Variation In Methane Emissions Research Articles

Tidal control on aerobic methane oxidation and mitigation of methane emissions from coastal mangrove sediments

Mangrove forests represent important sources of methane, partly thwarting their ecosystem function as an efficient atmospheric carbon dioxide sink. Many studies have focused on the spatial and temporal variability of methane emissions from mangrove ecosystems, yet little is known about the microbial and physical controls on the release of biogenic methane from tidally influenced mangrove sediments. Here, we show that aerobic methane oxidation is a key microbial process that effectively reduces methane emissions from mangrove sediments. We further demonstrate clear links between the tidal cycle and fluctuations in methane fluxes, with contrasting methane emission rates under different tidal amplitudes. Our data suggest that both the microbial methane oxidation activity and pressure-induced advective transport modulated methane fluxes in the mangrove sediments. Methane oxidation activity is limited by the availability of oxygen in the surface sediments, which in turn is controlled by tidal dynamics, further highlighting the interactive physico-biogeochemical controls on biological methane fluxes. Although we found some molecular evidence for anaerobic methanotrophs in the deeper sediments, anaerobic methane oxidation seems to play only a minor role in the mangrove sediments, with potential rates being two orders of magnitude lower than those of aerobic methane oxidation. Our findings confirmed the importance of surface sediments as biological barrier for methane. Specifically, when sediments were exposed to the air, methane consumption increased by ∼227%, and the methane flux was reduced by ∼62%, compared to inundated conditions. Our data demonstrate how tides can orchestrate the daily rhythm of methane consumption and production within mangrove sediments, thus explaining the temporal variability of methane emissions in the tidally influenced coastal mangrove systems.

Temporal Variations in Methane Emissions from a Restored Mangrove Ecosystem in Southern China

The role of coastal mangrove wetlands in sequestering atmospheric carbon dioxide has been increasingly investigated in recent years. While studies have shown that mangroves are weak sources of methane (CH4) emissions, measurements of CH4 fluxes from these ecosystems remain scarce. In this study, we examined the temporal variation and biophysical drivers of ecosystem-scale CH4 fluxes in China’s northernmost mangrove ecosystem based on eddy covariance measurements obtained over a 3-year period. In this mangrove, the annual CH4 emissions ranged from 6.15 to 9.07 g C m−2 year−1. The daily CH4 flux reached a peak of over 0.07 g C m−2 day−1 during the summer, while the winter CH4 flux was negligible. Latent heat, soil temperature, photosynthetically active radiation, and tide water level were the primary factors controlling CH4 emissions. This study not only elucidates the mechanisms influencing CH4 emissions from mangroves, strengthening the understanding of these processes but also provides a valuable benchmark dataset to validate the model-derived carbon budget estimates for these ecosystems.

Aircraft-derived CH4 emissions from surface and in-situ mining activities in the Alberta oil sands region

The identification and reduction of methane sources is considered an important part of the fight to stem greenhouse gas (GHG) emissions to the atmosphere. One of the largest industrial contributors to national GHG emissions in Canada is the Alberta Oil Sands Region. To quantify and investigate the spatial distribution and temporal variability of methane emissions from this region, airborne measurements were conducted in 2017 and 2018 with three aircraft. 59 flights were conducted in total to assess emissions for both open-pit and in-situ facilities, in both cold and warm seasons. Derived emission rates were higher than those reported in national inventories by 30%–96% depending on the facility. In-situ facilities had emission rates an order of magnitude lower than surface mining operations and differed significantly from inventory estimates. No statistical differences in CH4 emissions between cold and warm seasons were observed, substantiating the use of simple upscaling to annual emissions within inventories. Rather than confirming a reported decrease in emissions between 2013 and 2018, the measurements suggest essentially no change from the 18 t h−1 for the region observed in 2013. Overall, the results suggest that current methods of CH4 emission determination within the oil sands region, for use in reporting, require improvement.

Autochthonous organic matter input in reservoirs: Limited methane oxidation in sediments fails to suppress methane emission

The interception of rivers leads to the accumulation of substantial organic matter in reservoirs, exerting a significant influence on greenhouse gas emissions. The diverse imported organic matter, coupled with sedimentary heterogeneity and intricate microbial processes, gives rise to seasonal variations in methane emissions from reservoirs. In this study, sediment cores were supplemented with terrestrial or autochthonous carbon to emulate reservoir carbon input across different seasons, thereby investigating methane emission potential and associated microbial mechanisms within the sediment cores. Results demonstrated that autochthonous organic matter enhanced sediment organic content, thereby providing more substrates for the methanogenic process and fostering the proliferation of methanogens (with a relative abundance of 47.17 % to 60.66 %). Notably, the dominant genera of Methanosaeta, Methanosarcina, and Candidatus Methanomethylicus were boost on the surface layer of sediment. Concurrently, the introduction of autochthonous organic carbon spurred an increase in methane-oxidizing microbe, reaching up to 5.59 %, with Methylobacter and Candidatus Methanoperedens as the predominant species, which led to a downward migration of the functional microbial group in the sediment. Under the priming impact of autochthonous carbon, however, the methane oxidation probably doesn't consume the substantial methane produced in sediment. Consequently, the sediment functions as a hotspot for methane release into the overlying water, highlighting the necessity to include summer as critical periods for integrated assessments, particularly during algae bloom.

Drivers of intra-individual spatial variability in methane emissions from tree trunks in upland forest

Drivers of intra-individual spatial variability in methane emissions from tree trunks in upland forest

Diurnal variation in methane emission from a rice paddy due to ebullition.

Diurnal fluctuations in methane (CH4 ) emission are frequently observed in rice paddy fields, yet the driving mechanisms behind these variations are not fully understood. We posited that the observed diurnal patterns are predominantly due to temperature-dependent bubbling emissions (ebullition). To investigate this hypothesis, we conducted measurements of CH4 emission in a Japanese rice paddy using static chambers coupled with a high-time-resolution CH4 analyzer that allowed us to partition the total flux into its two principal components: plant-mediated transport and ebullition. Diurnal variation in the total flux was minimal during the early parts of reproductive growth (panicle formation and booting stages), when plant-mediated emission was dominant. At a later stage (heading period), ebullition accounted for 43%-70% of the total emission, and CH4 emission via both pathways varied diurnally; however, the diurnal range was much greater for bubbling emission (ratio of maximum to minimum=3.5) than for plant-mediated emission (ratio of maximum to minimum=1.4). The magnitude of emissions due to ebullition depended on the temperature, but was greater during the flux-increasing phase in the morning than the flux-decreasing phase in the afternoon even at the same temperature, forming a hysteresis in the flux versus temperature relationship. These findings support our hypothesis and indicate that abiotic processes, such as a change in the solubility of gases and the temperature dependence of the gas-phase volume (Charles's law), may play a central role in shaping the diurnal pattern of the CH4 ebullition.

SPATIAL VARIABILITY OF METHANE EMISSIONS FROM SOILS OF WET FORESTS: A BRIEF REVIEW

SPATIAL VARIABILITY OF METHANE EMISSIONS FROM SOILS OF WET FORESTS: A BRIEF REVIEW

Substituting ryegrass-based pasture with graded levels of forage rape in the diet of lambs reduced post-feeding variation in methane emissions

The temporal profile of methane (CH4) emissions from ruminants is affected by feed composition and forage type. This study aimed to 1) describe and compare the post-feeding pattern of CH4 emissions from lambs fed fresh ryegrass substituted with graded levels of forage rape, and 2) to evaluate the association between the magnitude of the variation in CH4 emission patterns and organic fermentation products quantified in pre- and- post- feeding rumen content samples. Methane emissions were measured at approximately 6 min intervals in respiration chambers from 70 lambs (n = 14 per treatment) fed ryegrass only (FR0) or as a proportion, 0.25 forage rape + 0.75 ryegrass (FR25), 0.50 forage rape + 0.50 ryegrass (FR50), 0.75 forage rape + 0.25 ryegrass (FR75), and forage rape only (FR100). The magnitude of the variability in CH4 emissions was defined as the ratio of maximum to minimum CH4 emissions in a 24 h period. This ratio was correlated (P < 0.05) with the proportions of major short chain fatty acids, and calculated hydrogen (H2) available per unit of glucose fermented (H2/GEF), quantified in rumen content samples collected pre- and- post- feeding. The CH4 emission profile after each feeding followed an asymmetrical positively skewed shape for FR0 to FR75, but not in lambs fed FR100. The lowest CH4 emission rates were observed before morning feeding which was followed by a CH4 peak within the 2 h after feeding, and then the CH4 emissions decreased until the next feeding. In FR100 lambs, the CH4 emission rate was, unexpectedly, relatively constant after feeding, without any clear CH4 peaks. The magnitude of the variability in the CH4 emission rate increased when the pH of rumen contents and H2/GEF measured in pre-feeding samples decreased. Including forage rape at 0.75, or greater proportion, in the diet of lambs drastically changed the CH4 emissions pattern, likely due to a continuous low pH in the rumen which is detrimental for methanogenesis. In conclusion, the daily variation in CH4 emissions decreased with increasing forage rape inclusion in the diet of sheep and this was associated with a decreasing acetate proportion in the rumen liquid.

Mechanistic Modeling of the Variability of Methane Emissions from an Artificial Reservoir

The mechanistic model LAKE2.3 was tested for its capability to predict of methane (CH4) emissions from reservoirs. Estimates of CH4 emissions from the Mozhaysk reservoir (Moscow region) provided by the model showed good agreement with instrumental in situ observations for several parameters of the water ecosystem. The average CH4 flux calculated by the model is 37.7 mgC-CH4 m−2 day−1, while according to observations, it is 34.4 mgC-CH4 m−2 day−1. Ebullition makes the largest contribution to the emissions from reservoirs (up to 95%) due to low methane solubility in water and the high oxidation rate of diffusive methane flux. During the heating period, an increase in methane emission is observed both in the model and empirical data, with a maximum before the onset of the autumn overturn. An effective parameter for calibrating the diffusive methane flux in the model is the potential rate of methane oxidation. For ebullition flux, it is the parameter q10 (an empirical parameter determining the relationship between methane generation and temperature) because methane production in bottom sediments is the most important. The results of this research can be used to develop mechanistic models and provide a necessary step toward regional and global simulations of lacustrine methane emission using LAKE2.3.

129 The Relationship Between Pulmonary Arterial Pressure Expected Progeny Differences and Methane Emissions

Abstract The objective of this study was to determine the relationship between pulmonary arterial pressure (PAP) expected progeny differences (EPD) and methane emissions. Pulmonary arterial pressure is an indicator of pulmonary arterial hypertension in cattle and is especially relevant at high altitudes, with increasing importance in moderate elevation feedlot and finishing systems. Enteric methane emissions from cattle are increasingly emphasized due to their contribution of greenhouse gases in the agricultural sector. As pulmonary arterial hypertension continues to be an issue in high altitudes and feedlots, understanding relationships with methane emission rates will be useful for both concomitant selection with the goal of improving environmental sustainability. The PAP EPD data were collected on 123 total heifers (n = 37) and steers (n = 86) located on a ranch in southern Wyoming at an elevation of 2,194.56 meters. Gas emissions were collected using a GreenFeed system (C-Lock, Rapid City, South Dakota), which measures the gas fluxes of individual animals at the CSU ARDEC facility at an elevation of 1,557.53 meters. Individual animal methane emissions were measured as grams/day. A generalized linear regression model was used to regress average daily methane emissions (mean = 163.08, SD = 38.74, min = 87.72, max = 247.03) on PAP EPD (mean = -0.464, SD = 1.565, min = -6.382, max = 4.286), pen, and age in days (mean = 258.2, SD = 20.009, min = 200, max = 298). The PAP EPD (P = 0.484) did not account for a significant amount of variation in methane emissions. Additionally, the effect of pen (P &amp;lt; 0.05) and age (P &amp;lt; 0.05) were significant sources of variation for methane production. The effects of pen had a range of 60.916 units of methane emission/day (P &amp;lt; 0.05). Age of the individual was evaluated as a linear covariate and had a solution of 0.678 units of methane emission/day (P &amp;lt; 0.05), suggesting that older individuals had increased methane emissions. These results show no statistical relationship between PAP EPD and methane in these data. While preliminary results suggest that selection for low PAP EPD individuals will have no adverse effects on sustainability metrics, future research is needed to fully understand the relationship between beef cattle production traits and sustainability metrics.

Hotspot Detection and Estimation of Methane Emissions from Landfill Final Cover

The main objectives of this study were to identify methane hotspots through spatial distribution tests of the surface methane concentration above a landfill final cover and to investigate the effects of rainfall, atmospheric pressure, ground temperature, and ambient methane concentration on methane emissions. A portable laser methane detector was used to measure the spatial distribution of methane concentrations. The methane concentration distribution showed a distinct spatial variability. The maximum methane concentration reached 3225 ppm, while 73.0% of the methane concentration values were below 10.0 ppm. Several meteorological factors were found to be associated with the variation in methane emissions. Rainfall limited gas transport in the cover, resulting in more significant methane hotspots. Atmospheric pressure was negatively correlated with methane emission. The ambient methane concentration and methane flux had a significant positive linear correlation. Based on a linear correlation equation, the spatial distribution of methane concentrations in the landfill could be converted into a methane emission distribution. The estimated average value for methane emissions in the test area was approximately 4.3 g m−2 d−1. This study provides an experimental basis for locating methane hotspots and assessing methane emissions in landfill final covers, and proposes supplementary means for detecting geomembrane damage in landfill covers.

Spatial and temporal variability of methane emissions and environmental conditions in a hyper-eutrophic fishpond

Abstract. Estimations of methane (CH4) emissions are often based on point measurements using either flux chambers or a transfer coefficient method, which may lead to strong underestimation of the total CH4 fluxes. In order to demonstrate more precise measurements of the CH4 fluxes from an aquaculture pond, using a higher resolution sampling approach we examined the spatiotemporal variability of CH4 concentrations in the water, related fluxes (diffusive and ebullitive) and relevant environmental conditions (temperature, oxygen, chlorophyll a) during three diurnal campaigns in a hyper-eutrophic fishpond. Our data show remarkable variance spanning several orders of magnitude while diffusive fluxes accounted for only a minor fraction of total CH4 fluxes (4.1 %–18.5 %). Linear mixed-effects models identified water depth as the only significant predictor of CH4 fluxes. Our findings necessitate complex sampling strategies involving temporal and spatial variability for reliable estimates of the role of fishponds in a global methane budget.

Water depth and productivity regulate methane (CH4) emissions from temperate cascade reservoirs in northern China

Reservoirs are important sources of emissions of the greenhouse gas methane, but the temporal and spatial variability of methane emissions and the underlying mechanisms driving methane flux in cascade reservoirs remain poorly understood. In this study, we used floating chambers to study methane emission fluxes and their temporal and spatial variations in three temperate cascade reservoirs with different nutrient levels and morphologies in northern China. We measured the methane production rate, sediment properties and water quality to identify key factors driving methane emissions. The results showed that the three cascade reservoirs were persistent sources of atmospheric methane during the ice-free period, and the methane flux ranged from 1.01 to 107.17 µmol m-2h−1, with a mean of 11.17 µmol m-2h−1. The mean methane flux of the eutrophic reservoir was more than 2 and 4 times that of the two mesotrophic reservoirs. Overall, methane flux showed high spatial and temporal variability within and among reservoirs, and the three reservoirs showed different patterns of spatial and temporal variability. Statistical analysis showed that the three reservoirs had different key drivers, indicating that the methane emissions of the three reservoirs were regulated by different underlying mechanisms. When all three reservoirs were pooled, we found that Chl-a was the most important driver of methane emissions. Our results suggest that to obtain an accurate estimate of whole-reservoir methane flux, different reservoir zones and seasons must be considered. Our results also highlight that regional and global methane budget upscaling should include considerations of reservoir diversity (different climatic zones, morphology and trophic levels).

Variability in methane emissions from stems and buttress roots of Bruguiera gymnorrhiza trees in a subtropical mangrove forest

AbstractTrees in wetlands emit a significant amount of methane (CH4) into the atmosphere which is produced in the soil where anaerobic conditions prevail. However, only very few studies have considered the role of trees in the CH4 budget of mangrove forests. How tree CH4 emissions () vary between and within individuals, and how these emissions vary with the day/night alternation and the tidal cycle, have not yet been elucidated. We measured from buttress roots and stems on individuals of Bruguiera gymnorrhiza of different sizes. We also measured from trees and sediments over a 24‐h cycle. Higher was measured on the buttress roots than on the stems, with a decreasing trend with height along the stems on a majority of trees. exhibited large variations over a 24‐h cycle which were observed for the first time in this study. from sediment was twice higher at low tide compared to just before or after the flood period. from non‐submerged stem positions at high tide was four times higher during the day than during the night. On buttress roots and on stem portion which were submerged, high were measured at the beginning of the ebb tide even at night. A better understanding of variability from mangrove trees and its controls, at different time scales and along spatial gradients is needed for accurate estimation of CH4 budgets of mangrove forests.

Combining host and rumen metagenome profiling for selection in sheep: prediction of methane, feed efficiency, production, and health traits

BackgroundRumen microbes break down complex dietary carbohydrates into energy sources for the host and are increasingly shown to be a key aspect of animal performance. Host genotypes can be combined with microbial DNA sequencing to predict performance traits or traits related to environmental impact, such as enteric methane emissions. Metagenome profiles were generated from 3139 rumen samples, collected from 1200 dual purpose ewes, using restriction enzyme-reduced representation sequencing (RE-RRS). Phenotypes were available for methane (CH4) and carbon dioxide (CO2) emissions, the ratio of CH4 to CH4 plus CO2 (CH4Ratio), feed efficiency (residual feed intake: RFI), liveweight at the time of methane collection (LW), liveweight at 8 months (LW8), fleece weight at 12 months (FW12) and parasite resistance measured by faecal egg count (FEC1). We estimated the proportion of phenotypic variance explained by host genetics and the rumen microbiome, as well as prediction accuracies for each of these traits.ResultsIncorporating metagenome profiles increased the variance explained and prediction accuracy compared to fitting only genomics for all traits except for CO2 emissions when animals were on a grass diet. Combining the metagenome profile with host genotype from lambs explained more than 70% of the variation in methane emissions and residual feed intake. Predictions were generally more accurate when incorporating metagenome profiles compared to genetics alone, even when considering profiles collected at different ages (lamb vs adult), or on different feeds (grass vs lucerne pellet). A reference-free approach to metagenome profiling performed better than metagenome profiles that were restricted to capturing genera from a reference database. We hypothesise that our reference-free approach is likely to outperform other reference-based approaches such as 16S rRNA gene sequencing for use in prediction of individual animal performance.ConclusionsThis paper shows the potential of using RE-RRS as a low-cost, high-throughput approach for generating metagenome profiles on thousands of animals for improved prediction of economically and environmentally important traits. A reference-free approach using a microbial relationship matrix from log10 proportions of each tag normalized within cohort (i.e., the group of animals sampled at the same time) is recommended for future predictions using RE-RRS metagenome profiles.

Evaluation of Methane Emission Reduction Potential of Water Management and Chinese Milk Vetch Planting in Hunan Paddy Rice Fields

In order to explore the methane reduction potential with two scenarios of water management and Chinese Milk Vetch return, we calculated the methane emissions of Hunan Province rice fields in 2019 using the SECTOR tool based on Excel and released by the International Rice Research Institute. Thus, we preliminarily established an agricultural carbon emissions monitoring, reporting, and verification (MRV) system. The results showed that: (1) There was significant spatial variation in methane emissions in Hunan rice fields, with higher emissions in both the south and north and lower emissions in the east and west. Late rice was the main contributor to methane emissions, and the cities of Changde, Hengyang, Yueyang, and Shaoyang were high-emission areas due to differences in rice planting types and areas. Compared with flooding (1275.75 Gg), optimized water management measures (mid-drainage and AWD irrigation) reduced methane emissions by 29~45% (905.79 and 701.66 Gg, respectively). (2) Under the same nitrogen input conditions, compared with a solely straw return (375.24 Gg), combining green manure with straw return could partially reduce methane emissions from Hunan super hybrid rice (327.63 Gg). Compared with the control fertilizers (404.28 Gg), the reduction rates of winter-planted Chinese Milk Vetch, the return of rice straw, and the incorporation of both Chinese Milk Vetch and straw were 7.19%, 13.01%, and 18.96%, respectively. Based on scientific accounting tools, a preliminary MRV system for rice field carbon emissions was established. Under the national demand for reducing fertilizer use and increasing efficiency, equal nitrogen organic amendments could effectively contribute to the development of green, low-carbon, and high-quality agriculture.

Relationship between daily variations of methane emissions and eructation peaks in dairy cows measured with an automated head-chamber system

Enteric methane (CH4) is primarily produced by methanogens during microbial fermentation in the rumen and is subsequently eructated through the oral and nasal passages of ruminants. As such, the present study hypothesized that the rates and daily amounts of CH4 emission may be closely associated with CH4 eructation peaks. To test this hypothesis, 12 dairy cows (6 lactating and 6 non-lactating) were selected as experimental animals with contrasting daily enteric CH4 emissions. The experiment was conducted in two phases, with six non-lactating dairy cows in phase 1 and six lactating dairy cows in phase 2. For each phase, CH4 eructation peaks were monitored every 2 h interval to calculate diurnal CH4 emissions, with 12 time points scattered in four consecutive days by an automated head-chamber (AHC) system. The measurements were repeated, and total period of measurements lasted for 16 consecutive day. The results showed that compared with non-lactating dairy cows, lactating dairy cows exhibited greater diurnal CH4 emissions and rates of CH4 emission, together with higher peak frequency (P = 0.002), peak height (P < 0.001) and peak area integral (P < 0.001). Moreover, the rates of CH4 emission was not constant and exhibited a fast increments after feeding, and had a positive correlation (R2 ≥ 0.54, P < 0.001) with peak height of CH4 eructation. In conclusion, these findings indicate that the varied rates of CH4 emission among dairy cows can be caused by altering peak frequency and peak area integral of CH4 eructation, while a diurnal change in rates of CH4 emission is associated with peak area integral through altering the peak height of CH4 eructation.

Assessment of landfill gases by LandGEM and energy recovery potential from municipal solid waste of Kanpur city, India

The world due to increased urbanization and globalization is facing major environmental challenges. Anthropogenic emissions of Greenhouse gases (GHG) like carbon dioxide and methane are on the rise and unsustainable which needs to be regulated. Open dumping of Municipal Solid Waste (MSW) contributes to generation of greenhouse gases like carbon dioxide and methane. This is because large fractions of the waste open dumped are organic in nature which undergoes anaerobic decomposition leading to generation of GHGs. In particular, methane has a high potential for energy generation and if utilized could be highly beneficial. The present study assesses the generation of landfill gases, primarily methane generation potential from MSW generated in Kanpur city using LandGEM 3.02 version model developed by USEPA for the period 2015–2030. It was observed from the study that the cumulative LFGs generation, methane emission and energy recovery potential estimated as 233.44 × 106 m3, 116 × 106 m3 and 858.14 × 106 MJ respectively. Uncertainty analysis carried out showed that variation in methane emissions maybe attributed to input parameters of k and Lo of the LandGEM model. The study shows that there exists high potential to control the greenhouse gas emissions by utilizing the methane generated for energy production.

Numerical Simulation of Temporal Variability of Methane Emissions from Mozhaysk Reservoir

Estimates of methane emission from the Mozhaysk reservoir surface were carried out using the mathematical model LAKE2.3. The average emission value is 361 tC per year, the average flux = 37.7 mgC–CH4 m–2 day–1. Comparison of the obtained estimates with in situ measurements revealed, that the methane emission and specific flux according to the model are in good agreement with the observations data. The ebullition makes the largest contribution to the total emission. During the heating period, an increase of methane emission is observed with a maximum before the autumn mixing stage. In the course of numerical experiments with the model, it was found that the amplitude of methane fluxes into the atmosphere is associated with fluctuations in atmospheric pressure, and the most significant emissions peaks associated with water level drawdowns. Effective method for calibrating the diffusion component of the methane flux into the atmosphere is the potential rate of methane oxidation in the Michaelis-Menten reaction, and for ebullition it is the methane generation parameter in bottom sediments — q10. For the described numerical experiments, the article presents the values of the annual emissions of methane into the atmosphere.

Spatial and temporal variation of methane emissions in the coastal Balearic Sea, Western Mediterranean

Methane (CH4) gas is the most important GHG after carbon dioxide, with open ocean areas acting as discreet CH4 sources and coastal regions as intense but variable CH4 sources to the atmosphere. Here, we report CH4 concentrations and air-sea fluxes in the coastal area of the Balearic Islands Archipelago (Western Mediterranean Basin). CH4 levels and related biogeochemical variables were measured in three coastal sampling sites between 2018 and 2021, with two located close to the densely populated island of Mallorca and one in a pristine area in the Cabrera Archipelago National Park. CH4 concentrations in seawater during the study period ranged from 2.7 to 10.9 nM, without significant differences between the sampling sites. Averaged estimated CH4 fluxes during the sampling period for the three stations oscillated between 0.2 and 9.7 μmol m−2 d−1 according to a seasonal pattern and in general all sites behaved as weak CH4 sources throughout the sampling period.