Ebullitive CH4 Flux Research Articles

Controlling Methane Ebullition Flux in Cascade Reservoirs of the Upper Yellow River by the Ratio of mcrA to pmoA Genes

Reservoirs are an important source of methane (CH4) emissions, but the relative contribution of CH4 ebullition and diffusion fluxes to total fluxes has received little attention in the past. In this study, we systematically monitored the CH4 fluxes of nine cascade reservoirs (Dahejia, Jishixia, Huangfeng, Suzhi, Kangyang, Zhiganglaka, Lijiaxia, Nina, and Longyangxia) in the upper reaches of the Yellow River in the dry (May 2023) and wet seasons (August 2023) using the static chamber gas chromatography and headspace equilibrium methods. We also simultaneously measured environmental physicochemical properties as well as the abundance of methanogens and methanotrophs in sediments. The results showed the following: (1) All reservoirs were sources of CH4 emissions, with an average diffusion flux of 0.08 ± 0.05 mg m−2 h−1 and ebullition flux of 0.38 ± 0.41 mg m−2 h−1. Ebullition flux accounted for 78.01 ± 7.85% of total flux. (2) Spatially, both CH4 diffusion and ebullition fluxes increased from upstream to downstream. Temporally, CH4 diffusion flux in the wet season (0.09 ± 0.06 mg m−2 h−1) was slightly higher than that in the dry season (0.08 ± 0.04 mg m−2 h−1), but CH4 ebullition flux in the dry season (0.38 ± 0.48 mg m−2 h−1) was higher than that in the wet season (0.32 ± 0.2 mg m−2 h−1). (3) qPCR showed that methanogens (mcrA gene) were more abundant in the wet season (5.43 ± 3.94 × 105 copies g−1) than that in the dry season (3.74 ± 1.34 × 105 copies g−1). Methanotrophs (pmoA gene) also showed a similar trend with more abundance found in the wet season (7 ± 2.61 × 105 copies g−1) than in the dry season (1.47 ± 0.92 × 105 copies g−1. (4) Structural equation modeling revealed that the ratio of mcrA/pmoA genes, water N/P, and reservoir age were key factors affecting CH4 ebullition flux. Variation partitioning further indicated that the ratio of mcrA/pmoA genes was the main factor causing the spatial variation in CH4 ebullition flux, explaining 35.69% of its variation. This study not only reveals the characteristics and influencing factors of CH4 emissions from cascade reservoirs on the Qinghai Plateau but also provides a scientific basis for calculating fluxes and developing global CH4 reduction strategies for reservoirs.

Methane ebullition fluxes and temperature sensitivity in a shallow lake

Inland waters are important sources of atmospheric methane (CH4), with a major contribution from the CH4 ebullition pathway. However, there is still a lack of CH4 ebullition flux (eFCH4) and their temperature sensitivity (Q10) in shallow lakes, which might lead to large uncertainties in CH4 emission response from aquatic to climate and environmental change. Herein, the magnitude and regulatory of two CH4 pathways (ebullition and diffusion) were studied in subtropical Lake Chaohu, China, using the real-time portable greenhouse gas (GHG) analyzer-floating chamber method at 18 sites over four seasons. eFCH4 (12.06 ± 4.10 nmol m−2 s−1) was the dominant contributing pathway (73.0 %) to the two CH4 emission pathways in Lake Chaohu. The whole-lake mass balance calculation demonstrated that 56.6 % of the CH4 emitted from the sediment escaped through the ebullition pathway. eFCH4 was significantly higher in the western (WL: 16.54 ± 22.22 nmol m−2 s−1) and eastern lake zones (EL: 11.89 ± 15.43 nmol m−2 s−1) than in the middle lake zone (ML: 8.86 ± 13.78 nmol m−2 s−1; p < 0.05) and were significantly higher in the nearshore lake zone (NL: 15.94 ± 19.58 nmol m−2 s−1) than in the pelagic lake zone (PL: 6.64 ± 12.37 nmol m−2 s−1; p < 0.05). eFCH4 was significantly higher in summer (32.12 ± 13.82 nmol m−2 s−1) than in other seasons (p < 0.05). eFCH4 had a strong temperature dependence. Sediment total organic carbon (STOC) is an important ecosystem level Q10 driver of eFCH4. The meta-analysis also verified that across ecosystems the ecosystem-level Q10 of eFCH4 was significantly positively correlated with STOC and latitude (p < 0.05). This study suggests that eFCH4 will become increasingly crucial in shallow lake ecosystems as climate change and human activities increase. The potential increase in ebullition fluxes in high-latitude lakes is of great importance.

Significant methane ebullition from large shallow eutrophic lakes of the semi-arid region of northern China

Eutrophic lakes are a major source of the atmospheric greenhouse gas methane (CH4), and CH4 ebullition emissions from inland lakes have important implications for the carbon cycle. However, the spatio-temporal heterogeneity of CH4 ebullition emission and its influencing factors in shallow eutrophic lakes of arid and semi-arid regions remain unclear. This study aimed to determine the mechanism of CH4 emission via eutrophication in Lake Ulansuhai, a large shallow eutrophic lake in a semi-arid region of China.To this end, monthly field surveys were conducted from May to October 2021, and gas chromatography was applied using the headspace equilibrium technique with an inverted funnel arrangement. The total CH4 fluxes ranged from 0.102 mmol m−2 d−1 to 59.296 mmol m−2 d−1 with an average value of 4.984 ± 1.82 mmol m−2 d−1. CH4 ebullition emissions showed significant temporal and spatial variations. The highest CH4 ebullition emission was observed in July with a grand mean of 9.299 mmol m−2 d−1, and the lowest CH4 ebullition emissions occurred in October with an average of 0.235 mmol m−2 d−1. Among seven sites (S1–S7), the maximum (3.657 mmol m−2 d−1) and minimum (1.297 mmol m−2 d−1). CH4 ebullition emissions were observed at S2 and S7, respectively. As the main route of CH4 emission to the atmosphere in Lake Ulansuhai, the CH4 ebullition flux during May to October accounted for 69% of the total CH4 flux. Statistical analysis showed that CH4 ebullition was positively correlated with temperature (R = 0.391, P < 0.01) and negatively correlated with air pressure (R = 0.286, P < 0.00). Temperature and air pressure were found to strongly regulate the production and oxidation of CH4. Moreover, nutritional status indicators such as TP and NH4+-N significantly affect CH4 ebullition emissions (R = 0.232, P < 0.01; R = −0.241, P < 0.01). This study reveals the influencing factors of CH4 ebullition emission in Lake Ulansuhai, and provides theoretical reference and data support for carbon emission from eutrophic lakes. Nevertheless, research on eutrophic shallow lakes needs to be further strengthened. Future research should incorporate improved flux measurement techniques with process-based models to improve the accuracy from regional to large-scale estimation of CH4 emissions and clarify the carbon budget of aquatic ecosystems. In this manner, the understanding and predictability of CH4 ebullition emission from shallow lakes can be improved.

Methane and carbon dioxide fluxes at high spatiotemporal resolution from a small temperate lake

Lakes are hotspots for CH4 and CO2 effluxes, but their magnitude and underlying drivers are still uncertain due to high spatiotemporal variation within and between lakes. We measured CH4 and CO2 fluxes at high temporal (hourly) and spatial resolution (approx. 13 m) using 24 automatic floating chambers equipped with continuously recording sensors that enabled the determination of diffusive and ebullitive gas fluxes. Additionally, we measured potential drivers such as weather patterns, water temperature, and O2 above the sediment. During five days in autumn 2021, we conducted measurements at 88 sites in a small, shallow eutrophic Danish Lake. CH4 ebullition was intense (mean 54.8 μmol m−2 h−1) and showed pronounced spatiotemporal variation. Ebullition rates were highest in deeper, hypoxic water (5–7 m). Diffusive CH4 fluxes were 4-fold lower (mean 15.0 μmol m−2 h−1) and spatially less variable than ebullitive fluxes, and significantly lower above hard sediments and submerged macrophyte stands. CO2 concentration in surface waters was permanently supersaturated at the mid-lake station, and diffusive fluxes (mean 919 μmol m−2 h−1) tended to be higher from deeper waters and increased with wind speed. To obtain mean whole-lake fluxes within an uncertainty of 20 %, we estimated that 72 sites for CH4 ebullition, 39 sites for diffusive CH4 fluxes and 27 sites for diffusive CO2 fluxes would be required. Thus, accurate whole-lake quantification of the dominant ebullitive CH4 flux requires simultaneous operation of many automated floating chambers. High spatiotemporal variability challenges the identification of essential drivers and current methods for upscaling lake CH4 and CO2 fluxes. We successfully overcame this challenge by using automatic floating chambers, which offer continuous CH4 and CO2 flux measurements at high temporal resolution and, thus, are an improvement over existing approaches.

Microbial Community Abundance Affects the Methane Ebullition Flux in Dahejia Reservoir of the Yellow River in the Warm Season

Reservoirs are an integral part of the global carbon cycle and generally considered to be methane (CH4) emission hot spots. Although remarkable research achievements have been made concerning CH4 ebullition from inland waters, such as rivers, lakes, and ponds, few have been devoted to CH4 ebullition from plateau reservoirs. The present study focused on CH4 ebullition from the Dahejia Reservoir located in the upper reaches of the Yellow River. We analyzed the spatial and temporal characteristics of CH4 ebullition flux across the water-atmosphere interface between July and August 2021. We also evaluated the influence of microbes on CH4 ebullition flux. The results showed that (1) CH4 ebullition was the dominant mode of CH4 emissions in the study site, which contributed to 78.85 ± 20% of total CH4 flux. (2) The mean CH4 ebullition flux in the nighttime (0.34 ± 0.21 mg m−2 h−1) was significantly higher than that in the daytime (0.19 ± 0.21 mg m−2 h−1). The mean CH4 ebullition flux first decreased and then increased from the upstream (0.52 ± 0.57 mg m−2 h−1) to the downstream (0.43 ± 0.3 mg m−2 h−1) of the Yellow River. (3) Sediment microbes affected the CH4 ebullition flux primarily by changing the microbial community abundance. The regression analysis showed that CH4 ebullition flux had a significantly linear negative correlation with microbial abundance in sediments. The redundancy analysis further showed CH4 ebullition flux was significantly positively correlated with the abundances of Firmicutes and Actinobacteria, and negatively with that of Proteobacteria and Chloroflexi. Among abiotic variables, CH4 ebullition flux was closely related to total phosphorus, total organic carbon, pH and nitrate nitrogen.

Ebullitive CH4 flux and its mitigation potential by aeration in freshwater aquaculture: Measurements and global data synthesis

Freshwater aquaculture ponds constitute one of the important anthropogenic sources of atmospheric methane (CH4). Nevertheless, estimates of global CH4 emissions from freshwater aquaculture have large uncertainties due to a lack of data from different aquaculture types. Furthermore, despite that ebullition is a major pathway of CH4 in aquatic systems, the quantification of ebullitive CH4 fluxes from typical freshwater aquaculture ponds has been poorly represented. Here, field measurements of CH4 fluxes over two years were taken to quantify ebullitive CH4 fluxes from inland freshwater fish and crab aquaculture ponds in subtropical China. Ebullitive CH4 fluxes averaged 15.97 ± 1.57 and 11.22 ± 1.26 mg m−2 d−1 in the fish and crab ponds in the first experimental year, respectively, and were 22.86 ± 2.30 and 21.95 ± 2.19 mg m−2 d−1 in the second year. During aquaculture period, ebullition dominated the emission pathways of CH4, accounting for 83% and 98% of the total CH4 emissions in the fish and crab ponds, respectively. Ebullitive CH4 fluxes exhibited considerable spatial variations, with the lowest flux rates captured at the aeration area due to aerator-use in both the fish and crab ponds. Dissolved oxygen and dissolved organic carbon were the two primary factors that drove ebullitive CH4 fluxes in both aquaculture ponds. By incorporating global measurement data, we further assessed the CH4 mitigation potential of aerator use in freshwater aquaculture and revealed the dominant role of ebullition in this mitigation contribution. Together with the rice-based aquaculture, aerator use could reduce CH4 emissions from freshwater aquaculture ponds globally by 71% and in China by 63%.

Diverse sediment microbiota shape methane emission temperature sensitivity in Arctic lakes

Northern post-glacial lakes are significant, increasing sources of atmospheric carbon through ebullition (bubbling) of microbially-produced methane (CH4) from sediments. Ebullitive CH4 flux correlates strongly with temperature, reflecting that solar radiation drives emissions. However, here we show that the slope of the temperature-CH4 flux relationship differs spatially across two post-glacial lakes in Sweden. We compared these CH4 emission patterns with sediment microbial (metagenomic and amplicon), isotopic, and geochemical data. The temperature-associated increase in CH4 emissions was greater in lake middles—where methanogens were more abundant—than edges, and sediment communities were distinct between edges and middles. Microbial abundances, including those of CH4-cycling microorganisms and syntrophs, were predictive of porewater CH4 concentrations. Results suggest that deeper lake regions, which currently emit less CH4 than shallower edges, could add substantially to CH4 emissions in a warmer Arctic and that CH4 emission predictions may be improved by accounting for spatial variations in sediment microbiota.

Large Seasonal and Habitat Differences in Methane Ebullition on the Amazon Floodplain

AbstractTropical floodplains are an important source of methane (CH4) to the atmosphere, and ebullitive fluxes are likely to be important. We report direct measurements of CH4 ebullition in common habitats on the Amazon floodplain over two years based on floating chambers that allowed detection of bubbles, and submerged bubble traps. Ebullition was highly variable in space and time. Of the 840 floating chamber measurements (equivalent to 8,690 min of 10‐min deployments), 22% captured bubbles. Ebullitive CH4 fluxes, measured using bubble traps deployed for a total of approximately 230 days, ranged from 0 to 109 mmol CH4 m−2 d−1, with a mean of 4.4 mmol CH4 m−2 d−1. During falling water, a hydroacoustic echosounder detected bubbles in 24% of the 70‐m segments over 34 km. Ebullitive flux increased as the water level fell faster during falling water periods. In flooded forests, highest ebullitive fluxes occurred during falling water, while in open water and herbaceous plant habitats, higher ebullitive fluxes were measured during low water periods. The contribution of diffusive plus ebullitive CH4 flux represented by ebullition varied from 1% (high and rising water in open water of the lake) to 93% (falling water in flooded forests) based on bubble traps. Combining ebullitive and diffusive fluxes among habitats in relation to variations in water depth and areal coverage of aquatic habitats provides the basis for improved floodplain‐wide estimates of CH4 evasion.

Ebullition Controls on CH4 Emissions in an Urban, Eutrophic River: A Potential Time-Scale Bias in Determining the Aquatic CH4 Flux.

Rivers and streams contribute significant quantities of methane (CH4) to the atmosphere. However, there is a lack of CH4 flux and ebullitive (bubble) emission data from urban rivers, which might lead to large underestimations of global aquatic CH4 emissions. Here, we conducted high-frequency surveys using the boundary layer model (BLM) supplemented with floating chambers (FCs) and bubble traps to investigate the seasonal and diurnal variability in CH4 emissions in a eutrophic urban river and to evaluate whether the contribution of bubbles is important. We found that ebullition contributed nearly 99% of CH4 emissions and varied on hourly to seasonal time scales, ranging from 0.83 to 230 mmol m-2 d-1, although diffusive emissions and CH4 concentrations in bubbles did not exhibit temporal variability. Ebullitive CH4 emissions presented high temperature sensitivity (r = 0.6 and p < 0.01) in this urban river, and eutrophication might have triggered this high temperature sensitivity. The ebullitive CH4 flux is more likely to be underestimated at low temperatures because capturing the bubble flux is more difficult, given the low frequency of ebullition events. This study suggests that future ebullition measurements on longer time scales are needed to accurately quantify the CH4 budgets of eutrophic urban rivers.

Coupling of stable carbon isotopic signature of methane and ebullitive fluxes in northern temperate lakes.

Stable isotopic analysis is a popular method to understand the mechanisms sustaining methane (CH4) emissions in various aquatic environments. Yet, the general lack of concurrent measurements of isotopes and fluxes impedes our ability to establish a connection between the variation in the rates of CH4 emission and isotopic signature. Here, we examine the magnitude of CH4 ebullition (bubbling) and stable carbon isotopic signature (δ13C-CH4) of bubble CH4 in four northern temperate lakes and evaluate the in-lake processes shaping their variability. The ebullitive CH4 flux and bubble δ13C-CH4 varied from 0.01 to 37.0mmolm-2d-1 and between -71.0‰ and -50.9‰, respectively. The high emission lakes in general and high fluxing shallow zones within each lake consistently showed enriched δ13C-CH4 signature. Subsequently, in addition to the temperature dependence (1.4±0.1eV), the rates of ebullition strongly correlated with the variability of δ13C-CH4 across our study lakes. Our results suggest that higher ebullitive emissions are sustained by acetoclastic methanogenesis, likely fueled by fresh organic matter inputs. Further, the annual whole-lake estimate of bubble isotopic flux alone showed depleted δ13C-CH4 values (-64.6±0.6‰ to -60.1±3.2‰), yet the signature of the total CH4 emission (ebullition+diffusion) was relatively enriched (-60.7‰ to -52.6‰) due to high methanotrophic activity in the water column. We show that δ13C-CH4 signature of bubbles can be linked to the magnitude of ebullition itself, yet we suggest there is a need to account for different emission pathways and their isotopic signature to allocate CH4 source signature to northern lakes.

Methane and nitrous oxide have separated production zones and distinct emission pathways in freshwater aquaculture ponds

Aquaculture systems receive intensive carbon (C) and nitrogen (N) loadings, and are therefore recognized as major anthropogenic sources of methane (CH4) and nitrous oxide (N2O) emissions. However, the extensively managed aquaculture ponds were identified as a hotspot of CH4 emission but just a weak N2O source. Here, we investigate annual CH4 and N2O fluxes from three earthen ponds used for crab culture, of different sizes, in southeast China. Our purposes are to identify the spatiotemporal variations of CH4 and N2O emissions and their components among ponds and to evaluate the zone for CH4 and N2O production. Static chamber-measured CH4 flux ranged from 0.03 to 64.7 mg CH4 m‒2 h‒1 (average: 9.02‒14.3 mg CH4 m‒2 h‒1), and temperature, followed by dissolved organic C (DOC) concentration, and redox potential, were the primary drivers of seasonal CH4 flux patterns. Annual mean diffusive CH4 flux was 1.80‒2.34 mg CH4 m‒2 h‒1, and that by ebullition was up to 7.20‒12.0 mg CH4 m‒2 h‒1 (79.1‒83.5% of the total CH4 flux). Annual CH4 emission was positively correlated with sediment DOC concentration but negatively (P < 0.05) correlated with water depth across ponds, with the highest CH4 emission occurred in a pond with low water depth and high DOC concentration. The calculated diffusive N2O flux by the gas transfer velocity was 0.32‒0.60 times greater than the measured N2O emission, suggesting that N2O in water column can not only evade as water-air fluxes but diffuse downwards and to be consumed in anaerobic sediments. This also indicates that N2O was primarily produced in water column. The highly reduced condition and depletion of NO3‒-N in sediments, can limit N2O production from both nitrification and denitrification but favor N2O consumption, leading the ponds to become a weak source of N2O annually and even a sink of N2O in summer. Our results highlight that the current global CH4 budget for inland waters is probably underestimated due to a lack of data and underestimation of the contribution of ebullitive CH4 flux in small lentic waters. The downwards N2O diffusion from the water column into sediment also indicates that the extensively-used model approach based on gas transfer velocity potentially overestimates N2O fluxes, especially in small eutrophic aquatic ecosystems.

Diurnal Pumped‐Storage Operation Minimizes Methane Ebullition Fluxes From Hydropower Reservoirs

AbstractHydropower is considered green energy and promoted to reduce greenhouse warming. However, hydropower is typically generated using reservoirs and reservoirs are known to emit substantial amounts of the greenhouse gas methane (CH4) to the atmosphere. In many reservoirs ebullition is the dominant pathway of CH4 emission. We show that continuous diurnal pumped‐storage operation, which combines water pumping into the reservoir typically during the night and water drawdown during high demand of electricity, is beneficial for reducing CH4 ebullition associated with hydropower generation. This conclusion is based on ebullition fluxes and water levels measured over 3 months in Schwarzenbach reservoir located in Germany. The reservoir was managed using three modes of operation: (1) diurnal pumping and turbination, (2) no pumping and no turbination, and (3) diurnal turbination. Cross‐correlation analysis indicates that ebullition fluxes predominantly occur during diurnal water level decrease associated with turbination. Consistently, average ebullition fluxes of CH4 were negligible during Mode (2) and substantial during Modes (1) and (3). During Mode (3) the average CH4 ebullition flux was ~197 mg m−2day−1, ~12 times larger than during Mode (1) (16 mg m−2day−1). Our data indicate that overall CH4 ebullition is about 3 times larger during 51 days of operation consisting of 38 days of no turbination followed by 13 days of diurnal turbination than during 51 days of continuous diurnal pumped‐storage operation. This suggests that continuous diurnal pumped‐storage operation leads to reduced CH4 ebullition from reservoirs and is therefore advantageous compared to modes of operations involving long‐term, large‐amplitude turbination cycles.

Intense methane ebullition from urban inland waters and its significant contribution to greenhouse gas emissions

The evasions of methane (CH4) and carbon dioxide (CO2) from inland waters represent substantial fluxes of greenhouse gases into the atmosphere, offsetting a large part of the continental carbon sink. However, the CH4 and CO2 emissions from urban inland waters are less constrained. In particular, ebullitive CH4 emissions from these waters are poorly understood. Here, we measured the concentrations and fluxes of CH4 and CO2 in rivers and lakes in the megacity of Beijing, China, between 2018 and 2019. The CH4 concentration ranged from 0.08 to 70.2 µmol L−1 with an average of 2.5 ± 5.9 µmol L−1. The average CH4 ebullition was 11.3 ± 30.4 mmol m−2 d−1 and was approximately 6 times higher than the global average. The average total CH4 flux (14.2 ± 35.1 mmol m−2 d−1) was 3 times higher than the global average, with ebullition accounting for 80% of the flux. The high surface water CH4 concentrations and ebullitive fluxes were caused by high sediment organic carbon/dissolved organic carbon contents, high aquatic primary productivity and shallow water depths in the urban inland waters. The CH4 emissions accounted for 20% of CO2 emissions in terms of the carbon release and were 1.7 times higher in terms of CO2 equivalent emissions from Beijing inland waters. Furthermore, the CH4 ebullition and its contribution to the total carbon gas emissions increased exponentially with the water temperature, suggesting a positive feedback probably occurs between the greenhouse gas emissions from urban inland waters and climate warming. This study confirms the major role of CH4 ebullition from urban inland waters in the global carbon budget under the rapid progress of global urbanization.

Environmental Controls of Diffusive and Ebullitive Methane Emissions at a Subdaily Time Scale in the Littoral Zone of a Midlatitude Shallow Lake

AbstractEnvironmental controls on methane (CH4) emission from lakes are poorly understood at subdaily time scales due to a lack of continuous data, especially for ebullition. We used a novel technique to partition eddy covariance CH4 flux observed in the littoral zone of a midlatitude shallow lake in Japan and examined the environmental controls on diffusion and ebullitive CH4 flux separately at a subdaily time scale in different seasons. Using the high‐frequency data, we investigated how CH4 accumulation in the water and sediment layers alters the dynamics and environmental controls of fluxes. The contribution of ebullitive flux to total flux was 57% on average. Environmental controls of diffusive and ebullitive fluxes known in the literature were confirmed. We further found that the environmental controls were different in different seasons and suggested that additional consideration of CH4 accumulation could explain the variability. The transfer of accumulated dissolved CH4 from the bottom water layer to the surface in summer and the accumulation of dissolved CH4 under surface ice in winter were suggested to be important for explaining the variability of diffusive flux. In summer, a higher ebullitive flux tended to occur following triggers such as a decrease in hydrostatic pressure. In winter, the impact of triggers was not obvious, and a higher ebullitive flux tended to occur in the morning. We suggested that the low CH4 production rate in winter slowed the replenishment of bubbles in the sediment, negating the effect of triggers on ebullition.

Ebullition was a major pathway of methane emissions from the aquaculture ponds in southeast China

Aquaculture ponds are hotspots of carbon cycling and important anthropogenic sources of the potent greenhouse gas methane (CH4). Despite the importance of CH4 ebullition in aquatic ecosystems, its magnitude and spatiotemporal variations in aquaculture ponds remain poorly understood. In this study, we determined the rates and spatiotemporal variations of ebullitive CH4 emissions from three mariculture ponds during the aquaculture period of two years at a subtropical estuary in southeast China. Our results showed that the mean ebullitive CH4 flux from the studied ponds was 14.9 mg CH4 m−2 h−1 during the aquaculture period and accounted for over 90% of the total CH4 emission, indicating the importance of ebullition as a major CH4 transport mechanism. Ebullitive CH4 emission demonstrated a clear seasonal pattern, with a peak value during the middle stage of aquaculture. Sediment temperature was found to be an important factor influencing the seasonal variations in CH4 ebullition. Ebullitive CH4 fluxes also exhibited considerable spatial variations within the ponds, with 49.7–71.8% of the whole pond CH4 ebullition being detected in the feeding zone where the large loading of sediment organic matter fueled CH4 production. Aquaculture ponds have much higher ebullitive CH4 effluxes than other aquatic ecosystems, which indicated the urgency to mitigate CH4 emission from aquaculture activities. Our findings highlighted that the importance of considering the large spatiotemporal variations in ebullitive CH4 flux in improving the accuracy of large-scale estimation of CH4 fluxes in aquatic ecosystems. Future studies should be conducted to characterize CH4 ebullitive fluxes over a greater number and diversity of aquaculture ponds and examine the mechanisms controlling CH4 ebullition in aquatic ecosystems.

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.

Methane Emission Characteristics and Its Influencing Factors over Aquaculture Ponds

Eutrophic aquaculture ponds are important methane (CH4) sources. In order to quantify CH4 emission characteristics and its influencing factors over aquaculture ponds, we conducted several intensive observations over two ponds located in Quanjiao County, Anhui Province, in 2018. The ebullition and diffusion flux of CH4 were measured in two seasons (winter and spring) using the inverted-funnel and bulk diffusion model, respectively. In winter, the CH4 ebullition flux during the daytime was higher than that at night (almost zero), whereas the diurnal pattern was found to be reversed in spring. Seasonally, the CH4 ebullition flux over the ponds was significantly lower in winter[3.92 mg·(m2·d)-1] than in spring[106.94 mg·(m2·d)-1], while the diffusion flux in winter[2.81 mg·(m2·d)-1] was slightly higher than in spring[0.87 mg·(m2·d)-1]. The CH4 ebullition and diffusion flux can be significantly controlled by natural factors such as water temperature and air pressure. It was revealed that the CH4ebullition flux in the studied ponds increased exponentially with increasing water temperature, and increased linearly with decreasing air pressure. Furthermore, we found that artificial management measures (i.e., winter drainage and spring manure treatment) could significantly enhance the CH4 ebullition flux rather than diffusion flux. For instance, the CH4 ebullition flux was found to increase with water depth decrease during the winter drainage period, while in the spring, the CH4 ebullition flux could reach as high as 1002.30 mg·(m2·d)-1 with chicken manure applicated. This study can provide data support for assessing the contribution of small ponds to the global carbon cycle.

Autochthonous dissolved organic matter potentially fuels methane ebullition from experimental lakes

Shallow lakes are hotspots for carbon processing and important natural sources of methane (CH4) emission. Ebullitive CH4 flux may constitute the overwhelming majority of total CH4 flux, but the episodic nature of ebullition events makes determining both quantity and the controlling factors challenging. Here we used the world's longest running shallow-lake mesocosm facility, where the experimental treatments are low and high nutrients crossed with three temperatures, to investigate the quantity and drivers of CH4 ebullition. The mean CH4 ebullition flux in the high nutrient treatment (41.5 ± 52.3 mg CH4–C m−2 d−1) mesocosms was significantly larger than in the low nutrient treatment (3.6 ± 5.4 mg CH4–C m−2 d−1) mesocosms, varying with temperature scenarios. Over eight weeks from June to August covered here warming resulted in a weak, but insignificant enhancement of CH4 ebullition. We found significant positive relationships between ebullition and chlorophyll-a, dissolved organic carbon (DOC), biodegradable DOC, δ2H, δ18O and δ13C-DOC, autochthonous dissolved organic matter (DOM) fluorescent components, and a fraction of lipids, proteins, and lignins revealed using ultrahigh-resolution mass spectrometry, and a negative relationship between ebullitive CH4 flux and the percentage volume inhabited of macrophytes. A 24 h laboratory bio-incubation experiment performed at room temperature (20 ± 2 °C) in the dark further revealed a rapid depletion of algal-DOM concurrent with a massive increased CH4 production, whereas soil-derived DOM had a limited effect on CH4 production. We conclude that eutrophication likely induced the loss of macrophytes and increase in algal biomass, and the resultant accumulation algal derived bio-labile DOM potentially drives enhanced outgassing of ebullitive CH4 from the shallow-lake mesocosms.

The Importance of CH4 Ebullition in Floodplain Fens

AbstractUncertainty in estimates of CH4 emissions from peatlands arise, in part, due to difficulties in quantifying the importance of ebullition. This is a particular concern in temperate lowland floodplain fens in which total CH4 emissions to the atmosphere (often measured as the sum of diffusive and plant‐mediated fluxes) are known to be high, but few direct measurements of CH4 ebullition fluxes have been made. Our study quantified CH4 fluxes (diffusion, plant‐mediated, and ebullition) from two temperate floodplain fens under conservation management (Norfolk, UK) over 176 days using funnels and static chambers. CH4 ebullition was a major component (&gt;38%) of total CH4 emissions over spring and summer. Seasonal variations in quantifiable CH4 ebullition fluxes were marked, covering six orders of magnitude (5 × 10−5 to 62 mg·CH4·m−2·hr−1). This seasonal variability in CH4 ebullition fluxes arose from changes in both bubble volume flux and bubble CH4 concentration, highlighting the importance of regular measurements of the latter for accurate assessment of CH4 ebullition using funnels. Soil temperature was the primary control on CH4 ebullition fluxes. Elevated water level was also associated with increased CH4 ebullition fluxes, with a distinct increase in CH4 ebullition flux when water level rose to within 10 cm of the peat surface. In contrast, CH4 ebullition flux decreased steadily with increasing plant cover (measured as vascular green area). Ebullition was both steady and episodic in nature, and drops in air pressure during the two‐day funnel deployments were associated with higher fluxes.

Diel methane flux from a subtropical eutrophic pond in November based on continuous monitoring

A field campaign was carried out to investigate continuous diel methane (CH4) flux from a subtropical eutrophic pond in November 2016. The diffusive methane flux of a single measurement had a range from 2.68 × 10−5 to 0.028 mmol·m−2·h−1 with an average of 0.011 ± 0.005 mmol·m−2·h−1. The diffusive methane flux of measurements from 9:00 to 10:30 and from 21:00 to 22:30 were very close to the average diffusive flux of all measurements. The bubble methane flux at different time measurements had much more variability than the diffusive methane flux. The bubble methane flux of a single measurement had a range from 0 to 0.312 mmol·m−2·h−1 with an average of 0.024 ± 0.054 mmol·m−2·h−1. For the eutrophic pond, the average bubble and diffusive CH4 flux were 0.56 ± 0.18 and 0.26 ± 0.04 mmol·m−2·day−1, respectively, and the CH4 ebullition flux accounted for 68.23% of the total flux. The maximum of the bubble CH4 flux was about 4.6 times of the minimum CH4 ebullition. The maximum of diffusive CH4 flux was ~ 1.7 times of the corresponding minimum. The diffusive methane fluxes in daytime and nighttime were almost equal. However, the bubble methane flux in daytime was 0.029 mmol·m−2·h−1, which was 1.6 times of that at night. Wind speed, the surface water temperature, and DO dominate methane effluxes from the pond, and the latter is in nature subjected to the metabolism of algae in the pond. However, key environmental factors which dominate gas flux processes vary with different weather conditions. Wind speed is unimportant when it is extremely low.