Ebullition Rates Research Articles

Is ebullition or diffusion more important as methane emission pathway in a shallow subsaline lake?

Methane (CH4) emissions via ebullition contribute significantly to greenhouse gas emissions from freshwater bodies. According to the literature, the ebullition pathway may even be the most important pathway in some cases, particularly in shallow lakes. Ebullition rates are not often estimated because of the high uncertainty associated with episodic releases, leading to difficulties in their determination. This study provides an estimate of such emissions in a large, shallow, subsaline lake in eastern Austria, Lake Neusiedl, and compares them to the diffusion pathway. Ebullition gas sampling was conducted every 5–10 days over a period of 107 days from late March to mid-July 2021, using ebullition traps placed in three distinct locations: Reed belt, Channel and Open water/Lake. The aim was to study the temporal and spatial heterogeneity of ebullition and its contribution to total emissions. At the same time, several water quality and other environmental parameters were measured and then tested against the CH4 ebullition rates to explore them as potential drivers for this pathway. The carbon isotope fractionation factor (αC) of the measured CH4 ebullition gas, ranging from 1.03 to 1.06, indicates a dominance of the acetoclastic methanogenesis in the sediments of Lake Neusiedl, regardless of the location. The Reed belt location showed the highest mean CH4 ebullition rate (17 ± 28 mg CH4 m−2 d−1), which is >340-fold higher than the mean of the other two locations, and demonstrated also a strong temperature dependency. In all locations at Lake Neusiedl, the median CH4 fluxes via diffusion are significantly higher than via ebullition. Our analyses do not confirm the dominance of the ebullition pathway in any of the studied locations. Whereas at the Reed belt, ebullition accounts for 48 % of the CH4 emissions, in the other two locations, is responsible only for about 1 %.

Methane Gas Ebullition Dynamics From Different Subtropical Wetland Vegetation Communities in Big Cypress National Preserve, Florida Are Revealed Using a Multi‐Method, Multi‐Scale Approach

AbstractMethane (CH4) emissions from peat soils are highly variable in space and time and are influenced by changes in biogeochemical controls and other environmental factors. Areas or times with disproportionally high CH4 emissions in wetlands may develop where conditions are especially conducive for microbial processes like methanogenesis. Currently, eddy covariance methods are employed to quantify CH4 exchanges over several extensive subtropical forested wetland communities in the Big Cypress National Preserve, Florida. In this work, we investigate the importance of multi‐scale measurements to characterize CH4 ebullition dynamics from subtropical wetlands. Our approach uses a combination of gas traps, time‐lapse photography, and capacitance probes to characterize ebullition dynamics from two different wetland vegetation communities for comparison to eddy covariance CH4 measurements at the site. Ground‐penetrating radar surveys and soil sampling are used to assess differences in subsurface properties between sites that influence ebullition. Our results show that the mean measurement bias between fluxes measured in this study and the eddy covariance measurements over the same period was 10–14 times larger during the wet season when ebullition rates were greatest, than the dry season, when ebullition rates were smallest. This suggests that eddy covariance measurements may underestimate the CH4 contribution of ebullition across heterogeneous wetland vegetation communities and that the comparability of CH4 fluxes from methods varying in spatio‐temporal scale changes in response to subtropical Florida seasonality. Our work suggests that these methods can be used to complement eddy covariance measurements and improve the characterization of ebullition dynamics in subtropical wetlands.

Spatial and temporal variability of greenhouse gas ebullition from temperate freshwater fish ponds

Fish ponds with their typically high carbon and nutrient inputs are relevant sources of greenhouse gases. However, not much is known about gas bubble emissions (ebullition) and their high spatiotemporal variability. This is the first study which quantified diffusive and ebullitive greenhouse gas emissions from temperate fish ponds. To improve greenhouse gas estimates, we investigated the diurnal and spatial variability of diffusive and ebullitive fluxes in 12 extensively to semi-intensively managed fish ponds near Bautzen, Germany. Emissions differed greatly between the different ponds but methane was consistently the predominant greenhouse gas. The feeding sites were hotspots with one order of magnitude higher ebullition rates compared to other parts of the ponds. At these hotspots, ebullitive fluxes of up to 38 L/m2d were measured with a mean bubble methane content of 79%, corresponding to a methane flux of 1.24 mol/m2d. Methane accounted for 90% of the global warming potential in one fish pond but carbon dioxide emissions of up to 242 mmol/m2d at the feeding sites were also significant. Nitrous oxide fluxes, in contrast, were low with 5 ± 9 μmol/m2d. Greenhouse gas ebullition decreased exponentially along a transect from the feeding site into the pond and showed some diurnal fluctuations. While diffusion was higher during night, ebullition rates increased in the morning, presumably caused by higher benthivorous fish activity. Our results highlight the potential of temperate fish ponds as significant greenhouse gas sources and ebullition as a significant pathway. For robust quantification, both small scale spatial and temporal variability as well as the hotspot of the feeding area must be considered.

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

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

The effect of biogas ebullition on ammonia emissions from animal manure-processing lagoons.

Various models have been developed to determine ammonia (NH3 ) emissions from animal manure-processing lagoons to enable relatively simple estimations of emissions. These models allow estimation of actual emissions without intensive field measurements or "one-size-fits-all" emission factors. Two mechanisms for lagoon NH3 emissions exist: (a) diffusive gas exchange from the water surface and (b) mass-flow (bubble transport) from NH3 contained within the ebullition gas bubble (as it rises to the surface) produced from anaerobic decomposition of organic matter. The purpose of this research is to determine whether gas ebullition appreciably affects NH3 emissions and therefore should be considered in emissions models. Specifically, NH3 mass-flow emissions were calculated and compared with calculated diffusive NH3 emissions. Mass-flow NH3 emissions were evaluated based on a two-film model, in connection with the acid dissociation constant of ammonium, to predict the degree of NH3 gas saturation within the bubbles. Average daily ammoniacal nitrogen concentration, pH, and measured biological gas production (ebullition) in conjunction with literature values for Henry's law constant were used to calculate emissions from NH3 saturation of ebullition gases. Ebullition enhancement of NH3 surface emissions due to increased turbulence was estimated from average lagoon ebullition rates and literature values of turbulence enhancement. Ebullition enhancement of NH3 surface emissions and ebullition mass-flow NH3 emissions was determined to be<10% and<0.052%, respectively, of total NH3 emissions. Therefore, because ebullition effects are small, they may be neglected when developing process models to estimate NH3 emissions from water surfaces of swine manure processing lagoons.

Acoustic Mapping of Gas Stored in Sediments of Shallow Aquatic Systems Linked to Methane Production and Ebullition Patterns

Bubble-mediated transport is the predominant pathway of methane emissions from inland waters, which are a globally significant sources of the potent greenhouse gas to the atmosphere. High uncertainties exist in emission estimates due to high spatial and temporal variability. Acoustic methods have been applied for the spatial mapping of ebullition rates by quantification of rising gas bubbles in the water column. However, the high temporal variability of ebullition fluxes can influence estimates of mean emission rates if they are based on reduced surveys. On the other hand, echo sounding has been successfully applied to detect free gas stored in the sediment, which provide insights into the spatial variability of methane production and release. In this study, a subtropical, midsize, mesotrophic drinking water reservoir in Brazil was investigated to address the spatial and temporal variability of free gas stored in the sediment matrix. High spatial resolution maps of gas content in the sediment were estimated from echo-sounding surveys. The gas content was analyzed in relation to water depth, sediment deposition, and organic matter content (OMC) available from previous studies, to investigate its spatial variability. The analysis was further supported by measurements of potential methane production rates, porewater methane concentration, and ebullition flux. The largest gas content (above average) was found at locations with high sediment deposition, and its magnitude depended on the water depth. At shallow water depth (&amp;lt;10 m), high methane production rates support gas-rich sediment, and ebullition is observed to occur rather continuously. At larger water depth (&amp;gt;12 m), the gas stored in the sediment is released episodically during short events. An artificial neural network model was successfully trained to predict the gas content in the sediment as a function of water depth, OMC, and sediment thickness (R2 = 0.89). Largest discrepancies were observed in the regions with steep slopes and for low areal gas content (&amp;lt;4 L m−2). Although further improvements are proposed, we demonstrate the potential of echo-sounding for gas detection in the sediment, which combined with sediment and water body characteristics provides insights into the processes that regulate methane emissions from inland waters.

Pond methane dynamics, from microbial communities to ecosystem budget, during summer in Alaska

AbstractPonds play a larger role in the global freshwater methane (CH4) budget than predicted from surface area alone. To improve our understanding of pond CH4 dynamics, we measured summer CH4 production, concentrations, and emissions to the atmosphere in nine Alaskan wetland ponds along with potential physical, chemical, and biological regulators. Pond CH4 production (0.64, 0.086–1.3 mmol m−2 d−1; median, interquartile range), as assessed with slurry incubations, was positively related to water‐column temperature and chlorophyll a (Chl a), negatively influenced by oxygen levels, and varied with microbial community structure. Average water‐column CH4 concentrations (0.39, 0.21–0.87 μmol L−1) were lower in deeper ponds and at higher oxygen levels, and as expected, they were correlated with diffusive emissions (0.055, 0.024–0.20 mmol m−2 d−1) assessed with flux chambers. Based on a mass balance approach, 39–99% of CH4 produced in ponds was oxidized. Pond ebullition (3.7, 0.60–24 mmol m−2 d−1) was higher and more variable than diffusive emissions. Additionally, pond ebullition rates were better correlated with production rates from the previous month. We also systematically compared the ratio of ebullition to diffusive CH4 emissions in our ponds and other northern lakes, which was negatively related to water depth (n = 71), but positively related to Chl a (n = 28). Our study sheds light on the factors that influence pond CH4 dynamics and demonstrates that pond ebullition is a significant CH4 source worthy of continued study.

Iterative Forecasting Improves Near-Term Predictions of Methane Ebullition Rates

Near-term, ecological forecasting with iterative model refitting and uncertainty partitioning has great promise for improving our understanding of ecological processes and the predictive skill of ecological models, but to date has been infrequently applied to predict biogeochemical fluxes. Bubble fluxes of methane (CH4) from aquatic sediments to the atmosphere (ebullition) dominate freshwater greenhouse gas emissions, but it remains unknown how best to make robust near-term CH4 ebullition predictions using models. Near-term forecasting workflows have the potential to address several current challenges in predicting CH4 ebullition rates, including: development of models that can be applied across time horizons and ecosystems, identification of the timescales for which predictions can provide useful information, and quantification of uncertainty in predictions. To assess the capacity of near-term, iterative forecasting workflows to improve ebullition rate predictions, we developed and tested a near-term, iterative forecasting workflow of CH4 ebullition rates in a small eutrophic reservoir throughout one open-water period. The workflow included the repeated updating of a CH4 ebullition forecast model over time with newly-collected data via iterative model refitting. We compared the CH4 forecasts from our workflow to both alternative forecasts generated without iterative model refitting and a persistence null model. Our forecasts with iterative model refitting estimated CH4 ebullition rates up to 2 weeks into the future [RMSE at 1-week ahead = 0.53 and 0.48 loge(mg CH4 m−2 d−1) at 2-week ahead horizons]. Forecasts with iterative model refitting outperformed forecasts without refitting and the persistence null model at both 1- and 2-week forecast horizons. Driver uncertainty and model process uncertainty contributed the most to total forecast uncertainty, suggesting that future workflow improvements should focus on improved mechanistic understanding of CH4 models and drivers. Altogether, our study suggests that iterative forecasting improves week-to-week CH4 ebullition predictions, provides insight into predictability of ebullition rates into the future, and identifies which sources of uncertainty are the most important contributors to the total uncertainty in CH4 ebullition predictions.

Spatial and temporal heterogeneity of methane ebullition in lowland headwater streams and the impact on sampling design

Headwater streams are known sources of methane (CH4) to the atmosphere, but their contribution to global scale budgets remains poorly constrained. While efforts have been made to better understand diffusive fluxes of CH4 in streams, much less attention has been paid to ebullitive fluxes. We examine the temporal and spatial heterogeneity of CH4 ebullition from four lowland headwater streams in the temperate northeastern United States over a 2‐yr period. Ebullition was observed in all monitored streams with an overall mean rate of 1.00 ± 0.23 mmol CH4 m−2 d−1, ranging from 0.01 to 1.79 to mmol CH4 m−2 d−1 across streams. At biweekly timescales, rates of ebullition tended to increase with temperature. We observed a high degree of spatial heterogeneity in CH4 ebullition within and across streams. Yet, catchment land use was not a simple predictor of this heterogeneity, and instead patches scale variability weakly explained by water depth and sediment organic matter content and quality. Overall, our results support the prevalence of CH4 ebullition from streams and high levels of variability characteristic of this process. Our findings also highlight the need for robust temporal and spatial sampling of ebullition in lotic ecosystems to account for this high level of heterogeneity, where multiple sampling locations and times are necessary to accurately represent the mean rate of flux in a stream. The heterogeneity observed likely indicates a complex set of drivers affect CH4 ebullition from streams which must be considered when upscaling site measurements to larger spatial scales.

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.

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

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

Seasonal patterns in greenhouse gas emissions from lakes and ponds in a High Arctic polygonal landscape

AbstractLakes and ponds can be hotspots for CO2 and CH4 emissions, but Arctic studies remain scarce. Here we present diffusive and ebullition fluxes collected over several years from 30 ponds and 4 lakes formed on an organic‐rich polygonal tundra landscape. Water body morphology strongly affects the mixing regime—and thus the seasonal patterns in gas emissions—with ice‐out and autumnal turnover periods identified as hot moments in most cases. The studied thermokarst lake maintained relatively high ebullition rates of millennia‐old CH4 (up to 3405 14C YBP). Larger and deeper kettle lakes maintained low fluxes of both gases (century to millennium‐old), slowly turning into a CO2 sink over the summer. During winter, lakes accumulated CO2, which was emitted during the ice‐out period. Coalescent polygonal ponds, influenced by photosynthesizing benthic mats, were continuous CO2 sinks, yet important CH4 emitters (modern carbon). The highest fluxes were recorded from ice‐wedge trough ponds (up to 96 mmol CO2 equivalent m−2 d−1). However, despite clear signs of permafrost carbon inputs via active shore erosion, these sheltered ponds emitted modern to century‐old greenhouse gases. As the ice‐free period lengthens, scenarios of warmer and wetter conditions could favor both the production of CO2 and CH4 from thawing permafrost carbon, and CH4 production from recently fixed carbon through an atmospheric CO2‐to‐CH4 shunt at sites in which primary production is stimulated. This must be carefully considered at the landscape scale, recognizing that older carbon stocks can be mineralized efficiently in specific locations, such as in thermokarst lakes.

Significant methane ebullition from alpine permafrost rivers on the East Qinghai–Tibet Plateau

Inland waters are large sources of methane to the atmosphere. However, considerable uncertainty exists in estimating the emissions of this potent greenhouse gas from global streams and rivers due, in part, to a lack of direct measurements in the high-altitude cryosphere and poor accounting for ebullition. Here we present methane concentrations and fluxes over three years in four basins on the East Qinghai–Tibet Plateau. Methane ebullition rates decrease exponentially whereas diffusion declines linearly with increasing stream order. Nonetheless, the average ebullition rate (11.9 mmolCH4 m−2 d−1) from these streams and rivers—which have large organic stocks in surrounding permafrost, abundant cold-tolerant methanogens, shallow water depths, and experience low air pressure—were six times greater than the global average and reached a maximum of 374.4 mmolCH4 m−2 d−1. Upscaled total emissions from sampled third- to seventh-order waterways of the East Qinghai–Tibet Plateau are estimated to be 0.20 TgCH4 yr−1, 79% of which was attributed to ebullition. These methane emissions are approximately 20% of CO2 emissions (2.70 TgCO2 yr−1) in terms of carbon release and two times greater in terms of CO2-equivalent emissions. When upscaled to first- to seventh-order waterways, we estimate emissions of 0.37–1.23 TgCH4 yr−1. Our findings demonstrate that high-elevation rivers on the Qinghai–Tibet Plateau are hotspots of methane delivery to the atmosphere. The large ebullitive fluxes, which constitute a substantial fraction of global fluvial methane emissions, reveal a positive feedback between climate warming, permafrost thaw and methane emissions. High-elevation rivers in permafrost of the East Qinghai–Tibet Plateau are hotspots of methane emissions, according to measurements of methane fluxes in the region.

The Magnitude and Drivers of Methane Ebullition and Diffusion Vary on a Longitudinal Gradient in a Small Freshwater Reservoir

Key Points CH4 ebullition rates decreased by 98% and CH4 diffusion decreased by 32% on a longitudinal gradient from reservoir inflow to dam Ebullition was driven by physical variables upstream and phytoplankton downstream; diffusion was most related to phytoplankton Despite large variation in CH4 emissions longitudinally, time series models well captured the dynamics of emissions from a small reservoir

Spatial and temporal variability of free gas content in shallow sediments: Lake Kinneret as a case study

Organic-rich aquatic sediments are a significant source of methane to the atmosphere. In situ remote quantifications of gas content in shallow sediment is a complex task due to its large spatiotemporal heterogeneity. The spatial and multiannual changes of free gas (methane) content (Θ) in shallow sediments were studied in deep subtropical Lake Kinneret. We implemented recently developed acoustic methodology that allows estimating Θ in sediment based on assessment of sound speed. This method is based on measurement of reflection coefficient of acoustic signal at low-to-mid frequencies from the water-sediment interface using geoacoustic inversion technique. The used approach provides an assessment of the mean gas content over certain bottom area near the sampling location. Analysis of acoustic measurements carried out in 2015–2018 shows distinct changes in Θ with bottom depth along the offshore transects. We found the inverse relationship between Θ and lake level. The observed patterns in sedimentary gas content are supported by previous observations showing (a) changes of acoustic sediment properties with depth and (b) inverse relationship between rate of gas ebullition from bottom and lake level. The obtained absolute values of Θ at different locations were the same order of magnitude as those estimated directly from frozen cores. The validity and reliability of the method and its further development are discussed.

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.

Ebullition of oxygen from seagrasses under supersaturated conditions.

Gas ebullition from aquatic systems to the atmosphere represents a potentially important fraction of primary production that goes unquantified by measurements of dissolved gas concentrations. Although gas ebullition from photosynthetic surfaces has often been observed, it is rarely quantified. The resulting underestimation of photosynthetic activity may significantly bias the determination of ecosystem trophic status and estimated rates of biogeochemical cycling from in situ measures of dissolved oxygen. Here, we quantified gas ebullition rates in Zostera marina meadows in Virginia, U.S.A. using simple funnel traps and analyzed the oxygen concentration and isotopic composition of the captured gas. Maximum hourly rates of oxygen ebullition (3.0 mmol oxygen m−2 h−1) were observed during the coincidence of high irradiance and low tides, particularly in the afternoon when oxygen and temperature maxima occurred. The daily ebullition fluxes (up to 11 mmol oxygen m−2 d−1) were roughly equivalent to net primary production rates determined from dissolved oxygen measurements indicating that bubble ebullition can represent a major component of primary production that is not commonly included in ecosystem‐scale estimates. Oxygen content comprised 20–40% of the captured bubble gas volume and correlated negatively with its δ18O values, consistent with a predominance of mixing between the higher δ18O of atmospheric oxygen in equilibrium with seawater and the lower δ18O of oxygen derived from photosynthesis. Thus, future studies interested in the metabolism of highly productive, shallow water ecosystems, and particularly those measuring in situ oxygen flux, should not ignore the bubble formation and ebullition processes described here.

IAMES: An inexpensive, Automated Methane Ebullition Sensor.

Atmospheric concentrations of methane have increased ∼2.4 fold since the industrial revolution with wetlands and inland waters representing the largest source of methane to the atmosphere. Substantial uncertainties remain in global methane budgets, due in part to the lack of adequate techniques and detailed measurements to assess ebullition in aquatic environments. Here, we present details of a low cost (∼$120 US per unit) ebullition sensor that autonomously logs both volumetric ebullition rate and methane concentrations. The sensor combines a traditional funnel bubble trap with an Arduino logger, a pressure sensor, thermal conductivity methane sensor, and a solenoid valve. Powered by three AA batteries, the sensor can measure autonomously for three months when programmed for a sampling frequency of 30 min. For field testing, four sensors were deployed for six weeks in a small lake. While ebullition was spatially and temporally variable, a distinct diurnal trend was observed with the highest rates from mid-morning to early afternoon. Ebullition rates were similar for all four sensors when integrated over the sampling period. The widespread deployment of low cost automated ebullition sensors such as the iAMES described here will help constrain one of the largest uncertainties in the global methane budget.

Rhizosphere to the atmosphere: contrasting methane pathways, fluxes, and geochemical drivers across the terrestrial–aquatic wetland boundary

Abstract. Although wetlands represent the largest natural source of atmospheric CH4, large uncertainties remain regarding the global wetland CH4 flux. Wetland hydrological oscillations contribute to this uncertainty, dramatically altering wetland area, water table height, soil redox potentials, and CH4 emissions. This study compares both terrestrial and aquatic CH4 fluxes in permanent and seasonal remediated freshwater wetlands in subtropical Australia over two field campaigns, representing differing hydrological and climatic conditions. We account for aquatic CH4 diffusion and ebullition rates and plant-mediated CH4 fluxes from three distinct vegetation communities, thereby examining diel and intra-habitat variability. CH4 emission rates were related to underlying sediment geochemistry. For example, distinct negative relationships between CH4 fluxes and both Fe(III) and SO42- were observed. Where sediment Fe(III) and SO42- were depleted, distinct positive trends occurred between CH4 emissions and Fe(II) ∕ acid volatile sulfur (AVS). Significantly higher CH4 emissions (p &lt; 0.01) in the seasonal wetland were measured during flooded conditions and always during daylight hours, which is consistent with soil redox potential and temperature being important co-drivers of CH4 flux. The highest CH4 fluxes were consistently emitted from the permanent wetland (1.5 to 10.5 mmol m−2 d−1), followed by the Phragmites australis community within the seasonal wetland (0.8 to 2.3 mmol m−2 d−1), whilst the lowest CH4 fluxes came from a region of forested Juncus spp. (−0.01 to 0.1 mmol m−2 d−1), which also corresponded to the highest sedimentary Fe(III) and SO42-. We suggest that wetland remediation strategies should consider geochemical profiles to help to mitigate excessive and unwanted methane emissions, especially during early system remediation periods.

Wetland methane emissions dominated by plant‐mediated fluxes: Contrasting emissions pathways and seasons within a shallow freshwater subtropical wetland

AbstractWetlands represent the largest natural source of methane; however, very few studies have simultaneously quantified the three main atmospheric flux pathways (i.e., diffusive, ebullition, and plant‐mediated). Unlike better‐studied northern hemisphere systems, many Australian subtropical wetlands undergo extreme wet/dry oscillations, which may strongly impact methane dynamics. We assessed diurnal methane emissions of multiple pathways during three distinct seasonal events within an Australian freshwater wetland. Six‐fold higher methane emissions occurred during summer compared to autumn floods (which followed an extensive dry‐period) and winter/cool conditions. Over three seasons, diffusion represented the highest average areal fluxes (25.9 ± 73.2 mmol m−2 d−1) but were within range of fluxes through water lily aerenchyma (20.8 ± 41.5 mmol m−2 d−1). Average ebullition rates were 5.5 ± 9.7 mmol m−2 d−1. Water column CH4 displayed high spatiotemporal variability, ranging from 55.0 to 253.5 μmol L−1. Time series δ13C–CH4 isotope measurements revealed an oxidation fraction of ~ 15% at night‐time and ~ 36% during day‐time, and night‐time diffusive fluxes were consistently ~ three‐fold higher than day‐time fluxes. By aggregating seasons and weighting for changes in lily coverage, plant‐mediated fluxes accounted for ~ 59% of the annual methane emissions, whereas ebullition and diffusion each accounted for ~ 20%. The up‐scaled annual area‐weighted wetland methane flux (combined pathways) was 27.3 ± 36.7 mmol m−2 d−1. We contend that water lilies (Nymphaea sp.) are the significant carbon source, mediator, and conduit for methane fluxes in this system, and the extremely large seasonal variability of methane emissions reflect dynamic redox oscillations driven by oscillating wet and dry conditions.