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