Methane Concentrations In Surface Waters Research Articles

Identifying Drivers Behind Spatial Variability of Methane Concentrations in East Siberian Ponds

Waterbody methane emissions per area are negatively correlated with the size of the emitting waterbody. Thus, ponds, defined here as having an area smaller than 8 · 104m2, contribute out of proportion to the aquatic methane budget compared to the total area they cover and compared to other waterbodies. However, methane concentrations in and methane emissions from ponds show more spatial variability than larger waterbodies. We need to better understand this variability to improve upscaling estimates of freshwater methane emissions. In this regard, the Arctic permafrost landscape is an important region, which, besides carbon-rich soils, features a high pond density and is exposed to above-average climatic warming. We studied 41 polygonal-tundra ponds in the Lena River Delta, northeast Siberia. We collected water samples at different locations and depths in each pond and determined methane concentrations using gas chromatography. Additionally, we collected information on the key properties of the ponds to identify drivers of surface water methane concentrations. The ponds can be categorized into three geomorphological types with distinct differences in drivers of methane concentrations: polygonal-center ponds, ice-wedge ponds and larger merged polygonal ponds. All ponds are supersaturated in methane, but ice-wedge ponds exhibit the highest surface water concentrations. We find that ice-wedge ponds feature a strong stratification due to consistently low bottom temperatures. This causes surface concentrations to mainly depend on wind speed and on the amount of methane that has accumulated in the hypolimnion. In polygonal-center ponds, high methane surface concentrations are mostly determined by a small water depth. Apart from the influence of water depth on mixing speed, water depth controls the overgrown fraction, the fraction of the pond covered by vascular plants. The plants provide labile substrate to the methane-producing microbes. This link can also be seen in merged polygonal ponds, which furthermore show the strongest dependence on area as well as an anticorrelation to energy input indicating that stratification influences the surface water methane concentrations in larger ponds. Overall, our findings underpin the strong variability of methane concentrations in ponds. No single driver could explain a significant part of the variability over all pond types suggesting that more complex upscaling methods such as process-based modeling are needed.

Methane fluxes from tropical coastal lagoons surrounded by mangroves, Yucatán, Mexico

AbstractMethane concentrations in the water column and emissions to the atmosphere were determined for three tropical coastal lagoons surrounded by mangrove forests on the Yucatán Peninsula, Mexico. Surface water dissolved methane was sampled at different seasons over a period of 2 years in areas representing a wide range of salinities and anthropogenic impacts. The highest surface water methane concentrations (up to 8378 nM) were measured in a polluted canal associated with Terminos Lagoon. In Chelem Lagoon, methane concentrations were typically lower, except in the polluted harbor area (1796 nM). In the relatively pristine Celestún Lagoon, surface water methane concentrations ranged from 41 to 2551 nM. Methane concentrations were negatively correlated with salinity in Celestún, while in Chelem and Terminos high methane concentrations were associated with areas of known pollution inputs, irrespective of salinity. The diffusive methane flux from surface lagoon water to the atmosphere ranged from 0.0023 to 15 mmol CH4 m−2 d−1. Flux chamber measurements revealed that direct methane release as ebullition was up to 3 orders of magnitude greater than measured diffusive flux. Coastal mangrove lagoons may therefore be an important natural source of methane to the atmosphere despite their relatively high salinity. Pollution inputs are likely to substantially enhance this flux. Additional statistically rigorous data collected globally are needed to better consider methane fluxes from mangrove‐surrounded coastal areas in response to sea level changes and anthropogenic pollution in order to refine projections of future atmospheric methane budgets.

Spatial variations in surface water methane super-saturation and emission in Lake Lugano, southern Switzerland

We measured methane concentrations in the surface water of the northern basin of Lake Lugano in spring (May 2012) and autumn (October 2011, 2012), and calculated turbulent diffusive methane fluxes to the atmosphere. Surface water methane concentrations were highly variable in space and time but always exceeded atmospheric equilibrium. Methane concentrations were significantly lower in spring (on average 16 nmol L−1) than during the autumn sampling campaigns (on average 57 nmol L−1 in 2011 and 45 nmol L−1 in 2012). This suggests methane accumulation in the surface mixed layer during the summer productive season. The origin of the methane in the lake’s surface waters requires further assessment, but the observed concentration profiles indicate that the excess methane originates from a near-surface source, rather than from the large deep-water methane pool in the anoxic monimolimnion. As a consequence of the higher surface water methane concentrations and increased buoyancy turbulence caused by autumnal cooling of the surface boundary layer, diffusive fluxes were much higher in October (average ~97 μmol m−2 day−1, compared to 7 μmol m−2 day−1 in May 2012). The increase in methane concentration in the surface water between spring and autumn suggests links between methane accumulation and the annual biological cycle, yet seasonal changes in wind and temperature forcing of methane emission likely play an important modulating role. While the relative importance of biological versus physical controls on methane emission in Lake Lugano awaits further investigations, our study underscores that lakes can act as an important source of methane to the atmosphere, even when the lake-internal microbial methane filter in the water column seems to work efficiently.

A water column study of methane around gas flares located at the West Spitsbergen continental margin

In the Arctic Seas, the West Spitsbergen continental margin represents a prominent methane seep area. In this area, free gas formation and gas ebullition as a consequence of hydrate dissociation due to global warming are currently under debate. Recent studies revealed shallow gas accumulation and ebullition of methane into the water column at more than 250 sites in an area of 665km2. We conducted a detailed study of a subregion of this area, which covers an active gas ebullition area of 175km2 characterized by 10 gas flares reaching from the seafloor at~245m up to 50m water depth to identify the fate of the released gas due to dissolution of methane from gas bubbles and subsequent mixing, transport and microbial oxidation.The oceanographic data indicated a salinity-controlled pycnocline situated ~20m above the seafloor. A high resolution sampling program at the pycnocline at the active gas ebullition flare area revealed that the methane concentration gradient is strongly controlled by the pycnocline. While high methane concentrations of up to 524nmolL−1 were measured below the pycnocline, low methane concentrations of less than 20nmolL−1 were observed in the water column above. Variations in the δ13CCH4 values point to a 13C depleted methane source (~−60‰ VPDB) being mainly mixed with a background values of the ambient water (~−37.5‰ VPDB). A gas bubble dissolution model indicates that ~80% of the methane released from gas bubbles into the ambient water takes place below the pycnocline. This dissolved methane will be laterally transported with the current northwards and most likely microbially oxidized in between 50 and 100 days, since microbial CH4 oxidation rates of 0.78nmold−1 were measured. Above the pycnocline, methane concentrations decrease to local background concentration of ~10nmolL−1.Our results suggest that the methane dissolved from gas bubbles is efficiently trapped below the pycnocline and thus limits the methane concentration in surface water and the air–sea exchange during summer stratification. During winter the lateral stratification breaks down and fractions of the bottom water enriched in methane may be vertically mixed and thus be potentially an additional source for atmospheric methane.

Methane in the Sevastopol coastal area, Black Sea

Concentrations of dissolved methane in seawater and bottom sediments, as well as of methane emanating from gas seeps were measured at 18 stations including several small bays in the Sevastopol coastal area (Black Sea) during 2007–2008. Methane concentrations in surface waters ranged from 10 to 2,970 nmol l−1, and correlated well with values recorded for sediments. Methane concentrations in the water column were influenced by water depth, as well as by air and water temperatures. In the spring and summer of 2008, in situ CH4 saturation relative to air was in the range of 970–71,900%. Maximum saturation was in summer. CH4 fluxes to the atmosphere from the Sevastopol coastal area were estimated to vary from 190 to 1,550 μmol m−2 day−1. Gas bubbles escaping from the seepages contained about 57 vol% methane. Radiocarbon dating of the methane revealed an age not exceeding 150 years, implying a biogenic origin.

Distribution, source and flux of methane in the western Pearl River Estuary and northern South China Sea

The distribution of methane in the western Pearl River Estuary (PRE) and northern South China Sea (SCS) was investigated during an oceanic cruise in September, 2006. Results show that the methane concentration in surface water ranged from 6.9 to 173.7 nmol L − 1 at the 27 stations sampled in the western PRE. The methane saturation ratio ranged from 329 to 7896%, and the average sea-to-air methane flux was about 63.5 ± 32.2 μmol m − 2 d − 1 . The methane concentration in surface water was high in the western margin of the upper estuary, near marshes and shoals. The methane concentration decreased rapidly from the estuarine margin to the open coastal ocean, resulting from salinity (sulfate) increase in the overlying water, increasing water depth, diffusion across the air–sea interface and dilution of seawater during estuarine mixing. The major methane source in the PRE is thought to be methanogenic degradation in organic-enriched sediments. In the northern SCS, the concentration of methane varied from 2.4 to 5.9 nmol L − 1 in surface water at the 55 testing stations. The methane saturation ratio was between 134 and 297 %, and the sea-to-air methane flux was estimated to be 15.6 ± 8.0 μmol m − 2 d − 1 . Two stations located at E7 (15° 59.405″; 17° 59.165″) and E6 (115° 0.065″; 17° 0.032″) showed abnormal methane concentrations of 9.5 and 13.8 nmol L − 1 in surface water, respectively, and were probably affected by pollution from allochthonous oil. A subsurface methane maximum (4–5 nmol L − 1 ) occurred in water that was 150–200 m deep within this area. This maximum may be due to accumulation of plankton productive particulates and limited sea–air exchange in the sea surface layer. Abnormally high methane values of 4.3 and 8.7 nmol L − 1 were measured in the bottom waters of stations B8 and E4, respectively. These can be interpreted as being due to methane seeps from seafloor sediments.

Considerable methane fluxes to the atmosphere from hydrocarbon seeps in the Gulf of Mexico

The flux of methane—a greenhouse gas—from submarine hydrocarbon seeps to the atmosphere is not well quantified. Direct measurements of methane concentrations and isotopic depth profiles in deepwater hydrocarbon plumes indicate that a significant amount of methane from deep-ocean sources could reach the surface ocean. The fluxes of the greenhouse gas methane from many individual sources to the atmosphere are not well constrained1. Marine geological sources may be significant2, but they are poorly quantified and are not included in the Intergovernmental Panel on Climate Change budget1. Previous results based on traditional indirect sampling techniques and modelling suggested bubble plumes emitted from marine seeps at depths greater than 200 m do not reach the surface mixed layer because of bubble dissolution and methane oxidation3,4,5. Here we report methane concentration and isotope-depth profiles from direct submersible sampling of deepwater (550–600 m) hydrocarbon plumes in the Gulf of Mexico. We show that bubble size, upwelling flows and the presence of surfactants inhibit bubble dissolution, and that methane oxidation is negligible. Consequently, methane concentrations in surface waters are up to 1,000 times saturation with respect to atmospheric equilibrium. We estimate that diffusive atmospheric methane fluxes from individual plumes are one to three orders of magnitude greater than estimates from shallow-water seeps6,7,8, greatly expanding the depth range from which methane seep emissions should be considered significant. Given the widespread occurrence of deepwater seeps, we suggest that current estimates of the global oceanic methane flux to the atmosphere1 may be too low.

Sources and emission of greenhouse gases in Danube Delta lakes

Production of methane and carbon dioxide as well as methane concentrations in surface waters and emissions to the atmosphere were investigated in two flow-through lake complexes (Uzlina-Isac and Puiu-Rosu-Rosulet) in the Danube Delta during post-flood conditions in May and low water level in September 2006. Retained nutrients fueled primary production and remineralization of bioavailable organic matter. This led to an observable net release of methane, particularly in the lakes Uzlina, Puiu and Rosu in May. Input from the Danube River, from redbuds and benthic release contributed to CH(4) concentrations in surface waters. In addition to significant river input of CO(2), this trace gas was released via aerobic remineralization within the water column and in top sediments. Emission patterns of CO(2) widely overlapped with those of CH(4). Generally, greenhouse gas emissions peaked in the lake complex adjacent to the Danube River in May due to strong winds and decreased with increasing hydrological distance from the Danube River. Intense remineralization of organic matter in the Danube Delta lakes results in a net source of atmospheric greenhouse gases.

Excess of bottom-released methane in an Arctic shelf sea polynya in winter

Latent heat polynyas are regions generating strong ice formation, convection and extensive water mass formation. Here we report on the effects of these processes on resuspension of sediments and subsequent methane release from the seafloor and on the resulting excess methane concentration in surface water on a polar shelf during winter. The study is based on measurements of concentration and δ 1 3C values of methane, water temperature, salinity, light transmission and sea ice data collected in March 2003 in Storfjorden, southern Svalbard. In winter, strong and persistent northeasterly winds create polynyas in eastern Storfjorden and cause ice formation. The resulting brine-enriched water cascades from the Storfjordbanken into the central depression thereby enhancing the turbulence near the seafloor. A distinct benthic nepheloid layer was observed reflecting the resuspension of sediments by the cascading dense bottom water. High concentrations of 13C-depleted methane suggest submarine discharge of methane with the resuspended sediments. As the source of the submarine methane, we propose recent bacterial methanogenesis near the sediment surface because of extremely high accumulation rates of organic carbon in Storfjorden. Convective mixing transports newly released methane from the bottom to the sea surface. This eventually results in an excess concentration in surface water with respect to the atmospheric equilibrium, and a sea-air flux of methane during periods of open water. When a new ice cover is formed, methane becomes trapped in the water column and subsequently oxidized. Thus, the residual methane is strongly enriched in 13C in relation to the δ 13 C CH 4 signature of atmospheric methane. Our results show that latent heat polynyas may induce a direct pathway for biogases like methane from sediments to the atmosphere through coupling of biogeochemical and oceanographic processes. Extrapolating these processes to all Arctic ocean polynyas, we estimate a transfer of CH 4 between 0.005 and 0.02 Tg yr −1. This is not a large contribution but the fluxes from the polynyas are 20–200 times larger than the ocean average and the methane evasion process in polynyas is certainly one that can be altered under climate change.

Methane and oxygen dynamics in a shallow floodplain lake: the significance of periodic stratification

Temperature, dissolved oxygen and dissolved methane profiles were measured during autumn and summer, in a shallow floodplain lake in south-eastern Australia to determine the effects of water-column stability on methane and oxygen dynamics. The water column was well mixed in autumn. Strong thermal stratification developed in the late afternoon in summer, with top-to-bottom temperature differences of up to ∼6 °C. Methane concentrations in surface waters varied over a daily cycle by an 18-fold range in summer, but only by a 2-fold range in autumn. The implication of short-term temporal variation is that static chambers deployed on the water surface for short times (less than a day) in summer will significantly underestimate the diffusive component of methane emissions across the water–atmosphere interface. There was a marked diel variation in dissolved oxygen concentrations in summer, with the highest oxygen values (commonly 5–8 mg l−1) occurring in the surface waters in late afternoon; the bottom waters were then devoid of oxygen (< 0.2 mg l−1). Because of high respiratory demands, even the surface water layers could be nearly anoxic by morning in summer. The concentration of dissolved oxygen in the surface waters was always less than the equilibrium value. When the water column became thermally stratified in summer, the dissolved oxygen and methane maxima were spatially separated, and planktonic methanotrophy would be limited to a moving zone, at variable depth, in the water column. In summer the whole-wetland rates of oxygen production and respiration, calculated from long-term (∼5 h) shifts in dissolved oxygen concentrations over a diel period, were approximately 6–10 and 3–6 mmol m−3 h−1, respectively. These values correspond to net and gross primary production rates of ∼0.7–1.2 and ∼1.0–1.9 g C m−3 day−1, respectively.

Methane emissions from an Amazon floodplain lake: Enhanced release during episodic mixing and during falling water

Methane fluxes to the troposphere were measured inLake Calado, a dendritic floodplain lake located inthe central Amazon Basin. Methane concentrations inthe surface water of the lake were less than 0.5 µM during a period of high and rising water inApril and May, except when episodic, deep mixingoccurred and surficial concentrations reached as highas 4.8 µM. Diffusive fluxes ranged from ca.2–20 mg CH4 m−2d−1 (mean: 6.6 mg CH4 m−2d−1) normally, but increased up to 220 mg CH4 m−2d−1 during the passageof a rare cold front. During six weeks of risingwater, the volume-weighted methane content of the lakeincreased during periods of stable thermalstratification (accumulation rates ranging from16–1411 mg CH4 m−2d−1) and decreasedduring episodes of partial mixing (rates of decreaseranging from 63–1792 mg CH4 m−2d−1). Surface water methane concentrations during low andfalling water in September varied by more than anorder of magnitude (range, 0.3–9 µM), anddiffusive fluxes ranged from 3–158 mg CH4m−2d−1 (mean, 54 mg CH4 m−2d−1). During falling water in September, total methane flux from the lake to the troposphere measured with a floating chamber rangedfrom 53–328 mg CH4 m−2d−1 (mean, 163 mgCH4 m−2d−1). Ebullitive flux averaged 69% of total flux. Total methane flux was highest when thermal stratification was lost due to decreasing lake depth.

Methane in surface waters of Oregon estuaries and rivers1

Methane concentrations in surface waters of Oregon rivers and estuaries were measured over a 4‐yr period. Geographic variations in riverine CH4 were observed. Results from undisturbed forest streams indicate that rivers can contain high natural levels of CH4 not attributable to pollution. Lateral diffusion and runoff from saturated forest and fertilized agricultural soils may be important in determining methane levels in rivers. Methane concentrations in well‐flushed estuaries appear to be controlled mainly by mixing between high CH4‐containing river water and low CH4‐containing seawater endmembers. Rivers and estuaries were found to be sources of methane to the atmosphere. Calculated daily fluxes to the atmosphere ranged from 1.2 to 71 mg CH4 m−2 for rivers and from 0.04 to 21 mg CH4 m−2 for estuarine samples.