Macroscopic cyanobacteria of the genusNostoc: a neglected and endangered constituent of European inland aquatic biodiversity

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Abstract
Nostoc caeruleum, N. commune, N. microscopicum, N. parmelioides, N. pruniforme, N. verrucosum and N. zetterstedtii are all cyanobacteria forming macroscopically visible colonies. They were selected for a nature conservation case study. Their specific identity is easily determined, and reliable historical records are available. Life conditions for these species in Europe in the ‘virgin’ postglacial biosphere, historical reports and the present-day situation are surveyed. Data concerning type materials, supplements to the protologues, and morphology are presented. The classic descriptions and diagnoses based only on herbarium material are supplemented by ontogenetic details. Global/regional changes in the biosphere (climate, regulation of lake levels and river flows, import of plant nutrients, water pollution, acidification, etc.) and their consequences for aquatic biocoenoses are reviewed. The analysis of man-made impacts on the environment enables us to evaluate the present state of habitats where these algae occur today or where they are now extinct. Documented changes in environmental conditions in the past serve as a basis for assessing the likely future developments of the biotopes and of the whole landscape where these cyanobacteria form part of the biocoenoses. However, there are few recent detailed ecological studies on benthic and terrestrial cyanobacteria. It is not yet clear which of the different components covered by the comprehensive term ‘eutrophication’ is the decisive factor for the changes in aquatic biocoenoses and the decline of cyanobacterial biodiversity.

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