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1. Large Seasonal and Habitat Differences in Methane Ebullition on the Amazon Floodplain

   https://doi.org/10.1029/2020JG005911  

   Barbosa, Pedro M.; Melack, John M.; Amaral, João H. F.; Linkhorst, Annika; Forsberg, Bruce R. (July 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract Tropical floodplains are an important source of methane (CH₄) to the atmosphere, and ebullitive fluxes are likely to be important. We report direct measurements of CH₄ebullition in common habitats on the Amazon floodplain over two years based on floating chambers that allowed detection of bubbles, and submerged bubble traps. Ebullition was highly variable in space and time. Of the 840 floating chamber measurements (equivalent to 8,690 min of 10‐min deployments), 22% captured bubbles. Ebullitive CH₄fluxes, measured using bubble traps deployed for a total of approximately 230 days, ranged from 0 to 109 mmol CH₄m⁻² d⁻¹, with a mean of 4.4 mmol CH₄m⁻² d⁻¹. During falling water, a hydroacoustic echosounder detected bubbles in 24% of the 70‐m segments over 34 km. Ebullitive flux increased as the water level fell faster during falling water periods. In flooded forests, highest ebullitive fluxes occurred during falling water, while in open water and herbaceous plant habitats, higher ebullitive fluxes were measured during low water periods. The contribution of diffusive plus ebullitive CH₄flux represented by ebullition varied from 1% (high and rising water in open water of the lake) to 93% (falling water in flooded forests) based on bubble traps. Combining ebullitive and diffusive fluxes among habitats in relation to variations in water depth and areal coverage of aquatic habitats provides the basis for improved floodplain‐wide estimates of CH₄evasion. 

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2. 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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3. The Effects of Engineered Aeration on Atmospheric Methane Flux From a Chesapeake Bay Tidal Tributary

   https://doi.org/10.3389/fenvs.2022.866152  

   Lapham, Laura L.; Hobbs, Edward A.; Testa, Jeremy M.; Heyes, Andrew; Forsyth, Melinda K.; Hodgkins, Casey; Szewczyk, Curtis; Harris, Lora A. (August 2022, Frontiers in Environmental Science)

   Engineered aeration is one solution for increasing oxygen concentrations in highly eutrophic estuaries that undergo seasonal hypoxia. Although there are various designs for engineered aeration, all approaches involve either destratification of the water column or direct injection of oxygen or air through fine bubble diffusion. To date, the effect of either approach on estuarine methane dynamics remains unknown. Here we tested the hypotheses that 1) bubble aeration will strip the water of methane and enhance the air-water methane flux to the atmosphere and 2) the addition of oxygen to the water column will enhance aerobic methane oxidation in the water column and potentially offset the air-water methane flux. These hypotheses were tested in Rock Creek, Maryland, a shallow-water sub-estuary to the Chesapeake Bay, using controlled, ecosystem-scale deoxygenation experiments where the water column and sediments were sampled in mid-summer, when aerators were ON, and then 1, 3, 7, and 13 days after the aerators were turned OFF. Experiments were performed under two system designs, large bubble and fine bubble approaches, using the same observational approach that combined discrete water sampling, long term water samplers (OsmoSamplers) and sediment porewater profiles. Regardless of aeration status, methane concentrations reached as high as 1,500 nmol L⁻¹in the water column during the experiments and remained near 1,000 nmol L⁻¹through the summer and into the fall. Since these concentrations are above atmospheric equilibrium of 3 nmol L⁻¹, these data establish the sub-estuary as a source of methane to the atmosphere, with a maximum atmospheric flux as high as 1,500 µmol m⁻²d⁻¹, which is comparable to fluxes estimated for other estuaries. Air-water methane fluxes were higher when the aerators were ON, over short time frames, supporting the hypothesis that aeration enhanced the atmospheric methane flux. The fine-bubble approach showed lower air-water methane fluxes compared to the larger bubble, destratification system. We found that the primary source of the methane was the sediments, however,in situmethane production or an upstream methane source could not be ruled out. Overall, our measurements of methane concentrations were consistently high in all times and locations, supporting consistent methane flux to the atmosphere. 

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4. Interannual, summer, and diel variability of CH ₄ and CO ₂ effluxes from Toolik Lake, Alaska, during the ice-free periods 2010–2015

   https://doi.org/10.1039/D0EM00125B  

   Eugster, Werner; DelSontro, Tonya; Shaver, Gaius R.; Kling, George W. (January 2020, Environmental Science: Processes & Impacts)

   Accelerated warming in the Arctic has led to concern regarding the amount of carbon emission potential from Arctic water bodies. Yet, aquatic carbon dioxide (CO 2 ) and methane (CH 4 ) flux measurements remain scarce, particularly at high resolution and over long periods of time. Effluxes of methane (CH 4 ) and carbon dioxide (CO 2 ) from Toolik Lake, a deep glacial lake in northern Alaska, were measured for the first time with the direct eddy covariance (EC) flux technique during six ice-free lake periods (2010–2015). CO 2 flux estimates from the lake (daily average efflux of 16.7 ± 5.3 mmol m −2 d −1 ) were in good agreement with earlier estimates from 1975–1989 using different methods. CH 4 effluxes in 2010–2015 (averaging 0.13 ± 0.06 mmol m −2 d −1 ) showed an interannual variation that was 4.1 times greater than median diel variations, but mean fluxes were almost one order of magnitude lower than earlier estimates obtained from single water samples in 1990 and 2011–2012. The overall global warming potential (GWP) of Toolik Lake is thus governed mostly by CO 2 effluxes, contributing 86–93% of the ice-free period GWP of 26–90 g CO 2,eq m −2 . Diel variation in fluxes was also important, with up to a 2-fold (CH 4 ) to 4-fold (CO 2 ) difference between the highest nighttime and lowest daytime effluxes. Within the summer ice-free period, on average, CH 4 fluxes increased 2-fold during the first half of the summer, then remained almost constant, whereas CO 2 effluxes remained almost constant over the entire summer, ending with a linear increase during the last 1–2 weeks of measurements. Due to the cold bottom temperatures of this 26 m deep lake, and the absence of ebullition and episodic flux events, Toolik Lake and other deep glacial lakes are likely not hot spots for greenhouse gas emissions, but they still contribute to the overall GWP of the Arctic. 

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5. Carbon Dioxide and Methane Emissions From A Temperate Salt Marsh Tidal Creek

   https://doi.org/10.1029/2019JG005558  

   Trifunovic, Branimir; Vázquez‐Lule, Alma; Capooci, Margaret; Seyfferth, Angelia L.; Moffat, Carlos; Vargas, Rodrigo (August 2020, Journal of Geophysical Research: Biogeosciences)

   Abstract Coastal salt marshes store large amounts of carbon but the magnitude and patterns of greenhouse gas (GHG; i.e., carbon dioxide (CO₂) and methane (CH₄)) fluxes are unclear. Information about GHG fluxes from these ecosystems comes from studies of sediments or at the ecosystem‐scale (eddy covariance) but fluxes from tidal creeks are unknown. We measured GHG concentrations in water, water quality, meteorological parameters, sediment CO₂efflux, ecosystem‐scale GHG fluxes, and plant phenology; all at half‐hour intervals over 1 year. Manual creek GHG flux measurements were used to calculate gas transfer velocity (k) and parameterize a model of water‐to‐atmosphere GHG fluxes. The creek was a source of GHGs to the atmosphere where tidal patterns controlled diel variability. Dissolved oxygen and wind speed were negatively correlated with creek CH₄efflux. Despite lacking a seasonal pattern, creek CO₂efflux was correlated with drivers such as turbidity across phenological phases. Overall, nighttime creek CO₂efflux (3.6 ± 0.63 μmol/m²/s) was at least 2 times higher than nighttime marsh sediment CO₂efflux (1.5 ± 1.23 μmol/m²/s). Creek CH₄efflux (17.5 ± 6.9 nmol/m²/s) was 4 times lower than ecosystem‐scale CH₄fluxes (68.1 ± 52.3 nmol/m²/s) across the year. These results suggest that tidal creeks are potential hotspots for CO₂emissions and could contribute to lateral transport of CH₄to the coastal ocean due to supersaturation of CH₄(>6,000 μmol/mol) in water. This study provides insights for modeling GHG efflux from tidal creeks and suggests that changes in tide stage overshadow water temperature in determining magnitudes of fluxes. 

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