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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. GRiMeDB: the Global River Methane Database of concentrations and fluxes

   https://doi.org/10.5194/essd-15-2879-2023  

   Stanley, Emily H.; Loken, Luke C.; Casson, Nora J.; Oliver, Samantha K.; Sponseller, Ryan A.; Wallin, Marcus B.; Zhang, Liwei; Rocher-Ros, Gerard (January 2023, Earth System Science Data)

   Abstract. Despite their small spatial extent, fluvial ecosystems play a significant role in processing and transporting carbon in aquatic networks, which results in substantial emission of methane (CH4) into the atmosphere. For this reason, considerable effort has been put into identifying patterns and drivers of CH4 concentrations in streams and rivers and estimating fluxes to the atmosphere across broad spatial scales. However, progress toward these ends has been slow because of pronounced spatial and temporal variability of lotic CH4 concentrations and fluxes and by limited data availability across diverse habitats and physicochemical conditions. To address these challenges, we present a comprehensive database of CH4 concentrations and fluxes for fluvial ecosystems along with broadly relevant and concurrent physical and chemical data. The Global River Methane Database (GriMeDB; https://doi.org/10.6073/pasta/f48cdb77282598052349e969920356ef, Stanley et al., 2023) includes 24 024 records of CH4 concentration and 8205 flux measurements from 5029 unique sites derived from publications, reports, data repositories, unpublished data sets, and other outlets that became available between 1973 and 2021. Flux observations are reported as diffusive, ebullitive, and total CH4 fluxes, and GriMeDB also includes 17 655 and 8409 concurrent measurements of concentrations and 4444 and 1521 fluxes for carbon dioxide (CO2) and nitrous oxide (N2O), respectively. Most observations are date-specific (i.e., not site averages), and many are supported by data for 1 or more of 12 physicochemical variables and 6 site variables. Site variables include codes to characterize marginal channel types (e.g., springs, ditches) and/or the presence of human disturbance (e.g., point source inputs, upstream dams). Overall, observations in GRiMeDB encompass the broad range of the climatic, biological, and physical conditions that occur among world river basins, although some geographic gaps remain (arid regions, tropical regions, high-latitude and high-altitude systems). The global median CH4 concentration (0.20 µmol L−1) and diffusive flux (0.44 mmolm-2d-1) in GRiMeDB are lower than estimates from prior site-averaged compilations, although ranges (0 to 456 µmol L−1 and −136 to 4057 mmolm-2d-1) and standard deviations (10.69 and 86.4) are greater for this larger and more temporally resolved database. Available flux data are dominated by diffusive measurements despite the recognized importance of ebullitive and plant-mediated CH4 fluxes. Nonetheless, GriMeDB provides a comprehensive and cohesive resource for examining relationships between CH4 and environmental drivers, estimating the contribution of fluvial ecosystems to CH4 emissions, and contextualizing site-based investigations. 

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3. Inundation, hydrodynamics and vegetation influences carbon dioxide concentrations in Amazon floodplain lakes

   https://doi.org/10.1007/s10021-021-00692-y  

   Amaral, J.H.F.; Melack, J.M.; Barbosa, P.M.; Borges, A.V.; Kasper, D.; Cortes, A.C.; Zhou, W.; MacIntyre, S.; Forsberg, B.R. (January 2021, Ecosystems)

   Extensive floodplains and numerous lakes in the Amazon basin are well suited to examine the role of floodable lands within the context of the sources and processing of carbon within inland waters. We measured diel, seasonal and inter-annual variations of carbon dioxide concentrations and related environmental variables in open water and flooded vegetation and estimated the extension of these habitats using remote sensing in a representative central Amazon floodplain lake, Lake Janauacá. Depth-averaged values of CO2 in the open water of the lake, 157± 91 µM (mean ± SD), were less than those in an embayment near aquatic vegetation, 285±116 µM, and were variable over 24-h periods at both sites. Within floating herbaceous plant mats, mean concentration (without one outlier) was 275±77 µM and in flooded forests mean concentration was 217±78 µM. Variability in CO2 concentrations in open water resulted from changes in the extent of inundation and exchange with vegetated habitats. The best statistical model, including CO2 in aquatic plant mats, Secchi depth, rate of change in water level and chlorophyll concentrations, explained around 90% of the variability in CO2 concentration. Three-dimensional hydrodynamic modeling demonstrated that diel differences in water temperature between plant mats and open water and basin-scale motions caused lateral exchanges of CO2 linking vegetated habitats to open water. Our findings extend understanding of CO2 in tropical lakes and floodplains with measurements and models that emphasize the importance of flooded forests and aquatic herbaceous plants fringing floodplain lakes as sources of carbon dioxide to the open waters. 

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4. BAWLD-CH₄: a comprehensive dataset of methane fluxes from boreal and arctic ecosystems

   https://doi.org/10.5194/essd-13-5151-2021  

   Kuhn, McKenzie A.; Varner, Ruth K.; Bastviken, David; Crill, Patrick; MacIntyre, Sally; Turetsky, Merritt; Walter Anthony, Katey; McGuire, Anthony D.; Olefeldt, David (January 2021, Earth System Science Data)

   Abstract. Methane (CH4) emissions from the boreal and arcticregion are globally significant and highly sensitive to climate change.There is currently a wide range in estimates of high-latitude annualCH4 fluxes, where estimates based on land cover inventories andempirical CH4 flux data or process models (bottom-up approaches)generally are greater than atmospheric inversions (top-down approaches). Alimitation of bottom-up approaches has been the lack of harmonizationbetween inventories of site-level CH4 flux data and the land coverclasses present in high-latitude spatial datasets. Here we present acomprehensive dataset of small-scale, surface CH4 flux data from 540terrestrial sites (wetland and non-wetland) and 1247 aquatic sites (lakesand ponds), compiled from 189 studies. The Boreal–Arctic Wetland and LakeMethane Dataset (BAWLD-CH4) was constructed in parallel with acompatible land cover dataset, sharing the same land cover classes to enablerefined bottom-up assessments. BAWLD-CH4 includes information onsite-level CH4 fluxes but also on study design (measurement method,timing, and frequency) and site characteristics (vegetation, climate,hydrology, soil, and sediment types, permafrost conditions, lake size anddepth, and our determination of land cover class). The different land coverclasses had distinct CH4 fluxes, resulting from definitions that wereeither based on or co-varied with key environmental controls. Fluxes ofCH4 from terrestrial ecosystems were primarily influenced by watertable position, soil temperature, and vegetation composition, while CH4fluxes from aquatic ecosystems were primarily influenced by watertemperature, lake size, and lake genesis. Models could explain more of thebetween-site variability in CH4 fluxes for terrestrial than aquaticecosystems, likely due to both less precise assessments of lake CH4fluxes and fewer consistently reported lake site characteristics. Analysisof BAWLD-CH4 identified both land cover classes and regions within theboreal and arctic domain, where future studies should be focused, alongsidemethodological approaches. Overall, BAWLD-CH4 provides a comprehensivedataset of CH4 emissions from high-latitude ecosystems that are usefulfor identifying research opportunities, for comparison against new fielddata, and model parameterization or validation. BAWLD-CH4 can bedownloaded from https://doi.org/10.18739/A2DN3ZX1R (Kuhn et al., 2021). 

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5. Modeling Thermal and Biogeochemical Dynamics in Two Ponds Within Alaska's Yukon–Kuskokwim Delta: Impacts of Climatic Variability on Greenhouse Gas Fluxes

   https://doi.org/10.1029/2024MS004441  

   Mullen, A_L; Jafarov, E_E; Hung, J_K_Y; Gurbanov, K.; Stepanenko, V.; Rogers, B_M; Watts, J_D; Natali, S_M; Poulin, B_A (April 2025, Journal of Advances in Modeling Earth Systems)

   Abstract Fluxes of carbon dioxide (CO₂) and methane (CH₄) from open water bodies are critical components of carbon‐climate feedbacks in high latitudes. Processes governing the spatial and temporal variability of these aquatic greenhouse gas (GHG) fluxes are still highly uncertain due to limited observational data sets and lack of modeling studies incorporating comprehensive thermal and biochemical processes. This research investigates how slight variations in climate propagate through the biogeochemical cycles of ponds and resulting impacts on GHG emissions. We examine the thermal and biogeochemical dynamics of two ponds in the Yukon–Kuskokwim Delta, Alaska, under varying climatic conditions to study the impacts on CO₂, CH₄, and oxygen (O₂) concentrations and fluxes. We performed multiple numerical experiments, using the LAKE process‐based model and field measurements, to analyze how these ponds respond to variations in air temperature, shortwave radiation, and snow cover. Our study demonstrates that ice cover duration and water temperature are primary climatic drivers of GHG fluxes. Climate experiments led to reductions in ice cover duration and increased water temperatures, which subsequently enhanced CH₄and CO₂gas emissions from two study ponds. On average, cumulative CH₄and CO₂emissions were 5% and 10% higher, respectively, under increases in air temperature and shortwave radiation. Additionally, we uncovered a need to incorporate groundwater influxes of dissolved gases and nutrients in order to fully represent processes governing aquatic biochemical activity. Our work highlights the importance of understanding local‐scale processes in predicting future Arctic contributions to GHG emissions. 

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