Abstract Arctic regions are experiencing rapid warming, leading to permafrost thaw and formation of numerous water bodies. Although small ponds in particular are considered hot spots for methane (CH 4 ) release, long‐term studies of CH 4 efflux from these surfaces are rare. We have collected an extensive data set of CH 4 ebullition (bubbling) measurements from eight small thaw ponds (<0.001 km 2 ) with different physical and hydrological characteristics over four summer seasons, the longest set of observations from thaw ponds to date. The measured fluxes were highly variable with an average of 20.0 mg CH 4 · m −2 · day −1 (median: 4.1 mg CH 4 · m −2 · day −1 , n = 2,063) which is higher than that of most nearby lakes. The ponds were categorized into four types based on clear and significant differences in bubble flux. We found that the amount of CH 4 released as bubbles from ponds was very weakly correlated with environmental variables, like air temperature and atmospheric pressure, and was potentially more related to differences in physical characteristics of the ponds. Using our measured average daily bubble flux plus the available literature, we estimate circumpolar thaw ponds <0.001 km 2 in size to emit between 0.2 and 1.0 Tg of CH 4 through ebullition. Our findings exemplify the importance of high‐frequency measurements over long study periods in order to adequately capture the variability of these water bodies. Through the expansion of current spatial and temporal monitoring efforts, we can increase our ability to estimate CH 4 emissions from permafrost pond ecosystems now and in the future.
Abstract Lake emissions of the climate forcing trace gas methane (CH 4 ) are spatiotemporally variable, but biases in flux measurements arising from undersampling are poorly quantified. We use a multiyear data set (2009–2017) of ice‐free CH 4 emissions from three subarctic lakes obtained with bubble traps ( n = 14,677), floating chambers ( n = 1,306), and surface concentrations plus a gas transfer model ( n = 535) to quantify these biases and evaluate corrections. Sampling primarily in warmer summer months, as is common, overestimates the ice‐free season flux by a factor 1.4–1.8. Temperature proxies based on Arrhenius functions that closely fit measured fluxes ( R 2 ≥ 0.93) enable gap filling the colder months of the ice‐free season and reduce sampling bias. Ebullition (activation energy 1.36 eV) expressed greater temperature sensitivity than diffusion (1.00 eV). Resolving seasonal and interannual variability in fluxes with proxies requires ∼135 sampling days for ebullition, and 22 and 14 days for diffusion via models and chambers, respectively.
Abstract. Lakes and reservoirs are important emitters of climate forcing trace gases. Various environmental drivers of the flux, such as temperature and wind speed, have been identified, but their relative importance remains poorly understood. Here we use an extensive field dataset to disentangle physical and biogeochemical controls on the turbulence-driven diffusive flux of methane (CH4) on daily to multi-year timescales. We compare 8 years of floating chamber fluxes from three small, shallow subarctic lakes (2010–2017, n = 1306) with fluxes computed using 9 years of surface water concentration measurements (2009–2017, n = 606) and a small-eddy surface renewal model informed by in situ meteorological observations. Chamber fluxes averaged 6.9 ± 0.3 mg m−2 d−1 and gas transfer velocities (k600) from the chamber-calibrated surface renewal model averaged 4.0 ± 0.1 cm h−1. We find robust (R2 ≥ 0.93, p
Abstract Quantification of the present and future contribution to atmospheric methane (CH 4 ) from lakes, wetlands, fluvial systems, and, potentially, coastal waters remains an important unfinished task for balancing the global CH 4 budget. Discriminating between these sources is crucial, especially across climate‐sensitive Arctic and subarctic landscapes and waters. Yet basic underlying uncertainties remain, in such areas as total wetland area and definitions of wetlands, which can lead to conflation of wetlands and small ponds in regional studies. We discuss how in situ sampling choices, remote sensing limitations, and isotopic signature overlaps can lead to unintentional double‐counting of CH 4 emissions and propose that this double‐counting can explain a pan‐Arctic bottom‐up estimate from published sources, 59.7 Tg yr −1 (range 36.9–89.4 Tg yr −1 ) greatly exceeding the most recent top‐down inverse modeled estimate of the pan‐Arctic CH 4 budget (23 ± 5 Tg yr −1 ).
Abstract. Lakes and wetlands, common ecosystems of the high northern latitudes, exchange large amounts of the climate-forcing gases methane (CH4) and carbon dioxide (CO2) with the atmosphere. The magnitude of these fluxes and the processes driving them are still uncertain, particularly for subarctic and Arctic lakes where direct measurements of CH4 and CO2 emissions are often of low temporal resolution and are rarely sustained throughout the entire year. Using the eddy covariance method, we measured surface-atmosphere exchange of CH4 and CO2 during 2.5 years in a thawed fen and a shallow lake of a subarctic peatland complex. Gas exchange at the fen exhibited the expected seasonality of a subarctic wetland with maximum CH4 emissions and CO2 uptake in summer, as well as low but continuous emissions of CH4 and CO2 throughout the snow-covered winter. The seasonality of lake fluxes differed, with maximum CO2 and CH4 flux rates recorded at spring thaw. During the ice-free seasons, we could identify surface CH4 emissions as mostly ebullition events with a seasonal trend in the magnitude of the release, while a net CO2 flux indicated photosynthetic activity. We found correlations between surface CH4 emissions and surface sediment temperature, as well as between diel CO2 uptake and diel solar input. At ice-out, the breakdown of thermal stratification following ice thaw triggered the degassing of both CH4 and CO2. This spring burst was observed in two consecutive years for both gases, with a large inter-annual variability in the magnitude of the CH4 degassing. On the annual scale, spring emissions converted the lake from a small CO2 sink to a CO2 source. 80 % of total annual carbon emissions from the lake were emitted as CO2. The annual total carbon exchange per unit area was highest at the fen, which was an annual sink of carbon with respect to the atmosphere. Continuous respiration during the winter partly counteracted the fen summer sink by accounting for, as both CH4 and CO2, 33 % of annual carbon exchange. Our study underlines (1) the importance of overturn periods (spring or fall) for the annual CH4 and CO2 emissions of northern lakes, (2) the significance of lakes as atmospheric carbon sources in subarctic landscapes while fens can be strong carbon sink and (3) the potential for ecosystem-scale eddy covariance measurements to improve the understanding of short-term processes driving lake-atmosphere exchange of CH4 and CO2.
Abstract Stable isotopes have emerged as popular study targets when investigating emission of methane (CH 4 ) from lakes. Yet little is known on how isotopic patterns conform to variations in emission magnitudes—a highly relevant question. Here, we present a large multiyear data set on stable isotopes of CH 4 ebullition (bubbling) from three small adjacent subarctic lakes. The δ 13 C‐CH 4 and δD‐CH 4 range from −78.4‰ to −53.1‰ and from −369.8‰ to −218.8‰, respectively, and vary greatly among the lakes. The signatures suggest dominant hydrogenotrophic methanogenesis, particularly in the deep zones, but there are also signals of seemingly acetoclastic production in some high fluxing shallow areas, possibly fueled by in situ vegetation, but in‐sediment anaerobic CH 4 oxidation cannot be ruled out as an alternative cause. The observed patterns, however, are not consistent across the lakes. Neither do they correspond to the spatiotemporal variations in the measured bubble CH 4 fluxes. Patterns of acetoclastic and hydrogenotrophic production plus oxidation demonstrate that gains and losses of sediment CH 4 are dominated by sub‐lake scale processes. The δD‐CH 4 in the bubbles was significantly different depending on measurement month, likely due to evaporation effects. On a larger scale, our isotopic data, combined with those from other lakes, show a significant difference in bubble δD‐CH 4 between postglacial and thermokarst lakes, an important result for emission inventories. Although this characteristic theoretically assists in source partitioning studies, most hypothetical future shifts in δD‐CH 4 due to high‐latitude lake area or production pathway are too small to lead to atmospheric changes detectable with current technology.
