Overlooked drivers of the greenhouse effect: The nutrient-methane nexus mediated by submerged macrophytes.

Submerged macrophytes remediation is a commonly used technique for improving water quality and restoring habitat in aquatic ecosystems. However, the drivers of success in the submerged macrophytes assembly process and their specific impacts on methane emissions are poorly understood. Thus, we conducted a mesocosm experiment to test the growth plasticity and carbon fixation of widespread submerged macrophytes (Vallisneria natans) under different nutrient conditions. A refined dynamic chamber method was utilized to concurrently collect and quantify methane emission fluxes arising from ebullition and diffusion processes. Significant correlations were found between methane flux and variations in the physiological activities of V. nantas by the fluorescence imaging system. Our results show that exceeding tolerance thresholds of ammonia in the water significantly interfered with the photosynthetic systems in submerged leaves and the radial oxygen loss in adventitious roots. The recovery process of V. natans accelerated the consumption of dissolved oxygen, leading to increase in the populations of methanogen (153.3 % increase of mcrA genes) and subsequently elevating CH4 emission fluxes (23.7 %) under high nutrient concentrations. Conversely, V. natans increased the available organic carbon under low nutrient conditions by radial oxygen loss, further increasing CH4 emission fluxes (94.7 %). Quantitative genetic and modeling analyses revealed that plant restoration processes drive ecological niche differentiation of methanogenic and methane oxidation microorganisms, affecting methane release fluxes within the restored area. The speciation process of V. natans is incapable of simultaneously meeting improved water purification and reduced methane emissions goals.

Ebullition dominates methane emissions in stratified coastal waters.

Coastal areas are an important source of methane (CH4). However, the exact origins of CH4 in the surface waters of coastal regions, which in turn drive sea-air emissions, remain uncertain. To gain a comprehensive understanding of the current and future climate change feedbacks, it is crucial to identify these CH4 sources and processes that regulate its formation and oxidation. This study investigated coastal CH4 dynamics by comparing water column data from six stations located in the brackish Tvärminne Archipelago, Baltic Sea. The sediment biogeochemistry and microbiology were further investigated at two stations (i.e., nearshore and offshore). These stations differed in terms of stratification, bottom water redox conditions, and organic matter loading. At the nearshore station, CH4 diffusion from the sediment into the water column was negligible, because nearly all CH4 was oxidized within the upper sediment column before reaching the sediment surface. On the other hand, at the offshore station, there was significant benthic diffusion of CH4, albeit the majority underwent oxidation before reaching the sediment-water interface, due to shoaling of the sulfate methane transition zone (SMTZ). The potential contribution of CH4 production in the water column was evaluated and was found to be negligible. After examining the isotopic signatures of δ13C-CH4 across the sediment and water column, it became apparent that the surface water δ13C-CH4 values observed in areas with thermal stratification could not be explained by diffusion, advective fluxes, nor production in the water column. In fact, these values bore a remarkable resemblance to those detected below the SMTZ. This supports the hypothesis that the source of CH4 in surface waters is more likely to originate from ebullition than diffusion in stratified brackish coastal systems.

Operational effects on aquatic carbon dioxide and methane emissions from the Belo Monte hydropower plant in the Xingu River, eastern Amazonia.

Operational demands and the natural inflow of water actively drive biweekly fluctuations in water levels in hydropower reservoirs. These daily to weekly fluctuations could have major effects on methane (CH4) and carbon dioxide (CO2) emissions via release of bubbles from reservoir bottom sediments (ebullition) or organic matter inputs, respectively. The impact of transient fluctuations in water levels on GHG emissions is poorly understood and particularly so in tropical run-of-the-river reservoirs. These reservoirs, characterized by high temperatures and availability of labile organic matter, are usually associated with extensive CH4 generation within bottom sediments. The aim of this study is to determine how water level fluctuations resulting from the operation of the Belo Monte hydropower plant on the Xingu River, eastern Amazon River Basin, affect local CO2 and CH4 emissions. Between February and December 2022, we monitored weekly fluxes and water concentrations of CO2 and CH4 in a site on the margin of the Xingu reservoir. Throughout the study period, fluxes of CO2 and CH4 were 118 ± 137 and 3.62 ± 8.47 mmol m-2 d-1 (average ± 1SD) while concentrations were 59 ± 29.77 and 0.30 ± 0.12 µM, respectively. The fluxes and water concentrations of CO2 were clearly correlated with the upstream discharge, and the variation observed was more closely associated with a seasonal pattern than with biweekly fluctuations in water level. However, CH4 fluxes were significantly correlated with biweekly water level fluctuations. The variations observed in CH4 fluxes occurred especially during the high-water season (February-April), when biweekly water level fluctuations were frequent and had higher amplitude, which increased CH4 ebullition. Reducing water level fluctuations during the high-water season could decrease ebullitive pulses and, consequently, total flux of CH4 (TFCH4) in the reservoir margins. This study underscores the critical role of water level fluctuations in near-shore CH4 emissions within tropical reservoirs and highlights significant temporal variability. However, additional research is necessary to understand how these findings can be applied across different spatial scales.

Including Methane Emissions from Agricultural Ponds in National Greenhouse Gas Inventories.

Agricultural ponds are a significant source of greenhouse gases, contributing to the ongoing challenge of anthropogenic climate change. Nations are encouraged to account for these emissions in their national greenhouse gas inventory reports. We present a remote sensing approach using open-access satellite imagery to estimate total methane emissions from agricultural ponds that account for (1) monthly fluctuations in the surface area of individual ponds, (2) rates of historical accumulation of agricultural ponds, and (3) the temperature dependence of methane emissions. As a case study, we used this method to inform the 2024 National Greenhouse Gas Inventory reports submitted by the Australian government, in compliance with the Paris Agreement. Total annual methane emissions increased by 58% from 1990 (26 kilotons CH4 year-1) to 2022 (41 kilotons CH4 year-1). This increase is linked to the water surface of agricultural ponds growing by 51% between 1990 (115 kilo hectares; 1,150 km2) and 2022 (173 kilo hectares; 1,730 km2). In Australia, 16,000 new agricultural ponds are built annually, expanding methane-emitting water surfaces by 1,230 ha yearly (12.3 km2 year-1). On average, the methane flux of agricultural ponds in Australia is 0.238 t CH4 ha-1 year-1. These results offer policymakers insights into developing targeted mitigation strategies to curb these specific forms of anthropogenic emissions. For instance, financial incentives, such as carbon or biodiversity credits, can mobilize widespread investments toward reducing greenhouse gas emissions and enhancing the ecological and environmental values of agricultural ponds. Our data and modeling tools are available on a free cloud-based platform for other countries to adopt this approach.

Organic Matter Accumulation and Hydrology as Drivers of Greenhouse Gas Dynamics in Newly Developed Artificial Channels.

Artificial channels, common features of inland waters, have been suggested as significant contributors to methane (CH4) and carbon dioxide (CO2) dynamics and emissions; however, the magnitude and drivers of their CH4 and CO2 emissions (diffusive and ebullitive) remain unclear. They are characterized by reduced flow compared to the donor river, which results in suspended organic matter (OM) accumulation. We propose that in such systems hydrological controls will be reduced and OM accumulation will control emissions by promoting methane production and outgassing. Here, we monitored summertime CH4 and CO2 concentrations and emissions on six newly constructed river-fed artificial channels, from bare riparian mineral soil to lotic channels, under two distinct flow regimes. Chamber-based fluxes were complemented with hydrology, total fluxes (diffusion + ebullition), and suspended OM accumulation assessments. During the first 6 weeks after the flooding, inflowing riverine water dominated the emissions over in-channel contributions. Afterwards, a substantial accumulation of riverine suspended OM (≥50% of the channel's volume) boosted in-channel methane production and led to widespread ebullition 10× higher than diffusive fluxes, regardless of the flow regime. Our finding suggests ebullition as a dominant pathway in these anthropogenic systems, and thus, their impact on regional methane emissions might have been largely underestimated.

Rainstorm and strong wind weathers largely increase greenhouse gases flux in shallow ponds.

Shallow-water ponds represent the hotspots of greenhouse gas (GHG) emissions. Most current studies focus on the temporal dynamics for GHGs in water, with little consideration given to the effects of weather changes. In this study, we measured and compared the concentrations and fluxes of CO2, CH4, and N2O from a pond in Northeast China under different meteorological conditions. Results showed that the rates of CO2, CH4, and N2O emissions from pond into the atmosphere during strong winds were 85.85 ± 7.55 mmol m-2 d-1, 22.05 ± 6.80 mmol m-2 d-1, and 10.87 ± 0.72 µmol m-2 d-1, respectively, significantly higher than those measured during non-rain weather. Among which, over 88 % of CH4 emissions were contributed by ebullition. Meanwhile, the CO2 and N2O flux were also significantly higher during heavy rainfall, reaching 100.05 ± 19.76 mmol m-2 d-1 and 5.90 ± 1.03 µmol m-2 d-1, respectively. Strong winds and precipitation induced sediment disturbances, high gas transport coefficients, reduced photosynthesis and oxygen greatly promoted the GHGs escape evasion. Wind speed, air pressure, solar radiation, and dissolved oxygen in water were important influencing factors. Our results emphasize the importance of capturing short-term weather disturbance events, especially rainstorm and strong winds, to accurately assess the annual GHG budget from these shallow water ecosystems.

Quantifying the contribution of methane diffusion and ebullition from agricultural ditches.

Agricultural ditches are significant methane (CH4) sources since substantial nutrient inputs stimulate CH4 production and emission. However, few studies have quantified the role of diffusion and ebullition pathways in total CH4 emission from agricultural ditches. This study measured the spatiotemporal variations of diffusive and ebullitive CH4 fluxes from a multi-level ditch system in a typical temperate agriculture area, and assessed their contributions to the total CH4 emission. Results illustrated that the mean annual CH4 flux in the ditch system reached 1475.1 mg m-2 d-1, among which 1376.7 mg m-2 d-1 was emitted via diffusion and 98.5 mg m-2 d-1 via ebullition. Both diffusive and ebullitive fluxes varied significantly across different types of ditches and seasons, with diffusion dominating CH4 emission in middle-size ditches and ebullition dominating in large-size ditches. Diffusion was primarily driven by large nutrient inputs from adjacent farmlands, while hydrological factors like water temperature and depth controlled ebullition. Overall, CH4 emission accounted for 86 % of the global warming potential across the ditch system, with 81 % attributed to diffusion and 5 % to ebullition. This study highlights the importance of agricultural ditches as hotspots for CH4 emissions, particularly the dominant role of the diffusion pathway.

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.

Temperature response of aquatic greenhouse gas emissions differs between dominant plant types.

Greenhouse gas (GHG) emissions from small inland waters are disproportionately large. Climate warming is expected to favor dominance of algae and free-floating plants at the expense of submerged plants. Through different routes these functional plant types may have far-reaching impacts on freshwater GHG emissions in future warmer waters, which are yet unknown. We conducted a 1,000 L mesocosm experiment testing the effects of plant type and warming on GHG emissions from temperate inland waters dominated by either algae, free-floating or submerged plants in controls and warmed (+4 °C) treatments for one year each. Our results show that the effect of experimental warming on GHG fluxes differs between dominance of different functional plant types, mainly by modulating methane ebullition, an often-dominant GHG emission pathway. Specifically, we demonstrate that the response to experimental warming was strongest for free-floating and lowest for submerged plant-dominated systems. Importantly, our results suggest that anticipated shifts in plant type from submerged plants to a dominance of algae or free-floating plants with warming may increase total GHG emissions from shallow waters. This, together with a warming-induced emission response, represents a so far overlooked positive climate feedback. Management strategies aimed at favouring submerged plant dominance may thus substantially mitigate GHG emissions.

Dynamic chamber as a more reliable technique for measuring methane emissions from aquatic ecosystems.

Aquatic ecosystems are the largest natural source of atmospheric methane ("CH4") worldwide. However, the current estimation of CH4 emissions from aquatic ecosystems still has extensive uncertainty due to large spatiotemporal variations in CH4 emissions as well as significant uncertainty in measurement methods. In this study, we initially investigated CH4 fluxes from a simulated eutrophic water body by using static chamber method ("SC") during an incubation period of 36 days. Approximately 23 % of the total flux measurements were unsuccessful because they lacked a linear correlation between the accumulation of CH4 concentrations and enclosure time. CH4 fluxes could be achieved for most measurements. However, 5 min after enclosing, the initial CH4 concentrations measured in the chambers were too high (up to 507.4 ppm) to greatly suppress CH4 emissions from the diffusion process. Therefore, a dynamic chamber method ("DC") was developed to overcome the shortcomings of the SC. To achieve the DC, air samples must be continuously collected at the inlet and outlet of the dynamic chamber at fixed flow rates. In contrast to the SC, effective CH4 flux data could be obtained by the DC for each measurement at different frequencies. The DC measured the diel and daily variations in CH4 fluxes and the displayed CH4 emissions from the simulated water were highly irregular. The displayed emissions had variations up to more than two orders of magnitude. These results implied that the SC measured few intermittent fluxes that were difficult to represent the actual CH4 emissions from eutrophic water. The DC developed in this study considers the temporal variations in CH4 emissions from aquatic ecosystems. Thus, the DC is expected to be applicable in the field flux measurements of CH4 as well as other greenhouse gases to reduce emissions uncertainties.

Ebullition-facilitated mobilization of trapped dense non-aqueous phase liquid at residual saturation from sandy sediments.

Gas ebullition can mobilize dense non-aqueous phase liquids (DNAPLs) from sediments to the overlying water column, increasing the DNAPL-impacted area and posing serious challenges to the remediation and management of contaminated sediments. Despite this, there have been few laboratory studies focused on gas ebullition-facilitated transport of DNAPL. In this study, bubble-facilitated transport was investigated by injecting gas (air or nitrogen) at 1 mL/min through a creosote source zone (∼25% saturation) capped with sand layers of different thicknesses. Three short-term experiments (8.3-8.7 h) were capped with 11.4, 7.0 or 4.5 cm of sand to estimate DNAPL flux. One long-term experiment (30 days) was capped with 8 cm of sand to investigate DNAPL removal. Heptane placed on a layer of water above the sand was used as a solvent trap and analyzed for petroleum hydrocarbons (PHCs). Results showed that creosote travelled as thin coatings and films surrounding gas bubbles migrating out of the source zone. Gas invasion was dominated by capillarity in the 11.4 cm-thick sand layer and by fracturing in the 7.0 and 4.5 cm-thick sand layers. Migration through these fractures often led to the formation of creosote tails on mobilized bubbles that drained towards the rear end of the bubble. The mass released decreased exponentially with sand cap thickness. In the long-term experiment, images showed significant depletion of the source zone in 30 days. Linear regression analysis showed that relationships with high predictive capabilities for ebullition-facilitated fluxes of hydrophobic organic contaminants can be obtained by incorporating gas ebullition flux and source strength, based on results from this study along with others from the field and laboratory. To our knowledge, this is the first study to compile and integrate data collected from laboratory and field studies to develop an assessment tool to facilitate the management of contaminated sediments affected by gas ebullition.

High temperature noble gas thermometry in Lake Kivu, East Africa.

Due to their biological and chemical inertness, noble gases in natural waters are widely used to trace natural waters and to determine ambient temperature conditions during the last intensive contact with the atmosphere (equilibration). Noble gas solubilities are strong functions of temperature, with higher temperatures resulting in lower concentrations. Thus far, only common environmental conditions have been considered, and hence investigated temperatures have almost never exceeded 35 °C, but environmental scenarios that generate higher surface-water temperatures (such as volcanism) exist nonetheless. Recently published measurements of noble gas concentrations in Lake Kivu, which sits at the base of the Nyiragongo volcano in East Africa, unexpectedly show that the deep waters are strongly depleted in noble gases with respect to in-situ conditions, and so far no quantitative explanation for this observation has been provided. We make use of recently published noble gas solubility data at higher temperatures to investigate our hypothesis that unusually high equilibration temperatures could have caused the low measured noble gas concentrations by applying various approaches of noble gas thermometry. Noble gas concentration ratios and least squares fitting of individual concentrations indicate that the data agrees best with the assumption that deep water originates from groundwater formed at temperatures of about 65 °C. Thus, no form of degassing is required to explain the observed noble gas depletion: the deep water currently contained in Lake Kivu has most probably never experienced a large scale degassing event. This conclusion is important as limnic eruptions were feared to threaten the lives of the local population.

Sediment Disturbance Negatively Impacts Methanogen Abundance but Has Variable Effects on Total Methane Emissions.

Methane emissions from aquatic ecosystems are increasingly recognized as substantial, yet variable, contributions to global greenhouse gas emissions. This is in part due to the challenge of modeling biologic parameters that affect methane emissions from a wide range of sediments. For example, the impacts of fish bioturbation on methane emissions in the literature have been shown to result in a gradient of reduced to enhanced emissions from sediments. However, it is likely that variation in experimental fish density, and consequently the frequency of bioturbation by fish, impacts this outcome. To explore how the frequency of disturbance impacts the levels of methane emissions in our previous work we quantified greenhouse gas emissions in sediment microcosms treated with various frequencies of mechanical disturbance, analogous to different levels of activity in benthic feeding fish. Greenhouse gas emissions were largely driven by methane ebullition and were highest for the intermediate disturbance frequency (disturbance every 7 days). The lowest emissions were for the highest frequency treatment (3 days). This work investigated the corresponding impacts of disturbance treatments on the microbial communities associated with producing methane. In terms of total microbial community structure, no statistical difference was observed in the total community structure of any disturbance treatment (0, 3, 7, and 14 days) or sediment depth (1 and 3 cm) measured. Looking specifically at methanogenic Archaea however, a shift toward greater relative abundance of a putatively oxygen-tolerant methanogenic phylotype (ca. Methanothrix paradoxum) was observed for the highest frequency treatments and at depths impacted by disturbance (1 cm). Notably, quantitative analysis of ca. Methanothrix paradoxum demonstrated no change in abundance, suggesting disturbance negatively and preferentially impacted other methanogen populations, likely through oxygen exposure. This was further supported by a linear decrease in quantitative abundance of methanogens (assessed by qPCR of the mcrA gene), with increased disturbance frequency in bioturbated sediments (1 cm) as opposed to those below the zone of bioturbation (3 cm). However, total methane emissions were not simply a function of methanogen populations and were likely impacted by the residence time of methane in the lower frequency disturbance treatments. Low frequency mechanical disruption results in lower methane ebullition compared to higher frequency treatments, which in turn resulted in reduced overall methane release, likely through enhanced methanotrophic activities, though this could not be identified in this work. Overall, this work contributes to understanding how animal behavior may impact variation in greenhouse gas emissions and provides insight into how frequency of disturbance may impact emissions.

Differences in ebullitive methane release from small, shallow ponds present challenges for scaling.

Small, shallow waterbodies are potentially important sites of greenhouse gas release to the atmosphere. The role of ebullition may be enhanced here relative to larger and deeper systems, due to their shallow water, but these features remain relatively infrequently studied in comparison to larger systems. Herein, we quantify ebullitive release of methane (CH4) in small shallow ponds in three regions of North America and investigate the role of potential drivers. Shallow ponds exhibited open-water season ebullitive CH4 release rates as high as 40 mmol m-2 d-1, higher than previously reported for similar systems. Ebullitive release of CH4 varied by four orders of magnitude across our 15 study sites, with differences in flux rates both within and between regions. What is less clear are the drivers responsible for these differences. There were few relationships between open water-season ebullitive flux and physicochemical characteristics, including organic matter, temperature, and sulphate. Temperature was only weakly related to ebullitive CH4 release across the study when considering all observation intervals. Only four individual sites exhibited significant relationships between temperature and ebullitive CH4 release. Other sites were unresponsive to temperature, and region-specific factors may play a role. There is some evidence that where surface water sulphate concentrations are high, CH4 production and release may be suppressed. Missouri sites (n = 5) had characteristically low ebullitive CH4 release; here bioturbation could be important. While this work greatly expands the number of open-water season ebullition rates for small and shallow ponds, more research is needed to disentangle the role of different drivers. Further investigation of the potential thresholding behaviour of sulphate as a control on ebullitive CH4 release in lentic systems is one such opportunity. What is clear, however, is that efforts to scale emissions (e.g., as a function of temperature) must be undertaken with caution.

Isotopic and Chemical Assessment of the Dynamics of Methane Sources and Microbial Cycling during Early Development of an Oil Sands Pit Lake.

Water-capped tailings technology (WCTT) is a key component of the reclamation strategies in the Athabasca oil sands region (AOSR) of northeastern Alberta, Canada. The release of microbial methane from tailings emplaced within oil sands pit lakes, and its subsequent microbial oxidation, could inhibit the development of persistent oxygen concentrations within the water column, which are critical to the success of this reclamation approach. Here, we describe the results of a four-year (2015-2018) chemical and isotopic (δ13C) investigation into the dynamics of microbial methane cycling within Base Mine Lake (BML), the first full-scale pit lake commissioned in the AOSR. Overall, the water-column methane concentrations decreased over the course of the study, though this was dynamic both seasonally and annually. Phospholipid fatty acid (PLFA) distributions and δ13C demonstrated that dissolved methane, primarily input via fluid fine tailings (FFT) porewater advection, was oxidized by the water column microbial community at all sampling times. Modeling and under-ice observations indicated that the dissolution of methane from bubbles during ebullition, or when trapped beneath ice, was also an important source of dissolved methane. The addition of alum to BML in the fall of 2016 impacted the microbial cycling in BML, leading to decreased methane oxidation rates, the short-term dominance of a phototrophic community, and longer-term shifts in the microbial community metabolism. Overall, our results highlight a need to understand the dynamic nature of these microbial communities and the impact of perturbations on the associated biogeochemical cycling within oil sands pit lakes.

Estimation of total flux of polycyclic aromatic hydrocarbons facilitated by methane ebullition into water column from global lake sediments.

Methane ebullition and contamination are two typical characteristics from lakes, however, these two are generally studied independently. In fact, the exchange of matter and energy between methane bubbles and their surrounding environment can be very active to enhance the contaminant transport. There is limited research on understanding the characteristics and trends of gas ebullition facilitated contaminant emissions in large areas considering water and air as receptors. We herein estimate the transport capacity of methane ebullition for polycyclic aromatic hydrocarbons (PAHs) out of the sediment from global lakes, which may reach an average of 71 (up to 159) t yr-1. Methane bubbles could transfer one third of the total PAH flux from sediments, or equivalent of 1.3-3.0 ng L-1 of additional PAHs, into the water column with the rest going into air, offsetting from 52 to 118% of dry PAH deposition flux into global lakes sediment per year. Given the PAH concentration in lake water is often in the range of 0.1-100 ng L-1, ebullition facilitated PAH flux may increase PAH concentration by a factor of 1.4 to 2.4 until 2,100, being a significant contributor for the PAH increment in lake waters.

High methane emissions from thermokarst lakes on the Tibetan Plateau are largely attributed to ebullition fluxes.

Ebullition has been shown to be an important pathway for methane (CH4) emissions from inland waters. However, the CH4 fluxes and their magnitudes in thermokarst lakes remain unclear due to limited research data, especially on the Tibetan Plateau (TP). The magnitude and regulation of two CH4 pathways, ebullition and diffusion, were investigated in 32 thermokarst lakes on the TP during the summer of 2020. CH4 emissions from thermokarst lakes on the TP showed significant spatiotemporal heterogeneity. Diffusion fluxes in lakes averaged 2.6 mmol m-2 d-1 (ranging from 0.003 to 48.4 mmol m-2 d-1), and ebullition fluxes in lakes averaged 6.6 mmol CH4 m-2 d-1 (ranging from 0.002 to 140.0 mmol m-2 d-1). Together, these ebullition fluxes contributed 66.1 ± 24.9% (ranging 5.4 to 100.0%) to the total (diffusion + ebullition) CH4 emissions, indicating the importance of ebullition as a major CH4 transport mechanism on the TP. In general, thermokarst lakes with higher CH4 diffusion fluxes and ebullition fluxes occurred in alpine meadows (2.5 ± 5.3 mmol m-2 d-1; 8.2 ± 20.6 mmol m-2 d-1), followed by alpine steppes (0.6 ± 5.3 mmol m-2 d-1; 0.7 ± 10.8 mmol m-2 d-1) and desert steppes (0.2 ± 0.2 mmol m-2 d-1; 0.6 ± 0.8 mmol m-2 d-1). The organic matter contents in water and sediment were found to be important factors influencing the seasonal variations in CH4 diffusion fluxes. However, the ebullition CH4 fluxes did not show a clear seasonal variation pattern. Our findings highlight the importance of considering the large spatiotemporal variations in ebullition CH4 fluxes to improve the accuracy of large-scale estimations of CH4 fluxes in thermokarst lakes on the TP. Greater insight into these aspects will increase the understanding of CH4 dynamics in thermokarst lakes on the TP, which is essential for forecasting and climate impact assessments and to better constrain feedback to climate warming.

Bioturbation frequency alters methane emissions from reservoir sediments.

Inland aquatic systems are major global contributors to the atmospheric carbon budget through greenhouse gas (GHG) emissions, although the amount and form of carbon released varies widely across and within systems. Bioturbation of aquatic sediments can impact biogeochemical conditions and physically release sediment-bound bubbles containing GHGs, but variation in the frequency of such disturbance may modify the rate and composition of resulting GHG emissions. We hypothesized that an intermediate bioturbation frequency would result in the greatest methane (CH4) releases due to mechanical release of trapped bubbles, while frequent disturbance would result in greater diffusive carbon dioxide (CO2) releases relative to CH4, due to increased aeration of the sediment. We tested this bioturbation frequency hypothesis using laboratory mesocosms containing homogenized reservoir sediment. We used mechanical disturbance to simulate bioturbation at 3, 7, 14, or 21-day intervals; a control treatment was undisturbed for the duration of the experiment. We measured GHG emission (ebullition and diffusion) rates. An intermediate frequency of disturbance (7 days) produced the highest total GHG emission rate, while the most frequent disturbance interval (3 days) and least frequent interval (0 days) reduced overall GHG emissions relative to weekly disturbance by 24% and 15%, respectively. These patterns were primarily driven by differences in CH4 ebullition. Contrary to our hypothesis, there was no relationship between disturbance frequency and diffusive CO2 emissions. For all disturbance treatments, the majority of ebullition occurred during disturbance events, suggesting mechanical release of entrapped bubbles is an important emission mechanism. The frequency of disturbance has variable effects on GHG emissions and may explain conflicting results in prior studies of bioturbation. Our study provides insight into bioturbation as a driver of within-system variation in GHG emissions and highlights that variable bioturbation frequency results in non-linear responses in CH4 emissions, a globally important GHG, from reservoir sediments.

Integrated approach towards quantifying carbon dioxide and methane release from waste stabilization ponds.

Accurate estimations of gaseous emissions and carbon sequestration in wastewater processing are essential for the design, operation and planning of treatment infrastructure, particularly considering greenhouse gas reduction targets. In this study, we look at the interplay between biological productivity, hydrodynamics and evasion of carbon-based greenhouse gases (GHG) through diffusion and ebullition in order to provide direction for more accurate assessments of their emissions from waste stabilization ponds (WSPs). The ponds stratified in the day and mixed at night. Buoyancy flux contributed between 40 and 75% to turbulence in the water column during nocturnal cooling events, and the associated mixing lead to increasing carbon dioxide (CO2) and methane (CH4) concentrations by up to an order of magnitude in the surface. The onset of stratification and phytoplankton surface blooms, associated with high pH as well as low and variable CO2 partial pressure resulted in an overall reduction of CO2 efflux. Ebullition represented between 40 and 99% of the total CH4 efflux, and up to 95% of the integrated GHG release during wastewater treatment (in CO2 equivalents). Hydrodynamic conditions, diurnal variability and ebullition need to be accounted for reliable assessments of GHG emissions from WSPs. Our study is an important step towards gaining a deeper understanding in the functioning of these hot spots of carbon processing. The contribution of WSPs to atmospheric GHG budget is likely to increase with population growth unless their performance is improved in this regard.