Freshwater diatom biogeography

Current diatom research is predominantly based on a paradigm of a cosmopoli tan geographic distribution of freshwater diatoms, a paradigm that has been almost without challenge. The accepted worldview of diatom biogeography ha s been based on dispersal and ecological selection however, it appears that such a perspective is not fully supported by observations. Studies of the regional distribution of diatoms, herbarium voucher material, local diatom populations, and eolian dispersal do not support cosmopolitanism. The distr ibution of diatoms may have more to do with historical distributions than w ith ecological tolerances. We advocate a phylogenetic view of biogeography, including the importance of historical distributions and evolutionary line ages. While a number of taxa may be confirmed to be cosmopolitan, we predic t that they are few compared to the total number of species. Furthermore, a nthropogenic impacts, including transport mechanisms and alterations of env ironments. may be responsible for a substantial number of true cosmopolitan taxa. As diatom distributions become better known, forgotten and overlooke d historical explanations for distributions will have implications for all areas of diatom research including: taxonomy, systematics, ecology, paleoli mnology, biomonitoring and paleontology.