Numerical modeling of vertical stratification of Lake Shira in summer

A one-dimensional numerical model and a two-dimensional numerical model of the hydrodynamic and thermal structure of Lake Shira during summer have been developed, with several original physical and numerical features. These models are well suited to simulate the formation and dynamics of vertical stratification and provide a basis for an ecological water-quality model of the lake. They allow for the quantification of the vertical mixing processes that govern not only the thermal structure but also the nutrient exchange, and more generally, the exchange of dissolved and particulate matter between different parts of the lake. The outcome of the calculations has been compared with the field data on vertical temperature and salinity distributions in Lake Shira. Lake Shira is meromictic and exhibits very stable annual stratification. The stratification is so stable because of the high salinity of the water. If the water in Lake Shira were fresh and other parameters (depth, volume, and meteorology) were the same, as now, the lake would be mixed in autumn. Using the newly developed models and using common meteorological parameters, we conclude that Lake Shira will remain stratified in autumn as long as the average salinity is higher than 3‰.     

A general one-dimensional vertical ecosystem model of Lake Shira (Russia,Khakasia): description,parametrization and analysis

A one-dimensional ecological model of the meromictic brackish Lake Shira (Russia, Khakasia) was developed. The model incorporates state-of-the-art knowledge about the functioning of the lake ecosystem using the most recent field observations and ideas from PCLake, a general ecosystem model of shallow freshwater lakes. The model of Lake Shira presented here takes into account the vertical dynamics of biomasses of the main species of algae, zooplankton and microbial community, as well as the dynamics of oxygen, detritus, nutrients and hydrogen sulphide from spring to autumn. Solar radiation, temperature and diffusion are modelled using real meteorological data. The parameters of the model were calibrated to the field data, after applying different methods of sensitivity analysis to the model. The resulting patterns of phytoplankton and nutrients dynamics show a good qualitative and quantitative agreement with the field observations during the whole summer season. Results are less satisfactory with respect to the vertical distribution of zooplankton biomass. We hypothesize that this is due to the fact that the current model does not take the sex and age structure of zooplankton into account. The dynamics of oxygen, hydrogen sulphide and the modelled positions of the chemocline and thermocline are again in good agreement with field data. This resemblance confirms the validity of the approach we took in the model regarding the main physical, chemical and ecological processes. This general model opens the way for checking various hypotheses on the functioning of the Lake Shira ecosystem in future investigations and for analysing options for management of this economically important lake.     

Lake Shira: Microzonality of the soil cover of lake depressions in the Shira steppe

Microzonality was studied on the model object—Shira lake depression. Geometry of the soil structures that comprise the soil cover has been revealed and characteristics of components of soil combinations have been analyzed.     

Microbiota within the perennial ice cover of Lake Vida, Antarctica

Lake Vida, located in the McMurdo Dry Valleys, Antarctica, is an 'ice-sealed' lake with approximately 19 m of ice covering a highly saline water column (approximately 245 ppt). The lower portions of the ice cover and the lake beneath have been isolated from the atmosphere and land for circa 2800 years. Analysis of microbial assemblages within the perennial ice cover of the lake revealed a diverse array of bacteria and eukarya. Bacterial and eukaryal denaturing gradient gel electrophoresis phylotype profile similarities were low (<59%) between all of the depths compared (five depths spanning 11 m of the ice cover), with the greatest differences occurring between surface and deep ice. The majority of bacterial 16S rRNA gene sequences in the surface ice were related to Actinobacteria (42%) while Gammaproteobacteria (52%) dominated the deep ice community. Comparisons of assemblage composition suggest differences in ice habitability and organismal origin in the upper and lower portions of ice cover. Specifically, the upper ice cover microbiota likely reflect the modern day transport and colonization of biota from the terrestrial landscape, whereas assemblages in the deeper ice are more likely to be persistent remnant biota that originated from the ancient liquid water column of the lake that froze.     

Vertical structure and photosynthetic activity of Lake Shira phytoplankton

This study was conducted to analyse vertical dynamics of phytoplankton distribution in Shira Lake during the summer stratification regime. From late June to September phytoplankton in Shira Lake were stratified with the maximum in the lower part of the thermocline, at a depth of 8–12 m, with a chlorophyll concentration up to 23 g and biomass up to 5 mg l^–1. Maxima of chlorophyll and biomass of cyanobacteria and green algae were in different layers. From June to September a major part of chlorophyll a was in green algae, while under ice – in cyanobacteria. The variable fluorescence proves high photosynthetic activity of algae in the depth assemblage. Epifluorescent analysis disclosed that additional light-harvesting pigments were better developed in cells from the depth maximum. The maximum of gross primary production calculated from fluorescence corresponded to the depth maximum of phytoplankton. Primary production over a season was 2.7 gO₂ m^–2. Formation mechanisms of the depth maximum of phytoplankton are discussed in this paper.     

Vertical and seasonal variation of phytoplankton photosynthesis in a brown-water lake with winter ice cover

SUMMARY. Unlike previously studied lakes with prolonged winter ice and snow cover, Lake Paajarvi, southern Finland, has a high humus content and consequently differs in both the quantity and quality of light penetration into its waters. Moreover, the range of temperature fluctuation and the degree of development of thermal stratification are greater in Paajarvi, and this increased environmental heterogeneity apparently stimulates diversity in the phytoplankton community, especially in the seasonal succession of species. Differences in the photosynthetic capacity of algae from different depths in the water column were not great. This is attributed to the extremely shallow euphotic zone, algae circulating freely through the steep light gradient and sedimenting rapidly once they pass through the thermocline into the hypolimnion. It is suggested that 'adaptation' of phytoplankton to the great seasonal changes in irradiance is achieved largely by successive growths of different species in the community, and that the adaptations and vertical migrations by individual algal species, which have been reported from polar and high alpine lakes, may be of secondary importance in Pääjärvi. The species successions in Pääjärvi produce changes in the pigment content of algae similar to those reported from polar and high alpine lakes, confirming that change in pigmentation is an important mechanism in light adaptation, whether at community or individual level. Algal pigment content was particularly high at the end of the long period of winter ice cover, indicating a degree of adaptation to the prolonged low-light conditions, which produced the extremely high photosynthetic capacities measured at this time. However, phytoplankton production at any irradiance was primarily determined by biomass.     

Growth of dominant zooplankton species feeding on plankton microflora in Lake Shira

Batch cultures and continuous flow cultures were used to study the growth rates of zooplankton species from Shira lake, the rotifer Brachionus plicatilis Muller and calanoid copepod Arctodiaptomus salinus Daday, which were fed on phytoplankton and bacterioplankton from the lake. Analyses of the birth and survival rates were used to demonstrate that the lake phytoplankton, consisting mostly of cyanobacteria and diatomaceous algae, is inadequotes for optimal realisation of the reproductive potential of B. plicatilis when compared with the bacterial diet. The study revealed that the kinetic growth characteristics of the two zooplankters were similar: B. plicatilis r _max, 0.120 d^–1; S ₀, 0.253; and K _s, 0.114 mg dry mass l^–1; and for A. salinus r _max, 0.129 d^–1; S ₀, 0.240; and K _s, 0.171 mg dry mass l^–1. Fluctuations in natural food concentration reduced the growth rate of both species. Even though the threshold concentration of food for B. plicatilis and A. salinus were quite similar, the copepods were less sensitive to food limitation.     

Geographical and seasonal distribution of multiple antibiotic resistance of heterotrophic bacteria of Lake Shira

From 1996 to 1999 heterotrophic bacteria of the brackish-water Lake Shira (Republic of Khakasia, Russia) were studied to understand the seasonal dynamics of their antibiotic resistance. During the winter, these bacteria were represented primarily by forms that could not be cultured and were psychrotolerant. In the summer period, heterotrophic, mesophilic bacteria increased in number. The percentages of isolates with multiple, antibiotic resistance isolated from the lake region near the resort area of the lake were 2–3 times higher than those from the central part of the lake. A decline in the bacterial numbers with multiple antibiotic resistance was observed during the cold period (February–March). Various mechanisms of multiple, antibiotic resistance of heterotrophic bacteria isolated from Shira lake are discussed.     

Data on the flora of bacillariophyta of Lake Shira (Khakassia,Russia)

This article deals with the results of the study of diatom algae of Lake Shira. The list of found algae contains 95 species (126, including infraspecific taxa). A brief ecological and geographical analysis of the algal flora is made.     

Mathematical models and computer programmes for the investigation of hydrophysical processes in Lake Shira

Physical processes determine to great extent the habitat of hydrobionts, as well as the transfer and sedimentation of substances, the intensity of pollution and rate of natural purification of water bodies. Mathematical models of different levels of complexity, developed to investigate hydrophysical processes in lakes, are discussed in this paper. The numerical algorithms and computer programmes described can be used to determine the influence of the morphometric characteristics and weather on the temperature regime of Lake Shira (Khakasia, Siberia) and the pattern of the wind currents. Examples of calculating the temperature regime in the context of a one- dimensional model and of calculating the parameters of wind currents in the water body of a simple geometrical form are given.The study suggests that the pattern of wind currents in Lake Shira is significantly affected by the density stratification, which depends not only on temperature but also on salinity. In order to construct a realistic pattern of currents a 3-D computer model of Lake Shira must be developed and used to estimate the validity of the two-dimensional and one-dimensional models. The present research can be further developed by extending the obtained algorithms to three-dimensional problems, taking into account the heat exchange, salinity and the geometry (bathymetric contours) of the water body. Calculations can be made for Lake Shira and the obtained data used in biophysical models.     

Microbial Processes of the Carbon and Sulfur Cycles in Lake Shira (Khakasia)

Microbiological and biogeochemical studies of the meromictic saline Lake Shira (Khakasia) were conducted. In the upper part of the hydrogen-sulfide zone, at a depth of 13.5–14 m, there was a pale pink layer of water due to the development of purple bacteria (6 × 10⁵ cells/ml), which were assigned by their morphological and spectral characteristics toLamprocystis purpurea (formerly Amoebobacter purpureus). In August, the production of organic matter (OM) in Lake Shira was estimated to be 943 mg C/(m²day). The contribution of anoxygenic photosynthesis was insignificant (about 7% of the total OM production). The share of bacterial chemosynthesis was still less (no more than 2%). In the anaerobic zone, the community of sulfate-reducing bacteria played a decisive role in the terminal decomposition of OM. The maximal rates of sulfate reduction were observed in the near-bottom water (114 g S/(l day)) and in the surface layer of bottom sediments (901 g S/(dm³ day)). The daily expenditure of C_org for sulfate reduction was 73% of C_org formed daily in the processes of oxygenic and anoxygenic photosynthesis and bacterial chemosynthesis. The profile of methane distribution in the water column and bottom sediments was typical of meromictic reservoirs. The methane content in the water column increased beginning with the thermocline (7–8 m) and reached maximum values in the near-bottom water (17 l/l). In bottom sediments, the greatest methane concentrations (57 l/l) were observed in the surface layer (0–3 cm). The integral rate of methane formation in the water column and bottom sediments was almost an order of magnitude higher than the rate of its oxidation by aerobic and anaerobic methanotrophic microorganisms.     

Microbial metabolism of the carbon and sulfur cycles in Shira Lake (Khakasia)

Microbiological and biogeochemical studies of the meromictic saline Lake Shira (Khakasia) were conducted. In the upper part of the hydrogen-sulfide zone, at a depth of 13.5-14 m, there was a pale pink layer of water due to the development of purple bacteria (6 x 10(5) cells/ml), which were assigned by their morphological and spectral characteristics to Lamprocystis purpureus (formerly Amoebobacter purpurea). In August, the production of organic matter (OM) in Lake Shira was estimated to be 943 mg C/(m2 day). The contribution of anoxygenic photosynthesis was insignificant (about 7% of the total OM production). The share of bacterial chemosynthesis was still less (no more than 2%). In the anaerobic zone, the community of sulfate-reducing bacteria played a decisive role in the terminal decomposition of OM. The maximal rates of sulfate reduction were observed in the near-bottom water (114 micrograms S/(1 day)) and in the surface layer of bottom sediments (901 micrograms S/(dm3 day)). The daily expenditure of Corg for sulfate reduction was 73% of Corg formed daily in the processes of oxygenic and anoxygenic photosynthesis and bacterial chemosynthesis. The profile of methane distribution in the water column and bottom sediments was typical of meromictic reservoirs. The methane content in the water column increased beginning with the thermocline (7-8 m), and reached maximum values in the near-bottom water (17 microliters/l). In bottom sediments, the greatest methane concentrations (57 microliters/l) were observed in the surface layer (0-3 cm). The integral rate of methane formation in the water column and bottom sediments was almost an order of magnitude higher than the rate of its oxidation by aerobic and anaerobic methanotrophic microorganisms.     

A dynamic model of hypothermia as an adaptive response by small birds to winter conditions

We present a dynamic programming model which is used to investigate hypothermia as an adaptive response by small passerine birds in winter. The model predicts that there is a threshold function of reserves during the night, below which it is optimal to enter hypothermia, and above which it is optimal to rest. This threshold function decreases during the night, with a particularly sharp drop at the end of the night, representing the time and energy costs associated with returning to normal body temperature. The results of the model emphasise the trade-off between energy and predation, not just between foraging options, but also between foraging during the day and entering hypothermia at night. The value of being able to use hypothermia represents not just energy savings, but also reduced predation risk due to changes in the optimal foraging strategy. Conditions which give a high value of hypothermia are short photoperiod, variable food supply, low temperatures, poor and scarce food supplies.     

A method of collecting water samples from immediately below an ice cover

A plexiglas box with cellular neoprene gaskets in two opposite faces which can be frozen in situ in a developing ice cover on a lake is described along with a water sampling system using evacuated glass tubes (Vacutainers) and blood collecting needles. This system allows collecting of water samples from directly beneath the ice without disturbing the ice cover. The box may be removed in an ice block at the end of the sampling period.     

Assessing the anthropogenic impact on Lake Shira from antibiotic resistance of heterotrophic bacteria by neural networks methods

A general approach to assessing the anthropogenic impact on lake ecosystems is proposed and exemplified for the case of Lake Shira (Republic of Khakasia, Russia). The impact strength is estimated by applying neural network-based methods to samples of data on interdependent marking features of autochthonous and allochthonous bacteria isolated from the lake in 1997–2001. The proposed combination of analysis methods makes it possible to determine the state of an ecosystem from both small-and large-size samples of data having complex interrelations.     

Dense winter bloom of the dinoflagellate Heterocapsa triquetra below the thick surface ice of brackish Lake Shihwa,Korea

We investigated the seasonal abundance of the dinoflagellate Heterocapsa triquetra (Ehrenberg) F. Stein, as well as the relevant in situ environmental factors, in brackish Lake Shihwa, Korea. We also examined the growth rates and morphological characteristics of the species in laboratory cultures. In the field, the population densities of H. triquetra remained at low levels from late spring to early summer, and then completely disappeared from August to November 2007. Interestingly, a dense bloom of H. triquetra appeared below the ice surface on 17 January 2008; identities of the cells were confirmed by rDNA sequence comparisons. The second peak reached a density of 672 × 10³ cells L^?1 on 28 March 2008, at a water temperature of 9.1°C. Laboratory experiments showed that growth rates of H. triquetra increased with incremental temperature increases within the range of 10 and 20°C. The highest growth rate reached by H. triquetra was 0.62 d^?1 at 20°C with a salinity of 30. Above 25°C, the dinoflagellate was unable to grow between salinities of 10 and 15, and reached only relatively low growth rates (<0.12 d^?1) under other salinity conditions. However, under continuous cultures at 5°C and 8°C, H. triquetra cells retained its growth capability for more than 12 days, implying that H. triquetra can survive and grows even at very low temperatures. The equivalent spherical diameter (ESD) of H. triquetra did not change markedly between 10 and 25°C, but the equivalent spherical diameter was significantly different at 5°C. The cell volume buildup of H. triquetra at low temperatures is one of the important survival strategies to overcome the harsh environmental conditions. These characteristics make H. triquetra a consistently dominant dinoflagellate in Lake Shihwa during the cold winter season.     

Impact of weather variability on spatial and seasonal dynamics of dissolved and suspended nutrients in water column of meromictic Lake Shira

Depths of thermocline and the redox zone, concentrations of dissolved and suspended carbon, and nitrogen and phosphorus in seston were measured in the pelagial of the saline meromictic Lake Shira (southern Siberia) in different years (2007–2011); the relationship of the values for those parameters with air-temperature variations was assessed. Positive correlations between both air temperatures in the previous year and the depth of the redox zone in winter and air temperature in April and the thermocline depth in summer were revealed. In the mixolimnion, the ratio of total nitrogen to total phosphorus almost always exceeded the Redfield ratio (16: 1); seston deficiency both in nitrogen and phosphorus was monitored in different seasons and at various depths. The amount of seston in the mixolimnion in summer almost doubled the amount of seston in winter and was directly related with the depth of the thermocline. In the monimolimnion, seston was rich in nitrogen and phosphorus. The amount of seston in the monimolimnion varied in different years and depended both on the air temperature in the previous year and the size of the zone.