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Abstract
The study investigated the taxonomic and local differentiation of diatom communities in two Alpine mountain streams in Vorarlberg (Austria). Over 16 months the diatom associations were explored, together with some important physical and chemical variables, and the fluctuation of discharge. The analyses of phytoassemblages are based on different samples and methods of mapping natural substrata. At three sections of the mountain stream Litz (calcareous) and four sections of the mountain stream Meng (deficient in lime) in different geological districts of Vorarlberg (Austria) the observation of primary production has been followed over one year. Special attention is paid to relative abundance of diatoms as a foundation for the vegetation analyses with the help of multivariate methods of analysis. The diatom assemblages are characteristic for alpine rivers (in the Litz: Achnanthes minutissima Kützing, Cymbella minuta Hilse and in the Meng: Achnanthes biasolettiana Grunow, Gomphonema olivaceum (Hornemann) Brébisson var. calcareum (Cleve) Cleve). The two mountain streams are primarily influenced by the geology of the catchment area and secondarily by fluctuations in discharge, temperature and light. The local differentiation of diatom assemblages (microhabitat structure) is marked in all seven sections of the river. The sessonal and longitudinal differentiation in both rivers, are as expected, relatively slight.