Abstract
Published checklists of freshwater diatoms that represent the American Northwest, Laurentian Great Lakes, Germany and the South Polar Region were compared systematically and the numbers of taxa shared by two or more of these regions were noted. There is a higher level of floristic correspondence between the American Northwest and Germany (71%) and between the American Northwest and the Laurentian Great Lakes (64%) than between the American Northwest and the South Polar Region (45%). These findings support a Holarctic Kingdom of diatoms that is parallel to the Holarctic Kingdom of flowering plants. Mountains and coastal areas and/or inland waters of high salinity may explain why the American Northwest and Germany have more taxa in common than the American Northwest and the Laurentian Great Lakes. Common riverine diatom taxa in the American Northwest are similar to those reported from nationwide monitoring stations. The number of truly cosmopolitan species – those found on all continents – is probably less than 300. The terms “cosmopolitan”, “endemic” and “native” are often misused when applied to diatoms and the first two terms always need to be qualified.
Highlights
Until the end of the last century, the prevailing view of freshwater diatom biogeography had been one of cosmopolitan distribution, where the majority of species occur worldwide in suitable habitats and endemism is exceptional
Light microscopy was the primary taxonomic tool and similar taxa were often “lumped” into a single species (e.g., Patrick and Reimer 1966, 1975, Krammer and Lange-Bertalot 1986, 1988, 1991a, 1991b). This era ended with the widespread availability of electron microscopy, which accelerated the description of new diatom species, most of which appear to be endemic
The combined American Northwest checklist was compared to a diatom dataset produced for rivers of the contiguous United States by the US Geological Survey’s National Water Quality Assessment (NAWQA) Program (Potapova and Charles 2002), which allowed for a comparison of the most frequently occurring taxa in the two datasets
Summary
Until the end of the last century, the prevailing view of freshwater diatom biogeography had been one of cosmopolitan distribution, where the majority of species occur worldwide in suitable habitats and endemism is exceptional. Diatoms and other eukaryotic microbes were dispersed globally with few restrictions (Finlay 2002, Finlay et al 2002) During this era, light microscopy was the primary taxonomic tool and similar taxa were often “lumped” into a single species (e.g., Patrick and Reimer 1966, 1975, Krammer and Lange-Bertalot 1986, 1988, 1991a, 1991b). Several lines of evidence have shown that dispersal of viable diatoms is problematic, that endemism is commonplace, and that geologic history and evolution are just as important as environmental factors in explaining the distribution of diatom species (Edlund and Jahn 2001, Kociolek and Spaulding 2000, Potapova and Charles 2002)
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