Sediment CH4 Production Research Articles

Impact of Reservoir Operation Policies on Spatiotemporal Dynamics of Sediment Methane Production and Release in a Large Reservoir

AbstractReservoir operation policies affect sediment methane (CH4) production and pathways through complex influence on hydrodynamic and biogeochemical processes in reservoir water and sediment. However, the underlying influencing mechanisms, especially the impact of operations on sediment CH4 production and pathways, remain poorly understood. This study combines a physical‐biogeochemical model with a reservoir operation model to evaluate operation impacts on spatiotemporal sediment CH4 production and release dynamics and to analyze affiliated processes and driving factors. Three typical operation policies (i.e., standard, hedging, eco‐friendly) are selected to create the operation scenarios, and a coupled physical‐biogeochemical model is adopted to simulate and compare sediment CH4 production, consumption, diffusion, and ebullition under these scenarios. The methods are applied in the second largest reservoir in China (i.e., Danjiangkou Reservoir) as a case study. Results show that annual total sediment CH4 production among the scenarios ranges from 285.9 to 298.3 Gg C year−1, while operation scenario outcomes significantly differ. Specifically, a significant CH4 percentage (34%–39%) is oxidized in surface sediment while the remaining escapes sediment through diffusion and ebullition. Although diffusion accounts for >60% of the annual sediment CH4 release, sediment ebullition ultimately dominates atmospheric CH4 emissions. Annual total atmospheric CH4 emissions range from 42.8 to 71.1 Gg C year−1 and account for 14%–25% of the annual total sediment production among the three scenarios. The differences demonstrate that reservoir operations significantly impact CH4 pathways and atmospheric emissions. This study provides implications for reservoir managers to coordinate socioeconomic and ecological protection while mitigating CH4 emission targets.

Influence of Vertical Hydrologic Exchange Flow, Channel Flow, and Biogeochemical Kinetics on CH4 Emissions From Rivers

AbstractCH4 emissions from inland water are highly uncertain in the current global CH4 budget, especially for rivers and streams due to sparse measurements and the uncertainty of measurements caused by turbulent water flow. A previous study has revealed that vertical hydrologic exchange flow (VHEF) is the main regulator of CH4 emissions from riverbed sediments. However, to what extent the understanding obtained from the plot‐scale can be extended to the reach scale and basin scale remains unknown. To address this challenge, we developed a process‐based model to estimate CH4 flux at the air‐water interface using the attributes available in the national hydrography data set. It calculates the annual mean flux of VHEF, CH4 production in sediments, and CH4 transport in the river channel in a sequential manner. Model performance is evaluated by CH4 efflux observed at the Hanford reach of the Columbia River. We show that reach‐wise sediment hydrologic and biogeochemical conditions estimated from the national hydrography data set could serve as a good indicator of CH4 emissions from rivers. Aerobic methane oxidation and export to the downstream are the dominant ways of total CH4 loss for the large lowland river. The hotspots of CH4 emissions are likely to be at the reaches with fine sediments and slow channel velocity. This study demonstrates the possibility of quantifying CH4 emissions at the reach scale and the modeling framework has the potential to be extended to the basin scale to improve estimates of CH4 emissions from lotic inland water.

Widespread dominance of methane ebullition over diffusion in freshwater aquaculture ponds

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

Evaluating the Feedback of the Reservoir Methane Cycle to Climate Warming under Hydrological Uncertainty

Freshwater reservoirs are widely recognized as methane (CH4) emission hotspots. Existing research has shown that temperature and hydrological conditions significantly affect wetland CH4 cycling processes. However, the feedback of the CH4 cycle to climate warming remains unclear for deep reservoirs where seasonal water thermal stratification exists. This study combined a reservoir CH4 cycling model and a Statistical DownScaling Model (SDSM) to evaluate reservoir CH4 cycling feedbacks under multiple climate change scenarios while accounting for hydrological uncertainty. Daily air temperatures in 2100 were predicted by the combination of the CanESM5 model and a SDSM. To address hydrological uncertainty, we selected three representative hydrological years (i.e., wet, normal, and dry) to create hydrological scenarios. Results showed that annual sediment CH4 production increased with warming, ranging 323.1–413.7 × 103 t C year−1 among multiple scenarios. Meanwhile, the CH4 oxidation percentage decreased with warming, which meant warming promoted sediment CH4 release non-linearly; 67.8–84.6% of sediment ebullient flux was ultimately emitted to the atmosphere (51.3–137.7 × 103 t C year−1), which showed ebullition was the dominant emission pathway. Higher air temperatures and drier conditions generally promote reservoir emissions. This study is helpful for predicting reservoir emissions while directing decision-making for reservoir sustainability.

Large increase in CH4 emission following conversion of coastal marsh to aquaculture ponds caused by changing gas transport pathways

Methane emissions from aquatic ecosystems play an important role in global carbon cycle and climate change. Reclamation of coastal wetlands for aquaculture use has been shown to have opposite effects on sediment CH4 production potential and CH4 emission flux, but the underlying mechanism remained unclear. In this study, we compared sediment properties, CH4 production potential, emission flux, and CH4 transport pathways between a brackish marsh and the nearby reclaimed aquaculture ponds in the Min River Estuary in southeastern China. Despite that the sediment CH4 production potential in the ponds was significantly lower than the marsh, CH4 emission flux in the ponds (17.4 ± 2.7 mg m−2 h−1) was 11.9 times higher than the marsh (1.3 ± 0.2 mg m−2 h−1). Plant-mediated transport accounted for 75% of the total CH4 emission in the marsh, whereas ebullition accounted for 95% of the total CH4 emission in the ponds. CH4 emission fluxes in both habitat types were highest in the summer. These results suggest that the increase in CH4 emission following the conversion of brackish marsh to aquaculture ponds was not caused by increased sediment CH4 production, but rather by eliminating rhizospheric oxidation and shifting the major transport pathway to ebullition, allowing sediment CH4 to bypass oxidative loss. This study improves our understanding of the impacts of modification of coastal wetlands on greenhouse gas dynamics.

Changes in sediment methanogenic archaea community structure and methane production potential following conversion of coastal marsh to aquaculture ponds

Widespread conversion of coastal wetlands into aquaculture ponds in coastal region often results in degradation of the wetland ecosystems, but its effects on sediment's potential to produce greenhouse gases remain unclear. Using field sampling, incubation experiments and molecular analysis, we studied the sediment CH4 production potential and the relevant microbial communities in a brackish marsh and the nearby aquaculture ponds in the Min River Estuary in southeastern China. Sediment CH4 production potential was higher in the summer and autumn months than in spring and winter months, and it was significantly correlated with sediment carbon content among all environmental variables. The mean sediment CH4 production potential in the aquaculture ponds (20.1 ng g−1 d−1) was significantly lower than that in the marsh (45.2 ng g−1 d−1). While Methanobacterium dominated in both habitats (41–59%), the overall composition of sediment methanogenic archaea communities differed significantly between the two habitats (p < 0.05) and methanogenic archaea alpha diversity was lower in the aquaculture ponds (p < 0.01). Network analysis revealed that interactions between sediment methanogenic archaea were much weaker in the ponds than in the marsh. Overall, these findings suggest that conversion of marsh land to aquaculture ponds significantly altered the sediment methanogenic archaea community structure and diversity and lowered the sediment's capacity to produce CH4.

Lake Sediment Methane Responses to Organic Matter are Related to Microbial Community Composition in Experimental Microcosms

Lake sediment microbial communities mediate carbon diagenesis. However, microbial community composition is variable across lakes, and it is still uncertain how variation in community composition influences sediment responses to environmental change. Sediment methane (CH4) production has been shown to be substantially elevated by increased lake primary productivity and organic matter supply. However, the magnitude of the response of CH4 production varies across lakes, and recent studies suggest a role for the microbial community in mediating this response. Here, we conducted sediment incubation experiments across 22 lakes to determine whether variation in sediment microbial community composition is related to the response of sediment CH4 production to increases in organic matter. We sampled the 22 lakes across a gradient of pH in order to investigate lakes with variable sediment microbial communities. We manipulated the incubations with additions of dried algal biomass and show that variation in the response of CH4 production to changes in organic matter supply is significantly correlated with metrics of sediment microbial community composition. Specifically, the diversity and richness of the non-methanogen community was most predictive of sediment CH4 responses to organic matter additions. Additionally, neither metrics of microbial abundance nor preexisting organic matter availability explained meaningful variation in the response. Thus, our results provide experimental support that differences in sediment microbial communities influences CH4 production responses to changes in organic matter availability.

Improved Membrane Inlet Mass Spectrometer Method for Measuring Dissolved Methane Concentration and Methane Production Rate in a Large Shallow Lake

Natural water bodies, such as lakes, rivers, and oceans, are important sources of atmospheric methane (CH4). Therefore, quantitative and accurate determination of the dissolved CH4 concentration in water is of great significance for studying CH4 emissions and providing an in-depth understanding of the carbon cycle. Headspace gas chromatography (HGC) is the traditional method for measuring CH4 in water. Despite its long success, it has a lot of problems in use, such as complex pretreatment and a long measurement time, and it is not suitable for the CH4 determination of a large number of samples. In view of these shortcomings, a more convenient and efficient method based on membrane inlet mass spectrometry (MIMS) for quantitative measurements of the dissolved CH4 concentration in water was established. In our study, the standard curves showed that the method had high accuracy, both at low and high CH4 concentrations. After a laboratory test, to evaluate the sensitivity of this method, samples were collected from a large shallow lake (Lake Taihu). Both the HGC method and MIMS method were used to determine the dissolved CH4 to compare these two methods. The small difference in CH4 concentration obtained from the MIMS and HGC methods and the significant correlation between the CH4 concentrations derived from the MIMS method with those derived from the HGC method showed that the MIMS method could replace the HGC method in the determination of dissolved CH4 in natural waters. In addition, we also measured the sediment CH4 production rates in three different areas of Lake Taihu using a laboratory incubation experiment. During the experiment, significant CH4 accumulations were observed, indicating that sediment CH4 production was an important source of dissolved CH4 in the water column. Our study concluded that the MIMS method was sufficient and a better alternative than the HGC method owing to its capacity to measure a broad range of values plus the fact that it was relatively easy to use with less manipulation of the samples.

An empirical model to predict methane production in inland water sediment from particular organic matter supply and reactivity

AbstractThe highest CH4 production rates can be found in anoxic inland water surface sediments however no model quantifies CH4 production following fresh particular organic matter (POM) deposition on anoxic sediments. This limits our capability of modeling CH4 emissions from inland waters to the atmosphere. To generate such a model, we quantified how the POM supply rate and POM reactivity control CH4 production in anoxic surface sediment, by amending sediment at different frequencies with different quantities of aquatic and terrestrial POM. From the modeled CH4 production, we derived parameters related to the kinetics and the extent of CH4 production. We show that the extent of CH4 production can be well predicted by the quality (i.e., C/N ratio) and the quantity of POM supplied to an anoxic sediment. In particular, within the range of sedimentation rates that can be found in aquatic systems, we show that CH4 production increases linearly with the quantity of phytoplankton‐derived and terrestrially derived POM. A high frequency of POM addition, which is a common situation in natural systems, resulted in higher peaks in CH4 production rates. This suggests that relationships derived from earlier incubation experiments that added POM only once, may result in underestimation of sediment CH4 production. Our results quantitatively couple CH4 production in anoxic surface sediment to POM sedimentation flux, and are therefore useful for the further development of mechanistic models of inland water CH4 emission.

Methanogenesis exceeds CH4 consumption in eutrophic lake sediments

AbstractLakes and reservoirs collectively contribute significant amounts of methane (CH4) to the global atmosphere. If CH4 production were not at least partially balanced by consumption (oxidation) in most of these systems, they could potentially emit an order of magnitude or more CH4. The impacts of environmental drivers such as trophic status, temperature, and latitude on CH4 production, CH4 oxidation, and the balance of the two processes influence current and future patterns of freshwater CH4 emissions. Using CH4 production and oxidation rates measured with a common methodology (incubations) from over 60 different lakes and reservoirs, we provide novel evidence for lower sediment CH4 oxidation efficiency at high sediment CH4 production rates. We also show a strong positive correlation between sediment CH4 production and lake trophic status. Our results suggest that less efficient CH4 consumption at high CH4 production rates could help explain greater surface emissions often observed in eutrophic lakes globally.

Variable sediment methane production in response to different source-associated sewer sediment types and hydrological patterns: Role of the sediment microbiome

Production of methane (CH4), an essential anthropogenic greenhouse gas, from municipal sewer sediment is a problem deserving intensive attention. Based on long-term laboratory batch tests in conjunction with 16 s rRNA gene sequencing and metagenomics, this study provides the first detailed assessment of the variable sediment CH4 production in response to different pollution source-associated sewer sediment types and hydrological patterns, while addressing the role of the sediment microbiome. The high CH4-production capability of sanitary sewer sediment is shaped by enriched biologically active substrate and dominated by acetoclastic methanogenesis (genus Methanosaeta). Moreover, it involves syntrophic interactions among fermentation bacteria, hydrogen-producing acetogens and methanogens. Distinct source-associated microbial species, denitrifying bacteria and sulfate-reducing bacteria occur in storm sewer and illicit discharge-associated (IDA) storm sewer sediments. This reveals their insufficient microbial function capabilities to support efficient methanogenesis. Hydrogenotrophic methanogenesis (genus Methanobacterium) prevails in both these sediments. In this context, storm sewer sediment has an extremely low CH4-production capability, while IDA storm sewer sediment still shows significant carbon emission through a possibly unique mechanism. Hydrological connections promote the sewer sediment biodegradability and CH4-production capability. In contrast, hydrological disconnection facilitates the prevalence of acetoclastic methanogenesis, sulfate-reducing enzymes, denitrification enzymes and the sulfur-utilizing chemolithoautotrophic denitrifier, which drastically decreases CH4 production. Turbulent suspension of sediments results in relative stagnation of methanogenesis. This work bridges the knowledge gap and will help to stimulate and guide the resolution of ‘bottom-up’ system-scale carbon budgets and GHG sources, as well as the target CH4 abatement interventions.

Spatial methane pattern in a deep freshwater lake: Relation to water depth and topography

Freshwater lakes are regarded as important methane (CH4) sources, accounting for ~20% of natural emission. To improve the assessment of the global greenhouse effect, it is necessary to consider spatial variability within lakes. Here, CH4 concentrations in the water column and sediment layers, as well as the sediment CH4 production potentials and diffusive fluxes, were studied in the littoral, intermediate, and profundal zones of the medium-sized (425 ha), deep (maximum depth 69.5 m) Lake Stechlin (Germany). Sediment CH4 concentrations, production potentials and sediment–water interface diffusive fluxes showed significant spatial heterogeneity and were highest in the profundal zone. CH4 concentrations in the surface water did not differ among the studied locations, indicating a decoupling from the production sites in the sediment. The high amount of CH4 in profundal sediments that might potentially be released to the atmosphere is either trapped or oxidized within the water column, while the surface water dissolved CH4 is more related to the dynamics in the epilimnion. The divergence in sediment physical (water content, grain size) and chemical (organic matter quantity or quality, sulfate) properties across the lake leads to variations in CH4 dynamics which are restricted to deeper habitats in this type of lake.

Spatial and temporal heterogeneity of geochemical controls on carbon cycling in a tidal salt marsh

Tidal salt marsh ecosystems store copious amounts of carbon (C) within sediments. In order to predict how these C stores may be affected by environmental change, it is critical to assess current CO2 and CH4 production and efflux from these ecosystems. Production and efflux of these greenhouse gases (GHGs) are governed by coupled geochemical, hydrological, physical and biological processes in sediments that are sensitive to local conditions, which can result in large spatial and temporal heterogeneity of GHGs dynamics within the ecosystem. To understand how the drivers of GHGs dynamics vary across salt marsh ecosystems, we coupled solid-phase geochemistry to measurements of porewater chemistry (to ∼1 m), CO2 and CH4 production in sediments and efflux to the atmosphere in a temperate tidal salt marsh for over one year to capture seasonal patterns within two vegetation zones of the marsh landscape that have distinct biogeochemical and hydrologic conditions: Tall Spartina (TS) and Short Spartina (SS). The SS vegetation zone experienced nearly constant inundation, low redox values (−200 to 200 mV), porewater pH 6–7 that did not vary with depth or time, an enrichment of pyrite and goethite with depth and up to 3 mM porewater sulfide. In contrast, the TS vegetation zone on the natural levee proximal to a tidal channel experienced large water level oscillations due to spring-neap tides that resulted in variable but higher redox values (0–700 mV), porewater pH 6–7 at depth but surface (0–3 cm) as low as 4 in the spring, an enrichment of ferrihydrite and a depletion of pyrite at ∼30 cm, and up to 0.8 mM ferrous Fe in porewater. At 50–56 cm, solid phase analyses (STXM-NEXAFS) revealed differential C speciation between the two vegetation zones, with stronger C-Fe spatial association at TS and stronger C-Ca co-association at SS despite both having similar soil pH of 3–4. These results suggest that soil pH may not be strongly predictive of C-mineral control in flooded marsh sediments. Both vegetation zones showed consistent CO2 and CH4 emissions from sediments to the atmosphere throughout the study period with TS having ∼60% higher median CO2 and SS having ∼55% higher median CH4 efflux. Using depth profiling, unexpectedly high concentrations of CO2 (>200 μM) and CH4 (>200 μM) were observed at depths 50–75 cm at both zones that were higher for SS in these sulfate-rich (up to 17 mM) sediments, which suggests methylotrophic methanogenesis occurs deep within the profile of salt marsh sediments away from the tidal channel. Moreover, if we extrapolate our median depth values of CH4 and CO2 to the 5.3 Mha of global salt marshes, this could account for a conservative estimate of ∼70 Gg of unaccounted C stored in gaseous form (i.e., CH4 and CO2) in marsh sediments, which should be considered when attempting to understand the current patterns and future responses of carbon dynamics from these ecosystems.

The significant contribution of lake depth in regulating global lake diffusive methane emissions

Global lakes have been identified as an important component of natural methane (CH4) sources. Given that lake CH4 emissions involve multiple, complex processes influenced by various environmental factors, estimates of global lake CH4 emissions are largely uncertain. In this study, we compiled global CH4 emission data on 744 lakes from published studies, and found a significantly negative correlation (R2 = 0.50, p < 0.01) between diffusive CH4 flux and lake maximum depth. Further analysis indicated that no significant differences in global sediment CH4 production were found for the different maximum depths investigated. Owing to the longer oxidation pathway, presence of oxycline layer, and the lower nutrient environment, deeper lakes yield less diffusive CH4 efflux compared to shallower lakes. Additionally, we also found that lake area was negatively correlated (R2 = 0.13, p < 0.01) to diffusive CH4 flux. Therefore, based on empirical correlations between lake morphometry (maximum depth and area) and diffusive CH4 emission, as well as the combination of two lake databases, we estimated that the annual diffusive CH4 emission from global lakes is approximately 11.2 (6.2–19.5) Tg CH4/yr, and greater than 84% is emitted from lakes with a mean depth of less than 5 m. Furthermore, two regions, 40–70° N (30.4%) and 20° S∼10° N (37.4%), were found to be the dominant contributors of global lake diffusive CH4 emissions, resulting from the considerable total lake area and the extensive shallow lakes in these regions. This study highlights the significance of the ‘depth-effect’ which controls the spatial distribution of lake diffusive CH4 flux and allows for the quantification of global lake diffusive CH4 emissions.

The control of sediment gas accumulation on spatial distribution of ebullition in Lake Kinneret

In freshwater lakes, ebullition is an important pathway for biogenic methane (CH4) to escape from sediment and reach the atmosphere. However, the high spatial and temporal variability of ebullition limits our ability to accurately measure or predict CH4 fluxes from lakes. To explore factors controlling the spatial distribution of ebullition, we investigated free gas accumulation in bottom sediment of Lake Kinneret, Israel. Sediment cores were collected from four sites at different water depth and distance from the shore. Sediment porewater was analyzed for dissolved CH4 concentration, pH, DOC, acetate, and sulfate. Anaerobic CH4 production rates in sediment were determined by incubating sediment samples. For characterizing in situ sediment volumetric gas content, hydroacoustic measurements at various frequencies were conducted at the coring sites and along multiple transects across the lake. A minimum in CH4 production was observed in the upper 30 cm of sediment, which coincided with enriched porewater sulfate. The depth-integrated sediment CH4 production provides a robust estimate for long-term ebullition of CH4 from sediment, while short-term variability was associated with seasonal lake level change. Acoustic measurements revealed the absence of free gas in sediments of the littoral zone and low ebullition rates in the shallow water zones. For the first time, this study reports the role of CH4 production in determining the spatial variability of free gas content in freshwater sediments. The results further demonstrate the importance of sediment gas content in explaining spatial variability of gas ebullition in lakes.

Methane formation and consumption by sediments in a cross-channel profile of a small river impoundment

Rivers are a natural source of methane (CH4) into the atmosphere and may contribute significantly to total CH4 emissions. Even though the details of sources of CH4 in rivers are not fully understood, weirs have been recognized as a hotspot of CH4 emissions. In this study, we investigated CH4 production and consumption in air-exposed river sediments along a cross-channel transect located upstream of a weir. Stable carbon isotopes were used for determination of individual methanogenic pathways. In order to understand the relationship between physicochemical and biological processes, additional parameters such as organic matter, grain median size, and carbon and nitrogen content were characterized as well. Generally, samples from the surface sediment layer (0-10 cm) had higher CH4 production than sediments from the deeper layer (10-20 cm) during the incubation experiments. Sediments near the bank zones and in the mid-channel were characterized by the highest organic carbon content (6.9 %) as well the highest methanogenic activity (2.5 mmol g-1 DW d-1). The CH4 production was predominated by H2/CO2 dependent methanogenesis in the surface sediment layer (0-10 cm), while the proportion of acetoclastic and hydrogenotrophic methanogenesis in the deeper sediment layer (10-20 cm) was balanced. The CH4 oxidation potential of sediments showed the same spatial pattern as observed for the CH4 production. Our results showed high spatial variability of sediment CH4 production and oxidation in the cross-channel profile upstream of the weir, whereas the highest CH4 dynamics were observed in the littoral zones. This variability was closely linked with the carbon and nitrogen content in the sediment samples.

What the bubble knows: Lake methane dynamics revealed by sediment gas bubble composition

AbstractAtmospheric methane (CH4) concentrations have more than doubled in the past ~ 250 yr, although the sources of this potent greenhouse gas remain poorly constrained. Freshwaters contribute ~ 20% of natural CH4 emissions, about half attributed to ebullition. Estimates remain uncertain as ebullition is stochastic, making measurements difficult, time consuming, and costly with current methods (e.g., floating chambers, funnel gas traps, and hydroacoustics). We present a novel approach to quantify basin‐wide hypolimnetic CH4 fluxes at the sediment level based on measurements of bubble gas content and modeling of dissolved pore‐water gases. We show that the relative ebullition flux pathway can be resolved by knowledge of only bubble gas content. As sediment CH4 production, diffusion, and ebullition are interrelated, the addition of a second observation allows closing the entire sediment CH4 balance. Such measurements could include bubble formation depth, sediment diffusive fluxes, ebullition, sediment CH4 production, or the hypolimnetic CH4 mass balance. The measurement of bubble gas content is particularly useful for identifying local ebullitive hotspots and integrating spatial heterogeneity of CH4 fluxes. Our results further revealed the crucial effect of water column depth, production rates, and hypolimnetic dissolved CH4 concentrations on sediment CH4 dynamics. Although we apply the model to cohesive sediments in an anoxic hypolimnion, the model can be applied to shallow, oxic settings by altering the CH4 production rate curve to account for oxidation. Utilizing our approach will provide a deeper understanding of in‐lake CH4 budgets, and thus improve CH4 emission estimates from inland freshwaters at the regional and global scales.

Trimethylamine stimulated and dissolved organic matter inhibited methane production in sediment from the Poyang Lake, China

ABSTRACTMethane (CH4) emitted from wetlands contributes significantly to the greenhouse effect. The Poyang Lake, the largest freshwater lake in China, is fed by five rivers and connects to the Yangtze River. The area of the lake fluctuates dramatically between drawdown and flood periods with large areas of wetlands. In order to understand the CH4 production capacity and factors that influence CH4 production in the wetland, a static closed chamber combined with a gas chromatograph technique was used to investigate the influence of substrates and electron acceptors on methanogenesis. The results showed that CH4 production capacity of sediments from the Poyang Lake was and it was stimulated by trimethylamine (TMA) to a great extent. Incubation temperature played a vital role on CH4 production in sediments and the optimum temperature for methanogenesis was 35°C. Minimum CH4 production capacity occurred with the addition of FeCl3, and the inhibitory effects of electron acceptors decreased in the sequence: FeCl3 > MnO2 > DOM > Fe2O3. In this study, DOM was demonstrated as one of the inhibitors to methanogenesis and TMA was the main substrate of methanogens in the sediments of the Poyang Lake whose pH value is 7.83.

The role of filamentous algae Spirogyra spp. in methane production and emissions in streams

Periphyton is an important primary producer in freshwater ecosystems. However, their effects on methane (CH4) production and emissions in streams and rivers are not fully understood. The purpose of this study was to investigate the effects of filamentous algae Spirogyra spp. on CH4 flux and to characterize the factors responsible for these effects. Sediments cores were collected from a stream in the suburbs of Shanghai and either inoculated with Spirogyra spp. in the laboratory or left uninoculated. CH4 flux, dissolved CH4 concentration, potential CH4 production rate and organic carbon in sediments and overlying water were measured for 2 months. Study results indicated that CH4 fluxes in algal treatment columns were significantly higher than in unvegetated sediments during initial algal decomposition, while potential CH4 production rates in algal treatment columns were significantly higher than in controls during both algal bloom and initial decomposition. The relationship between CH4 flux and environmental parameters was assessed and CH4 flux was found to exhibit a strongly positive relationship with dissolved organic carbon (DOC) concentration and potential CH4 production rate during experimental periods. It was concluded that Spirogyra spp. increased CH4 emissions in streams by providing algal carbon sources for methanogenesis and stimulating the CH4 production in sediments. These findings may facilitate understanding the roles of periphyton in aquatic methane dynamics and global greenhouse gas emissions.

Substrate limitation of sediment methane flux, methane oxidation and use of stable isotopes for assessing methanogenesis pathways in a small arctic lake

Among predicted impacts of climate change in the Arctic are greater thaw depth and shifts in vegetation patterns and hydrology that are likely to increase organic carbon and nutrient loading to lakes. We measured substrate limitation of sediment methane (CH4) flux, examined pathways of methanogenesis, and potential CH4 oxidation using stable isotope labeled acetate in intact sediment cores from arctic lake GTH 112 (68°40′20″N, 149°14′57″W). We hypothesized that the acetoclastic pathway would dominate methanogenesis, reflecting dissolved organic carbon supply from the surrounding landscape, and that sediment CH4 flux would be stimulated by addition of acetate. Experiments demonstrated acetate limitation of sediment CH4 flux with short-term CH4 flux response to availability of acetate, high rates of CH4 oxidation, and strong dominance of the acetoclastic over the hydrogenotrophic methanogenic pathway. The experiments also indicated that isotopic fractionation effects during isotope enrichment experiments are large during methanogenesis and can alter the methanogenic pathways being investigated. Under oxic conditions, CH4 oxidation at the sediment–water interface or in the water column is likely to account for much of diffusive CH4 flux, but under anoxic hypolimnetic conditions and increased substrate availability, conditions that are likely to occur with climate change, sediment CH4 flux will likely increase, with oxidation utilizing a smaller portion of sediment CH4 production.