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

Abstract

The paper summarises the results of a three-year research study (European Union Grant: INTAS 97-O519) aimedat investigating the planktonic populations and trophic organization of the Lake Shira ecosystem – a saline lake inKhakasia, Siberia. The lake exhibits a stable summer-autumn stratification of the chemical-biological components.The mechanisms responsible for the ‘blooming’ of phytoplankton in the deeper layers were investigated in greaterdetail, using data from both field and laboratory experiments. The spectra of nutrition were examined to estimatethe 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 explainthe development in the deeper waters of Lyngbya contorta. The scheme of trophic interactions was put up, basedon the assessment of the carbon pools and carbon flows in the pelagic zone of the lake. A mathematical modelof the vertical structure of the lake's plankton populations was constructed, using the ecosystem description anddata of vertical turbulent diffusion. The role of light and nutrient limitations and grazing mortality in formingthe vertical inhomogeneities, particularly in lowering the depth of the maximal cyanobacterial biomass, has beendemonstrated. The theoretical curves for the stratification of chemical and biological parameters have been broughtin conformity with the field observations, e.g. for the different patterns of the peaks, and for the biomass maximaof cyanobacteria, purple and green sulphur bacteria, oxygen, and hydrogen sulphide. The calculations revealedthat for an adequate assessment of the parameters for the hydrogen sulphide zone it is necessary to introduce flowsof allochthonous organic matter. Based on the form of the sulphur distribution curve, the allochthonous input oforganic matter and the inflow of hydrogen sulphide from the bottom have been theoretically discriminated for thefirst time. It has also been ascertained that irrespective of the depth the allochthonous substances limiting bacterialgrowth, the bacteria are uniformly distributed over depth and can serve as an indicator of the presence of limitation(the effect of autostabilisation in space).