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1. Low Diffusive Methane Emissions From the Main Channel of a Large Amazonian Run-of-the-River Reservoir Attributed to High Methane Oxidation

   https://doi.org/10.3389/fenvs.2021.655455  

   Sawakuchi, Henrique O.; Bastviken, David; Enrich-Prast, Alex; Ward, Nicholas D.; Camargo, Plínio B.; Richey, Jeffrey E. (April 2021, Frontiers in Environmental Science)

   The global development of hydropower dams has rapidly expanded over the last several decades and has spread to historically non-impounded systems such as the Amazon River’s main low land tributaries in Brazil. Despite the recognized significance of reservoirs to the global methane (CH 4 ) emission, the processes controlling this emission remain poorly understood, especially in Tropical reservoirs. Here we evaluate CH 4 dynamics in the main channel and downstream of the Santo Antônio hydroelectric reservoir, a large tropical run-of-the-river (ROR) reservoir in Amazonia. This study is intended to give a snapshot of the CH 4 dynamics during the falling water season at the initial stage after the start of operations. Our results show substantial and higher CH 4 production in reservoirs’ littoral sediment than in the naturally flooded areas downstream of the dam. Despite the large production in the reservoir or naturally flooded areas, high CH 4 oxidation in the main channel keep the concentration and fluxes of CH 4 in the main channel low. Similar CH 4 concentrations in the reservoir and downstream close to the dam suggest negligible degassing at the dam, but stable isotopic evidence indicates the presence of a less oxidized pool of CH 4 after the dam. ROR reservoirs are designed to disturb the natural river flow dynamics less than traditional reservoirs. If enough mixing and oxygenation remain throughout the reservoir’s water column, naturally high CH 4 oxidation rates can also remain and limit the diffusive CH 4 emissions from the main channel. Nevertheless, it is important to highlight that our results focused on emissions in the deep and oxygenated main channel. High emissions, mainly through ebullition, may occur in the vast and shallow areas represented by bays and tributaries. However, detailed assessments are still required to understand the impacts of this reservoir on the annual emissions of CH 4 . 

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2. Response of CO ₂ and CH ₄ emissions from Arctic tundra soils to a multifactorial manipulation of water table, temperature and thaw depth

   https://doi.org/10.1088/2752-664X/ad089d  

   Best, K.; Zona, D.; Briant, E.; Lai, Chun-Ta; Lipson, D. A.; McEwing, K. R.; Davidson, S. J.; Oechel, W. C. (November 2023, Environmental Research: Ecology)

   Abstract Significant uncertainties persist concerning how Arctic soil tundra carbon emission responds to environmental changes. In this study, 24 cores were sampled from drier (high centre polygons and rims) and wetter (low centre polygons and troughs) permafrost tundra ecosystems. We examined how soil CO₂and CH₄fluxes responded to laboratory-based manipulations of soil temperature (and associated thaw depth) and water table depth, representing current and projected conditions in the Arctic. Similar soil CO₂respiration rates occurred in both the drier and the wetter sites, suggesting that a significant proportion of soil CO₂emission occurs via anaerobic respiration under water-saturated conditions in these Arctic tundra ecosystems. In the absence of vegetation, soil CO₂respiration rates decreased sharply within the first 7 weeks of the experiment, while CH₄emissions remained stable for the entire 26 weeks of the experiment. These patterns suggest that soil CO₂emission is more related to plant input than CH₄production and emission. The stable and substantial CH₄emission observed over the entire course of the experiment suggests that temperature limitations, rather than labile carbon limitations, play a predominant role in CH₄production in deeper soil layers. This is likely due to the presence of a substantial source of labile carbon in these carbon-rich soils. The small soil temperature difference (a median difference of 1 °C) and a more substantial thaw depth difference (a median difference of 6 cm) between the high and low temperature treatments resulted in a non-significant difference between soil CO₂and CH₄emissions. Although hydrology continued to be the primary factor influencing CH₄emissions, these emissions remained low in the drier ecosystem, even with a water table at the surface. This result suggests the potential absence of a methanogenic microbial community in high-centre polygon and rim ecosystems. Overall, our results suggest that the temperature increases reported for these Arctic regions are not responsible for increases in carbon losses. Instead, it is the changes in hydrology that exert significant control over soil CO₂and CH₄emissions. 

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3. Diverse sediment microbiota shape methane emission temperature sensitivity in Arctic lakes

   https://doi.org/10.1038/s41467-021-25983-9  

   Emerson, Joanne B.; Varner, Ruth K.; Wik, Martin; Parks, Donovan H.; Neumann, Rebecca B.; Johnson, Joel E.; Singleton, Caitlin M.; Woodcroft, Ben J.; Tollerson, II, Rodney; Owusu-Dommey, Akosua; et al (October 2021, Nature Communications)

   Abstract Northern post-glacial lakes are significant, increasing sources of atmospheric carbon through ebullition (bubbling) of microbially-produced methane (CH₄) from sediments. Ebullitive CH₄flux correlates strongly with temperature, reflecting that solar radiation drives emissions. However, here we show that the slope of the temperature-CH₄flux relationship differs spatially across two post-glacial lakes in Sweden. We compared these CH₄emission patterns with sediment microbial (metagenomic and amplicon), isotopic, and geochemical data. The temperature-associated increase in CH₄emissions was greater in lake middles—where methanogens were more abundant—than edges, and sediment communities were distinct between edges and middles. Microbial abundances, including those of CH₄-cycling microorganisms and syntrophs, were predictive of porewater CH₄concentrations. Results suggest that deeper lake regions, which currently emit less CH₄than shallower edges, could add substantially to CH₄emissions in a warmer Arctic and that CH₄emission predictions may be improved by accounting for spatial variations in sediment microbiota. 

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4. Methane production controls in a young thermokarst lake formed by abrupt permafrost thaw

   https://doi.org/10.1111/gcb.16151  

   Pellerin, André; Lotem, Noam; Walter Anthony, Katey; Eliani Russak, Efrat; Hasson, Nicholas; Røy, Hans; Chanton, Jeffrey P.; Sivan, Orit (March 2022, Global Change Biology)

   Abstract Methane (CH₄) release to the atmosphere from thawing permafrost contributes significantly to global CH₄emissions. However, constraining the effects of thaw that control the production and emission of CH₄is needed to anticipate future Arctic emissions. Here are presented robust rate measurements of CH₄production and cycling in a region of rapidly degrading permafrost. Big Trail Lake, located in central Alaska, is a young, actively expanding thermokarst lake. The lake was investigated by taking two 1 m cores of sediment from different regions. Two independent methods of measuring microbial CH₄ production, long term (CH₄accumulation) and short term (¹⁴C tracer), produced similar average rates of 11 ± 3.5 and 9 ± 3.6 nmol cm⁻³ d⁻¹, respectively. The rates had small variations between the different lithological units, indicating homogeneous CH₄production despite heterogeneous lithology in the surface ~1 m of sediment. To estimate the total CH₄production, the CH₄production rates were multiplied through the 10–15 m deep talik (thaw bulb). This estimate suggests that CH₄ production is higher than emission by a maximum factor of ~2, which is less than previous estimates. Stable and radioactive carbon isotope measurements showed that 50% of dissolved CH₄in the first meter was produced further below. Interestingly, labeled¹⁴C incubations with 2‐¹⁴C acetate and¹⁴C CO₂indicate that variations in the pathway used by microbes to produce CH₄depends on the age and type of organic matter in the sediment, but did not appear to influence the rates at which CH₄ was produced. This study demonstrates that at least half of the CH₄produced by microbial breakdown of organic matter in actively expanding thermokarst is emitted to the atmosphere, and that the majority of this CH₄is produced in the deep sediment. 

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5. Investigations of Aerobic Methane Oxidation in Two Marine Seep Environments: Part 2—Isotopic Kinetics

   https://doi.org/10.1029/2019JC015603  

   Chan, E. W.; Shiller, A. M.; Joung, D. J.; Arrington, E. C.; Valentine, D. L.; Redmond, M. C.; Breier, J. A.; Socolofsky, S. A.; Kessler, J. D. (November 2019, Journal of Geophysical Research: Oceans)

   Abstract During aerobic oxidation of methane (CH₄) in seawater, a process which mitigates atmospheric emissions, the¹²C‐isotopologue reacts with a slightly greater rate constant than the¹³C‐isotopologue, leaving the residual CH₄isotopically fractionated. Prior studies have attempted to exploit this systematic isotopic fractionation from methane oxidation to quantify the extent that a CH₄pool has been oxidized in seawater. However, cultivation‐based studies have suggested that isotopic fractionation fundamentally changes as a microbial population blooms in response to an influx of reactive substrates. Using a systematic mesocosm incubation study with recently collected seawater, here we investigate the fundamental isotopic kinetics of aerobic CH₄oxidation during a microbial bloom. As detailed in a companion paper, seawater samples were collected from seep fields in Hudson Canyon, U.S. Atlantic Margin, and atop Woolsey Mound (also known as Sleeping Dragon) which is part of lease block MC118 in the northern Gulf of Mexico, and used in these investigations. The results from both Hudson Canyon and MC118 show that in these natural environments isotopic fraction for CH₄oxidation follows a first‐order kinetic process. The results also show that the isotopic fractionation factor remains constant during this methanotrophic bloom once rapid CH₄oxidation begins and that the magnitude of the fractionation factor appears correlated with the first‐order reaction rate constant. These findings greatly simplify the use of natural stable isotope changes in CH₄to assess the extent that CH₄is oxidized in seawater following seafloor release. 

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