Druhové zloženie subfosílnych spoločenstiev rozsievok (Bacillariophyceae) Nižného Terianskeho plesa
In the framework of the international project AL:PE 2 (Acidification of Mountain Lakes: Paleolimnology and Ecology) focused on analyses of glacial lake pollution caused by atmospheric imissions, species composition of subfossil testaceous algae (Bacillariophyceae) assemblages in the bottom sediments was studied in order to detect changes in water quality in the past time. Nižné Terialnske pleso (glacial lake) was selected to study current species structure of testaceous algae assemblages. The lake is located in the Valley of Nefcerka, the High Tatra Mts., in the elevation 1 941 m a.s.l. (size 4.8 ha, length 360 m, width 235 m, depth 44.4 m). In 1993 samples of profundal sediments (height cca 35 cm, width 7.4 cm) were collected from the deepest part of the lake by depth probes and split into 1 mm cuts. Sedimentation rate was estimated on 0.043 cm per year until 1 850. The samples were dated by radionuclide method, e.g. layer 7.25 cm corresponded to the period 1837±19 years, and chemically processed for further analyses of selected systematic groups. 500 shells of testaceous algae were counted in each sample together with the number of microspheres (polystyrene grains 5–30 µm in size). Ratio between shells and microspheres in each sample was used to estimate the relative abundance of algae in each layer. Totally, 20 samples of various depths corresponding to the certain time period were analyzed. Individual layers from the upper part to the depth of 35 cm differed in the qualitative and quantitative structure of the assemblage. In total, 106 species of 23 genera were detected in the analyzed sediments. In the upper 2 cm layer (the period 1955–1993), species, e.g. Achnanthes marginulata, A. subatomoides, A. austriaca var. helvetica, A. altaica, Synedra minuscula, that were not found or had very low abundance in the deeper layers occurred. In the layers 17–2 cm, relative abundance of several species drastically decreased, yet abundance of other species, e.g. Fragilaria construens, Navicula schmassmannii, Achnanthes minutissima, A. curtissima, significantly increased. The lower layers between 35–17 cm had different assemblage structure. Both planktonic and non-planktonic species, e.g. Asterionella formosa, Synedra tenera, Fragilaria brevistriata, F. pinnata, Denticula tenuis, Navicula schmassmannii, Achnanthes minutissima, A. curtissima occurred. In summary, subfossil testaceous algae assemblages were very sensitive to water quality changes in the history of lake and may be used as sensitive indicators of water quality in the global environmental monitoring schemes.
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