Methane Ebullition Research Articles

Detection of methane ebullition in shelf waters of the Laptev Sea in the Eastern Arctic Region

Detection of methane ebullition in shelf waters of the Laptev Sea in the Eastern Arctic Region

The acoustic quantification of fish in the presence of methane bubbles in the stratified Lake Kinneret, Israel

Abstract Ostrovsky, I. 2009. The acoustic quantification of fish in the presence of methane bubbles in the stratified Lake Kinneret, Israel. – ICES Journal of Marine Science, 66: 1043–1047. Methane ebullition from bottom sediments is common in many productive bodies of water. During acoustic surveys, the methane bubbles can be mistaken as fish. In the stratified Lake Kinneret, the densities and target-strength (TS) values of acoustic targets were measured with a 120-kHz, dual-beam echosounder at night, when the fish schools were dispersed and single targets could be detected. The density and TS of methane bubbles were estimated in the anoxic hypolimnion, where there were no fish. A discrete-bubble model was applied in combination with a TS vs. methane bubble-size relationship to estimate the methane bubble TS at different depths. This information was used to quantify the densities of methane bubbles and fish, and the fish TS in the epilimnion where both types of target were found. During periods of rapid decrease in water level, methane bubbles could be the principal acoustic targets in the epilimnion, and their TS strongly overlapped that of fish. In some strata, they contributed up to 90–95% of all targets. The amount of methane bubbles in the water column depends on the type of sediment, bottom depth, and the history of sediment degassing. Their presence could be a serious obstacle to the accurate assessment of fish abundance in lakes and reservoirs with varying water levels, where intensive gas emission is a common feature. For such situations, backscattering by methane bubbles must be taken into account to avoid the erroneous quantification of fish abundance.

Hydroacoustic assessment of fish abundance in the presence of gas bubbles

Methane ebullition from bottom sediments is abundant in many productive water bodies. The occurrence of the methane bubbles in the water column may cause erroneous quantification of fish densities in aquatic ecosystems when using hydroacoustic methods, since the target strength (TS) of bubbles strongly overlaps with that of fish. In this study, an approach that allows separation of TS distributions of fish and bubbles is proposed. The method was applied to data collected in a shallow reservoir with a 120‐kHz echo sounder. Separation between fish and bubbles was based on the difference in their trajectories on echograms. To distinguish the target trajectories, the water column was sampled at low boat speed at restricted locations. Sampling was also carried out at high boat speed to assess the abundance and TS distribution of all targets in the entire reservoir. The information from the two different sampling modes was integrated in a simple mathematical algorithm, which allowed calculating bubble and fish densities in the reservoir. The total abundance of gas bubbles was higher than the density of fish. The elaborated method can be useful for quantification of dominant objects when spatial distributions are heterogeneous.

Tide‐driven deep pore‐water flow in intertidal sand flats

Sulfidic seeps with methane ebullition were observed at the low‐water line of intertidal sand flats at a number of locations in the Wadden Sea. Bioturbating fauna was absent in the seep areas but abundant in the more central areas of the tidal flat. At one site, the vertical methane and sulfate distribution in pore water was determined along transects from the low‐water line toward the interior of the sand flat. The resulting two‐dimensional distributions showed a plume of methane‐rich and sulfate‐depleted pore water reaching from a depth below 1.2 m beneath the sand surface up to the sediment surface at the low‐water line. The δ13C of methane released at the seeps was −68.6‰, indicating a biological origin. The 14C signature of methane was clearly elevated by anthropogenic radiocarbon, which shows that the methane was formed less than 50 yr ago. The observations indicate an internal circulation, where water enters the sand flats in the central area and exits at the low‐water line. Pore‐water flow patterns in the sand flat during the tidal cycle were calculated from the surface topography and from the pressure distribution at the flat surface across the tidal cycle. The calculated flow patterns explain the measured methane and sulfate distributions and predict a residence time of the seepage water of about 30 yr. Intertidal sand flats act as one‐way valves, passing water from the central surface through the interior of the flat to an outflow zone at and below the low‐water line with a velocity of millimeters to centimeters per day. The flow causes permeable tidal flats to emit methane to the surface water and atmosphere in substantial amounts.

Introduction to Water Use From Arctic Lakes: Identification, Impacts, and Decision Support1

Introduction to Water Use From Arctic Lakes: Identification, Impacts, and Decision Support¹

The Potential Use of Synthetic Aperture Radar for Estimating Methane Ebullition From Arctic Lakes1

Abstract: Arctic lakes are significant emitters of methane (CH4), a potent greenhouse gas, to the atmosphere; yet no rigorous quantification of the magnitude and variability of pan‐Arctic lake emissions exists. In this study, we demonstrate the potential for a new method using synthetic aperture radar (SAR) imagery to detect methane bubbles in lake ice to scale up whole‐lake measurements of CH4 ebullition (bubbling) to regional scales. We estimated ebullition from lakes, which is often the dominant mode of lake emissions, by mapping the distribution of bubble clusters frozen in early winter ice across surfaces of seven tundra lakes and one boreal forest lake in Alaska. Applying previously measured ebullition rates associated with four distinct classes of bubble clusters found in lake ice, we estimated whole‐lake emissions from individual lakes. The percent surface area of lake ice covered with bubbles (R2 = 0.68) and CH4 ebullition rates from lakes (R2 = 0.59) and were correlated with radar return values from RADARSAT‐1 Standard Beam mode 3 for the tundra lakes, suggesting that with appropriate scaling and consideration for variability in lake‐ice conditions, this technique has the potential to be used for estimating broader‐scale regional and pan‐Arctic lake methane emissions.

Quantifying gas ebullition with echosounder: the role of methane transport by bubbles in a medium‐sized lake

In lakes and reservoirs with variable water level, gas ebullition can play a substantial role in methane transport in the water column and to the atmosphere. However, measuring methane ebullition from sediment is difficult as releases are highly heterogeneous and intermittent on macro‐ and micro‐scales. In contrast to conventional gas traps and optical methods, hydroacoustic technology allows rapid scanning over large volumes of the water column synoptically to quantify gas bubble abundance. A 120‐kHz dual beam downward‐looking echosounder was used to measure the size distributions of bubbles that do not resonate at the sonar frequency. Data obtained with this sonar permit accurate calculation and evaluation of ebullition flux from the bottom. A robust relationship was established between gas volumes and backscattering cross‐section of individual bubbles in experimental conditions, and rise velocities of bubbles were precisely measured. The volume backscattering coefficient was shown to be a good gauge of the total volume of bubbles per cubic meter of water, allowing the use of a single‐beam sonar for measuring volumetric bubble concentrations. Data obtained from hydroacoustic surveys on Lake Kinneret, where gaseous methane is emitted from randomly dispersed sediment sources, indicated that ~90% of bubbles escaping from soft sediments ranged from 1.3 mm to 4.5 mm and ~50% ranged from 2.0 mm to 3.2 mm in equivalent radius. In summer‐fall 2001, the gaseous methane fluxes from hypolimnetic sediments was ~10 mmol m−2 d−1, accounting for one‐third of the observed methane accumulation in the hypolimnion. This relatively high ebullition rate could be attributed to the gradual decreasing of the mean water level in preceding years.

Falling atmospheric pressure as a trigger for methane ebullition from peatland

Peatlands are widely regarded as a significant source of atmospheric CH4, a potent greenhouse gas. At present, most of the information on environmental emissions of CH4 comes from infrequent, temporally discontinuous ground‐based flux measurements. Enormous efforts have been made to extrapolate measured emission rates to establish seasonal or annual averages using relevant biogeochemical factors, such as water table positions or peat temperatures, by assuming that the flux was stationary during a substantial nonsampling period. However, this assumption has not been explicitly verified, and little is known about the continuous variation of the CH4 flux in a timescale of individual flux measurement. In this study, we show an abrupt change in the CH4 emission rate associated with falling atmospheric pressure. We found that the CH4 flux can change by 2 orders of magnitude within a matter of tens of minutes owing to the release of free‐phase CH4 triggered by a drop in air pressure. The contribution of the ebullition to the total CH4 flux during the measurements was significant (50–64%). These results clearly indicated that field campaigns must be designed to cover this rapid temporal variability caused by ebullition, which may be especially important in intemperate weather. Process‐based CH4 emission models should also be modified to include air pressure as a key factor for the control of ebullient CH4 release from peatland.

Extreme event dynamics in methane ebullition fluxes from tropical reservoirs

Tropical hydroelectric reservoirs generally constitute an appreciable source of CH4 (methane), a potent greenhouse gas. In this letter, we investigate the statistical characteristics of methane ebullition fluxes in hydroelectric reservoirs. To this end, we use CH4 flux measurements obtained in Manso (wet season, 2004) and Corumbá (dry and wet seasons, 2005) reservoirs, located respectively in Mato Grosso and Goiás, Brazil. Methane ebullition fluxes were measured using open dynamic chambers, connected to an infrared photo‐acoustic trace gas analyzer (TGA). Our main result indicates that when properly rescaled, all methane ebullition data collapse into a single statistic well described by a Generalized Pareto distribution, with shape parameter well above zero. The approach presented here, which combines high‐frequency CH4 ebullition data and Extreme Value theory analytical tools, shows that, although bubbling patterns appear to be highly complex and unpredictable, they may still be described by a rather simple (but non trivial) dynamics.

Effect of temperature and atmospheric pressure on methane (CH4) ebullition from near‐surface peats

Recent studies suggest that ebullition of biogenic gas bubbles is an important process of CH4 transfer from northern peatlands into the atmosphere and, as such, needs to be better described by models of peat carbon dynamics. We develop and test a simple ebullition model in which a threshold gas volume in the peat has to be exceeded before ebullition occurs. The model assumes that the gas volume varies because of gas production and variations in pressure and temperature. We incubated peat cores in the laboratory for 190 days and measured their volumetric gas contents and the ebullition flux. The laboratory results support the threshold concept and, considering the simplicity of the model, the calculated ebullition compared well with measured fluxes during the final 120 days with an r2 of 0.66. An improved, more realistic description would also include temporal and spatial variations in gas production and bubble retention terms.

Cultivation of methanogens from shallow marine sediments at Hydrate Ridge, Oregon

Little is known about the methanogenic degradation of acetate, the fate of molecular hydrogen and formate or the ability of methanogens to grow and produce methane in cold, anoxic marine sediments. The microbes that produce methane were examined in permanently cold, anoxic marine sediments at Hydrate Ridge (44 degrees 35' N, 125 degrees 10' W, depth 800 m). Sediment samples (15 to 35 cm deep) were collected from areas of active methane ebullition or areas where methane hydrates occurred. The samples were diluted into enrichment medium with formate, acetate or trimethylamine as catabolic substrate. After 2 years of incubation at 4 degrees C to 15 degrees C, enrichment cultures produced methane. PCR amplification and sequencing of the rRNA genes from the highest dilutions with growth suggested that each enrichment culture contained a single strain of methanogen. The level of sequence similarity (91 to 98%) to previously characterized prokaryotes suggested that these methanogens belonged to novel genera or species within the orders Methanomicrobiales and Methanosarcinales. Analysis of the 16S rRNA gene libraries from DNA extracted directly from the sediment samples revealed phylotypes that were either distantly related to cultivated methanogens or possible anaerobic methane oxidizers related to the ANME-1 and ANME-2 groups of the Archaea. However, no methanogenic sequences were detected, suggesting that methanogens represented only a small proportion of the archaeal community.

Physical Controls on Methane Ebullition from Reservoirs and Lakes

Understanding the nature and extent of methane production and flux in aquatic sediments has important geochemical, geotechnical, and global climate change implications. Quantifying these processes is difficult, because much of the methane flux in shallow sediments occurs via ebullition (bubbling). Direct observation of bubble formation is not possible, and bubbling is episodic and dependent upon a number of factors. Whereas previous studies have correlated methane flux with surface wind intensity, detailed study of Lake Gatun in Panama and Lago Loiza in Puerto Rico suggest that methane flux is more closely correlated with the shear stress in sediments caused by bottom currents. Bottom currents in turn are a complex function of wind, internal pressure gradients, and lake bathymetry. A simple physical model of bottom currents and sediments in these lakes suggests that most methane ebullition originated from the upper 10–20 cm of the sediment column. Our data reaffirm previous studies showing that ebullitive methane flux is minor in water deeper than ∼5 m.

Methane bubbles in Lake Kinneret: Quantification and temporal and spatial heterogeneity

The amount of methane bubbles rising from the bottom of Lake Kinneret was quantified by using a dual‐beam echo sounder. Both echo‐counting (EC) and echo integration (EI) techniques were implemented. Bubbles can confound the identification of fish targets because their acoustic sizes strongly overlap. Analysis of vertical changes in densities in the anoxic hypolimnion (with no fish) indicated that multiple targets were most abundant near the bottom, which caused an essential bias of bubble density estimates by EC. EI was a reliable tool for assessment of bubble density near the bottom. In the upper part of the water column, both techniques provided similar estimates of density of targets (mainly bubbles) because they were measured between dense fish schools during the daytime. The mean acoustic size of rising bubbles decreased in the hypolimnion but increased in the epilimnion, suggesting change in the relative importance of factors controlling gas volume. In summer and fall 2001, the bubbles became predominant echo‐reflecting objects in the epilimnion. Temporal and spatial changes in bubble densities were highly heterogeneous, suggesting strong variability in factors affecting the gas ebullition. This variability should be taken into consideration when attempting to quantify the methane ebullition and assessing fish abundance in aquatic systems.

Redox processes in recent sediments of the river Meuse, The Netherlands

Pore-water concentrations of inorganic solutes were measured at four locations in a recent sedimentation area of the river Meuse in The Netherlands. The pore-water concentration profiles were interpreted using the steady state one-dimensional reaction/transport model STEADYSED1. This model explicitly accounts for the organic matter degradation pathways and secondary redox reactions. Results show that the model reproduces the measured pore-water profiles of redox species reasonably well, although significant divergence is observed for pH. The latter is due to the absence of pH buffering by CaCO3 in the model. At all locations, methanogenesis is the major pathway of organic matter degradation below 3 cm from the sediment-water interface. However, organic matter degradation rates by methanogenesis may be overestimated, because methane ebullition is not included. Differences in profiles of redox-sensitive ions among the four locations are explained by differences in depositional conditions, in particular the sediment accumulation rate and supply of organic matter.

Trapped methane volume and potential effects on methane ebullition in a northern peatland

A novel way of estimating the gas bubble volume in the floating mat sediment of a peatland was developed at Thoreau’s Bog in Concord, Massachusetts. Statistically significant relationships between the buoyancy of the floating Sphagnum mat and atmospheric pressure were observed, and these relationships were used to estimate the gas bubble volume. The mass of CH4 stored in gas bubbles is estimated to be as much as 3 times the mass of dissolved CH4, depending on the time of year. The gas bubble volume is frequently large enough to serve as a significant buffer between microbial production of CH4 and the release of CH4 to the atmosphere. Changes in atmospheric pressure, temperature, and water‐table elevation may result in modulation of the ebullitive CH4 flux. Periods of rapidly rising atmospheric pressure or equivalent pressure changes due to water‐table elevation are capable of arresting bubble volume growth, thereby halting CH4 ebullition. Periods of rapid cooling of the bog could also temporarily halt ebullition, as thermally induced contraction of bubbles and dissolution of CH4 offset bubble volume growth due to methanogenesis.

Simulation of the effect of methane bubble plumes on vertical mixing in Mono Lake

A one-dimensional vertical mixing model modified for application to hypersaline Mono Lake reproduced mixed layer dynamics well but hypolimnetic heating was underestimated. One possible source of increased hypolimnetic heating is vertical mixing caused by bubble plumes of methane rising from the sediments. Estimates of vertical mixing from methane seepage in Mono Lake were made with the inclusion of a bubble plume algorithm. A methane ebullition rate three hundred times greater than the maximum estimate for Mono Lake was required to simulate the observed hypolimnetic heating.

Methane emissions from the Amazon Floodplain: Characterization of production and transport

Methane production, transport and emission in a floodplain lake in central Amazonia were investigated by isotopic studies and gas exchange measurements. Samples of sediment free gas were depleted in δ13CCH4, δ13DCH4,and δ13CCO2 values. The isotopic composition of the sediment free methane clearly demonstrated a methane production by methyl fermentation. This finding was strengthened by the coexisting δ13CCO2 and δ13CCO2 values in the sediment free gas. The flux rates of methane ebullition and diffusion were measured during a complete annual cycle using the static chamber method. Significant differences were observed in the release of methane from individual vegetation types, i.e., phytoplankton, floating grass mats, and flooded forest. Each vegetation type showed a distinct seasonal pattern. The highest ebullition rates (mean value, 69 mg CH4 m−2d−1) were recorded in the flooded forest, covering the higher areas of the floodplains with a long subaerial period. Significantly lower averages of the gas bubble flux were recorded in the permanently aquatic areas of the lake (mean value, 29 mg CH4 m−2d−1) and in the intermediate area with floating grass mats (mean value, 23 mg CH44 m−2d−1. Ebullition was the predominant mechanism for the methane transport from the varzea sediment into the atmosphere with maximum values of up to 200 mg CH4 m−2d−1. The diffusive flux remained below 29 mg CH4 m−2d−1 at all sites throughout the entire annual cycle. The variation of the ebullutive flux was found to determine the spatial and temporal variation of the total methane flux in the varzea. We estimate that ebullition accounts for 80% of the total methane emission from the varzea.

Biogeochemistry of billabong sediments. I. The effect of macrophytes

SUMMARY. We examined the effects of an emergent macrophyte (Eleocharis sphacelata R. Br., Cyperaceae) and a submerged macrophyte (Vallisneria gigantea Graeb., Hydrocharitaceae) on the biogeochemistry of the sediments of a billabong in south‐eastern Australia. Sediments from an E. sphacelata bed had significantly lower concentrations of exchangeable phosphorus than did sediments from a nearby bare area or a V. gigantea bed, but neither macrophyte had a measureable effect on their sediment's exchangeable ammonium content. The redox potential in the upper 10cm of E. sphacelata sediments was about 100 mV higher than that of bare sediments, or of sediments colonized by V. gigantea. There were few consistent differences between vegetated and bare sediments in terms of the activity of extracellular enzymes, such as α‐amylase, protease, β‐d glucosidase, lipase or alkaline phosphatase. Rates of alkaline phosphatase activity (235–306μmol (g dry wt)−1 day−1) were markedly higher than those commonly reported for sediments or soils. Rates of gas release were higher from bare sediments (21–93 ml m−2 h−1) than from E. sphacelata or V. gigantea sediments (17–23 and 21‐24ml m−2 h−1, respectively). Gas bubbles consisted mainly of methane (26–66%) and nitrogen (15–68%). Rates of methane ebullition varied from 5 to 60ml m−2 h−1. In‐vitro methanogenesis was most rapid in samples of the upper flocculent sediment. Methanogenesis was slower in V. gigantea sediments than in bare area or E. sphacelata sediments, but was markedly accelerated by additions of acetate and/or H2/CO2 in all sites. Profiles of total extractable fatty acids and phospholipid fatty acids demonstrated that material derived from higher plants dominated the sediment organic matter in all sites. Bacteria were also a significant component of sediment organic matter, as fatty acids for which bacteria can be assumed the sole source accounted for 18–30% of total fatty acid content. Biomarkers for sulphate‐reducing bacteria (Desulfobacter spp.) were detected, and for type II methanotrophic bacteria.

Methanogenesis from Methylated Amines in a Hypersaline Algal Mat

Methane ebullition and high rates of methane production were observed in sediments of a hypersaline pond (180 per thousand) which contained sulfate in excess of 100 mM. The highest rates of methane production were observed in surface sediments associated with an algal mat dominated by a Spirulina sp. The mat contained a methylated amine, glycine betaine (GBT), at levels which accounted for up to 20% of the total mat nitrogen. GBT was apparently the source of trimethylamine (TMA), which was also present in the sediment at relatively high concentrations. Patterns of substrate metabolism by the methanogenic populations in sediment slurries suggested that TMA was a major methane precursor. Neither exogenous hydrogen nor acetate stimulated methanogenesis, while addition of a variety of amines including TMA, trimethylamine oxide, GBT, and choline resulted in substantial increases with yields of >70%. The temperature optimum for methanogenesis in this system was 45 to 55 degrees C, which coincided with the observed sediment temperature. Patterns and rates of methane production in this and other hypersaline algal mats may be determined by a complex interaction between salinity, the use of methylated amines for osmoregulation by algae, and the formation of TMA by fermentation.