Understanding the mechanisms of blooming of phytoplankton in Lake Shira, a saline lake in Siberia (the Republic of Khakasia)

The paper summarises the results of a three-year research study (European Union Grant: INTAS 97-O519) aimed at investigating the planktonic populations and trophic organization of the Lake Shira ecosystem – a saline lake in Khakasia, Siberia. The lake exhibits a stable summer-autumn stratification of the chemical-biological components. The mechanisms responsible for the ‘blooming[lsquor] of phytoplankton in the deeper layers were investigated in greater detail, using data from both field and laboratory experiments. The spectra of nutrition were examined to estimate the relationships between the specific growth rates of the hydrobionts and the influence of the limiting factors: light, nutrients. The observed heterotrophic capability of a metalimnetic phytoplankton population might help explain the development in the deeper waters of Lyngbya contorta. The scheme of trophic interactions was put up, based on the assessment of the carbon pools and carbon flows in the pelagic zone of the lake. A mathematical model of the vertical structure of the lake's plankton populations was constructed, using the ecosystem description and data of vertical turbulent diffusion. The role of light and nutrient limitations and grazing mortality in forming the vertical inhomogeneities, particularly in lowering the depth of the maximal cyanobacterial biomass, has been demonstrated. The theoretical curves for the stratification of chemical and biological parameters have been brought in conformity with the field observations, e.g. for the different patterns of the peaks, and for the biomass maxima of cyanobacteria, purple and green sulphur bacteria, oxygen, and hydrogen sulphide. The calculations revealed that for an adequate assessment of the parameters for the hydrogen sulphide zone it is necessary to introduce flows of allochthonous organic matter. Based on the form of the sulphur distribution curve, the allochthonous input of organic matter and the inflow of hydrogen sulphide from the bottom have been theoretically discriminated for the first time. It has also been ascertained that irrespective of the depth the allochthonous substances limiting bacterial growth, the bacteria are uniformly distributed over depth and can serve as an indicator of the presence of limitation (the effect of autostabilisation in space). Of indisputable interest to limnology are the specific methods developed for understanding the functioning of Lake Shira ecosystem. These include the autostabilisation of the limiting factors, the on-the-spot fluorescent method of determining the three classes of microalgae, the algal mixotrophy and the planktonic population interactions and feedbacks, and development of a more sensitive, bioluminescent method for mapping the non-homogeneities. Owing to a balanced combination of classical approa [KEYWORDS: meromictic lakes, stratification, mathematical models of stratification, trophic scheme, carbon budget, hydrogen sulphide, heterotrophic bacteria, microbial loop, cyanobacteria]

Special Issue: Integrated Empirical and Modelling Studies on the Ecology of Two Saline Lakes in South Siberia (Khakasia, Russia) / Guest Edited by Ramesh D. Gulati, Wolf M. Mooij, Andrey G. Degermendzhy

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Preface to the Siberian lakes special issue

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Ecology of Meromictic Lakes

This volume presents recent advances in the research on meromictic lakes and a state-of-the art overview of this area. After an introduction to the terminology and geographic distribution of meromictic lakes, three concise chapters describe their physical, chemical and biological features. The following eight chapters present case studies of more than a dozen meromictic lakes, showing the variety of physical and biochemical processes that promote meromixis. The result is a broad picture of the ecology and biochemistry of meromictic lakes in tropical and cold regions, in man-made pit lakes and euxinic marine lakes, and in freshwater as well as hypersaline lakes. In the final chapter the editors provide a synthesis of the topic and conclude that the study of meromictic lakes also offers new insights into the limnology of inland lakes. The book appeals to researchers in the fields of ecology, limnology, environmental physics and biophysics

Some generalizations based on stratification and vertical mixing in meromictic Lake Shira, Russia, in the period 2002–2009

In a brackish, temperate, 24-m-deep Lake Shira, the profiles of salinity, temperature, oxygen and sulfide concentrations were measured on a seasonal basis from 2002 to 2009. The lake was shown to be meromictic with autumnal overturn restricted to mixolimnion. The depth of mixolimnion and position of oxic–anoxic interface varied annually. The spring mixing processes contribute to the formation of mixolimnion in autumn. The exceptionally windy spring of 2007 caused the deepening of mixolimnion in the winter of 2008. The winter position of oxic–anoxic interface was affected by the position of lower boundary of mixolimnion in all winters. The salinity in the winter mixolimnion increased compared with the autumn because of freezing out of salts from the upper water layers meters during ice formation and their dissolution in water below. The profiles of salinity and temperature were simulated by the mathematical 1-D model of temperature and salinity conditions taking into account ice formation. The simulated profiles generally coincided with the measured ones. The coincidence implies that simplified one-dimensional model can be applied to roughly describe salinity and density profiles and mixing behavior of Lake Shira.

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

Formation of the vertical heterogeneity in the Lake Shira ecosystem: the biological mechanisms and the mathematical model

Data on the seasonal changes in vertical heterogeneity of the physical-chemical and biological parameters of the thermally stratified Shira Lake ecosystem (Khakasia, Siberia) in 1996–2000 have been analyzed. The interaction mechanisms involving: (1) The plankton populations in aerobic and anaerobic zones, involving the cycling of carbon and sulphur, (2) the primary production limitation (by light and phosphorus) and inhibition (by light), and (3) the kinetic characteristics of plankton populations have been elucidated. A mathematical model of the vertical structure of the lake's plankton populations, based on the ecosystem description and on vertical turbulent diffusion of the matter, has been constructed. The green alga Dictyosphaerium tetrachotomum (Chlorophyta) and the cyanobacterium Lyngbya contorta (Cyanophyta), which dominated the phytoplankton biomass, were taken as oxygen producers. Arctodiaptomus salinus (a calanoid copepod) has been assumed as the main grazer in Shira Lake as it dominated the zooplankton biomass. Four groups of microorganisms involved in the sulphur cycle formation have been distinguished: sulphur, sulphur purple, sulphur green and SRB. H2S is oxidized to sulphate (only the green sulphur bacteria oxidize it to sulphur), and sulphate is reduced to H2S, forming neither sulphur nor its water-soluble compounds. The role of grazing, light and nutrient limitation, in forming the vertical inhomogeneities, particularly in lowering the depth of the maximal cyanobacterial biomass, has been demonstrated. When the model takes into account both light limitation and nutrient limitation of algal growth by P and consumption of algae by crustaceans: (a) in the scenario where the P is formed only by the cycling and decomposition of autochthonous organic matter, both the green algae and cyanobacteria are eliminated; (b) in the scenario involving an additional P flux in the deep water layers the peak of the cyanobacteria is at a depth of 10 m, and its amplitude is close to the one observed in the lake. The position of the peak remains stable owing to the `double' limitation mechanism: light `from above' and P `from below'. Another mechanism responsible for the deep position of the peak of cyanobacteria was analyzed mathematically based on the model involving the experimentally proven assumption of the growth inhibition by light in the epilimnion and the light limitation in the hypolimnion. The main result is: the peak is positioned stable at its depth and does not change with time. The analytical and numerical calculations made for this positioning mechanism yielded the formulae relating the depth of the maximum of algal biomass, the `width' of the peak base and the peak amplitude and a number of parameters (algae elimination, turbulent diffusion coefficient, sedimentation rate, light extinction coefficient and light intensity). The theoretical curves for the stratification of chemical and biological parameters have been brought in conformity with field observations, e.g. for the different patterns for the peaks, and the biomass maxima of cyanobacteria, purple and green sulphur bacteria, oxygen, and hydrogen sulphide. The calculations revealed that for an adequate assessment of the parameters for the hydrogen sulphide zone it is necessary to introduce flows of allochthonous organic matter. For the first time, theoretically, based on the form of the sulphur distribution curve, the allochthonous input of organic matter [KEYWORDS: phytoplankton; sulphur cycle; vertical model; stratification control; sulphate-reducing bacteria; hydrogen sulphide

Effect of winter conditions on distributions of anoxic phototrophic bacteria in two meromictic lakes in Siberia, Russia

The year-to-year variations of vertical distribution and biomass of anoxic phototrophic bacteria were studied during ice periods 2003–2005 and 2007–2008 in meromictic lakes Shira and Shunet (Southern Siberia, Russian Federation). The bacterial layers in chemocline of both lakes were sampled with a thin-layer hydraulic multi-syringe sampler. In winter, biomass of purple sulphur bacteria varied considerably depending on the amount of light penetrating into the chemocline through the ice and snow cover. In relatively weakly stratified, brackish Shira Lake, the depth of chemocline varied between winters, so that light intensity for purple sulphur bacteria inhabiting this zone differed. In Shira Lake, increased transparency of mixolimnion in winter, high chemocline position and absence of snow resulted in light intensity and biomass of purple sulphur bacteria exceeding the summer values in the chemocline of the lake. We could monitor snow cover at the lake surface using remote sensing and therefore estimate dynamics and amount of light under ice and its availability for phototrophic organisms. In Shunet Lake, the light intensities in the chemocline and biomasses of purple sulphur bacteria were always lower in winter than in summer, but the biomasses of green sulphur bacteria were similar.

A one-dimensional model of vertical stratification of Lake Shira focussed on winter conditions and ice cover

In meromictic lakes such as Lake Shira, horizontal inhomogeneity is small in comparison with vertical gradients. To determine the vertical distribution of temperature, salinity, and density of water in a deep zone of a Lake Shira, or other saline lakes, a one-dimensional (in vertical direction) mathematical model is presented. A special feature of this model is that it takes into account the process of ice formation. The model of ice formation is based on the one-phase Stefan problem with the linear temperature distribution in the solid phase. A convective mixed layer is formed under an ice cover due to salt extraction in the ice formation process. To obtain analytical solutions for the vertical distribution of temperature, salinity, and density of water, we use a scheme of vertical structure in the form of several layers. In spring, the ice melts as top and bottom. These processes are taken into account in the model. The calculated profiles of salinity and temperature of Shira Lake are in good agreement with field measurement data for each season. Additionally, we focussed on the redox zone, which is the zone in which the aerobic layers of a water column meet the anaerobic ones. Hyperactivity of plankton communities is observed in this zone in lakes with hydrogen sulphide monimolimnion, and Lake Shira is among them. The location of the redox zone in the lake, which is estimated from field measurements, coincides with a sharp increase in density (the pycnocline) during autumn and winter. During spring and summer, the redox zone is deeper than the pycnocline. The location of pycnocline calculated with the hydro physical model is in good agreement with field measurement data.