Sulfate reduction and methanogenesis in the Shira and Shunet meromictic lakes (Khakasia, Russia)

The biogeochemical and molecular biological study of the chemocline and sediments of saline meromictic lakes Shira and Shunet (Khakasia, Russia) was performed. A marked increase in the rates of sulfate reduction and methanogenesis was revealed at the medium depths of the chemocline. The rates of these processes in the bottom sediments decreased with depth. The numbers of the members of domains Bacteria, Archaea, and of sulfate-reducing bacteria (SRB) were determined by fluorescence in situ hybridization with rRNA specific oligonucleotide probes labeled with horseradish peroxidase and subsequent tyramide signal amplification. In the chemocline, both the total microbial numbers and those of Bacteria were shown to increase with depth. The archaea and SRB were present in almost equal numbers. In the lake sediments, a drastic decrease in microbial numbers with depth was revealed. SRB were found to prevail in the upper sediment layer and archaea in the lower one. This finding correlated with the measured rates of sulfate reduction and methanogenesis.     

Vertical distribution and rotifer concentrations in the chemocline of meromictic lakes

The vertical distribution of planktonic rotifers has been analysed in relation to season in several meromictic lakes; a coastal lagoon with sea-water intrusion and three dissolution lakes from two karstic systems. Two species, Filinia hofmanni and a form of Anuraeopsis fissa have been found to be more or less restricted to the chemocline or adjacent strata any time they occurred. Many species common in the upper water layers developed large populations near or in the chemocline and more strikingly in summer. Some species had two vertical maxima (one in the surface or the thermocline and another near the chemocline), while others successively shifted their maxima between the upper layers and the chemocline. It is hypothetized that these rotifers are either very versatile or are differentiated as ecotypes, one of them adapted to the chemocline environment. This distribution in a peculiar fluctuating, anoxic, H₂S-rich environment poses questions about the biology of those rotifers which there develop extraordinary populations.     

Photosynthetic bacteria in meromictic lakes and stratified fjords of the Vestfold Hills,Antarctica

Thirteen meromictic lakes and two permanently stratified fjords in the Vestfold Hills, Antarctica, were surveyed in 1983 for photosynthetic bacteria. Burton Lake and Ellis Fjord were sampled throughout the year to determine seasonal variations. Physical and chemical parameters were recorded and related to the species present. The dominant species in waters with salinities of 100.7 g kg^–1 were Chlorobium vibrioforme and Chlorobium limicola with populations at the O₂–H₂S interface in the range 0.3 to 6.7 × 10⁶ ml^–1. Neither of these species was found at higher salinities. Thiocapsa roseopersicina and a Chromatium sp. were found in low numbers (< 10⁵ ml^–1) in most of the same waters as the Chlorobium spp. These bacterial phototrophs developed in a narrow band below the O₂–H₂S interface where both light and H₂S were available. Very low numbers (< 10² ml^–1) of Rhodopseudomonas palustris were found in both oxic and anoxic waters having salinity 148 g kg^–1. The dominance of the Chlorobium spp. is ascribed to their more efficient maintenance metabolism during the darkness, their faster growth at low light intensities (< 1 µE m^–2 s^–1) and the lack of selective filtering of incident light. The Chlorobium spp. grew well at –2 °C, but not –5°C in hypersaline waters. The concentration of H₂S had no apparent effect on the development of the bacterial flora. Viable cells were found to depths of 100 m in Ellis Fjord indicating that viability in total darkness could have been maintained for periods of the order of 1700 days.     

Seasonal changes in the chemistry and biology of a meromictic lake (Big Soda Lake,Nevada, U.S.A.)

Big Soda Lake is an alkaline, saline lake with a permanent chemocline at 34.5 m and a mixolimnion that undergoes seasonal changes in temperature structure. During the period of thermal stratification, from summer through fall, the epilimnion has low concentrations of dissolved inorganic nutrients (N, Si) and CH₄, and low biomass of phytoplankton (chlorophyll a ca. 1 mgm ^-3). Dissolved oxygen disappears near the compensation depth for algal photosynthesis (ca. 20 m). Surface water is transparent so that light is present in the anoxic hypolimnion, and a dense plate of purple sulfur photosynthetic bacteria (Ectothiorhodospira vacuolata) is present just below 20 m (Bchl a ca. 200 mgm^-3). Concentrations of N H₄ ⁺, Si, and CH₄ are higher in the hypolimnion than in the epilimnion. As the mixolimnion becomes isothermal in winter, oxygen is mixed down to 28 m. Nutrients (NH₄ ⁺, Si) and CH₄ are released from the hypolimnion and mix to the surface, and a diatom bloom develops in the upper 20 m (chlorophyll a > 40 mgm^-3). The deeper mixing of oxygen and enhanced light attenuation by phytoplankton uncouple the anoxic zone and photic zone, and the plate of photosynthetic bacteria disappears (Bchl a ca.10mgm^-3). Hence, seasonal changes in temperature distribution and mixing create conditions such that the primary producer community is alternately dominated by phytoplankton and photosynthetic bacteria: the phytoplankton may be nutrient-limited during periods of stratification and the photosynthetic bacteria are light-limited during periods of mixing.     

A three-year study of controls on methane emissions from two Michigan peatlands

We investigate temporal changes in methane emissions over a three-year period from two peatlands in Michigan. Mean daily fluxes ranged from 0.6–68.4 mg CH₄ m^–2d^–1 in plant communities dominated by Chamaedaphne calyculata, an eficaceous shrub, to 11.5–209 mg CH₄ m^–2d^–1 in areas dominated by plants with aerenchymatous tissues, such as Carex oligosperma and Scheuchzeria palustris. Correlations between methane flux and water table position were significant at all sites for one annual cycle when water table fluctuations ranged from 15 cm above to 50 cm below the peat surface. Correlations were not significant during the second and third annual periods with smaller water table fluctuations. Methane flux was strongly correlated with peat temperatures at –5 to –40 cm (r _s = 0.82 to 0.98) for all three years at sites with flora acting as conduits for methane transport. At shrub sites, the correlations between methane flux and peat temperature were weak to not significant during the first two years, but were strong in the third year.Low rates of methane consumption (–0.2 to –1.5 mg CH₄ m^–2 d^–1 ) were observed at shrub sites when the water table was below –20 cm, while sites with plants capable of methane transport always had positive net fluxes of methane. The methane oxidizing potential at both types of sites was confirmed by peat core experiments. The results of this study indicate that methane emissions occur at rates that cannot be explained by diffusion alone; plant communities play a significant role in altering methane flux from peatland ecosystems by directly transporting methane from anaerobic peat to the atmosphere.     

Taxonomy and ecology of the phytoplankton of Lake Fidler and Sulphide Pool,meromictic Tasmanian lakes

A floristic list of 89 freshwater phytoplanktonic algae occurring in two neighbouring, dystrophic, meromictic Tasmanian lakes is given. In both lakes the preponderance of desmids and phytoflagellates, especially chrysophytes, is a characteristic in keeping with their dystrophic nature. All the alga must be adapted to low levels of red light and some habitually inhabit crepuscular depths rather than executing diel vertical migrations such as happens in many dystrophic waters. Floristic differences between the two lakes are related to morphometric differences and the degree of entrainment of tychoplankton. The photosynthetic biomass of both lakes is predominantly monimolimnetic, made up of few species. The floristically-rich mixolimnion contributes little to biomass. The ecology of the dystrophic flora is discussed in relation to the special circumstances of meromixis.     

Upscaling methane fluxes from closed chambers to eddy covariance based on a permafrost biogeochemistry integrated model

Northern peatlands are a major natural source of methane (CH₄) to the atmosphere. Permafrost conditions and spatial heterogeneity are two of the major challenges for estimating CH₄ fluxes from the northern high latitudes. This study reports the development of a new model to upscale CH₄ fluxes from plant communities to ecosystem scale in permafrost peatlands by integrating an existing biogeochemical model DeNitrification‐DeComposition (DNDC) with a permafrost model Northern Ecosystem Soil Temperature (NEST). A new ebullition module was developed to track the changes of bubble volumes in the soil profile based on the ideal gas law and Henry's law. The integrated model was tested against observations of CH₄ fluxes measured by closed chambers and eddy covariance (EC) method in a polygonal permafrost area in the Lena River Delta, Russia. Results from the tests showed that the simulated soil temperature, summer thaw depths and CH₄ fluxes were in agreement with the measurements at the five chamber observation sites; and the modeled area‐weighted average CH₄ fluxes were similar to the EC observations in seasonal patterns and annual totals although discrepancy existed in shorter time scales. This study indicates that the integrated model, NEST–DNDC, is capable of upscaling CH₄ fluxes from plant communities to larger spatial scales.     

Sulfate reduction rates and some aspects of the limnology of four lakes and a fjord in the Vestfold Hills,Antarctica

Sulfate reduction rates were measured in waters and sediments from four antarctic lakes and an antarctic fjord basin by a radiometric technique. There was generally a linear correlation between the period of incubation and sulfate reduced; the average of the correlation coefficients was 0.76 ± 0.1. The rates at 6 °C were very low (0.0–1.1 µmol kg^–1 d^–1) when compared to most other marine and non-marine environments for which sulfate reduction rates have been reported. Lactate and acetate did not stimulate sulfate reduction. Temperatures of the sediments selected from the different sites varied from –0.4 to 4.5 °C and the chloride and sulfate concentrations of the sediments varied from 0.19 to 0.83 mol kg^–1 and 0.04 to 41.01 mmol kg^–1 respectively. Sulfate reduction rates did not correlate with the chlorosity of sediment porewaters.     

Winter fluxes of methane from Minnesota peatlands

Winter fluxes of methane were investigated in northern Minnesota during 1988–89 and 1989–90. Two bogs and a fen emitted methane throughout the snow-covered season (November through March). Fluxes decreased to a low level of 3–16 mg CH₄ m^–2 d^–1 in late March, reflecting decreasing peat temperatures and (in 1989–90) increasing depth of frost in the peat. Winter fluxes calculated by integration for an open poor fen, an open bog, a forested bog hollow, and a hummock site in the forested bog averaged 49, 12, 13, and 5 mg m^–2 d^–1, respectively, in 1989–1990 (the year most measurements were made). These comprised 11%, 4%, 15%, and 21% of total annual flux.     

Plant transport and methane production as controls on methane flux from arctic wet meadow tundra

The roles of plant transport and CH₄ production in controlling CH₄ flux from wet meadow tundra communities were investigated. Plant transport was the dominant pathway of CH₄ flux from this ecosystem. Most CH₄ production (measured within situ anaerobic incubations) occurred well below the water table, and C supply (estimated by anaerobic CO₂ production) was the best single predictor of CH₄ production rates. Plant transport of CH₄ was controlled both by CH₄ supply and the plant species.Eriophorum angustifolium transported substantially more CH₄ than didCarex aquatilis, due to differences in size and structure of the two species. The composition of the plant community was a greater control on CH₄ flux from the site than either water table height (which varied only slightly) or CH₄ production rates, indicating the importance of species-specific plant dynamics in controlling CH₄ flux from arctic wetlands.     

Seasonal changes in fluxes of methane and volatile sulfur compounds from rice paddies and their concentrations in soil water

Rice paddies emit not only methane but also several volatile sulfur compounds such as dimethyl sulfide (DMS: CH₃SCH₃). However, little is known about DMS emission from rice paddies. Fluxes of methane and DMS, and the concentrations of methane and several volatile sulfur compounds including hydrogen sulfide (H₂S), carbonyl disulfide (CS₂), methyl mercaptan (CH₃SH) and DMS in soil water and flood water were measured in four lysimeter rice paddies (2.5 × 4 m, depth 2.0 m) once per week throughout the entire cultivation period in 1995 in Tsukuba, Japan. The addition of exogenous organic matter (rice straw) was also examined for its influence on methane or DMS emissions. Methane fluxes greatly differed between treatments in which rice straw had been incorporated into the paddy soil (rice straw plot) and plots without rice straw (mineral fertilizer plot). The annual methane emission from the rice straw plots (37.7 g m^-2) was approximately 8 times higher than that from the mineral fertilizer plots (4.8 g m^-2). Application of rice straw had little influence on DMS fluxes. Significant diurnal and seasonal changes in DMS fluxes were observed. Peak DMS fluxes were found around noon. DMS was emitted from the flood water in the early growth stage of rice and began to be emitted from rice plants during the middle stage. DMS fluxes increased with the growth of rice plants and the highest flux, 15.1 µg m^-2 h^-1, was recorded before heading. DMS in the soil water was negligible during the entire cultivation period. These facts indicate that the DMS emitted from rice paddies is produced by metabolic processes in rice plants. The total amount of DMS emitted from rice paddies over the cultivated period was estimated to be approximately 5–6 mg m^-2. CH₃SH was emitted only from flood water during the first month after flooding.     

Methane production and methane consumption: a review of processes underlying wetland methane fluxes

Potential rates of both methane production and methane consumptionvary over three orders of magnitude and their distribution is skew.These rates are weakly correlated with ecosystem type, incubationtemperature, in situ aeration, latitude, depth and distanceto oxic/anoxic interface. Anaerobic carbon mineralisation is amajor control of methane production. The large range in anaerobicCH₄:CO₂ production rates indicate that a largepart of the anaerobically mineralised carbon is used for reduction ofelectron acceptors, and, hence, is not available for methanogenesis.Consequently, cycling of electron acceptors needs to be studied tounderstand methane production. Methane and oxygen half saturationconstants for methane oxidation vary about one order of magnitude.Potential methane oxidation seems to be correlated withmethanotrophic biomass. Therefore, variation in potential methaneoxidation could be related to site characteristics with a model ofmethanotrophic biomass.     

Spatio-temporal variability and environmental controls of methane fluxes at the forest–tundra ecotone in the Fennoscandian mountains

We report on temporal and spatial variability in net methane (CH₄) fluxes measured during the thaw period of 1999 and 2000 at three study sites along a c. 8° latitudinal gradient in the Fennoscandian mountain range and across the mountain birch‐tundra ecotone. All of the sites studied here were underlain by well‐drained mesic soils. In addition, we conducted warming experiments in the field to simulate future climate change. Our results show significant CH₄ uptake at mesic sites spanning the forest‐tundra ecotone: on average 0.031 and 0.0065 mg CH₄ m^?2 h^?1 during the 1999 and 2000 thaw periods, respectively, in Abisko (Sweden), and 0.019 and 0.032 mg CH₄ m^?2 h^?1 during 2000 in Dovrefjell and Joatka (Norway), respectively. These values were both temporally and spatially highly variable, and multiple regression analysis of data from Abisko showed no consistent relationship with soil‐moisture status and temperature. Also, there was no consistent difference in CH₄ fluxes between forest and tundra plots; our data, therefore, provide no support for the hypothesis that conversion of tundra to mountain birch forest, or vice versa, would result in a systematic change in the magnitude or direction of net CH₄ fluxes in this region. Experimental warming treatments were associated with a 2.4 °C increase in soil temperatures (5 cm depth) in 1999 in Abisko, but no consistent soil warming was noted at any of the three field locations during 2000. In spite of this, there were significant treatment effects, principally early during the thaw period, with increased CH₄ uptake compared with control (ambient) plots. These results suggest that direct effects of air warming on vegetation processes (e.g. transpiration, root exudation and nutrient assimilation) can influence CH₄ fluxes even in predominantly methanotrophic environments. We conclude that net CH₄ oxidation is significant in these cold, mesic soils and could be strengthened in an environmental change scenario involving a combination of (i) an increase in the length of the thaw period and (ii) increased mean temperatures during this period in combination with decreased soil‐moisture content.     

The importance of floating peat to methane fluxes from flooded peatlands

The effect of flooding on methane (CH₄) fluxes was studied through the construction of an experimental reservoir in a boreal forest wetland at the Experimental Lakes Area in northwestern Ontario. Prior to flooding, the peatland surface was a small source of CH₄ to the atmosphere (1.0± SD of 2.3 mg CH₄ m^–2 d^–1). After flooding, CH₄ fluxes from the submerged peat surface increased to 64±68 mg CH4 m^–2 d^–1 CH₄ bubbles within the submerged peat caused about 1/3 of the peat to float. Fluxes from these floating peat islands were much higher (440±350 mg CH₄ m^–2 d^–2) than from both the pre-flood (undisturbed) and the post-flood (submerged) peat surfaces.The high fluxes of CH₄ from the floating peat surfaces may be explained by a number of factors known to affect the production and consumption of CH₄ in peat. In floating peat, however, these factors are particularly enhanced and include decreased oxidation of CH₄ due to the loss of aerobic habitat normally found above the water table of undisturbed peat and to increased peat temperatures. The extremely high fluxes associated with newly lifted peat may decrease as the islands age. However, CH₄ flux rates from floating peat islands that were several years old still far exceeded those from undisturbed peat surfaces and from the water surface of a newly created reservoir.     

Meromixis in an antarctic fjord: a precursor to meromictic lakes on an isostatically rising coastline

Physico-chemical data and isotopic studies (utilising H¹⁴CO₃, H₂ ¹⁸O, ³H₂O) suggest that hypersaline meromixis in Ellis Fjord (Vestfold Hills, Antarctica) was initiated during the middle Holocene period, when hypersaline brine, excluded during the annual formation of sea-ice, gravitated in a density current to the bottom. The application of this contemporary information to the genesis of the meromictic lakes found today in the Vestfold Hills, suggest that their meromixis may have developed prior to isolation from the sea. Comparison of physico-chemical data from the meromictic basins of Ellis Fjord with that of the Vestfold Hills saline lake allows some determination of their evolutionary pathways initiated before, during and after isolation from the sea. Further evolution of each lake can be explained through the individual interaction between climate, the catchment size and basin morphology.     

High carbon emissions from thermokarst lakes and their determinants in the Tibet Plateau

Thermokarst lakes are potentially important sources of methane (CH₄) and carbon dioxide (CO₂). However, considerable uncertainty exists regarding carbon emissions from thermokarst lakes owing to a limited understanding of their patterns and motivators. In this study, we measured CH₄ and CO₂ diffusive fluxes in 163 thermokarst lakes in the Qinghai–Tibet Plateau (QTP) over 3 years from May to October. The median carbon emissions from the QTP thermokarst lakes were 1440 mg CO₂ m⁻² day⁻¹ and 60 mg CH₄ m⁻² day⁻¹, respectively. The diffusive rates of CO₂ and CH₄ are related to the catchment land cover type. Sediment microbial abundance and hydrochemistry explain 51.9% and 38.3% of the total variance in CH₄ diffusive emissions, respectively, while CO₂ emissions show no significant relationship with environmental factors. When upscaling carbon emissions from the QTP thermokarst lakes, the annual average CH₄ release per lake area is equal to that of the pan-Arctic region. Our findings highlight the importance of incorporating in situ observation data with different emission pathways for different land cover types in predicting carbon emissions from thermokarst lakes in the future.     

Seasonal variation in methane emission from a temperate Phragmites-dominated marsh: effect of growth stage and plant-mediated transport

The eddy covariance technique was employed with a tunable diode laser spectrometer to quantify methane flux from a prairie marsh dominated by Phragmites australis in north-central Nebraska, USA. The observations spanned the entire growing season (April to October) and a wide range of weather conditions, allowing a quantitative assessment of the physical and biological controls of methane emission in this ecosystem. Diel patterns in methane emission varied markedly depending on plant growth stage. Prior to plant emergence above water, the rate of methane emission from the marsh was fairly constant throughout the day. After emergence above water, there was a gradual increase in methane emission after sunrise with a peak in late afternoon. Significant changes in diel patterns were observed after tillering. Then, the diel pattern was characterized by a mid- to late-morning peak and a 2-to 4-fold increase in methane emissions from night to daytime. In early stages of plant growth, molecular diffusion through dead/live plants and the standing water column seemed to be the primary transport mechanism. After tillering, a transition occurred in the transport mechanism from a molecular diffusion to a convective throughflow, which is a rapid and active gas transport driven by pressure differences. The role of convective throughflow became less important as the plants senesced. Integrated methane emission over the six-month measurement period (April–October) was about 64 g CH₄ m^–2. On an annual basis, we estimate the annual methane emission from this ecosystem to be ≈ 80 g CH₄ m^–2 and that about 80% of the total methane emission occurred between late April and late October.     

Effects of land-use change on solute fluxes to floodplain lakes of the central Amazon

A time-series analysis of airborne photographs and Landsat thematic mapper (TM and ETM+) images and hydrochemical data were used to examine the effects of land-use change from 1930 to 2001 on solute inputs to Lake Calado, a floodplain lake in the central Amazon. Deforestation from slash-and-burn agricultural activities has dramatically decreased the amount of primary growth upland and flooded forests in the basin. The increasing area that is converted to agricultural plots and pasture in the Lake Calado basin has increased solute loading to the lake from upland tributaries (storm and base flow), bank seepage and overland flow, and decreased throughfall inputs. Whereas solute concentrations in stream water were generally higher in 1992 than 1930, Na⁺ and Cl^– concentrations were also considerably higher in 2001 than 1992, likely because of an increase in the number of humans and cattle in the watershed. Estimates of solute inputs to Lake Calado via throughfall indicate that the mass transfer of some major solutes in the throughfall of undisturbed flooded forests can be larger than that from a combination of all other sources in areas that do not have a strong influence from the Solimões River. Chemical gains in rain as it passed through the forest canopy occurred for most major ions and relatively large gains were observed for and Ca²⁺. Although often neglected in studies of tropical forest ecosystems, throughfall can be an important source of solutes to relatively undisturbed lake environments in the central Amazon.     

Potential activity of methane and ammonium oxidation by methanotrophic communities from the soda lakes of southern Transbaikal

Radioisotopic measurements of the methane consumption by mud samples taken from nine Southern Transbaikal soda lakes (pH 9.5–10.6) showed an intense oxidation of methane in the muds of Lakes Khuzhirta, Bulamai Nur, Gorbunka, and Suduntuiskii Torom, with the maximum oxidation rate in the mud of Lake Khuzhirta (33.2 nmol/(ml day)). The incorporation rate of the radioactive label from¹⁴CH₄ into¹⁴CO₂ was higher than into acid-stable metabolites. Optimum pH values for methane oxidation in water samples were 7–8, whereas mud samples exhibited two peaks of methane oxidation activity (at pH 8.15–9.4 and 5.8–6.0). The majority of samples could oxidize ammonium to nitrites; the oxidation was inhibited by methane. The PCR amplification analysis of samples revealed the presence of genes encoding soluble and paniculate methane monooxygenase and methanol dehydrogenase. Three alkaliphilic methanotrophic bacteria of morphotype I were isolated from mud samples in pure cultures, one of which, B5, was able to oxidize ammonium to nitrites at pH 7–11. The data obtained suggest that methanotrophs are widely spread in the soda lakes of Southern Transbaikal, where they can actively oxidize methane and ammonium.     

Soil properties and sediment accretion modulate methane fluxes from restored wetlands

Wetlands are the largest source of methane (CH₄) globally, yet our understanding of how process‐level controls scale to ecosystem fluxes remains limited. It is particularly uncertain how variable soil properties influence ecosystem CH₄ emissions on annual time scales. We measured ecosystem carbon dioxide (CO₂) and CH₄ fluxes by eddy covariance from two wetlands recently restored on peat and alluvium soils within the Sacramento–San Joaquin Delta of California. Annual CH₄ fluxes from the alluvium wetland were significantly lower than the peat site for multiple years following restoration, but these differences were not explained by variation in dominant climate drivers or productivity across wetlands. Soil iron (Fe) concentrations were significantly higher in alluvium soils, and alluvium CH₄ fluxes were decoupled from plant processes compared with the peat site, as expected when Fe reduction inhibits CH₄ production in the rhizosphere. Soil carbon content and CO₂ uptake rates did not vary across wetlands and, thus, could also be ruled out as drivers of initial CH₄ flux differences. Differences in wetland CH₄ fluxes across soil types were transient; alluvium wetland fluxes were similar to peat wetland fluxes 3 years after restoration. Changing alluvium CH₄ emissions with time could not be explained by an empirical model based on dominant CH₄ flux biophysical drivers, suggesting that other factors, not measured by our eddy covariance towers, were responsible for these changes. Recently accreted alluvium soils were less acidic and contained more reduced Fe compared with the pre‐restoration parent soils, suggesting that CH₄ emissions increased as conditions became more favorable to methanogenesis within wetland sediments. This study suggests that alluvium soil properties, likely Fe content, are capable of inhibiting ecosystem‐scale wetland CH₄ flux, but these effects appear to be transient without continued input of alluvium to wetland sediments.