Diffusive CH4 Emissions Research Articles

Magnitude of and Hydroclimatic Controls on CO2 and CH4 Emissions in the Subtropical Monsoon Pearl River Basin

AbstractRivers are important ecosystems for carbon emissions and play a crucial role in the global carbon cycle. However, CO2 and CH4 emissions from subtropical rivers are substantially under‐represented in global‐scale estimates. Here, we explored the regional patterns of riverine CO2 and CH4 dynamics in the Pearl River basin with a subtropical monsoon climate. We found that its CO2 and diffusive CH4 emissions showed a decreasing trend with increasing stream order. Seasonality in CO2 and diffusive CH4 emissions was primarily driven by variations in partial pressure of CO2 (pCO2) and CH4 (pCH4) and gas transfer velocities, which were strongly regulated by hydrology and climate. We further estimated the basin‐wide CO2 and diffusive CH4 fluxes at 17.8 ± 7.4 Tg C yr−1 and 191.5 ± 139.9 Gg C yr−1, respectively. When normalized to the water surface, the mean diffusive fluxes were 790.1 and 8.5 mmol m−2 d−1 for CO2 and CH4, respectively, which were 1.3 and 2.5 times higher than the global mean riverine CO2 and CH4 fluxes, respectively. This suggests that the global significance of subtropical rivers is probably underestimated because their substantially higher CH4 fluxes are unaccounted for. Furthermore, compared with measured pCO2, the alkalinity‐based pCO2 could introduce significant errors by 20% at ∼30% of the sampling sites, underscoring the necessity of direct measurements to reduce uncertainty. This study provides the first estimate of basin‐wide CO2 and diffusive CH4 emissions in the PRB through direct pCO2 and pCH4 measurements, and highlights the role of hydrologic and climatic factors in governing riverine carbon emissions.

Greenhouse gas emission flux (CO2, CH4, N2O) from marine shrimp (Litopenaeus vannamei) monoculture cultures in the Brazilian semi-arid region

Greenhouse gases (GHG) and their rising atmospheric concentrations have been one of the most important international subjects due to their huge impact on climate change (IPCC, 2022). Aquaculture is of crucial importance to the human food supply and nutrition since it is one of the major sectors providing animal protein, besides reducing the pressure on the fishery stocks from natural resources worldwide (Cai, Zhou, 2019; FAO, 2022). On the other hand, it represents a threat to the environment by generating waste. Thus, this research aims to evaluate the GHG flux dynamics in marine shrimp (Litopenaeus vannamei) monoculture cultures. The study was carried out in two aquaculture farms, both located in the Brazilian semi-arid region. In order to measure the production of greenhouse gases, three cultures of marine shrimp were monitored. The dynamics of the diffusive and ebullitive emission of CO2, CH4, and N2O were monitored in the three units during the daytime (6:00 AM) and nighttime (6:00 PM). The results showed the stocking density as the main regulator for ebullitive CO2 emissions. A decreasing trend in diffusive CO2 emissions was observed. Aeration played a crucial role in controlling the diffusive and ebullitive emissions of CH4 and N2O, effectively reducing their release. The diffusive CH4 emissions exceeded ebullitive emissions and the oxygen content suppressed the anoxic pathway for N2O, and the ponds acted as a sink for this gas. This research aims to evaluate the dynamics of greenhouse gas flow in marine shrimp (Litopenaeus vannamei) monoculture cultures in the Brazilian semi-arid region and to verify whether lower stocking densities emit less greenhouse gases when compared to higher densities.

Widespread dominance of methane ebullition over diffusion in freshwater aquaculture ponds

An ever-increasing demand for protein-rich food sources combined with dwindling wild fish stocks has caused the aquaculture sector to boom in the last two decades. Although fishponds are potentially strong emitters of the greenhouse gas methane (CH4), little is known about the magnitude, pathways, and drivers of these emissions. We measured diffusive CH4 emissions at the margin and in the center of 52 freshwater fishponds in Brazil. In a subset of ponds (n = 31) we additionally quantified ebullitive CH4 fluxes and sampled water and sediment for biogeochemical analyses. Sediments (n = 20) were incubated to quantify potential CH4 production. Ebullitive CH4 emissions ranged between 0 and 477 mg m−2 d−1 and contributed substantially (median 85%) to total CH4 emissions, surpassing diffusive emissions in 81% of ponds. Diffusive CH4 emissions were higher in the center (median 11.4 mg CH4 m−2 d−1) than at the margin (median 6.1 mg CH4 m−2 d−1) in 90% of ponds. Sediment CH4 production ranged between 0 and 3.17 mg CH4 g C−1 d−1. We found no relation between sediment CH4 production and in situ emissions. Our findings suggest that dominance of CH4 ebullition over diffusion is widespread across aquaculture ponds. Management practices to minimize the carbon footprint of aquaculture production should focus on reducing sediment accumulation and CH4 ebullition.

Quantification of Diffusive Methane Emissions from a Large Eutrophic Lake with Satellite Imagery.

Lakes are major emitters of methane (CH4); however, a longstanding challenge with quantifying the magnitude of emissions remains as a result of large spatial and temporal variability. This study was designed to address the issue using satellite remote sensing with the advantages of spatial coverage and temporal resolution. Using Aqua/MODIS imagery (2003-2020) and in situ measured data (2011-2017) in eutrophic Lake Taihu, we compared the performance of eight machine learning models to predict diffusive CH4 emissions and found that the random forest (RF) model achieved the best fitting accuracy (R2 = 0.65 and mean relative error = 21%). On the basis of input satellite variables (chlorophyll a, water surface temperature, diffuse attenuation coefficient, and photosynthetically active radiation), we assessed how and why they help predict the CH4 emissions with the RF model. Overall, these variables mechanistically controlled the emissions, leading to the model capturing well the variability of diffusive CH4 emissions from the lake. Additionally, we found climate warming and associated algal blooms boosted the long-term increase in the emissions via reconstructing historical (2003-2020) daily time series of CH4 emissions. This study demonstrates the great potential of satellites to map lake CH4 emissions by providing spatiotemporal continuous data, with new and timely insights into accurately understanding the magnitude of aquatic greenhouse gas emissions.

Factors influencing submerged macrophyte presence in fresh and brackish eutrophic waters and their impact on carbon emissions

In agricultural landscapes of North-Western Europe, the majority of water bodies do not meet the targets set by the European Water Framework Directive due to a lack of submerged macrophytes and associated biodiversity. These eutrophic waters can also be a substantial source of methane (CH4) and carbon dioxide (CO2) to the atmosphere. Here we present a two-year field experiment on the island of Goeree-Overflakkee (southwest Netherlands), conducted in six drainage ditches varying in salinity, where we monitored four permanent plots per ditch and varied the presence of both fish and macrophytes. We aimed to: 1) investigate factors limiting submerged macrophyte growth, focussing on exclusion of grazing pressure and bioturbation by fish; and 2) quantify the CO2 and CH4 emission under these conditions. Even in highly eutrophic, semi turbid ditches with fluctuating salinity levels and sulphide presence in the root zone, submerged macrophytes established successfully after introduction when the influence of grazing and bioturbation by fish was excluded. In the exclosures, diffusive CH4 and CO2 emissions, but not ebullitive CH4 emissions were significantly reduced. The spontaneous development of submerged macrophytes in the exclosures without macrophyte introduction underlined the effect of grazing and bioturbation by fish and suggest that abiotic conditions did not hamper submerged macrophyte development. Our results provide important insights into the influential factors for submerged macrophyte development and potential for future management practices. Large-scale fish removal may stimulate submerged macrophyte growth and reduce methane emissions, albeit that the macrophyte diversity will likely stay low in our study region due to fluctuating salinity and eutrophic conditions.

Wetland plant development overrides nitrogen effects on initial methane emissions after peat rewetting

Growing productive wetland species on rewetted peatland (paludiculture) is a promising solution to offset carbon loss from drained peatlands. The inlet of nitrogen (N) rich surface water, a proposed method to improve productivity of vegetation, may affect methane (CH4) emissions. This study aims to compare initial CH4 emissions from newly rewetted peat with different types of vegetation and N loading simulating diffuse N inlet. Diffusive CH4 emissions were measured in peat mesocosms during one growing season. Peat cores were either planted with Typha latifolia or Phragmites australis or they were left bare. Mesocosms received 0, 50, 150 or 450 kg ha−1 year−1 N. Plants affected CH4 emissions from rewetted peat soil, leading to stable fluxes over time of 133 mg m−2 day−1 CH4 at 20 °C. Biomass harvesting lead to a 153% increase of CH4 emissions. With increasing N load, CH4 emissions from mesocosms with Typha and Phragmites decreased up to a load of 150 kg ha−1 N, but this was only significant for the Phragmites treatment. Emissions of unvegetated mesocosms increased with increasing N load but not significantly. In conclusion, our mesocosm study suggests that vegetation can reduce or prevent an increase in CH4 emissions from rewetted peatlands compared to only rewetting, possibly due to an increased oxygenation of the sediments by macrophyte roots preventing excessive CH4 formation, while added N does not provoke great changes in emissions at N concentrations up to 150 kg ha−1.

Estimating Drivers and Pathways for Hydroelectric Reservoir Methane Emissions Using a New Mechanistic Model

AbstractHydroelectric reservoirs can emit significant quantities of methane, particularly through degassing at turbine outlets. Improved understanding of processes affecting hydroelectric reservoir CH4 emissions is thus important as the world economy transitions to renewable forms of energy production. Here we develop and evaluate a new mechanistic model of CH4 emissions: ResME ([Res]ervoir [M]ethane [E]missions), which estimates carbon inputs and methanogenesis to predict CH4 release via ebullition and diffusion, plant emissions, and downstream emissions. ResME results demonstrate that the relative importance of allochthonous and autochthonous carbon input to methane emissions varies by latitude, with allochthonous carbon contributions typically being higher in tropical reservoirs. Results also demonstrate that total reservoir emissions are highly dependent on turbine intake depths, which are not typically reported. Potential maximum degassing emissions from existing hydroelectric reservoirs are estimated as 11 ± 4 Tg C/yr, if all reservoirs had deep turbine intakes and stratified for 5 months per year. In comparison, the estimated diffusive, ebullitive, and plant CH4 emissions are estimated to be 2.8 ± 0.2 Tg C/yr (where the true uncertainty is much higher than the model standard error). Future work should focus on improving estimates of reservoir carbon inputs and decomposition rates, as well as surveying turbine intake depths. Satellite measurements from missions such as TROPOMI may also help constrain hydropower methane emissions.

Dominance of Diffusive Methane Emissions From Lowland Headwater Streams Promotes Oxidation and Isotopic Enrichment

Inland waters are the largest natural source of methane (CH4) to the atmosphere, yet the contribution from small streams to this flux is not clearly defined. To fully understand CH4 emissions from streams and rivers, we must consider the relative importance of CH4 emission pathways, the prominence of microbially-mediated production and oxidation of CH4, and the isotopic signature of emitted CH4. Here, we construct a complete CH4 emission budgets for four lowland headwater streams by quantifying diffusive CH4 emissions and comparing them to previously published rates of ebullitive emissions. We also examine the isotopic composition of CH4 along with the sediment microbial community to investigate production and oxidation across the streams. We find that all four streams are supersaturated with respect to CH4 with diffusive emissions accounting for approximately 78–100% of total CH4 emissions. Isotopic and microbial data suggest CH4 oxidation is prevalent across the streams, depleting approximately half of the dissolved CH4 pool before emission. We propose a conceptual model of CH4 production, oxidation, and emission from small streams, where the dominance of diffusive emissions is greater compared to other aquatic ecosystems, and the impact of CH4 oxidation is observable in the emitted isotopic values. As a result, we suggest the CH4 emitted from small streams is isotopically heavy compared to lentic ecosystems. Our results further demonstrate streams are important components of the global CH4 cycle yet may be characterized by a unique pattern of cycling and emission that differentiate them from other aquatic ecosystems.

Pond methane dynamics, from microbial communities to ecosystem budget, during summer in Alaska

AbstractPonds play a larger role in the global freshwater methane (CH4) budget than predicted from surface area alone. To improve our understanding of pond CH4 dynamics, we measured summer CH4 production, concentrations, and emissions to the atmosphere in nine Alaskan wetland ponds along with potential physical, chemical, and biological regulators. Pond CH4 production (0.64, 0.086–1.3 mmol m−2 d−1; median, interquartile range), as assessed with slurry incubations, was positively related to water‐column temperature and chlorophyll a (Chl a), negatively influenced by oxygen levels, and varied with microbial community structure. Average water‐column CH4 concentrations (0.39, 0.21–0.87 μmol L−1) were lower in deeper ponds and at higher oxygen levels, and as expected, they were correlated with diffusive emissions (0.055, 0.024–0.20 mmol m−2 d−1) assessed with flux chambers. Based on a mass balance approach, 39–99% of CH4 produced in ponds was oxidized. Pond ebullition (3.7, 0.60–24 mmol m−2 d−1) was higher and more variable than diffusive emissions. Additionally, pond ebullition rates were better correlated with production rates from the previous month. We also systematically compared the ratio of ebullition to diffusive CH4 emissions in our ponds and other northern lakes, which was negatively related to water depth (n = 71), but positively related to Chl a (n = 28). Our study sheds light on the factors that influence pond CH4 dynamics and demonstrates that pond ebullition is a significant CH4 source worthy of continued study.

Characterizing Methane Emission Hotspots From Thawing Permafrost

AbstractMethane (CH4) emissions from climate‐sensitive ecosystems within the northern permafrost region represent a potentially large but highly uncertain source, with current estimates spanning a factor of seven (11–75 Tg CH4 yr−1). Accelerating permafrost thaw threatens significant increases in pan‐Arctic CH4 emissions, amplifying the permafrost carbon feedback. We used airborne imaging spectroscopy with meter‐scale spatial resolution and broad coverage to identify a previously undiscovered CH4 emission hotspot adjacent to a thermokarst lake in interior Alaska. Hotspot emissions were confined to <1% of the 10 ha lake study area. Ground‐based chamber measurements confirmed average daily fluxes from the hotspot of 1,170 mg CH4 m−2 d−1, with extreme daily maxima up to 24,200 mg CH4 m−2 d−1. Ground‐based geophysical measurements revealed thawed permafrost directly beneath the CH4 hotspot, extending to a depth of ∼15 m, indicating that the intense CH4 emissions likely originated from recently thawed permafrost. Hotspot emissions accounted for ∼40% of total diffusive CH4 emissions from the lake study site. Combining study site findings with hotspot statistics from our 70,000 km2 airborne survey across Alaska and northwestern Canada, we estimate that pan‐Arctic terrestrial thermokarst hotspots currently emit 1.1 (0.1–5.2) Tg CH4 yr−1, or roughly 4% of the annual pan‐Arctic wetland budget from just 0.01% of the northern permafrost land area. Our results suggest that significant proportions of pan‐Arctic CH4 emissions originate from disproportionately small areas of previously undetermined thermokarst emissions hotspots, and that pan‐Arctic CH4 emissions may increase non‐linearly as thermokarst processes increase under a warming climate.

Rivers draining contrasting landscapes exhibit distinct potentials to emit diffusive methane (CH4)

Methane (CH4) is the second most important greenhouse gas, contributing approximately 17% of radiative forcing, and CH4 emissions from river networks due to intensified human activities have become a worldwide issue. However, there is a dearth of information on the CH4 emission potentials of different rivers, especially those draining contrasting watershed landscapes. Here, we examined the spatial variability of diffusive CH4 emissions and discerned the roles of environmental factors in influencing CH4 production in different river reaches (agricultural, urban, forested and mixed-landscape rivers) from the Chaohu Lake Basin in eastern China. According to our results, the urban rivers most frequently exhibited extremely high CH4 concentrations, with a mean concentration of 5.46 μmol L−1, equivalent to 4.1, 9.7, and 7.2 times those measured in the agricultural, forested, and mixed-landscape rivers, respectively. The availability of carbon sources and total phosphorus were commonly identified as the most important factors for CH4 production in agricultural and urban rivers. Dissolved oxygen and oxidation-reduction potential were separately discerned as important factors for the forested and mixed-landscape rivers, respectively. Monte Carlo flux estimations demonstrated that rivers draining contrasting landscapes exhibit distinct potentials to emit CH4. The urban rivers had the highest CH4 emissions, with a flux of 9.44 mmol m−2 d−1, which was 5.1–10.4 times higher than those of the other river reaches. Overall, our study highlighted that management actions should be specifically targeted at the river reaches with the highest emission potentials and should carefully consider the influences of different riverine environmental conditions as projected by their watershed landscapes.

Increase of methane emission linked to net cage fish farms in a tropical reservoir

This study aimed to evaluate the influence of Nile tilapia (Oreochromis niloticus) net cage farming on methane (CH4) emission in a Brazilian tropical reservoir through the quantification of spatial and seasonal variations of diffusive and ebullitive CH4 emissions. To explain the variability of CH4 emission, a hierarchical identification of limnologic and atmospheric parameters was used. A significant increase in diffusive emissions from net cage sites (33.4 ± 54.9 mg CCH4 m−2 d−1) was observed when compared to the sites before (4.4 ± 9.4 mg CCH4 m−2 d−1) and after (6.2 ± 15.6 mg CCH4 m−2 d−1) the net cages. Also, significant increase of ebullitive emissions was also observed from the net cage sites (2,417.1 ± 2,040.7 mg CCH4 m−2 d−1) when compared to the average emissions from sites before (1.1 ± 2.1 mg CCH4 m−2 d−1) and after (88.7 ± 92.6 mg CCH4 m−2 d−1). Results showed a decrease in average CH4 emissions towards the sites after the net cages in all sampled areas which suggests that the influence of tilapia fish farming is local. The highest averages in the reservoir areas for diffusive and ebullitive CH4 emissions were observed in areas with higher production of tilapia. Finally, seasonal variation was observed on diffuse CH4 emission. High concentrations of carbon and phosphorus in water and sediment were the main drivers for increasing diffusive and ebullitive methane emissions. Our findings provide valuable information that tilapia net cage farming in reservoirs can be associated with local and dispersive increased methane emissions.

A new modelling framework to assess biogenic GHG emissions from reservoirs: The G-res tool

Human-made reservoirs are now recognized as potentially significant sources of greenhouse gases, comparable to other anthropogenic sources, yet efforts to estimate these reservoir emissions have been hampered by the complexity of the underlying processes and a lack of coherent budgeting approaches. Here we present a unique modelling framework, the G-res Tool, which was explicitly designed to estimate the net C footprint of reservoirs across the globe. The framework involves the development of statistically robust empirical models describing the four major emission pathways for carbon-based greenhouse gases (GHG) from reservoirs: diffusive CO2 and CH4 emissions, bubbling CH4 emissions from the reservoir surface, and CH4 emissions due to degassing downstream the reservoir, based on an extensive meta-analysis of published data from the past three decades. These empirical models allow the prediction of reservoir-specific emissions, how they may shift over time and account for naturally occurring GHG generating pathways in aquatic networks.

Drainage Ditches Contribute Considerably to the CH4 Budget of a Drained and a Rewetted Temperate Fen

Small water bodies including drainage ditches can be hotspots for methane (CH4) emissions from peatlands. We assessed the CH4 emissions of a drained and a rewetted temperate fen including emissions of managed and unmanaged drainage ditches over the course of 2.5 years, covering three vegetation periods. Ditch CH4 emissions in the rewetted fen were significantly higher than in the drained fen. In the rewetted fen ditches contributed up to 91% of the annual CH4 budget, despite covering only 1.5% of the area. In the drained fen CH4 emissions were solely made up of ditch emissions. When including CH4 uptake by the peat soil, the CH4 balance of the drained fen was neutral. Dissolved organic carbon concentrations likely had an enhancing effect on CH4 emissions while nitrate and sulfate in the ditch water seem to have had an inhibitory effect. Air and water temperature controlled seasonal variability of ebullitive as well as diffusive CH4 emissions. Ebullition contributed less than 10% to the overall CH4 budget in the ditches. Drainage ditches represent a hotspot of CH4 emissions and need therefore be taken into account when assessing the success of rewetting projects of peatlands.

Methane oxidation in the waters of a humic-rich boreal lake stimulated by photosynthesis, nitrite, Fe(III) and humics

Abstract. Small boreal lakes are known to contribute significantly to global CH4 emissions. Lake Lovojärvi is a eutrophic lake in southern Finland with bottom water CH4 concentrations up to 2 mM. However, the surface water concentration, and thus the diffusive emission potential, was low (< 0.5 µM). We studied the biogeochemical processes involved in CH4 removal by chemical profiling and through incubation experiments. δ13C-CH4 profiling of the water column revealed a methane-oxidation hotspot just below the oxycline and zones of CH4 oxidation within the anoxic water column. In incubation experiments involving the addition of light and/or oxygen, CH4 oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobic CH4 oxidation. We observed a distinct peak in CH4 concentration at the chlorophyll-a maximum, caused by either in situ CH4 production or other CH4 inputs such as lateral transport from the littoral zone. In the dark anoxic water column at 7 m depth, nitrite seemed to be the key electron acceptor involved in CH4 oxidation, yet additions of Fe(III), anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic CH4 oxidation. Surprisingly, nitrite seemed to inhibit CH4 oxidation at all other depths. Overall, this study shows that photosynthetically fueled CH4 oxidation can be a key process in CH4 removal in the water column of humic, turbid lakes, thereby limiting diffusive CH4 emissions from boreal lakes. Yet, it also highlights the potential importance of a whole suite of alternative electron acceptors, including humics, in these freshwater environments in the absence of light and oxygen.

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

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 (CH4) emission, the processes controlling this emission remain poorly understood, especially in Tropical reservoirs. Here we evaluate CH4 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 CH4 dynamics during the falling water season at the initial stage after the start of operations. Our results show substantial and higher CH4 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 CH4 oxidation in the main channel keep the concentration and fluxes of CH4 in the main channel low. Similar CH4 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 CH4 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 CH4 oxidation rates can also remain and limit the diffusive CH4 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 CH4.

Hotspots of Diffusive CO2 and CH4 Emission From Tropical Reservoirs Shift Through Time

AbstractThe patterns of spatial and temporal variability in CO2 and CH4 emission from reservoirs are still poorly studied, especially in tropical regions where hydropower is growing. We performed spatially resolved measurements of dissolved CO2 and CH4 surface water concentrations and their gas‐exchange coefficients (k) to compute diffusive carbon flux from four contrasting tropical reservoirs across Brazil during different hydrological seasons. We used an online equilibration system to measure dissolved CO2 and CH4 concentrations; we estimated k from floating chamber deployments in conjunction with discrete CO2 and CH4 water concentration measurements. Diffusive CO2 emissions were higher during dry season than during rainy season, whereas there were no consistent seasonal patterns for diffusive CH4 emissions. Our results reveal that the magnitude and the spatial within‐reservoir patterns of diffusive CO2 and CH4 flux varied strongly among hydrological seasons. River inflow areas were often characterized by high seasonality in diffusive flux. Areas close to the dam generally showed low seasonal variability in diffusive CH4 flux but high variability in CO2 flux. Overall, we found that reservoir areas exhibiting highest emission rates (“hotspots”) shifted substantially across hydrological seasons. Estimates of total diffusive carbon emission from the reservoir surfaces differed between hydrological seasons by a factor up to 7 in Chapéu D'Úvas, up to 13 in Curuá‐Una, up to 4 in Furnas, and up to 1.8 in Funil, indicating that spatially resolved measurements of CO2 and CH4 concentrations and k need to be performed at different hydrological seasons in order to constrain annual diffusive carbon emission.

The Role of Methane Transport From the Active Layer in Sustaining Methane Emissions and Food Chains in Subarctic Ponds

AbstractGroundwater discharge from the seasonally thawed active layer is increasingly recognized as an important pathway for delivering methane (CH4) into Arctic lakes and streams, but its contribution to CH4 emissions from thaw ponds and its influence on the trophic support and nutritional quality of pond food chains remains unexplored. We quantified the transport of CH4 from the active layer through groundwater discharge into thaw ponds in a subarctic catchment in northern Sweden, using radon (222Rn) as groundwater tracer. We analyzed stable isotopes and fatty acids of pond macroinvertebrates to evaluate the potential effects of groundwater‐mediated CH4 inputs on the aquatic food chains. Our results indicate that active layer groundwater discharge flows are nontrivial (range 6%–46% of pond volume per day) and the associated CH4 fluxes (median 339 mg C m−2day−1, interquartile range [IQR]: 179–419 mg C m−2 day−1) can sustain the diffusive CH4 emissions from most of the ponds (155 mg C m−2 day−1, IQR: 55–234 mg C m−2 day−1). Consumers in ponds receiving greater CH4 inputs from the active layer had lower stable carbon (C) isotope signatures that indicates a greater trophic reliance on methane oxidizing bacteria (MOB), and they had lower nutritional quality as indicated by their lower tissue concentrations of polyunsaturated fatty acids. Overall, this work links physical (CH4 transport from the active layer), biogeochemical (CH4 emission), and ecological (MOB‐consumer interaction) processes to provide direct evidence for the role of active layer groundwater discharge in CH4 cycling of subarctic thaw ponds.

Spatial variations in diffusive methane fluxes and the role of eutrophication in a subtropical shallow lake

Shallow lakes account for most of the diffusive CH4 emissions from global lakes, and they also suffer from eutrophication worldwide. Determining the effect of eutrophication on diffusive CH4 fluxes is fundamental to understanding CH4 emissions in shallow lakes. This study aimed to investigate the spatial variations in diffusive CH4 fluxes and explore the role of eutrophication in Lake Chaohu, a large and shallow eutrophic lake in the lower reaches of the Yangtze River. A one-year field observation was carried out to examine CH4 concentrations in the sediment and water and the diffusive fluxes of CH4 across the sediment-water interface (Fs-w) and water-air interface (Fw-a). Both Fs-w (0.306–1.56 mmol m−2 d−1) and Fw-a (0.097–0.529 mmol m−2 d−1) were upward and showed significant spatial heterogeneity and were significantly positively correlated. Parameters related to eutrophication had significant positive relationships with Fw-a, and the total phosphorus distribution in the water explained the greatest proportion of the spatial variation in Fw-a. Distance to shore and water depth were inversely correlated with Fw-a and modified the effects of eutrophication. Overall, the results provide direct evidence of the key role of eutrophication in shaping the spatial distribution of diffusive CH4 fluxes and a scientific basis for predicting changes in CH4 emissions with future eutrophication changes in shallow lakes in subtropical zones.

Large Spatial Variations in Diffusive CH4 Fluxes from a Subtropical Coastal Reservoir Affected by Sewage Discharge in Southeast China

Coastal reservoirs are potentially CH4 emission hotspots owing to their biogeochemical role as the sinks of anthropogenic carbon and nutrients. Yet, the fine-scale spatial variations in CH4 concentrations and fluxes in coastal reservoirs remain poorly understood, hampering an accurate determination of reservoir CH4 budgets. In this study, we examined the spatial variability of diffusive CH4 fluxes and their drivers at a subtropical coastal reservoir in southeast China using high spatial resolution measurements of dissolved CH4 concentrations and physicochemical properties of the surface water. Overall, this reservoir acted as a consistent source of atmospheric CH4, with a mean diffusive flux of 16.1 μmol m-2 h-1. The diffusive CH4 flux at the reservoir demonstrated considerable spatial variations, with the coefficients of variation ranging between 199 and 426% over the three seasons. The shallow water zone (comprising 23% of the reservoir area) had a disproportionately high contribution (56%) to the whole-reservoir diffusive CH4 emissions. Moreover, the mean CH4 flux in the sewage-affected sectors was significantly higher than that in the nonsewage-affected sectors. The results of bootstrap analysis further showed that increasing the sample size from 10 to 100 significantly reduced the relative standard deviation of mean diffusive CH4 flux from 73.7 to 3.4%. Our findings highlighted the role of sewage in governing the spatial variations in reservoir CH4 emissions and the importance of high spatial resolution data to improve the reliability of flux estimates for assessing the contribution of reservoirs to the regional and global CH4 budgets.