Water CH4 Concentrations Research Articles

Summer methane emissions from sewage water-fed tropical shallow aquaculture ponds characterized by different water depths.

Aquaculture practices are steadily increasing to meet the fish demand, especially in tropical countries like India. However, efforts to characterize the contribution of these aquaculture ponds towards greenhouse gas emission like CH4 are still very few. CH4 concentration in water [pCH4(water)] and air-water CH4 fluxes were estimated (during the summer months) in two sewage-fed ponds having different depths situated in the East Kolkata Wetlands, India (a Ramsar site). pCH4(water) in both of these ponds showed significant positive correlation with water temperature (R2 = 0.68 and 0.71, p < 0.05). Daily mean chlorophyll-a, turbidity, biochemical oxygen demand (BOD) and gross primary productivity (GPP) also showed positive correlation with pCH4(water). This indicated that higher primary production and presence of turbid materials acted as substrates for methanogenesis, which favoured air-water CH4 effluxes towards atmosphere. Mean air-water CH4 fluxes in the ponds having depth of 1.1m and 0.6m were observed to be 24.79 ± 12.02mgm-2h-1 and 6.05 ± 3.14mgm-2h-1 respectively. Higher depth facilitated net heterotrophic conditions, which led to lower dissolved oxygen levels, which, in turn, led to lower rate of CH4 oxidation. Moreover, under reduced photosynthetically active radiation (in the pond having greater depth), the pH values were comparatively lower (~7.7), which further facilitated a favourable condition for the methanogens to grow. On the whole, it was inferred that apart from pre-established physicochemical factors, depth was also found to play a decisive role in regulating the air-water CH4 fluxes from these aquaculture ponds. In future, continuous sampling should be carried out (by chamber method) to take into account the ebullition CH4 fluxes, and more number of ponds should be sampled throughout a complete annual cycle to have a more holistic understanding about this cluster of sewage-fed aquaculture ponds.

Dissolved methane concentrations and fluxes to the atmosphere from a tropical floodplain lake

Large uncertainties in estimates of methane (CH4) emissions from tropical inland waters reflect the paucity of information at appropriate temporal and spatial scales. CH4 concentrations, diffusive and ebullitive fluxes, and environmental parameters in contrasting aquatic habitats of Lake Janauaca, an Amazon floodplain lake, measured for two years revealed patterns in temporal and spatial variability related to different aquatic habitats and environmental conditions. CH4 concentrations ranged from below detection to 96 µM, CH4 diffusive fluxes from below detection to 2342 µmol m−2 h−1, and CH4 ebullitive fluxes from 0 to 190 mmol m−2 d−1. Vegetated aquatic habitats had higher surface CH4 concentrations than open water habitats, and no significant differences in diffusive CH4 fluxes, likely due to higher k values measured in open water habitats. CH4 emissions were enhanced after a prolonged low water period, when the exposed sediments were colonized by herbaceous plants that decomposed after water levels rose, possibly fueling CH4 production. Statistical models indicated the importance of variables related to CH4 production (temperature, dissolved organic carbon) and consumption (dissolved nitrogen, oxygenated water column), as well as maximum depth, in controlling surface water CH4 concentrations.

The influence of Carex aquatilis and Juncus balticus on methane dynamics: A comparison with water sourced from a natural and a constructed fen

As fen peatlands have been heavily disturbed by resource extraction in northeastern Alberta, Canada, fen construction has been completed. In order to optimize biogeochemical function of future constructed fens, it is beneficial to understand methane (CH4) cycling of newly constructed fens, and how revegetation strategies influence CH4 dynamics. Here, we investigate the effects of two vascular species used for fen construction on CH4 dynamics. A factorial greenhouse experiment using peat columns and a laboratory incubation experiment were used to understand differences in CH4 production, emissions, pore water concentration, and oxidation between Carex aquatilis Wahlenb. and Juncus balticus Willd. The experiment also considered how water sourced from the constructed fen influenced CH4 dynamics compared to natural rich fen water. Higher pore water CH4 concentration and potential CH4 production were found at C. aquatilis columns, possibly associated with higher labile carbon throughout the column. In columns with J. balticus, evidence to support radial oxygen loss reducing CH4 concentration and production was found. Water sampled from peat columns with constructed fen water had higher Fe (all cation forms), Mn (all cation forms), SO42−, and NO3− compared to columns with rich fen water, which corresponded to lower CH4 emissions and pore water concentration. Results from this study could be used to inform revegetation designs of future constructed fens that consider greenhouse gas emissions, including CH4, as a reclamation goal.

Large Fine‐Scale Spatiotemporal Variations of CH4 Diffusive Fluxes From Shrimp Aquaculture Ponds Affected by Organic Matter Supply and Aeration in Southeast China

AbstractMariculture shrimp ponds are important CH4 sources to the atmosphere. However, the spatiotemporal variations of CH4 concentration and flux at fine spatial scales in mariculture ponds are poorly known, particularly in China, worlds largest aquaculture producer. In this study, the plot‐scale spatiotemporal variations of water CH4 concentration and flux, both within and among ponds, were researched in shrimp ponds in Shanyutan wetland, Min River Estuary, Southeast China. The average water CH4 concentration and diffusion flux across the water‐air interface in the shrimp ponds over the shrimp aquaculture period varied from 2.29 ± 0.29 to 50.48 ± 20.91 μM and from 0.09 ± 0.01 to 2.32 ± 0.95 mmol·m−2·hr−1, respectively. The CH4 emissions from the estuarine ponds varied greatly between seasons, with peaks in August and September, which was similar to the trend of water temperature and dissolved oxygen concentrations. There was no remarkable difference in CH4 concentration and flux between shrimp ponds but significantly spatiotemporal differences in CH4 concentration and flux within the ponds. Significantly higher emissions occurred in the feeding zone, accounting for approximately 60% of total CH4 emission flux, while much lower CH4 emissions appeared in aeration zone, contributing 14% to total flux. This study suggests the importance of considering spatiotemporal variation in the whole‐pond estimates of CH4 concentration and flux. In light of such high spatial variation within ponds, improving aeration and feed utilization efficiency would help to mitigate CH4 emissions from mariculture ponds.

To shake or not to shake: 13C-based evidence on anaerobic methane oxidation in paddy soil

The anaerobic oxidation of methane (AOM) removes most of the biologically produced methane (CH4) from marine ecosystems before it enters the atmosphere and thus mitigates greenhouse gas emissions. As compared to marine environments, surprisingly little is known about the role of AOM in terrestrial ecosystems. Particularly, how AOM controls the CH4 budget of paddy soils is unexplored, partly reflecting analytical difficulties in analyzing CH4 turnover. To date, the most commonly used method to study AOM in soils is in vitro incubation of microcosms with CH4 injection into the headspace with/without shaking of slurry. Shaking, however, introduces various errors and disturbances. Here we measured AOM in rice paddy soil using a new alternative approach that introduced 13C-labelled CH4 directly into soil slurry via a silicone tube without shaking. The results were compared to those obtained by the classical approaches (i.e., with and without tubes and/or shaking). In all batches, 13C enrichment of CO2 after 13CH4 injection clearly confirmed the occurrence of AOM in paddy soil. The cumulative AOM during 84 days reached 0.16–0.24 μg C g−1 dry soil without shaking, but it was 33–80% lower with shaking. Unexpectedly, the effect of silicone tubes on AOM was insignificant either with or without shaking, suggesting that the CH4 concentration in water (slurry) was not the main limiting factor for AOM. Without shaking, the controls without CH4 addition revealed a steady increase of CH4 in the headspace/tube, whereas the CH4 concentration in jars with shaking was constantly low during 59 days. This suggests that shaking inhibited methanogenesis. There was a strong linear correlation between the amount of CH4 oxidized and CH4 produced with shaking (R2 = 0.91), whereas without shaking this relationship followed a power growth regression. Based on the current and reported AOM rates, rough upscale to paddy soils in China showed recycling of ca. 2.0 Tg C of CH4 each year, making AOM a crucial terrestrial CH4 sink.

Methane Emissions From the Salt Marshes of Doñana Wetlands: Spatio-Temporal Variability and Controlling Factors

Coastal wetlands are significant sources of methane to the atmosphere, but emissions in these ecosystems are still poorly quantified, as in situ data are limited. In this study, we present the first assessment of spatio-temporal changes in air-water CH4 fluxes in the salt marshes of Donana wetlands (SW Spain), one of the most emblematic protected areas in Europe due to its high biotic diversity and unique importance for aquatic wildfowl. The marshes are flooded by estuarine waters from the adjacent Guadalquivir River by tidal intrusion, which must influence aquatic CH4 dynamics through changes in salinity and water chemistry that affects sedimentary methanogenesis. During sixteen samplings conducted between March 2016 and March 2018, surface water CH4 concentrations were measured by using static-head space equilibration gas chromatography in seven sites representing salt marshes located in the land strip close to the estuary. Because of meteorological conditions and tide variations, salinity markedly changed across the salt marshes, although sites located closer to the river mouth could be categorized as polyhaline marshes whereas upstream sites formed a group of mesohaline marshes. The CH4 saturation range was 252-36,735 % (average 5,170%) and 374-620,007 % (average 31,541%) in polyhaline and mesohaline marshes respectively, suggesting inhibitory effect of sulfate on methanogenesis, although a linear trend between dissolved CH4 concentration and salinity was not observed. In contrast, water temperature and chlorophyll a were significantly and positively correlated with methane, indicating sedimentary methanogenesis control by temperature and organic matter availability boosted by primary productivity. This does not exclude the possibility that some CH4 might come from estuarine inputs. Air-water CH4 fluxes ranged from 2.6 to 720 μmol m−2d−1 (average 104 μmol m−2 d−1) in the polyhaline marshes and from 5.6 to 12,715 μmol m−2d−1 (average 637 μmol m−2 d−1) in the mesohaline marshes, with the higher emissions being measured during the summer months in all sites. Even though the strongest environmental drivers of dissolved CH4 accumulation seemed to be temperature and productivity, the spatio-temporal patterns observed suggest that methane dynamics in Donana salt marshes are controlled by a mosaic of processes rather than by a single environmental force.

Factors controlling seasonal CO2 and CH4 emissions in three tropical mangrove-dominated estuaries in Australia

CO2 and CH4 emissions from estuaries of the Southern Hemisphere are greatly under-represented in global estuary emission estimates. This study quantifies seasonal pCO2 and CH4 concentrations and emissions along the salinity gradient of three tropical mangrove-dominated estuaries in Australia. A combination of approaches (i.e. carbon stable isotopes, groundwater inputs, riverine contribution, freshwater flushing times) was used to assess the spatial and seasonal variable factors that controlled the estuary surface water CO2 and CH4 concentrations and hence emissions. Overall, CO2 and CH4 emissions ranged from 21.6 to 110.4 mmol m−2 d−1 and 40.3 to 1047.1 μmol m−2 d−1, respectively, and were within or at the high end compared to estuaries in Australia and globally. In the Johnstone River estuary, high emissions were predominantly driven by groundwater and riverine carbon inputs with exports of CO2 and CH4 to the ocean expected in the wet season. In the Fitzroy River estuary and Constant Creek estuary, in situ production and terrestrial carbon inputs were likely the main factors controlling CO2 and CH4 emissions. The contribution of riverine CO2 may be more important to overall CO2 emissions than the riverine CH4 to overall CH4 emissions in estuaries. The relative contribution of in situ production, the exchange with adjacent coastal habitats (i.e. mangroves, inter-tidal flats), and terrestrial, riverine and groundwater inputs in the wet and in the dry season determined whether CO2 and CH4 were fully ventilated within the estuaries or exported to the open ocean. The revised global estimate for estuary CO2 emission of the latitude 0 to 23.5 °S is 52.1 ± 16.1 mmol m−2 d−1, which is 15% higher than a recent estimate of this latitudinal region.

Spatio-temporal variability and controls on methane and nitrous oxide in the Guadalquivir Estuary, Southwestern Europe

Estuaries are significant methane (CH4) and nitrous oxide (N2O) emitters, although dynamics of both greenhouse gases in these ecosystems are regulated by complex processes. In this work, we aimed at characterizing the spatio-temporal distribution of CH4 and N2O in the Guadalquivir river estuary (SW Spain), the southernmost European estuary. During eight sampling cruises conducted between 2016 and 2017, surface water CH4 and N2O concentrations were measured along the salinity gradient of the estuary by using static-head space equilibration gas chromatography. The CH4 and N2O saturation ranges over the estuarine transect were 520–30,800% (average 2285%) and 40–390% (average 183%), respectively and air–water fluxes ranged from 13 to 1000 µmol m− 2 day− 1(average 66.2 µmol m− 2 day− 1) for CH4 and from − 7 to 35 µmol m− 2 day− 1 (average 8.5 µmol m− 2 day− 1) for N2O. A slight increase in the emissions was detected upstream and no seasonal trends were observed. Mixing between freshwater and oceanic waters influenced biogeochemistry of estuarine waters, affecting CH4 and N2O fluxes. In order to identify potential sources of CH4 and N2O, biogeochemical parameters involved in the formation pathways of both gases, such as salinity, dissolved oxygen, nutrients and organic matter were analyzed. Results suggested that sulfate inhibition and microbial oxidation played a relevant role in dissolved CH4 accumulation in the water column whereas associations found between N2O, nitrate and oxygen indicated that nitrification was a major source of this gas. Therefore, the influence of the tidal-fluvial interaction on ecosystem metabolism regulates trace gas dynamics in the Guadalquivir estuary.

Factors affecting methane loss from a water scrubbing based biogas upgrading system

Biogas upgrading is a vital step to produce high quality fuel called biomethane with above 90% methane (CH4). Among the various technologies available for biomethane production, water scrubbing is the most extensively implemented technology around the world. However, during the process of biogas upgrading, a some amount of CH4 is separated as CH4 loss through the water flowing out of the water scrubbing column. In this paper, various factors affecting CH4 loss from water scrubbing method are analysed. Some factors such as pressure, water flow rate and CH4 concentration in input gas are dependent upon the solubility and partial pressures of the gases are generally known. Apart from these factors CH4 losses due to bubble entrainment due to high pressure difference between water scrubbing column and desorption tank and gas short circuiting of the gas through the bottom section of the column due to no water sealing and water level maintencence also contribute to CH4 losses. Therefore, CH4 losses during the water scrubbing process due to these factors have been experimentally studied in this paper. A pilot scale water scrubbing system for biogas upgradation was used for the study. It was observed that CH4 % (v/v) in the upgraded biogas and CH4 loss % from the desorbed gas increased with the increase in pressure and increase in the concentration of CH4 in the input gas. Increase in water flow rates caused removal of larger quantities of water containing more absorbed CH4 and CO2 from the scrubbing column, thereby increasing CH4 loss of the system. Highest CH4 loss % of 9.9% (±0.1%), was obtained with raw biogas sample when water was desorbed at atmospheric pressures in the desorption tank, i.e. when pressure difference between the water scrubbing column and desorption tank was highest. A pressure vessel was installed in between scrubbing column and desorption tank to reduce the pressure difference for water leaving the column by varying the pressure in the pressure vessel from 1 to 9bar. With the increase in pressure in the pressure vessel, the pressure difference for water decreased which led to a saving in overall CH4 loss of the system. Water sealing and water level maintenance in the bottom section of the column also affected CH4 losses of the system. All the factors discussed in the article contribute to the CH4 losses from the scrubbing column and cannot be solely credited to a single factor.

Methane emissions dynamics from a constructed fen and reference sites in the Athabasca Oil Sands Region, Alberta.

Recently, fen construction projects on surface mines in northeastern Alberta have been attempted as a reclamation strategy to reintroduce peatlands into the region where industry disturbs a substantial amount of wetland ecosystems. Knowledge of carbon cycling and greenhouse gas (GHG) dynamics, including methane (CH4), is one way to understand the biogeochemical function of newly constructed fen ecosystems. In this study we monitored CH4 emissions and CH4 pore water concentration, as well as ecological and soil chemistry controls on CH4 emissions and pore water concentration, from a constructed fen. The same variables were also monitored at two natural reference fens that had similar vascular vegetation to the constructed fen. Methane emissions were lower at the constructed fen compared to the reference poor fen, but similar to the reference saline fen. However, CH4 concentration in pore water at 0.2m and 0.7m depth was lower at the constructed fen than either of the natural reference sites. The supply rate of sulfur (all mobile forms) was the most dominant control on CH4 emission and CH4 pore water concentration. While low CH4 emissions may be beneficial for constructed fens from a GHG perspective, this condition indicates that peat and carbon accumulation at these reclaimed sites may ensue slowly. Therefore, a clear statement of goals is required to determine how CH4 dynamics from constructed fen ecosystems relate to the reclamation outcome.

Effect of plant functional type on methane dynamics in a restored minerotrophic peatland

Peatland methane (CH4) fluxes may vary between plant types; however, in mixed communities, the specific role of each species is difficult to distinguish. The goal of this study was to determine the individual and interacting effect of moss, graminoid and shrub plant functional types on CH4 dynamics of experimentally planted plots in a rewetted minerotrophic peatland. We measured CH4 flux, pore water CH4 concentration and CH4 production and oxidation potential in pure stands of reintroduced Tomenthypnum nitens (Hedw.) Loeske, Carex aquatilis Wahlenb, or Myrica gale L., as well as mixtures of T. nitens + C. aquatilis and T. nitens + M. gale. Methane flux was also measured on bare peat plots. The presence of both the graminoid C. aquatilis and the shrub M. gale resulted in the highest CH4 production potential in near surface peat (10 cm). The presence of moss (T. nitens) and C. aquatilis significantly increased CH4 oxidation potential. Water table position was a significant control on CH4 flux, but the presence of C. aquatilis maintained higher flux even at dry plots. Plots including C. aquatilis had significantly lower pore water CH4 concentration at 30 cm depth, likely reflecting CH4 oxidation and transport. Management of restored sites aiming to reduce CH4 flux should focus on hydrology, i.e. water table position. The presence of graminoids enhances CH4 flux, while moss presence may result in lower CH4 emission.

Low methane (CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;) emissions downstream of a monomictic subtropical hydroelectric reservoir (Nam Theun 2, Lao PDR)

Abstract. Methane (CH4) emissions from hydroelectric reservoirs could represent a significant fraction of global CH4 emissions from inland waters and wetlands. Although CH4 emissions downstream of hydroelectric reservoirs are known to be potentially significant, these emissions are poorly documented in recent studies. We report the first quantification of emissions downstream of a subtropical monomictic reservoir. The Nam Theun 2 Reservoir (NT2R), located in the Lao People's Democratic Republic, was flooded in 2008 and commissioned in April 2010. This reservoir is a trans-basin diversion reservoir which releases water into two downstream streams: the Nam Theun River below the dam and an artificial channel downstream of the powerhouse and a regulating pond that diverts the water from the Nam Theun watershed to the Xe Bangfai watershed. We quantified downstream emissions during the first 4 years after impoundment (2009–2012) on the basis of a high temporal (weekly to fortnightly) and spatial (23 stations) resolution of the monitoring of CH4 concentration. Before the commissioning of NT2R, downstream emissions were dominated by a very significant degassing at the dam site resulting from the occasional spillway discharge for controlling the water level in the reservoir. After the commissioning, downstream emissions were dominated by degassing which occurred mostly below the powerhouse. Overall, downstream emissions decreased from 10 GgCH4 yr−1 after the commissioning to 2 GgCH4 yr−1 4 years after impoundment. The downstream emissions contributed only 10 to 30 % of total CH4 emissions from the reservoir during the study. Most of the downstream emissions (80 %) occurred within 2–4 months during the transition between the warm dry season (WD) and the warm wet season (WW) when the CH4 concentration in hypolimnic water is maximum (up to 1000 µmol L−1) and downstream emissions are negligible for the rest of the year. Emissions downstream of NT2R are also lower than expected because of the design of the water intake. A significant fraction of the CH4 that should have been transferred and emitted downstream of the powerhouse is emitted at the reservoir surface because of the artificial turbulence generated around the water intake. The positive counterpart of this artificial mixing is that it allows O2 diffusion down to the bottom of the water column, enhancing aerobic methane oxidation, and it subsequently lowered downstream emissions by at least 40 %.

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.

An inter-regional assessment of concentrations and δ13C values of methane and dissolved inorganic carbon in small European lakes

Methane (CH4) and carbon dioxide emissions from lakes are relevant for assessing the greenhouse gas output of wetlands. However, only few standardized datasets describe concentrations of these gases in lakes across different geographical regions. We studied concentrations and stable carbon isotopic composition (δ13C) of CH4 and dissolved inorganic carbon (DIC) in 32 small lakes from Finland, Sweden, Germany, the Netherlands, and Switzerland in late summer. Higher concentrations and δ13C values of DIC were observed in calcareous lakes than in lakes on non-calcareous areas. In stratified lakes, δ13C values of DIC were generally lower in the hypolimnion due to the degradation of organic matter (OM). Unexpectedly, increased δ13C values of DIC were registered above the sediment in several lakes. This may reflect carbonate dissolution in calcareous lakes or methanogenesis in deepwater layers or in the sediments. Surface water CH4 concentrations were generally higher in western and central European lakes than in Fennoscandian lakes, possibly due to higher CH4 production in the littoral sediments and lateral transport, whereas CH4 concentrations in the hypolimnion did not differ significantly between the regions. The δ13C values of CH4 in the sediment suggest that δ13C values of biogenic CH4 are not necessarily linked to δ13C values of sedimentary OM but may be strongly influenced by OM quality and methanogenic pathway. Our study suggests that CH4 and DIC cycling in small lakes differ between geographical regions and that this should be taken into account when regional studies on greenhouse gas emissions are upscaled to inter-regional scales.

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.

Microbial methane cycling in the bed of a chalk river: oxidation has the potential to match methanogenesis enhanced by warming

Summary Many rivers are oversaturated in methane (CH4) and carbon dioxide (CO2) relative to the atmosphere, but we know little about the biological controls on the balance between these two important greenhouse gases and how they might respond to warming. We characterise the potential response to temperature in the biological production of CO2 and CH4 and the subsequent microbial oxidation of that CH4, that is the sink and source components of the CH4 cycle, in contrasting river bed sediments: fine sediments, which are largely anoxic, and oxic, coarse gravels. In the fine sediments, anaerobic production of both CH4 and CO2 increased with temperature, with apparent activation energies for each being 0.51 eV and 0.24 eV, respectively. The difference between the two resulted in a 4% increase in the ratio of CH4:CO2 production for a 1 °C increase in temperature. In the coarse gravels, microbial CH4 oxidation showed no response to temperature at CH4 concentrations characteristic of these gravel beds (30–200 nmol CH4 L−1), due to strong substrate limitation. In contrast, at higher (although still rate limiting) CH4 concentrations, more characteristic of the fine sediment patches (2–4 μmol CH4 L−1), CH4 oxidation exhibited an increasingly strong response to temperature, eventually exceeding that for CH4 production. In the fine sediment, the surface layers had a CH4 oxidation capacity over 100 times greater than the gravels and the kinetic response to differing pore water CH4 concentrations meant CH4 was oxidised some 2000 times faster in the fine sediment patches compared with the coarse gravels. The calculated kinetic and temperature responses showed that with warming, methanogenesis is unlikely to outstrip methanotrophy and the ratio of CO2 to CH4 emitted could be conserved. Consequently, any changes in the efflux ratio of CH4 to CO2 are unlikely to be due to the incapacity of methanotrophy to respond to CH4 production, but rather to a physical bypassing of the methanotrophic community (e.g. through ebullition or transport via plant stems) or contraction of the oxic layer.

The origin of sub-surface source waters define the sea–air flux of methane in the Mauritanian Upwelling, NW Africa

Concentrations and flux densities of methane were determined during a Lagrangian study of an advective filament in the permanent upwelling region off western Mauritania. Newly upwelled waters were dominated by the presence of North Atlantic Central Water and surface CH4 concentrations of 2.2±0.3nmolL−1 were largely in equilibrium with atmospheric values, with surface saturations of 101.7±14%. As the upwelling filament aged and was advected offshore, CH4 enriched South Atlantic Central Water from intermediate depths of 100–350m was entrained into the surface mixed layer of the filament following intense mixing associated with the shelf break. Surface saturations increased to 198.9±15% and flux densities increased from a mean value over the shelf of 2.0±1.1μmolm−2d−1 to a maximum of 22.6μmolm−2d−1. Annual CH4 emissions for this persistent filament were estimated at 0.77±0.64Gg which equates to a maximum of 0.35% of the global oceanic budget. This raises the known outgassing intensity of this area and highlights the importance of advecting filaments from upwelling waters as efficient vehicles for air-sea exchange.

Methane and carbon dioxide emissions from inland waters in India – implications for large scale greenhouse gas balances

Inland waters were recently recognized to be important sources of methane (CH4 ) and carbon dioxide (CO2 ) to the atmosphere, and including inland water emissions in large scale greenhouse gas (GHG) budgets may potentially offset the estimated carbon sink in many areas. However, the lack of GHG flux measurements and well-defined inland water areas for extrapolation, make the magnitude of the potential offset unclear. This study presents coordinated flux measurements of CH4 and CO2 in multiple lakes, ponds, rivers, open wells, reservoirs, springs, and canals in India. All these inland water types, representative of common aquatic ecosystems in India, emitted substantial amounts of CH4 and a major fraction also emitted CO2 . The total CH4 flux (including ebullition and diffusion) from all the 45 systems ranged from 0.01 to 52.1mmolm(-2) d(-1) , with a mean of 7.8±12.7 (mean±1 SD) mmolm(-2) d(-1) . The mean surface water CH4 concentration was 3.8±14.5μm (range 0.03-92.1μm). The CO2 fluxes ranged from -28.2 to 262.4mmolm(-2) d(-1) and the mean flux was 51.9±71.1mmolm(-2) d(-1) . The mean partial pressure of CO2 was 2927±3269μatm (range: 400-11467μatm). Conservative extrapolation to whole India, considering the specific area of the different water types studied, yielded average emissions of 2.1 Tg CH4 yr(-1) and 22.0 Tg CO2 yr(-1) from India's inland waters. When expressed as CO2 equivalents, this amounts to 75 Tg CO2 equivalentsyr(-1) (53-98 Tg CO2 equivalentsyr(-1) ; ±1 SD), with CH4 contributing 71%. Hence, average inland water GHG emissions, which were not previously considered, correspond to 42% (30-55%) of the estimated land carbon sink of India. Thereby this study illustrates the importance of considering inland water GHG exchange in large scale assessments.

Influence of weather variables on methane and carbon dioxide flux from a shallow pond

Freshwaters are important sources of the greenhouse gases methane (CH4) and carbon dioxide (CO2) to the atmosphere. Knowledge about temporal variability in these fluxes is very limited, yet critical for proper study design and evaluating flux data. Further, to understand the reasons for the variability and allow predictive modeling, the temporal variability has to be related to relevant environmental variables. Here we analyzed the effect of weather variables on CH4 and CO2 flux from a small shallow pond during a period of 4 months. Mean CH4 flux and surface water CH4 concentration were 8.0 [3.3–15.1] ± 3.1 mmol m−2 day−1 (mean [range] ± 1 SD) and 1.3 [0.3–3.5] ± 0.9 µM respectively. Mean CO2 flux was 1.1 [−9.8 to 16.0] ± 6.9 mmol m−2 day−1. Substantial diel changes in CO2 flux and surface water CH4 concentration were observed during detailed measurements over a 24 h cycle. Thus diel patterns need to be accounted for in future measurements. Significant positive correlations of CH4 emissions with temperature were found and could include both direct temperature effects as well as indirect effects (e.g. related to the growth season and macrophyte primary productivity providing organic substrates). CO2 flux on the other hand was negatively correlated to temperature and solar radiation, presumably because CO2 consumption by plants was higher relative to CO2 production by respiration during warm sunny days. Interestingly, CH4 fluxes were comparable to ponds with similar morphometry and macrophyte abundance in the tropics. We therefore hypothesize that CH4 and CO2 summer emissions from ponds could be more related to the morphometry and dominating primary producers rather than latitude per se. Data indicate that CH4 emissions, given the system characteristic frameworks, is positively affected by increased temperatures or prolonged growth seasons.

Determination of methane content in NaCl–H2O fluid inclusions by Raman spectroscopy. Calibration and application to the external part of the Central Alps (Switzerland)

The reconstruction of the entrapment conditions of geological fluids requires determining the volumetric and composition properties of the inclusions containing these fluids. In some cases, the analytical data necessary for PVTX determination cannot be obtained from microthermometry. The quantification of the dissolved gases in aqueous fluid inclusions by Raman spectroscopy, following proper calibration of the instrument and methodology, can provide alternative data. In the present study, the intensity of the Raman signal of the symmetric stretching vibrational mode of methane was calibrated in order to (1) determine CH4 concentration in pure and saline water and (2) quantify the molar fractions of H2O and CH4 in the gas phase. High-pressure optical cell (HPOC), i.e. a pressurization system connected to a silica microcapillary heated on a customized heating–cooling stage, reveals to be much more convenient and accurate than synthetic fluid inclusions. Moreover, a wide range of pressure, temperature, and salinity can be covered by this methodology. Over 1000 measurements were produced to define the calibration curves, covering the ranges in temperature, pressure and salinity of 60–180°C, 30–1000bar, and 0–4mol.kg−1 NaCl, respectively, which corresponds to a CH4 molality up to 0.6mol.kg−1. The CH4 solubility vs. CH4/H2O Raman peak area ratio was fitted by a second-order polynomial curve (R2=0.996). In the gas phase, the molar fraction of H2O vs. Raman peak area ratio is fitted by a straight line (R2=0.990). The calibration was applied to a set of natural fluid inclusions trapped within late quartz Alpine fissure of the external part of the Central Alps (Switzerland). The determination of CH4 concentration in the studied fluids provided valuable insight on conditions of trapping and on the pressure regimes prevailing in this low-grade metamorphic setting.