Water CH4 Concentrations Research Articles

Salinity decreases methane concentrations in Chinese lakes

Lakes are important sources of methane (CH4), and understanding the influence of environmental factors on CH4 concentration in lake water is crucial for accurately assessing CH4 emission from lakes. In this study, we investigated CH4 concentration in two connected Tibetan Plateau lakes, Lake Keluke (an open freshwater lake) and Lake Tuosu (a closed saline lake), through in-situ continuous measurements taken in different months from 2021 to 2023. The results show substantial spatial and seasonal variations in CH4 concentrations in the two lakes, while the CH4 concentrations in Lake Keluke are consistently higher than those in Lake Tuosu for each month. Despite sharing similar environmental conditions due to connected (e.g. pH, water temperature, dissolved oxygen content, and total organic carbon content), the critical difference between the two lakes is their salinity. This implies that salinity is the critical factor contributing to the decrease in CH4 concentrations in Lake Tuosu, possibly due to the changes in microbial species between freshwater and brackish/saline lakes. Additionally, to further validate the effect of salinity on CH4 concentrations in lake water, we compared the CH4 concentrations of 33 lakes (including 5 saline lakes and 28 freshwater lakes) from the Tibetan Plateau, Chinese Loess Plateau, and Yangtze Plain, and found that saline lakes consistently exhibit lower CH4 concentrations (avg. 0.08 μmol/L), while freshwater lakes generally display higher CH4 concentrations (avg. 1.25 μmol/L) with considerable fluctuations. Consequently, freshwater and saline lakes exhibit distinct CH4 emissions, which could be used for more accurate estimation of global CH4 emission from lakes.

Impacts of vertical mixing and ice-melt on N2O and CH4 concentrations in the Canadian Arctic Ocean

The sources and sinks of methane (CH4) and nitrous oxide (N2O) in the Arctic Ocean are subject to significant uncertainty, due to a combination of high spatial and temporal variability, and limited observations over relevant scales. To address this, an underway ship-board system was developed to simultaneously and continuously measure surface water CH4 and N2O concentrations in the Canadian Arctic Ocean, providing high-resolution data over a range of hydrographic regimes. Across our survey region during late summer, 2021, CH4 and N2O saturation ranged from 94% to 231% and 96%–120%, respectively. Localized CH4 and N2O maxima were observed along the cruise track, including high concentrations over the continental shelf of eastern Baffin Island, and in some waters influenced by sea ice cover or recent glacier retreat. We use our high-resolution data to examine the relationship between N2O and CH4 concentrations and a variety of oceanographic variables, demonstrating both positive correlations with salinity, which reflects the likely influence of vertical mixing, and negative correlations with salinity, indicative of ice-melt and freshwater inputs. These results build upon previous discrete CH4 and N2O observations in the Canadian Arctic, providing new information on the fine-scale distribution of these gases, and the potential underlying biogeochemical factors driving their variability. Results from our work have implications for understanding the biogeochemical cycling of Arctic Ocean CH4 and N2O under a rapidly warming climate.

Methanogens limited to lower rhizosphere and to an atypical salt marsh niche along a pristine intertidal mangrove continuum

AbstractMangroves are valuable ecosystems that facilitate primary production, carbon sequestration, and regulation of greenhouse gas (GHG) cycles in coastal sediments, with microorganisms playing key roles. Specialized bacteria and archaea compete for energy and resources in mangrove sediments to inhabit optimal ecological niches and can produce or consume methane (CH4)—a potent GHG—in the process. CH4 cycling in mangroves has gained growing attention, yet uncertainties regarding functional and spatial distributions of microorganisms remain. Here, we demonstrate that in a pristine mangrove forest, CH4 concentrations and methanogen communities are concentrated within lower or below rhizosphere depths. We also reveal atypical niches for methanogens in the upper tidal salt marsh zone where vegetation is sparse and highest methanogens abundances were detected at deepest depths (4715 reads g−1) despite relatively high redox potentials (> 250 mV). Pore water CH4 concentrations were highest at the deepest depth within the mangrove forest (max. 3.40 ± 0.21 μM) and coincided with the highest sediment CH4 fluxes (276.4 ± 54.2 μmol m−2d−1) and methanotroph abundances at the surface (1309 reads g−1). Sediment CH4 oxidation fractions between the deepest (60 cm) and shallowest (5 cm) depths were estimated between 18.8% and 64.9%. Positive correlation between crab burrows and CH4 fluxes suggests that CH4 from deeper sediment and salt marsh niches can be transported via conduits to the atmosphere. The spatial data from this study highlights the importance of investigating CH4 dynamics across estuarine ecosystem gradients to better understand the complex roles of vital coastal vegetation zones in the face of a changing climate.

Large methane emission during ice-melt in spring from thermokarst lakes and ponds in the interior Tibetan Plateau

The magnitudes of annual CO2 and CH4 emissions from thermokarst lakes and ponds are uncertain due to scarce measurements on the Tibetan Plateau (TP) and limited data on emissions during ice-melt. To evaluate the importance of CO2 and CH4 emissions during late winter and spring among the whole year, surface water CO2 and CH4 concentrations were measured with the headspace method in ten thermokarst lakes and ponds in 2020–2021 on the TP. The concentrations of CH4 and CO2 under ice in winter (CH4, 1.05–127.16 μmol/L, CO2, 13.24–95.57 μmol/L) were 3–4 orders of magnitude and several times higher than those in summer (CH4, 0.02–2.34 μmol/L, CO2, 7.26–49.18 μmol/L), respectively. The CH4 concentrations and fluxes during ice-melt increased with as the depth of the thermokarst lakes and ponds increased. However, the variation in CO2 concentration was not significant in depth. Snowfall events played an important role in influencing gas exchange during ice-melt. In the ten lakes researched, 42.2% and 19.2% of the annual CH4 and CO2 fluxes occurred during ice-melt, respectively. Our results indicated that CH4 and CO2 accumulation in winter should be considered when evaluating annual carbon budgets in thermokarst lakes and ponds on the TP and similarly high fluxes during ice-melt might occur in other thermokarst lakes and ponds.

Characterisation of Methane Production Pathways in Sediment of Overwashed Mangrove Forests

Methane (CH4) emissions in mangrove ecosystems may complicate the ecosystem’s potential carbon offset for climate change mitigation. Microbial processes and the mass balance of CH4 in mangrove sediment are responsible for the emissions from the ecosystems. This is the follow up of our previous research which found the super saturation of CH4 in the pore water of mangrove sediment compared to atmospheric CH4 and the lack of a correlation between pore water sulphate and CH4 concentrations. This study is going to investigate methane production pathways in the sediment of overwashed mangrove forests. Two approaches were used to study methanogens here: (1) the spread plate count method and the quantitative polymerase chain reaction (qPCR) method, and (2) laboratory experiments with additional methanogenic substrates (methanol, acetate, and hydrogen) to determine which substrates are more conducive to methane production. According to the qPCR method, methanogen abundance ranged from 72 to 6 × 105 CFU g−1 sediment, while SRB abundance ranged from 2 × 102 to 2 × 105 CFU g−1 sediment. According to the plate count method, the abundance of methylotrophic methanogens (the only group of methanogens capable of competing with SRBs) ranged from 8.3 × 102 to 5.1 × 104 CFU g−1, which is higher than the abundance of the other group of methanogens (0 to 7.7 × 102 CFU g−1). The addition of methanol to the sediment slurry, a substrate for methylotropic methanogens, resulted in a massive production of CH4 (up to 9 × 104 ppm) and intriguingly the control treatments with autoclaving did not kill methanogens. These findings suggested that mangrove ecosystems in the marine environment provide favourable conditions for methanogens and further characterisation of the methanogen involved in the process is required. As a result, future research in this ecosystem should include methane production in carbon offset calculations, particularly due to methylotropic methanogenesis.

Frequent algal blooms dramatically increase methane while decrease carbon dioxide in a shallow lake bay

Freshwater ecosystems play a key role in global greenhouse gas estimations and carbon budgets, and algal blooms are widespread owing to intensified anthropological activities. However, little is known about greenhouse gas dynamics in freshwater experiencing frequent algal blooms. Therefore, to explore the spatial and temporal variations in methane (CH4) and carbon dioxide (CO2), seasonal field investigations were performed in the Northwest Bay of Lake Chaohu (China), where there are frequent algal blooms. From the highest site in the nearshore to the pelagic zones, the CH4 concentration in water decreased by at least 80%, and this dynamic was most obvious in warm seasons when algal blooms occurred. CH4 was 2–3 orders of magnitude higher than the saturated concentration, with the highest in spring, which makes this bay a constant source of CH4. However, unlike CH4, CO2 did not change substantially, and river mouths acted as hotspots for CO2 in most situations. The highest CO2 concentration appeared in winter and was saturated, whereas at other times, CO2 was unsaturated and acted as a sink. The intensive photosynthesis of rich algae decreased the CO2 in the water and increased dissolved oxygen and pH. The increase in CH4 in the bay was attributed to the mineralization of autochthonous organic carbon. These findings suggest that frequent algal blooms will greatly absorb more CO2 from atmosphere and increasingly release CH4, therefore, the contribution of the bay to the lake's CH4 emissions and carbon budget will be major even though it is small. The results of this study will be the same to other shallow lakes with frequent algal bloom, making lakes a more important part of the carbon budget and greenhouse gases emission.

Evaluating alternative ebullition models for predicting peatland methane emission and its pathways via data–model fusion

Abstract. Understanding the dynamics of peatland methane (CH4) emissions and quantifying sources of uncertainty in estimating peatland CH4 emissions are critical for mitigating climate change. The relative contributions of CH4 emission pathways through ebullition, plant-mediated transport, and diffusion, together with their different transport rates and vulnerability to oxidation, determine the quantity of CH4 to be oxidized before leaving the soil. Notwithstanding their importance, the relative contributions of the emission pathways are highly uncertain. In particular, the ebullition process is more uncertain and can lead to large uncertainties in modeled CH4 emissions. To improve model simulations of CH4 emission and its pathways, we evaluated two model structures: (1) the ebullition bubble growth volume threshold approach (EBG) and (2) the modified ebullition concentration threshold approach (ECT) using CH4 flux and concentration data collected in a peatland in northern Minnesota, USA. When model parameters were constrained using observed CH4 fluxes, the CH4 emissions simulated by the EBG approach (RMSE = 0.53) had a better agreement with observations than the ECT approach (RMSE = 0.61). Further, the EBG approach simulated a smaller contribution from ebullition but more frequent ebullition events than the ECT approach. The EBG approach yielded greatly improved simulations of pore water CH4 concentrations, especially in the deep soil layers, compared to the ECT approach. When constraining the EBG model with both CH4 flux and concentration data in model–data fusion, uncertainty of the modeled CH4 concentration profiles was reduced by 78 % to 86 % in comparison to constraints based on CH4 flux data alone. The improved model capability was attributed to the well-constrained parameters regulating the CH4 production and emission pathways. Our results suggest that the EBG modeling approach better characterizes CH4 emission and underlying mechanisms. Moreover, to achieve the best model results both CH4 flux and concentration data are required to constrain model parameterization.

High spatiotemporal variability of methane concentrations challenges estimates of emissions across vegetated coastal ecosystems.

Coastal methane (CH4) emissions dominate the global ocean CH4 budget and can offset the “blue carbon” storage capacity of vegetated coastal ecosystems. However, current estimates lack systematic, high‐resolution, and long‐term data from these intrinsically heterogeneous environments, making coastal budgets sensitive to statistical assumptions and uncertainties. Using continuous CH4 concentrations, δ13C‐CH4 values, and CH4 sea–air fluxes across four seasons in three globally pervasive coastal habitats, we show that the CH4 distribution is spatially patchy over meter‐scales and highly variable in time. Areas with mixed vegetation, macroalgae, and their surrounding sediments exhibited a spatiotemporal variability of surface water CH4 concentrations ranging two orders of magnitude (i.e., 6–460 nM CH4) with habitat‐specific seasonal and diurnal patterns. We observed (1) δ13C‐CH4 signatures that revealed habitat‐specific CH4 production and consumption pathways, (2) daily peak concentration events that could change >100% within hours across all habitats, and (3) a high thermal sensitivity of the CH4 distribution signified by apparent activation energies of ~1 eV that drove seasonal changes. Bootstrapping simulations show that scaling the CH4 distribution from few samples involves large errors, and that ~50 concentration samples per day are needed to resolve the scale and drivers of the natural variability and improve the certainty of flux calculations by up to 70%. Finally, we identify northern temperate coastal habitats with mixed vegetation and macroalgae as understudied but seasonally relevant atmospheric CH4 sources (i.e., releasing ≥ 100 μmol CH4 m−2 day−1 in summer). Due to the large spatial and temporal heterogeneity of coastal environments, high‐resolution measurements will improve the reliability of CH4 estimates and confine the habitat‐specific contribution to regional and global CH4 budgets.

Modeling the spatial and temporal variability in surface water CO2 and CH4 concentrations in a newly created complex of boreal hydroelectric reservoirs

Hydroelectric reservoirs emit carbon dioxide (CO2) and methane (CH4) to the atmosphere, yet there is still much uncertainty concerning the magnitude and drivers of these greenhouse gas (GHG) emissions. This uncertainty is particularly large over the initial years after flooding and in complex, cascade reservoir systems where studies are rare. We assessed the spatial and temporal patterns of CO2 and CH4 concentrations in the newly created La Romaine complex, which is composed of three consecutive reservoirs (RO1, RO2, RO3) along the La Romaine River. Dissolved CO2 and CH4 concentrations were intensively measured over three seasons for four years. Results show elevated CH4 and especially CO2 concentrations in surface waters of all three reservoirs upon flooding, with strong seasonality and high spatial heterogeneity within reservoirs. There was a strong seasonal decoupling of surface water CO2 and CH4 concentrations. Contrary to expectations, surface water CO2 and CH4 concentrations were relatively stable over the initial years of flooding, with exception of the decrease in CO2 concentrations in the shallower RO1 reservoir. Further, individual reservoir characteristics, notably reservoir morphometry and pre-flood land cover, together with climatic factors were the main drivers of CO2 and CH4 concentrations, and the reservoir position in the cascade played a minor role. Models differed for CO2 and CH4, and also between reservoirs highlighting the need to capture these specificities in reservoir functioning. We establish a modeling framework to effectively fill the spatial and temporal gaps that inevitably exist in the sampling coverage of large and heterogeneous reservoirs, which combined with appropriately modeled gas transfer velocities, will serve as a platform to derive robust estimates of diffusive fluxes. This modeling framework can be transposed to other reservoirs, and will contribute to more accurate and representative estimates of diffusive carbon emissions from hydroelectric reservoirs.

Patterns and drivers of CH4 concentration and diffusive flux from a temperate river–reservoir system in North China

Freshwater reservoirs are regarded as an important anthropogenic source of methane (CH4) emissions. The temporal and spatial variability of CH4 emissions from different reservoirs results in uncertainty in the estimation of the global CH4 budget. In this study, surface water CH4 concentrations were measured and diffusive CH4 fluxes were estimated via a thin boundary layer model in a temperate river–reservoir system in North China, using spatial (33 sites) and temporal (four seasons) monitoring; the system has experienced intensive aquaculture disturbance. Our results indicated that the dissolved CH4 concentration in the reservoir ranged from 0.07 to 0.58 µmol/L, with an annual average of 0.13 ± 0.10 µmol/L, and the diffusive CH4 flux across the water–air interface ranged from 0.66 to 3.61 μmol/(m2•hr), with an annual average of 1.67 ± 0.75 μmol/(m2•hr). During the study period, the dissolved CH4 concentration was supersaturated and was a net source of atmospheric CH4. Notably, CH4 concentration and diffusive flux portrayed large temporal and spatial heterogeneity. The river inflow zone was determined to be a hotspot for CH4 emissions, and its flux was significantly higher than that of the tributary and main basin; the CH4 flux in autumn was greater than that in other seasons. We also deduced that the CH4 concentration/diffusive flux was co-regulated mainly by water temperature, water depth, and water productivity (Chla, trophic status). Our results highlight the importance of considering the spatiotemporal variability of diffusive CH4 flux from temperate reservoirs to estimate the CH4 budget at regional and global scales.

Analysis of the effect mechanism of water and CH4 concentration on gas explosion in confined space

In order to study the effect of water and CH4 concentration on gas explosion, a 20L spherical explosive device was used to carry out a water-containing gas explosion experiment, and the explosion simulation was carried out with CHEMKIN-PRO, the mechanism of water on gas explosion was analyzed from the perspective of free radicals and energy. The results showed that the upper limit of gas explosion, maximum explosion pressure and temperature decreased significantly with the increase of water content. The higher the concentration of CH4, the more obvious the inhibitory effect of water on gas explosion pressure, and the optimal explosion concentration of CH4 decreased with the increase of water content. As the water content and CH4 concentration increase, the residual CH4 content increases after the explosion, the O2 content decreases, and the CO content produced increases. When the CH4 concentration is lower than the optimal concentration, water promotes the formation of CO2 to a certain extent; when the CH4 concentration is higher than the optimal explosive concentration, the CO2 content decreases with the increase of water content. Overall, water inhibits methane explosion, the addition of water on the one hand reduces the concentration of active free radicals H, O, OH, on the other hand, it interferes with the generation of gas explosion energy and consumes the kinetic energy of the gas explosion flame shock wave through heat absorption, thus inhibiting the intensity of gas explosion.

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.

Methane oxidation in the waters of a humic-rich boreal lake stimulated by photosynthesis, nitrite, Fe(III) and humics

Abstract. Small boreal lakes are known to contribute significantly to global CH4 emissions. Lake Lovojärvi is a eutrophic lake in southern Finland with bottom water CH4 concentrations up to 2 mM. However, the surface water concentration, and thus the diffusive emission potential, was low (< 0.5 µM). We studied the biogeochemical processes involved in CH4 removal by chemical profiling and through incubation experiments. δ13C-CH4 profiling of the water column revealed a methane-oxidation hotspot just below the oxycline and zones of CH4 oxidation within the anoxic water column. In incubation experiments involving the addition of light and/or oxygen, CH4 oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobic CH4 oxidation. We observed a distinct peak in CH4 concentration at the chlorophyll-a maximum, caused by either in situ CH4 production or other CH4 inputs such as lateral transport from the littoral zone. In the dark anoxic water column at 7 m depth, nitrite seemed to be the key electron acceptor involved in CH4 oxidation, yet additions of Fe(III), anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic CH4 oxidation. Surprisingly, nitrite seemed to inhibit CH4 oxidation at all other depths. Overall, this study shows that photosynthetically fueled CH4 oxidation can be a key process in CH4 removal in the water column of humic, turbid lakes, thereby limiting diffusive CH4 emissions from boreal lakes. Yet, it also highlights the potential importance of a whole suite of alternative electron acceptors, including humics, in these freshwater environments in the absence of light and oxygen.

Diffusive CH4 fluxes from aquaculture ponds using floating chambers and thin boundary layer equations

Static floating chambers (FCs) are the conventional method to measure CH4 fluxes across the water-air interface in ponds, while thin boundary layer (TBL) modelling is increasingly used to estimate CH4 fluxes. In this study, both FCs measurements and TBL models of gas transfer velocity were used to determine CH4 evasion from aquaculture ponds in southeastern China. The surface water CH4 concentrations ranged from 0.4 to 9.1 μmol L-1 with an average of 4.8 ± 0.8 μmol L-1. CH4 flux was always positive, indicating the ponds as a persistent CH4 source to air. Mean CH4 flux based on different TBL models showed large variations, ranging between 19 and 316 μmol m−2 h−1. Compared against the direct measurement FCs, three TBL models developed for the open sea, flowing estuarine system and lentic ecosystem (TBLW92a, TBLRC01, and TBLCL98, respectively) overestimated CH4 emission by 40–200%, while the wind tunnel-based TBL model (TBLLM86) underestimated CH4 emission. Two TBL models developed for lakes (TBLW92b and TBLCW03) gave estimates similar to FCs.

Dramatic temporal variations in methane levels in black bloom prone areas of a shallow eutrophic lake

Global lakes serve as a key natural source of methane (CH4) and suffer from increasing hypoxia due to unprecedented anthropogenic activities and climate change. A black bloom is a temporary hypoxia triggered by a longstanding algal bloom, which facilitates CH4 production by creating reducing conditions and abundant algae-sourced organic carbon. One-year investigations were conducted to examine temporal CH4 dynamics in the water and sediment pore water in black bloom prone areas (BBPAs) in Lake Taihu, China, where there had been at least two recorded black bloom events. The CH4 in the water changed significantly with time (p < 0.001), with the highest concentrations appearing in warm months when an abnormal lower dissolved oxygen content was observed at different sites, which were one to two orders of magnitude higher than other months. Compared with the control site, there were significantly higher CH4 concentrations in BBPA waters (p < 0.001), which was consistent with the higher CH4 in the sediment pore water. Methane dynamics in the water showed significant positive correlations with temperature, total phosphorus, total nitrogen, ammonia-N, and soluble reactive phosphorus (p < 0.05), but showed a significant inverse correlation with dissolved oxygen (p < 0.01). Redundancy analysis indicated dissolved oxygen made the largest contribution to CH4 dynamics in the BBPAs. A significant increase in the CH4 in water will turn BBPAs into temporary hot spots with substantial CH4 emissions with the appearance of black blooms. The results provide new insights into understanding future CH4 dynamics under globally prevailing algal blooms and climate change.

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.

Methane Levels of a River Network in Wuxi City, China and Response to Water Governance

The majority of rivers are a CH4 source that accounts for an important proportion of annual global emissions. However, CH4 evasion from urban river networks has received disproportionately less attention than their contribution. The effect of water governance on water quality and CH4 emission in urban areas remains unclear. Water quality, CH4 concentrations, and fluxes from a river network in Binhu District, Wuxi City, and their response to water governance were analyzed in this study. CH4 concentrations in the investigated rivers ranged from 0.05 μmol L−1 to 16.37 μmol L−1 (2.47 ± 4.5 μmol L−1, medium 0.23 μmol L−1), and CH4 diffusive fluxes were 75.55 ± 171.78 μmol m−2 h−1 with a medium of 6.50 μmol m−2 h−1. CH4 concentration showed a significant correlation with water quality parameters, especially for NH3–N (r = 0.84, p &lt; 0.001). Significant differences in water quality and CH4 levels were found between sites that had conducted water management and those that continued to exhibit poor water quality. Our analysis showed that rivers under water governance have a positive tendency toward water ecological restoration, and a significant decrease in CH4 efflux to the air can be achieved after extensive and intensified water governance.

Filamentous green algae Spirogyra regulates methane emissions from eutrophic rivers.

Excessive growth of filamentous green algae in rivers has attracted much attention due to their functional importance to primary production and carbon cycling. However, comprehensive knowledge of how filamentous green algae affect carbon cycling, especially the CH4 emissions from river ecosystems, remains limited. In this study, incubation experiments were conducted to examine the factors regulating CH4 emissions from a eutrophic river with dense growth of filamentous green algae Spirogyra through combinations of biogeochemical, molecular biological, and stable carbon isotope analyses. Results showed that although water dissolved oxygen (DO) in the algae+sediment (A+S) incubation groups increased up to 19 mg L-1, average CH4 flux of the groups was 13.09 μmol m-2 day-1, nearly up to two times higher than that from sediments without algae (S groups). The significant increase of sediment CH4 oxidation potential and methanotroph abundances identified the enhancing sediment CH4 oxidation during Spirogyra bloom. However, the increased water CH4 concentration was consistent with depleted water [Formula: see text] and decreased apparent fractionation factor (αapp), suggesting the important contribution of Spirogyra to the oxic water CH4 production. It can thus be concluded that high DO concentration during the algal bloom promoted the CH4 consumption by enhancing sediment CH4 oxidation, while algal-linked oxic water CH4 production as a major component of water CH4 promoted the CH4 emissions from the river. Our study highlights the regulation of Spirogyra in aquatic CH4 fluxes and will help to estimate accurately CH4 emissions from eutrophic rivers with dense blooms of filamentous green algae. Graphical abstract.

Spatial and temporal variability of methane emissions from cascading reservoirs in the Upper Mekong River

Reservoirs are an important source of atmospheric methane (CH4), a potent greenhouse gas. The Mekong, one of the largest Asian rivers, has been heavily dammed and can be a potential hotspot for CH4 emissions. While low diffusive CH4 flux was previously reported from cascading reservoirs in the Upper Mekong, the contribution of ebullition (bubbling) remained unexplored. To better constrain the magnitude and drivers of ebullition from these reservoirs, automated bubble traps were deployed in four reservoirs, allowing for continuous measurement of the ebullitive flux with high temporal resolution for a period of six months. To characterize the spatial variability of CH4 fluxes mediated by ebullition and diffusion, whole-reservoir surveys were conducted using a scientific echo sounder for bubble observations together with a gas equilibrator for mapping dissolved CH4 concentration in surface water from which diffusive fluxes were estimated. Potential production and anaerobic oxidation rates of CH4 were estimated in laboratory incubations of sediment cores collected near the bubble trap deployment sites. The CH4 production potential in sediments increased strongly along the reservoir cascade, with mostly minor reduction by anaerobic oxidation. Surface CH4 fluxes, in contrast, showed high spatial variability in both ebullitive and diffusive pathways (ranging 0.05–44 and 1.8–6.4 mg m−2 d−1, respectively, among all reservoirs). Ebullitive fluxes were about one order of magnitude higher than diffusive fluxes and remained significant at sites as deep as 30–45 m. The repeated spatial pattern of ebullition (higher fluxes at the dam area than in upstream sections) suggests the possible control of emission rates by sediment transport and deposition.

Heterogeneous CO2 and CH4 patterns across space and time in a small boreal lake

ABSTRACT Small boreal lakes emit large amounts of carbon dioxide (CO2) and methane (CH4) to the atmosphere. Yet emissions of these greenhouse gases are variable in space and time, in part due to variable within-lake CO2 and CH4 concentrations. To determine the extent and the underlying drivers of this variation, we measured lake water CO2 and CH4 concentrations and estimated associated emissions using spatially discrete water samples collected every 2 weeks from a small boreal lake. On select dates, we also collected groundwater samples from the surrounding catchment. On average, groundwater draining a connected peat mire complex had significantly higher CO2 and CH4 concentrations compared to waters draining forest on mineral soils. However, within the lake, only CH4 concentrations nearshore from the mire complex were significantly elevated. We observed little spatial variability in surface water CO2; however, bottom water CO2 in the pelagic zone was significantly higher than bottom waters at nearshore locations. Overall, temperature, precipitation, and thermal stratification explained temporal patterns of CO2 concentration, whereas hydrology (discharge and precipitation) best predicted the variation in CH4 concentration. Consistent with these different controls, the highest CO2 emission was related to lake turnover at the end of August while the highest CH4 emission was associated with precipitation events at the end of June. These results suggest that annual carbon emissions from small boreal lakes are influenced by temporal variation in weather conditions that regulate thermal stratification and trigger hydrologic land–water connections that supply gases from catchment soils to the lake.