Permanent Thermal and Chemical Stratification in a Restored Urban Meromictic Lake

Meromictic lakes are unique aquatic ecosystems that occur extremely rarely. The phenomenon of meromixis can result from both natural and anthropogenic factors. The aim of this study was to analyse thermal and chemical stratification in a small, deep (6 ha, H max = 24.5 m) lake. The evaluated lake had a typical summer thermal profile with a shallow epilimnion, a sharp thermocline, and a distinct monimolimnion layer in the hypolimnion, which was also maintained during circulation. The lake had a clinograde oxygen profile, with an oxygen deficit in the metalimnion and permanent anoxic conditions in the deeper layers, including during circulation. A redox zone was identified during summer stagnation. The monimolimnion formed a thermally isolated layer at a depth of around 15 m, and the chemocline was situated above the monimolimnion. In the chemocline, the EC gradient ranged from 61 to 77 μS·cm−1 per meter of depth in the summer and from 90 to 130 μS·cm−1 per meter of depth during circulation. EC was significantly correlated with Ca2+ concentration (r2 = 0.549). Chemical stratification, particularly with regard to organic matter distribution, was observed in the chemocline. The monimolimnion severely limited nutrient internal loading.

The New Method for Determining the Composition of Wood Gas Produced in Gas Generators of the Inverted Gasification Process

The article presents a new method for determining the composition of wood generator gas produced in gas generators of the inverted gasification process. The shortcomings of the existing calculation methods are analyzed, the main of which is the insufficient harmonization of the calculation results with the experimental data. The authors substantiate the priority of the main chemical reactions occurring during gasification of wood fuel. There are three active zones of gasification, viz.: a redox zone, a reduction zone and a zone of interaction of gasification products with each other and with the carbon of the fuel. In general, a redox zone consists of two subzones: in the first one reactions of water gas formation occur, and the second one appears when excess air is supplied to the gas generator. The proposed method for calculating the components of the generator gas is a set of a modified balance method and an added method for calculating the concentrations of chemical reaction products by the equilibrium constants of these reactions in the active gasification zones with different temperatures. The modified balance method considers the primary processes of wood and moist air transformation into components of the generator gas in the first subzone of the redox zone. The modified balance method is based on the equations of material balance of carbon, hydrogen, oxygen, moisture, nitrogen and thermal balance of the system. The added method determines the concentrations of the components of the generator gas in the second subzone of the redox zone, as well as in the reduction zone and the zone of interaction of the gasification products with each other and with the fuel carbon. The combination of these two methods makes it possible to calculate with greater accuracy the output of the generator gas, the concentration of its components, fuel and air consumption, as well as a number of other characteristics of the gas generator.

Study of microbial communities in redox zone of meromictic lakes isolated from the White Sea using spectral and molecular methods

Due to postglacial isostatic uplift many stratified lakes, at different stages of isolation, are located along the shores of the White Sea. In five lakes, located near the White Sea Biological Station of Moscow State University, salinity, temperature, pH, concentration of dissolved oxygen, redox potential, and illuminance were measured. Distribution of microorganisms and spectral properties of water layers were also studied. All the lakes had a narrow bright coloured layer in the redox zone caused by mass development of phototropic microorganisms. Light absorption and fluorescence spectra indicated algae containing chlorophyll a predominate in the red water layers while the colouration of green and brown layers is caused by green sulphur bacteria with bacteriochlorophylls d and e. Sunlight is completely absorbed in the redox zone because of the high density of algae and/or bacteria, resulting in aphotic conditions below. Coloured layers act as a specific biotope for special communities of microorganisms. Eukaryotes identified by the 18S rRNA gene included different species of mixotrophic algae and ciliates resistant to anoxia. The water layer colour and spectral characteristics (i.e. light absorption and fluorescence) of water in the redox zone can be considered indicators of the stage of lake isolation from the sea, with the red colour caused by cryptophyte alga Rhodomonas sp. bloom found in earlier stages and brown and green colours caused by green sulphur bacteria in later stages.

Comparative studies of pelagic microbial methane oxidation within the redox zones of the Gotland Deep and Landsort Deep (central Baltic Sea)

Pelagic methane oxidation was investigated in dependence on differing hydrographic conditions within the redox zone of the Gotland Deep (GD) and Landsort Deep (LD), central Baltic Sea. The redox zone of both deeps, which indicates the transition between oxic and anoxic conditions, was characterized by a pronounced methane concentration gradient between the deep water (GD: 1233 nM, 223 m; LD: 2935 nM, 422 m) and the surface water (GD and LD < 10 nM). This gradient together with a 13C CH4 enrichment (δ13C CH4 deep water: GD −84‰, LD −71‰; redox zone: GD −60‰, LD −20‰; surface water: GD −47‰, LD −50‰; δ13C CH4 vs. Vienna Pee Dee Belemnite standard), clearly indicating microbial methane consumption within the redox zone. Expression analysis of the methane monooxygenase identified one active type I methanotrophic bacterium in both redox zones. In contrast, the turnover of methane within the redox zones showed strong differences between the two basins (GD: max. 0.12 nM d−1, LD: max. 0.61 nM d−1), with a nearly four-times-lower turnover time of methane in the LD (GD: 455 d, LD: 127 d). Vertical mixing rates for both deeps were calculated on the base of the methane concentration profile and the consumption of methane in the redox zone (GD: 2.5 × 10–6 m2 s−1, LD: 1.6 × 10–5 m2 s−1). Our study identified vertical transport of methane from the deep-water body towards the redox zone as well as differing hydrographic conditions (lateral intrusions and vertical mixing) within the redox zone of these deeps as major factors that determine the pelagic methane oxidation.

Comparative studies of pelagic microbial methane oxidation within two anoxic basins of the central Baltic Sea (Gotland Deep and Landsort Deep)

Pelagic methane oxidation was investigated in dependence on differing environmental conditions within the redox zone of the Gotland Deep (GD) and Landsort Deep (LD), central Baltic Sea. The redox zone of both deeps, which indicates the transition between oxic and anoxic conditions, was characterized by a pronounced methane concentration gradient between the deep water (GD: 1233 nM, LD: 2935 nM) and the surface water (GD and LD < 10 nM), together with a 13C CH4 enrichment (δ13C CH4 deep water: GD −84‰, LD −71‰ ; redox zone: GD −60‰, LD −20‰ ; δ13C CH4 vs. Vienna Pee Dee Belemnite standard), clearly indicating microbial methane consumption in that specific depth interval. Expression analysis of the methane monooxygenase identified one active type I methanotrophic bacterium in both redox zones. In contrast, the turnover of methane within the redox zones showed strong differences between the two basins (GD: max. 0.12 nM d–1 and LD: max. 0.61 nM d–1), with a four times higher turnover rate constant (k) in the LD (GD: 0.0022 d–1, LD: 0.0079 d–1). Vertical mixing rates for both deeps were calculated on the base of the methane concentration profile and the consumption of methane in the redox zone (GD: 2.5 × 10–6 m2 s–1 LD: 1.6 × 10–5 m2 s–1). Our study identified vertical transport of methane from the deep water body towards the redox zone as well as differing hydrographic conditions within the oxic/anoxic transition zone of these deeps as major factors that determine the pelagic methane oxidation.

Aerobic methanotrophy within the pelagic redox-zone of the Gotland Deep (central Baltic Sea)

Water column samples taken in summer 2008 from the stratified Gotland Deep (central Baltic Sea) showed a strong gradient in dissolved methane concentrations from high values in the saline deep water (max. 504 nM) to low concentrations in the less dense, brackish surface water (about 4 nM). The steep methane-gradient (between 115 and 135 m water depth) within the redox-zone, which separates the anoxic deep part from the oxygenated surface water (oxygen concentration 0–0.8 mL L−1), implies a methane consumption rate of 0.28 nM d−1. The process of microbial methane oxidation within this zone was evident by a shift of the stable carbon isotope ratio of methane between the bottom water (δ13C CH4 = −82.4‰ and the redox-zone (δ13C CH4 = −38.7‰. Water column samples between 80 and 119 m were studied to identify the microorganisms responsible for the methane turnover in that depth interval. Notably, methane monooxygenase gene expression analyses for water depths covering the whole redox-zone demonstrated that accordant methanotrophic activity was probably due to only one phylotype of the aerobic type I methanotrophic bacteria. An imprint of these organisms on the particular organic matter was revealed by distinctive lipid biomarkers showing bacteriohopanepolyols and lipid fatty acids characteristic for aerobic type I methanotrophs (e.g., 35-aminobacteriohopane-30,31,32,33,34-pentol), corroborating their role in aerobic methane oxidation in the redox-zone of the central Baltic Sea.