The freshwater and terrestrial diatom composition of the Subantarctic island Ile de la Possession is investigated in detail. The island is part of the Crozet Archipelago, situated in the southern Indian Ocean. A total of 220 diatom taxa, belonging to 44 genera have been identified after surveying more than 500 samples, taken from all over the island. The most diverse genera are Pirmularia (41 taxa), Nitzschia (19 taxa), Psammothidium (13 taxa) and Diadesmis (11 taxa). Detailed SEM analysis of all observed taxa resulted in the description of several new species and varieties, especially in the genera Diadesmis and Pinnularia, whereas several others need further observations to be described later on as new taxa. In addition, the taxonomy of the so-called Subantarctic taxa has been thoroughly revised based on the most recent insights. All taxa are morphologically and ecologically characterized and elaborately documented with both LM and SEM micrographs.weiterlesen