Diffusive CH4 Flux Research Articles

Lakes on the Tibetan Plateau as Conduits of Greenhouse Gases to the Atmosphere

AbstractLakes play an important role in the global carbon cycle, and littoral zones of lakes are potential hotspots of greenhouse gas production. In this study, we measured the partial pressures of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in the littoral zones of 17 lakes on the Tibetan Plateau. The littoral zones of lakes on the Tibetan Plateau were supersaturated and acted as sources of CO2, CH4, and N2O to the atmosphere. The average partial pressures of CO2, CH4, and N2O in the surface lake water were 664.8 ± 182.5, 139.8 ± 335.6, and 0.3 ± 0.1 μatm, respectively. The average diffusive fluxes (and uncentainty intervals) of these three gases were 73.7 (0.9–295.3) mmol · m−2 · day−1, 5.2 (0.0008–45.9) mmol · m−2 · day−1, and 6.5 (0.07–20.9) μmol · m−2 · day−1, respectively. The diffusive fluxes of CO2 in lakes were significantly correlated with dissolved organic carbon, dissolved organic nitrogen, salinity, and water temperature. The diffusive fluxes of N2O were significantly correlated with lake water depth. However, no relationships were found between environmental factors and the CH4 diffusive flux at the scale of this study. CO2 exchange with the atmosphere from saline lakes was found to be higher than from freshwater lakes with equivalent CO2 concentrations by a factor of 2.5 due to chemical enhancement of the gas transfer velocity. Therefore, further study with enhanced spatiotemporal resolution and breadth is needed to better understand the important role played by lakes on the Tibetan Plateau in both regional and global carbon cycles.

Diel and seasonal methane flux across water–air interface of a subtropic eutrophic pond

Methane, which is an important greenhouse gas, has received less attention regarding its flux in ponds. Small ponds, whose area only occupies approximately 8.6%, comprise the bulk of CH4 efflux from lakes and ponds on a global scale. However, temporal and spatial variability, as well as consequences of CH4 fluxes from ponds, remains unknown. The aim of this study was to examine using 4 field experiments diel methane (CH4) fluxes from a subtropic eutrophic pond in different seasons. For the eutrophic pond, the mean CH4 efflux for all seasons was 1.772 mg/m2/h, and CH4 emissions in summer were approximately three-fold higher than total of winter, spring, and autumn. Methane diffusive emissions were positively correlated with water temperature, dissolved oxygen (DO) and air temperature but negatively related to pH and to the difference between water temperature and air temperature. The diel diffusive CH4 flux among different seasons varied significantly. The CH4 bubble flux did not differ markedly in winter, spring and autumn, but the quantity in summer was significantly different from all other seasons. Bubble is the main pathway for CH4 emissions. The CH4 ebullition flux accounts for 66, 71, 97 and 98% of the total in winter, spring, summer and autumn, respectively. On an annual scale, the CH4 ebullition flux accounts for 77% of the total fluxes (diffusive + ebullitive). Our results show that further investigations need to be carried out to probe temporal variability of CH4 fluxes in ponds located in different climate zones for better understanding of the global carbon budget, which is critical to predict future climate changes.

Chaoborus spp. Transport CH4 from the Sediments to the Surface Waters of a Eutrophic Reservoir, But Their Contribution to Water Column CH4 Concentrations and Diffusive Efflux Is Minor.

Chaoborus spp. (midge larvae) live in the anoxic sediments and hypolimnia of freshwater lakes and reservoirs during the day and migrate to the surface waters at night to feed on plankton. It has recently been proposed that Chaoborus take up methane (CH4) from the sediments in their tracheal gas sacs, use this acquired buoyancy to ascend into the surface waters, and then release the CH4, thereby serving as a CH4 "pump" to the atmosphere. We tested this hypothesis using diel surveys and seasonal monitoring, as well as incubations of Chaoborus to measure CH4 transport in their gas sacs at different depths and times in a eutrophic reservoir. We found that Chaoborus transported CH4 from the hypolimnion to the lower epilimnion at dusk, but the overall rate of CH4 transport was minor, and incubations revealed substantial variability in CH4 transport over space and time. We calculated that Chaoborus transport ∼0.1 mmol CH4 m-2 yr-1 to the epilimnion in our study reservoir, a very low proportion (<1%) of total CH4 diffusive flux during the summer stratified period. Our data further indicate that CH4 transport by Chaoborus is sensitive to water column mixing, Chaoborus density, and Chaoborus species identity.

Spatially Resolved Measurements of CO2 and CH4 Concentration and Gas-Exchange Velocity Highly Influence Carbon-Emission Estimates of Reservoirs.

The magnitude of diffusive carbon dioxide (CO2) and methane (CH4) emission from man-made reservoirs is uncertain because the spatial variability generally is not well-represented. Here, we examine the spatial variability and its drivers for partial pressure, gas-exchange velocity (k), and diffusive flux of CO2 and CH4 in three tropical reservoirs using spatially resolved measurements of both gas concentrations and k. We observed high spatial variability in CO2 and CH4 concentrations and flux within all three reservoirs, with river inflow areas generally displaying elevated CH4 concentrations. Conversely, areas close to the dam are generally characterized by low concentrations and are therefore not likely to be representative for the whole system. A large share (44–83%) of the within-reservoir variability of gas concentration was explained by dissolved oxygen, pH, chlorophyll, water depth, and within-reservoir location. High spatial variability in k was observed, and kCH4 was persistently higher (on average, 2.5 times more) than kCO2. Not accounting for the within-reservoir variability in concentrations and k may lead to up to 80% underestimation of whole-system diffusive emission of CO2 and CH4. Our findings provide valuable information on how to develop field-sampling strategies to reliably capture the spatial heterogeneity of diffusive carbon fluxes from reservoirs.

Seasonal variability in methane and nitrous oxide fluxes from tropical peatlands in the western Amazon basin

Abstract. The Amazon plays a critical role in global atmospheric budgets of methane (CH4) and nitrous oxide (N2O). However, while we have a relatively good understanding of the continental-scale flux of these greenhouse gases (GHGs), one of the key gaps in knowledge is the specific contribution of peatland ecosystems to the regional budgets of these GHGs. Here we report CH4 and N2O fluxes from lowland tropical peatlands in the Pastaza–Marañón foreland basin (PMFB) in Peru, one of the largest peatland complexes in the Amazon basin. The goal of this research was to quantify the range and magnitude of CH4 and N2O fluxes from this region, assess seasonal trends in trace gas exchange, and determine the role of different environmental variables in driving GHG flux. Trace gas fluxes were determined from the most numerically dominant peatland vegetation types in the region: forested vegetation, forested (short pole) vegetation, Mauritia flexuosa-dominated palm swamp, and mixed palm swamp. Data were collected in both wet and dry seasons over the course of four field campaigns from 2012 to 2014. Diffusive CH4 emissions averaged 36.05 ± 3.09 mg CH4–C m−2 day−1 across the entire dataset, with diffusive CH4 flux varying significantly among vegetation types and between seasons. Net ebullition of CH4 averaged 973.3 ± 161.4 mg CH4–C m−2 day−1 and did not vary significantly among vegetation types or between seasons. Diffusive CH4 flux was greatest for mixed palm swamp (52.0 ± 16.0 mg CH4–C m−2 day−1), followed by M. flexuosa palm swamp (36.7 ± 3.9 mg CH4–C m−2 day−1), forested (short pole) vegetation (31.6 ± 6.6 mg CH4–C m−2 day−1), and forested vegetation (29.8 ± 10.0 mg CH4–C m−2 day−1). Diffusive CH4 flux also showed marked seasonality, with divergent seasonal patterns among ecosystems. Forested vegetation and mixed palm swamp showed significantly higher dry season (47.2 ± 5.4 mg CH4–C m−2 day−1 and 85.5 ± 26.4 mg CH4–C m−2 day−1, respectively) compared to wet season emissions (6.8 ± 1.0 mg CH4–C m−2 day−1 and 5.2 ± 2.7 mg CH4–C m−2 day−1, respectively). In contrast, forested (short pole) vegetation and M. flexuosa palm swamp showed the opposite trend, with dry season flux of 9.6 ± 2.6 and 25.5 ± 2.9 mg CH4–C m−2 day−1, respectively, versus wet season flux of 103.4 ± 13.6 and 53.4 ± 9.8 mg CH4–C m−2 day−1, respectively. These divergent seasonal trends may be linked to very high water tables (&gt; 1 m) in forested vegetation and mixed palm swamp during the wet season, which may have constrained CH4 transport across the soil–atmosphere interface. Diffusive N2O flux was very low (0.70 ± 0.34 µg N2O–N m−2 day−1) and did not vary significantly among ecosystems or between seasons. We conclude that peatlands in the PMFB are large and regionally significant sources of atmospheric CH4 that need to be better accounted for in regional emissions inventories. In contrast, N2O flux was negligible, suggesting that this region does not make a significant contribution to regional atmospheric budgets of N2O. The divergent seasonal pattern in CH4 flux among vegetation types challenges our underlying assumptions of the controls on CH4 flux in tropical peatlands and emphasizes the need for more process-based measurements during periods of high water table.

Mangrove Methane Biogeochemistry in the Indian Sundarbans: A Proposed Budget

Biogeochemical cycling of CH4 was investigated at Lothian Island, one of the relatively pristine islands of Indian Sundarbans and its adjacent Saptamukhi estuary, during June 2010 to December 2012. Intertidal mangrove sediments were highly anoxic and rich in organic carbon. Mean rates of methanogenesis were 3547 and 48.88 µmol m-3 wet sediment d-1, for intertidal (up to 25 cm depth) and sub-tidal sediments (first 5 cm depth), respectively. CH4 in pore-water was 53.4 times more supersaturated than in adjacent estuarine waters. This resulted in significant CH4 efflux from sediments to estuarine waters - via advective and diffusive transport. About 8.2% of the total CH4 produced in intertidal mangrove sediments was transported to the adjacent estuary through advective flux, which was 20 times higher than diffusive CH4 flux. Mean CH4 concentrations in estuarine surface and sub-surface waters were 69.9 and 56.1 nM, respectively, with a dissolved CH4 oxidation rate in estuarine surface waters of 20.5 nmol L-1 d-1. An estimated 0.09 Gg yr-1 of CH4 is released from estuaries of Sundarbans to the regional atmosphere. The mean CH4 mixing ratio over the forest atmosphere was 2 ppmv. On annual basis, only 2.75% of total supplied CH4 to the forest atmosphere was transported to the upper atmosphere via biosphere-atmosphere exchange. Mean CH4 photo-oxidation rate over the forest atmosphere was 3.25 x 10-9 mg cm-3 d-1. Using new and and previously published data we present for the first time, a CH4 budget for Sundarbans mangrove ecosystem which in part, revealed the existence of anaerobic CH4 oxidation in the mangrove sediment column.

Estimates of GHG emissions by hydroelectric reservoirs: The Brazilian case

This article reports and discusses the application of an analysis of reservoir greenhouse gas (GHG) emission using the net emission approach (difference between post-impoundment and pre-impoundment emissions) to assessments of GHG emissions from hydropower reservoirs compared with GHG emissions from thermal plants producing electrical energy. Reservoirs were chosen from a representative set of different Brazilian biomes and climatic regimes. A field campaign program was performed between 2011 and 2013 to estimate net GHGs emissions from 8 Brazilian representative hydropower plants. Four field campaigns to each hydropower plant were scheduled within two-month intervals to measure CH4, CO2 and N2O diffusive fluxes on air-water interfaces (reservoir surface area and downstream river, CH4 and CO2 ebullitive fluxes on the air-water interfaces of the reservoir and degassing rates at powerhouse turbines). Permanent carbon burial rates were also measured in the reservoirs. Statistical analyses of the data were representative of post-impoundment annual values for GHG emissions for each pathway and for permanent carbon burial rates. Values for pre-impoundment representative annual GHG emission were calculated taking into account previous land cover mapping data from the period of the creation of the reservoir, and reference GHG flux values established by the literature for each land cover class. To compare with thermopower generation, annual values obtained were expressed as GHG intensity in g of CO2eq.kWh−1 considering a Global Warming Potential value for 100 years; the factor 44/12 was considered as the permanent carbon burial rate necessary to transform carbon into carbon dioxide; and production of plant firm energy was also considered. Annual net GHG emissions estimates were expressed as GHG intensity in g of CO2eq kWh−1.In all cases analyzed, hydroelectric plants emit less greenhouse gases than natural gas and coal-fired thermoelectric plants. The exception was the Balbina hydroelectric dam, which has a low annual power generation and a very large reservoir: it emits more gas per unit of energy than the two thermoelectric plants abovementioned.

Characteristics of Methane Flux Across the Water-air Interface in Subtropical Shallow Ponds

Five shallow ponds of Yichang were selected to illustrate the characteristics of methane(CH4) in subtropical eutrophic shallow ponds. CH4 flux across the water-air interface was quantified with static floating chamber method for one year. Annual CH4 fluxes of the five ponds were 4.495, 12.702, 6.827, 8.920, 17.560 mg·(m2·h)-1 respectively. Diffusive CH4 fluxes were 0.075, 0.087, 0.118, 0.086, 0.151 mg·(m2·h)-1 respectively and bubble emissions were 4.420, 12.616, 6.709, 8.834, 17.409 mg·(m2·h)-1 respectively. Over 98% of total CH4 flux was bubble emission and CH4 flux was apparently higher than other aquatic ecosystems. So the CH4 flux of shallow waters was high and bubble emission was the dominant way. CH4 emission would be largely underestimated if the research only focus on the diffusion discharge and ignore the bubble emission.

Management effects on greenhouse gas dynamics in fen ditches

Globally, large areas of peatland have been drained through the digging of ditches, generally to increase agricultural production. By lowering the water table it is often assumed that drainage reduces landscape-scale emissions of methane (CH4) into the atmosphere to negligible levels. However, drainage ditches themselves are known to be sources of CH4 and other greenhouse gases (GHGs), but emissions data are scarce, particularly for carbon dioxide (CO2) and nitrous oxide (N2O), and show high spatial and temporal variability. Here, we report dissolved GHGs and diffusive fluxes of CH4 and CO2 from ditches at three UK lowland fens under different management; semi-natural fen, cropland, and cropland restored to low-intensity grassland. Ditches at all three fens emitted GHGs to the atmosphere, but both fluxes and dissolved GHGs showed extensive variation both seasonally and within-site. CH4 fluxes were particularly large, with medians peaking at all three sites in August at 120–230mgm−2d−1. Significant between site differences were detected between the cropland and the other two sites for CO2 flux and all three dissolved GHGs, suggesting that intensive agriculture has major effects on ditch biogeochemistry. Multiple regression models using environmental and water chemistry data were able to explain 29–59% of observed variation in dissolved GHGs. Annual CH4 fluxes from the ditches were 37.8, 18.3 and 27.2gCH4m−2yr−1 for the semi-natural, grassland and cropland, and annual CO2 fluxes were similar (1100 to 1440gCO2m−2yr−1) among sites. We suggest that fen ditches are important contributors to landscape-scale GHG emissions, particularly for CH4. Ditch emissions should be included in GHG budgets of human modified fens, particularly where drainage has removed the original terrestrial CH4 source, e.g. agricultural peatlands.

Spatial variation in flux, δ13C and δ2H of methane in a small Arctic lake with fringing wetland in western Greenland

Small lakes in northern latitudes represent a significant source of CH4 to the atmosphere that is predicted to increase with warming in the Arctic. Yet, whole-lake CH4 budgets are lacking as are measurements of δ13C-CH4 and δ2H-CH4. In this study, we quantify spatial variability of diffusive and ebullitive fluxes of CH4 and corresponding δ13C-CH4 and δ2H-CH4 in a small, Arctic lake system with fringing wetland in southwestern Greenland during summer. Net CH4 flux was highly variable, ranging from an average flux of 7 mg CH4 m−2 d−1 in the deep-water zone to 154 mg CH4 m−2 d−1 along the lake margin. Diffusive flux accounted for ~8.5 % of mean net CH4 flux, with plant-mediated and ebullitive flux accounting for the balance of the total net flux. Methane content of emitted ebullition was low (mean ± SD 10 ± 17 %) compared to previous studies from boreal lakes and wetlands. Isotopic composition of net CH4 emissions varied widely throughout the system, with δ13C-CH4 ranging from −66.2 to −55.5 ‰, and δ2H-CH4 ranging from −345 to −258 ‰. Carbon isotope composition of CH4 in ebullitive flux showed wider variation compared to net flux, ranging from −69.2 to −49.2 ‰. Dissolved CH4 concentrations were highest in the sediment and decreased up the water column. Higher concentrations of CH4 in the hypoxic deep water coincided with decreasing dissolved O2 concentrations, while methanotrophic oxidation dominated in the epilimnion based upon decreasing concentrations and increasing values of δ13C-CH4 and δ2H-CH4. The most depleted 13C- and 2H-isotopic values were observed in profundal bottom waters and in subsurface profundal sediments. Based upon paired δ13C and δ2H observations of CH4, acetate fermentation was likely the dominant production pathway throughout the system. However, isotopic ratios of CH4 in deeper sediments were consistent with mixing/transition between CH4 production pathways, indicating a higher contribution of the CO2 reduction pathway. The large spatial variability in fluxes of CH4 and in isotopic composition of CH4 throughout a single lake system indicates that the underlying mechanisms controlling CH4 cycling (production, consumption and transport) are spatially heterogeneous. Net flux along the lake margin dominated whole-lake flux, suggesting the nearshore littoral area dominates CH4 emissions in these systems. Future studies of whole-lake CH4 budgets should consider this significant spatial heterogeneity.

Summer fluxes of methane and carbon dioxide from a pond and floating mat in a continental Canadian peatland

Abstract. Ponds smaller than 10 000 m2 likely account for about one-third of the global lake perimeter. The release of methane (CH4) and carbon dioxide (CO2) from these ponds is often high and significant on the landscape scale. We measured CO2 and CH4 fluxes in a temperate peatland in southern Ontario, Canada, in summer 2014 along a transect from the open water of a small pond (847 m2) towards the surrounding floating mat (5993 m2) and in a peatland reference area. We used a high-frequency closed chamber technique and distinguished between diffusive and ebullitive CH4 fluxes. CH4 fluxes and CH4 bubble frequency increased from a median of 0.14 (0.00 to 0.43) mmol m−2 h−1 and 4 events m−2 h−1 on the open water to a median of 0.80 (0.20 to 14.97) mmol m−2 h−1 and 168 events m−2 h−1 on the floating mat. The mat was a summer hot spot of CH4 emissions. Fluxes were 1 order of magnitude higher than at an adjacent peatland site. During daytime the pond was a net source of CO2 equivalents to the atmosphere amounting to 0.13 (−0.02 to 1.06) g CO2 equivalents m−2 h−1, whereas the adjacent peatland site acted as a sink of −0.78 (−1.54 to 0.29) g CO2 equivalents m−2 h−1. The photosynthetic CO2 uptake on the floating mat did not counterbalance the high CH4 emissions, which turned the floating mat into a strong net source of 0.21 (−0.11 to 2.12) g CO2 equivalents m−2 h−1. This study highlights the large small-scale variability of CH4 fluxes and CH4 bubble frequency at the peatland–pond interface and the importance of the often large ecotone areas surrounding small ponds as a source of greenhouse gases to the atmosphere.

Spatiotemporal patterns in methane flux and gas transfer velocity at low wind speeds: Implications for upscaling studies on small lakes

AbstractLakes contribute significantly to the global natural emissions of methane (CH4) and carbon dioxide. However, to accurately incorporate them into the continental carbon balance more detailed surveys of lacustrine greenhouse gas emissions are needed, especially in respect to spatiotemporal variability and to how this affects the upscaling of results. We investigated CH4 flux from a small, wind‐shielded lake during 10 field trips over a 14 month period. We show that floating chambers may be used to calibrate the relationship between gas transfer velocity (k) and wind speed at 10 m height (U10) to the local system, in order to obtain more accurate estimates of diffusive CH4 flux than by applying general models predicting k based on U10. We confirm earlier studies indicating strong within‐lake spatial variation in this relationship and in ebullitive CH4 flux within the lake basin. However, in contrast to the pattern reported in other studies, ebullitive CH4 flux was highest in the central parts of the lake. Our results indicate positive relationships between k and U10 at very low U10 (0–3 m s−1), which disagrees with earlier suggestions that this relationship may be negligible at low U10 values. We estimate annually averaged open water CH4 emission from Lake Gerzensee to be 3.6–5.8 mmol m−2 d−1. Our data suggest that estimates of greenhouse gas emissions from aquatic systems to the atmosphere based on the upscaling of short‐term and small‐scale measurements can be improved if both spatial and temporal variabilities of emissions are taken into account.

Methane dynamics in subtropical freshwater reservoirs and the mediating microbial communities

Methane (CH4) is a potent greenhouse gas of primarily biogenic origin. Aquatic systems are significant CH4 sources but uncertainty surrounds their estimated source strengths. There is limited understanding of CH4 concentrations variability in both space and time (dynamics) and the responsible drivers. Water column CH4 dynamics were investigated in three subtropical freshwater reservoirs in Southeast Queensland, Australia, during four seasons in 2011–2012. The reservoirs (Little Nerang Dam-LND, Lake Baroon-LB and Lake Wivenhoe-LW) are of different sizes and characteristics. Overall, CH4 concentrations were significantly higher in the strongly stratified LND (0.03–361 µM) with a more forested catchment and sheltered landscape, intermediate in LB (0.05–220 µM) and lowest in the relatively well-mixed LW (0.04–150 µM) in a more open grassland landscape. CH4 concentrations were highest in summer and spring when the reservoirs were strongly stratified and lowest in winter when the reservoirs were more mixed with deeper dissolved oxygen penetration. Concentrations were also highest in the anoxic water column and often had minima at the oxycline when stratification occurred. Highest CH4 concentrations were predominantly at the sediment–water interface but there were indications of high CH4 production within the anoxic water column in both LB and LND. Moderate CH4 peaks were consistently observed in the oxic subsurface water in all reservoirs likely increasing diffusive CH4 fluxes. DO was a strong proxy for methanotrophy while methanogenesis was best indicated by the oxidation–reduction potential of the water column. Microbial community analysis revealed methanotrophs throughout the water column but methanogenic Archaea mainly proliferated below the oxycline. This is the first comprehensive field-based study towards understanding of CH4 dynamics in subtropical freshwater systems.

Oxidative mitigation of aquatic methane emissions in large Amazonian rivers.

The flux of methane (CH4 ) from inland waters to the atmosphere has a profound impact on global atmospheric greenhouse gas (GHG) levels, and yet, strikingly little is known about the dynamics controlling sources and sinks of CH4 in the aquatic setting. Here, we examine the cycling and flux of CH4 in six large rivers in the Amazon basin, including the Amazon River. Based on stable isotopic mass balances of CH4 , inputs and outputs to the water column were estimated. We determined that ecosystem methane oxidation (MOX) reduced the diffusive flux of CH4 by approximately 28-96% and varied depending on hydrologic regime and general geochemical characteristics of tributaries of the Amazon River. For example, the relative amount of MOX was maximal during high water in black and white water rivers and minimal in clear water rivers during low water. The abundance of genetic markers for methane-oxidizing bacteria (pmoA) was positively correlated with enhanced signals of oxidation, providing independent support for the detected MOX patterns. The results indicate that MOX in large Amazonian rivers can consume from 0.45 to 2.07 Tg CH4 yr(-1) , representing up to 7% of the estimated global soil sink. Nevertheless, climate change and changes in hydrology, for example, due to construction of dams, can alter this balance, influencing CH4 emissions to atmosphere.

Spatio‐temporal variability of lake CH4 fluxes and its influence on annual whole lake emission estimates

AbstractLakes are major sources of methane (CH4) to the atmosphere that contribute significantly to the global budget. Recent studies have shown that diffusive fluxes, ebullition and surface water CH4 concentrations can differ significantly within lakes—spatially and temporally. CH4 fluxes may be affected at longer scales in response to seasons, temperature, lake mixing events, short term weather events like pressure variations, shifting winds and diel cycles. Frequent measurements of fluxes in the same system and integrated assessments of the impacts of the spatio‐temporal variability are rare. Thereby, large scale assessments frequently lack information on this variability which can potentially lead to biased estimates. In this study, we analysed the variability of CH4 fluxes and surface water CH4 concentrations across open water areas of lakes in a small catchment in southwest Sweden over two annual cycles. Significant patterns in CH4 concentrations, diffusive fluxes, ebullition and total fluxes were observed in space (between and within lakes) and in time (over diel cycles to years). Differences observed among the lakes can be associated with lake characteristics. The spatial variability within lakes was linked to depth or distance to stream inlets. Temporal variability was observed at diel to seasonal scales and was influenced by weather events. The fluxes increased exponentially with temperature in all three lakes, with stronger temperature dependence with decreasing depth. By comparing subsets of our data with estimates using all data we show that considering the spatio‐temporal variability in CH4 fluxes is critical when making whole lake or annual budgets.

Diffusive emission of methane and carbon dioxide from two hydropower reservoirs in Brazil.

The role of greenhouse gas emissions from freshwater reservoirs and their contribution to increase greenhouse gas concentrations in the atmosphere is currently under discussion in many parts of the world. We studied CO2 and CH4 diffusive fluxes from two large neotropical hydropower reservoirs with different climate conditions. We used floating closed-chambers to estimate diffusive fluxes of these gaseous species. Sampling campaigns showed that the reservoirs studied were sources of greenhouse gases to the atmosphere. In the Serra da Mesa Reservoir, the CH4 emissions ranged from 0.530 to 396.96 mg.m(-2).d(-1) and CO2 emissions ranged from -1,738.33 to 11,166.61 mg.m(-2).d(-1) and in Três Marias Reservoir the CH4 fluxes ranged 0.720 to 2,578.03 mg.m(-2).d(-1) and CO2 emission ranged from -3,037.80 to 11,516.64 to mg.m(-2).d(-1). There were no statistically significant differences of CH4 fluxes between the reservoirs, but CO2 fluxes from the two reservoirs studied were significantly different. The CO2 emissions measured over the periods studied in Serra da Mesa showed some seasonality with distinctions between the wet and dry transition season. In Três Marias Reservoir the CO2 fluxes showed no seasonal variability. In both reservoirs, CH4 emissions showed a tendency to increase during the study periods but this was not statistically significant. These results contributed to increase knowledge about the magnitude of CO2 and CH4 emission in hydroelectric reservoirs, however due to natural variability of the data future sampling campaigns will be needed to better elucidate the seasonal influences on the fluxes of greenhouse gases.

Greenhouse gas emission and storage in a small shallow lake

Small lakes are likely to show considerable temporal variability in greenhouse gas emissions given their transient stratification and short residence time. To determine the extent that CO2 and CH4 emission and storage depends on surface meteorology, we studied a shallow lake during 2 years with contrasting rainfall and thermal stratification. Gas fluxes were estimated with wind-based and surface renewal models and compared to direct measurements obtained with floating chambers. The assessment of greenhouse gases storage revealed that the lake gained CO2 in association with rainfall in both the rainier (2011) and drier summer (2012). In 2011, stratification was less extensive and disrupted frequently. The lake was a source of CO2 and CH4, and ebullition exceeded diffusive fluxes of CH4. In 2012, stratification was more persistent, the lake was a sink for CO2 during dry periods, CO2 and CH4 accumulated in the hypolimnion later in the summer when rainfall increased, diffusive fluxes of CH4 were similar to those in 2011 mid-summer and over four times higher during overturn. Ebullition was lower in the drier summer. Fluxes measured with chambers were closer to estimations from the surface renewal model and about two times values estimated with wind-based models.

Physicochemical controls of diffusive methane fluxes in the Okavango Delta, Botswana

Atmospheric methane (CH4) is one of the three key greenhouse gases (GHGs) driving global climate change. The atmospheric concentration of CH4 has increased by about 150 % above pre-industrial levels of 400–700 ppb due to anthropogenic activities. Although tropical wetlands account for 50–60 % of the global wetland CH4 emissions, the biogeochemistry of these wetlands, including controls of CH4 emissions from the systems, is poorly understood compared to temperate wetlands. This has resulted in large inter-model variations of the magnitude and distribution of CH4 emission estimates from these tropical wetlands. A recent study in the Okavango Delta, Botswana, estimated diffusive CH4 flux at 1.8 ± 0.2 Tg year−1, accounting for 2.8 ± 0.3 % of the total CH4 emission from tropical wetlands. In this paper we present an assessment of relationships between diffusive CH4 flux rates and physicochemical variables in the overlying water column to identify and understand regulatory variables for the diffusive CH4 fluxes in the Delta. The results show that diffusive CH4 flux rates from the Delta seem to be controlled by a combination of physicochemical variables. Although site specific fluxes seem to be controlled by different combinations of factors, temperature was the primary predictor of CH4 flux rates at almost all the sampled habitats and sites in the Delta. Most physicochemical variables, especially in the permanent swamps, were correlated with temperature implying that their regulatory effect on diffusive CH4 fluxes could be modified by climate change feedback as well.

Summer methane ebullition from a headwater catchment in Northeastern Siberia

Streams and rivers are active processors of terrestrial carbon and significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. Recent studies suggest that ebullition may represent a sizable yet overlooked component of the total CH4 flux from these systems; however, there are no published CH4 ebullition estimates for streams or rivers in subarctic or arctic biomes, regions that store vast quantities of vulnerable, old organic carbon in permafrost soils. We quantified CH4 ebullition from headwater streams in a small arctic watershed in Northeastern Siberia. Ebullitive emissions were 0.64 mmol m−2 d−1, which is lower than the global average but approximately 2 times greater than the pan-arctic diffusive CH4 flux estimate reported in a recent synthesis of global freshwater CH4 emissions. The high CO2:CH4 of sediment bubbles (0.52) suggests that methane emissions may currently be constrained by resource competition between methanogens and microbes using more efficient metabolic strategies. Furthermore, the magnitude and frequency of ebullition events were greater as temperatures increased, suggesting that ebullition from streams could become a more prominent component of the regional CH4 flux in a warmer future.

Biogeochemical dynamics of exogenous methane in an estuary associated to a mangrove biosphere; The Sundarbans, NE coast of India

The distribution, sources and fate of dissolved CH4 were monitored in a mangrove associated estuary of the Sundarbans from January 2011 to December 2012. This tropical estuary was polyhaline (salinity range between 19.06±4.33 and 27.09±0.59), well mixed and net heterotrophic. Average dissolved CH4 concentrations in estuarine surface and bottom waters were 69.90 and 56.17nM, respectively, showing a very high degree of super-saturation (≈2869±571%). Dissolved CH4 was significantly correlated with salinity but not with other physicochemical and biological properties of the estuarine water, indicating its exogenous origin. Average dissolved CH4 concentration in sediment pore water was 53.4 times higher than estuarine water, suggesting significant CH4 influx from the sediment to the estuary via advective and diffusive transports. Average advective and diffusive CH4 fluxes from intertidal and sub-tidal sediments were 159.52 and 8.45μmolm−2d−1, respectively. The average dissolved CH4 oxidation rate in the estuarine water column was 20.59nmolL−1d−1. Being highly supersaturated with respect to atmospheric equilibrium the estuary acts as a source of 0.093Gg CH4 annually to the atmosphere with an average flux of 8.88μmolm−2d−1. In the estuarine CH4 cycle the advective flux and CH4 oxidation were recorded as the predominant input and output processes, respectively. Finally, a box model representing the CH4 biogeochemical cycle in this estuarine mangrove habitat derived a CH4 export flux from the estuary to the adjacent continental shelf of 0.485Ggyr−1.