic lakes. The lakes accumulated consistent and high amounts of CO₂ and CH₄ (56–97% as CO₂) over the winter, resulting in a high flux during ice thaw. The CO₂ flux during ice thaw increased with increasing mean depth of the lakes, while the CH₄ flux was high in lakes surrounded by mires. The ice thaw period was quantitatively important to the annual gas balances of the lakes. For nine of the lakes, 11 to 55% of the annual flux occurred during thaw. For three of the lakes with an apparent net annual CO₂ uptake, including the thaw period reversed the balance from sink to source. Our results suggest that the ice thaw period is critically important for the emissions of CO₂ and CH₄ in small lakes.

Northern inland waters emit CO₂ and CH₄ to the atmosphere but the importance of these emissions is poorly understood due to a lack of integrated catchment‐scale estimates of carbon (C) emissions from lakes and streams. In this study we quantified the annual emission of CO₂ and CH₄ from 27 lakes and 23 stream segments in a 15 km² subarctic catchment in northern Sweden. All lakes and streams were net sources of C to the atmosphere on an annual basis. Streams dominated (96%) the aquatic CO₂ emission while lakes (61%) dominated the aquatic CH₄ emission. Total aquatic C emission from the catchment was estimated to be 9.1 g C m⁻² yr⁻¹ (98% as CO₂). Although streams only accounted for 4% of the aquatic area in the catchment, they accounted for 95% of the total emission. The C emissions from lakes and streams were considerably larger than previously reported downstream waterborne export of C from the catchment, indicating that the atmospheric losses of C in the aquatic systems are an important component in the catchment C balance.

Low‐order streams are suggested to dominate the atmospheric CO₂ source of all inland waters. Yet, many large‐scale stream estimates suffer from methods not designed for gas emission determination and rarely include other greenhouse gases such as CH₄. Here, we present a compilation of directly measured CO₂ and CH₄ concentration data from Swedish low‐order streams (> 1600 observations across > 500 streams) covering large climatological and land‐use gradients. These data were combined with an empirically derived gas transfer model and the characteristics of a ca. 400,000 km stream network covering the entire country. The total stream CO₂ and CH₄ emission corresponded to 2.7 Tg C yr⁻¹ (95% confidence interval: 2.0–3.7) of which the CH₄ accounted for 0.7% (0.02 Tg C yr⁻¹). The study highlights the importance of low‐order streams, as well as the critical need to better represent variability in emissions and stream areal extent to constrain future stream C emission estimates.

Northern regions have received considerable attention not only because the effects of climate change are amplified at high latitudes but also because this region holds vast amounts of carbon (C) stored in permafrost. These carbon stocks are vulnerable to warming temperatures and increased permafrost thaw and the breakdown and release of soil C in the form of carbon dioxide (CO₂) and methane (CH₄). The majority of research has focused on quantifying and upscaling the effects of thaw on CO₂ and CH₄ emissions from terrestrial systems. However, small ponds formed in permafrost wetlands following thawing have been recognized as hotspots for C emissions. Here, we examined the importance of small ponds for C fluxes in two permafrost wetland ecosystems in northern Sweden. Detailed flux estimates of thaw ponds during the growing season show that ponds emit, on average (±SD), 279 ± 415 and 7 ± 11 mmol C m⁻² d⁻¹ of CO₂ and CH₄, respectively. Importantly, addition of pond emissions to the total C budget of the wetland decreases the C sink by ~39%. Our results emphasize the need for integrated research linking C cycling on land and in water in order to make correct assessments of contemporary C balances.

Climate warming raises the question whether high‐latitude landscape still function as net carbon (C) sinks. By compiling an integrated C balance for an intensely studied subarctic catchment, we show that this catchment's C balance is not likely to be a strong current sink of C, a commonly held assumption. In fact, it is more plausible (71% probability) that the studied catchment functions as a C source (−11 ± 20 g C m⁻² yr⁻¹). Analyses of individual fluxes indicate that soil and aquatic C losses offset C sequestering in other landscape components (e.g., peatlands and aboveground forest biomass). Our results stress the importance of fully integrated catchment C balance estimates and highlight the importance of upland soils and their interaction with the aquatic network for the catchment C balance.

Accounting for temporal changes in carbon dioxide (CO₂) effluxes from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of flux measurements of CO₂ based on the eddy covariance method from 13 lakes and reservoirs in the Northern Hemisphere, and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO₂ flux at all sites. By accounting for diel variation, only 11% of site-months were net daily sinks of CO₂. Annual CO₂ emissions had an average of 25% (range 3%–58%) interannual variation. Similar to studies on streams, nighttime emissions regularly exceeded daytime emissions. Biophysical regulations of CO₂ flux variability were delineated through mutual information analysis. Sample analysis of CO₂ fluxes indicate the importance of continuous measurements. Better characterization of short- and long-term variability is necessary to understand and improve detection of temporal changes of CO₂ fluxes in response to natural and anthropogenic drivers. Our results indicate that existing global lake carbon budgets relying primarily on daytime measurements yield underestimates of net emissions.