Three New Species of Navicula (Bacillariophyta) from Oregon and Montana and a Review of Diatom Endemism in the Northwest

Three new species of Navicula sensu stricto are described from samples deposited in the Montana Diatom Collection. Navicula galloae and N. sovereignii live in streams in the Cascade Mountains of Oregon. N. flatheadensis lives in lakes in the headwaters of the Flathead River Basin in western Montana. At present, 80 species and varieties of endemic diatoms in 20 genera define the Northwest as a distinct biogeographical region, and many endemic species remain to be described. Other diatom taxa in the region indicate floristic affinities with northeast Asian, Arctic, and European alpine floras. Factors contributing to the large number of endemic taxa and to observed floristic affinities are discussed.