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Using mathematical modeling methods, the work investigates possible existence of a phytoflagellate population in the ecosystem of a stratified lake (Lake Shira, Khakasia, Russia). Until this study was carried out, no phytoflagellate population had been detected in the lake, although there had been indirect evidence of the possible presence of a cryptomonad population in this water body. To answer the question if this population may be present in this water body, as the first step of investigation, a mathematical model was developed describing the microorganism biomass dynamics and phytoflagellate relationship with the lake food web. The calculations showed that cryptophytic algae were abundant in the water column layers above the lake chemocline at the beginning of summer but showed low biomass concentrations during other periods of time. In the summer following the construction of the model, samples were collected from Lake Shira, as the next step of the study. A cryptophytic algae population was found in the water column, and the pattern of its distribution in the ecosystem was similar to that predicted by the model. In addition to that, we studied theoretically possible reasons for the early summer development of the population in the deep water layers and found that it was the result of the mixotrophic feeding strategy of cryptomonads and trophic interactions in the food web. The study shows that preliminary modeling of the locational and temporal distribution of populations whose presence in the ecosystem is only suggested by indirect evidence may be useful as a tool for searching for the species in natural ecosystems and as an approach enabling a more accurate description of the food web structure of a water body.

During two sampling seasons we analyzed on weekly basis fatty acid (FA) composition of seston fraction 130μm, and compared them using a multivariate canonical correlation analysis (CCA). Besides, we evaluated a possible impact of water temperature and inorganic nutrients on FA composition of the seston and the zooplankton.In spite of significant differences in percentages of several individual FAs, we found very strong canonical correlation (cross-correlation, 1-week lag) between FA composition of the seston and the zooplankton. The most important factor, providing the overall canonical cross-correlation between FA profiles of the seston and the zooplankton fractions was eicosapentaenoic acid (20:5ω3, EPA). FA composition of the zooplankton fraction had comparatively poor correlations with taxonomic composition of the zooplankton. Thus, seasonal variations of FA composition of the zooplankton were determined primarily by seasonal changes in FA composition of the seston, rather than by taxonomic differences of FA profiles between rotifers, cyclopoids and cladocerans. FA composition of the seston was strongly affected by its taxonomic composition, namely by that of phytoplankton. According to CCA, the highest factor loadings pertained to diatoms interacting with their marker acids, including EPA, and cyanobacteria and greens, interacting with their marker acids. Ciliates and small rotifers composed considerable and sometimes major part of the seston biomass, but according to CCA their contributions to seasonal variations of the total FA profile of the seston were insignificant. This finding indirectly support the conclusion of the other authors, that the main source of FAs presented in ciliates and rotifers must be sought in algae and that they do not modify FA composition of food consumed, apart from repackaging it.Water temperature was the principal environmental parameter which drove the overall variations of FA composition. Factor loadings for the inorganic nutrients were comparatively negligible. The main contribution in the seasonal variation of FA composition of the seston was given by negative interaction between water temperature and percentage of EPA in the seston.

Fatty acid composition of triacylglycerols (TAG) and polar lipids (PL) of seston from the Siberian Reservoir Bugach was analyzed to reveal the spectra of nutrition of microand mesozooplankton (protozoa and rotifers). The seasonal changes of microand mesozooplankton nutrition were observed. Bacteria comprised relatively constant part in feeding protozoa and rotifers during the whole vegetation season. The maximum consumption of higher plants detritus occurred at the beginning of August. Phytoplankton was consumed more intensively in spring during the rapid growth of diatoms (Stephanodiscus sp.) and from the middle to the end of June when cyanobacteria (Anabaena flos-aquae, Aphanizomenon flos-aquae, Planktothrix agardhii), dynophytes (Peridinium sp.) euglenas (Euglena proxima) and diatoms (Stephanodiscus sp.) were dominated in Bugach Reservoir. The dominant species of protozoa differed in their nutrition spectra: Holophrya sp. was feeding on diatoms in spring and bacteria in the middle of summer; Tintinidium sp. was consuming diatoms and cyanobacteria; Nassula picta was feeding on cyanobacteria.

A feature of meromictic lakes is that several physicochemical and biological gradients affect the vertical distribution of different organisms. The vertical stratification of physical, chemical and biological components in saline, fishless meromictic lakes Shira and Shunet (Siberia, Russia) is quite different mainly because both mean depth and maximum depth of lakes differ as well as their salinity levels differ. The chemocline of the Lake Shira, as in many meromictic lakes, is inhabited by bacterial community consisting of purple sulphur and heterotrophic bacteria. As the depth of the chemocline is variable, the bacterial community does not attain high densities. The mixolimnion in Lake Shira, which is thermally stratified in summer, also creates different habitat for various species. The distribution of phytoplankton is non-uniform with its biomass peak in the metalimnion. The distribution of zooplankton is also heterogeneous with rotifers and juvenile copepods inhabiting the warmer epilimnion and older copepods found in the cold but oxic hypolimnion. The amphipod Gammarus lacustris which can be assigned to the higher trophic link in the fishless lake’s ecosystem, such as Lake Shira, is also distributed non-uniformly, with its peak density generally observed in the thermocline region. The chemocline in Lake Shunet is located at the depth of 5 m, and unlike in Lake Shira, due to a sharp salinity gradient between the mixolimnion and monimolimnion, this depth is very stable. The mixolimnion in Lake Shunet is relatively shallow and the chemocline is inhabited by (1) an extremely dense bacterial community; (2) a population of Cryptomonas sp.; and (3) ciliate community comprising several species. As the mixolimnion of Lake Shunet is not thermally stratified for long period, the phytoplankton and zooplankton populations are not vertically stratified. The gammarids, however, tend to concentrate in a narrow layer located 1–2 m above the chemocline. We believe that in addition to vertical inhomogeneities of both physicochemical parameters, biological and physical factors also play a role in maintaining these inhomogeneities. We conclude that the stratified distributions of the major food web components will have several implications for ecosystem structure and dynamics. Trophic interactions as well as mass and energy flows can be significantly impacted by such heterogeneous distributions. Species spatially separated even by relatively short distances, say a few centimetres will not directly compete. Importantly, we demonstrate that not only bacteria, phytoflagellates and ciliate tend to concentrate in thin layers but also larger-sized species such Gammarus (amphipods) can also under certain environmental conditions have stratified distribution with maxima in relatively thin layer. As the vertical structure of the lake ecosystem is rather complex in such stratified lakes as ours, the strategy of research, including sampling techniques, should consider potentially variable and non-homogeneous distributions.