At the sub-genus level, the ANW shares from 676 (SPR) to 1017 (GER) taxa with the three other checklists that were compared in this study (Table 2). Notably, the 1017 taxa shared with Germany represent 71% of the taxa reported from the ANW. Curiously, the ANW shares about 100 more taxa with Germany than it does with the much closer Laurentian Great Lakes region (916 taxa shared).

Although the ANW, LGL, and SPR checklists are nearly equal in size (the SPR list is slightly larger), there were many more matches between ANW and LGL (916) than between ANW and SPR (676). Matches between ANW and LGL represent 64% of the ANW flora, whereas matches between ANW and SPR represent only 45% of the ANW flora. This 45% agreement might seem large given that the ANW and South Polar regions are on nearly opposite ends of the Earth, however the SPR checklist extends to 35°south latitude and includes temperate as well as sub-Antarctic and Antarctic habitats (Kellogg and Kellogg 2002). Also, researchers working on samples from the Southern Hemisphere rely mostly on floras prepared for the Northern Hemisphere and may be inclined to “force” southern specimens into northern taxa.

At the genus level there is much more agreement among checklists (Table 2). When the GER and ANW checklists are compared, the 114 genera they have in common represent 95% of the 120 genera reported from the American Northwest. The 109 genera in common between LGL and ANW represent 91% of the 120 genera reported from ANW and the 106 genera shared by SPR and ANW represent 88% of the ANW genera. Even when all four checklists are compared, the 96 shared genera represent 80% of the genera present in the ANW checklist. This suggests a lower rate of endemism at the genus level than at the sub-genus level.

When all three checklists from the Northern Hemisphere are compared, there is a considerable decline in the number of shared taxa to 472 or 33% of the taxa in the ANW checklist (Table 2). These may be core or “signature” taxa that define the Holarctic Kingdom of diatoms. When all four checklists are compared, the number of shared taxa declines to 309, which is only 21% of the sub-generic taxa in the ANW checklist. These taxa are widespread in temperate regions of both the Northern and Southern Hemispheres. If comparable checklists of taxa from temperate regions of Africa, Asia and South America were also included, the list of truly cosmopolitan taxa (found on all continents) would be shorter, perhaps much shorter.

The NAWQA dataset includes 1167 named taxa and 381 unknowns based on 2735 samples collected at sites scattered across the contiguous United States but concentrated in the eastern part of the country (Potapova and Charles 2002). The combined ANW checklist contains 1581 named taxa plus 334 unknowns based on more than 8,500 samples collected across the ANW, but mostly in Montana (Bahls 2009). All of the NAWQA sites are flowing-water sites, whereas the ANW checklist includes taxa from a wide variety of habitats, including springs, seeps, ephemeral pools, rivers, streams, lakes and wetlands.

The most frequently occurring diatom taxa in the ANW and their corresponding frequency and rank in NAWQA samples are presented in Table 3. Most of these taxa are widespread in temperate regions worldwide and all of them are addressed in the Freshwater Flora of Middle Europe (Krammer and Lange-Bertalot 1986, 1988, 1991a, 1991b). As would be expected of taxa with wide geographic ranges, the taxa in Table 3 have broad ecological amplitudes. Twenty-four of these same taxa are among the 30 most frequently occurring taxa at NAWQA sampling sites. Differences between the two lists may be expected given the different geographic scopes of the two datasets and additional habitats represented by the ANW list.

Many of the unknown taxa from both the NAWQA and ANW datasets are probably undescribed and new to science. Most of these will likely have limited distributions and be endemic at one scale or another (local, regional, continental or hemispheric).

Two lines of evidence presented here support a Holarctic distribution model for freshwater diatoms: (1) a high level of floristic affinity between the ANW and Germany, with shared sub-generic taxa accounting for 71% of the ANW flora; and (2) a much lower level of floristic affinity between the ANW and the South Polar Region (45% of sub-generic taxa) than between the ANW and the Laurentian Great Lakes (64%) or between the ANW and Germany. The percentages of genera shared by the regions are larger, ranging from a high of 95% (ANW vs. GER) to a low of 88% (ANW vs. SPR).

There are three possible explanations why the ANW shares more taxa with Germany than it does with the Great Lakes region: (1) The German checklist includes 160 more taxa than the LGL checklist and therefore provides more opportunities for matches; (2) Germany and the ANW both have mountains higher than 2,000 m elevation, which allow for more southerly range extensions of northern-alpine taxa (the Laurentian Great Lakes region lacks high mountains); and (3) Germany and the ANW both have marine coasts and many brackish to hyper-saline inland waters, features that are either not present or not as common in the Great Lakes region. Many salt-tolerant taxa that are common to both Germany and the ANW are addressed in a monograph on diatoms of marine coasts (Witkowski et al. 2000).

Potapova and Charles (2002) note that 1016 (70%) of the 1461 non-planktonic diatom taxa in the NAWQA dataset have been recorded on continents other than North America, mainly Europe. In addition, 49 taxa known only from North America and 15 taxa known only from the New World are included in the NAWQA dataset (Potapova and Charles 2002). A similar but undetermined number of North American and New World endemics also occur in the ANW checklist, including many of those described by Sovereign (1958, 1963) and Kociolek and Stoermer (1986, 1988).

Reinforcing the relative dissimilarity of floras from the Northern and Southern Hemispheres are the findings of Kociolek and Spaulding (2000). They compared lists of diatoms from the Arctic and Subarctic Region of North America to lists from the Antarctic that were presented at a workshop on Arctic-Antarctic diatoms (e.g., Håkansson and Jones 1994, Hamilton et al. 1994). The Arctic and Antarctic regions have aquatic habitats with similar physical conditions at comparable latitudes and may be expected to have similar diatom floras under a cosmopolitan model for diatom distributions. However, only 80 (8.9%) of 897 taxa reported from the two regions are common to both regions (Kociolek and Spaulding 2000).

A Holarctic model for the distribution of diatoms is further supported by established Holarctic distributions for flowering plants (Takhtajan 1986). Historically, dispersal of flowering plants, and probably diatoms, was aided by land bridges that connected Europe with North America (through Greenland) until the late Eocene, and Asia with North America (Bering Land Bridge) off and on until the end of the Pleistocene (Cox and Moore 2005). Finally, all areas in the Holarctic region share a common history of glaciation. Since the end of the Pliocene, several advancing continental glaciers essentially stripped the Holarctic of its preexisting flora and resulted in a “drastic modernization” of the flora across the region (Cox and Moore 2005, Pielou 1991). The Holarctic region thereby became common ground for the establishment of a modern, cold-tolerant flora.

Rappaport’s Rule holds that high-latitude organisms have broader geographic ranges than those from the low latitudes (Cox and Moore 2005). The occurrence of broad-range species at higher latitudes could be a consequence of (1) a common tectonic history (Laurasia giving rise to Laurentia and Eurasia); (2) a common history of glaciation in the Northern Hemisphere, leaving only the most adaptable species behind; or (3) greater seasonal fluctuations at higher latitudes, which select for species with broad ecological amplitudes (Cox and Moore 2005), or a combination of these. Whatever the underlying cause or causes, the broad ranges of many diatom taxa in the northern Hemisphere conform to Rappaport’s Rule.

The term “cosmopolitan” is often used offhandedly to describe freshwater diatom taxa that are widely distributed. Webster’s New World Dictionary defines cosmopolitan as “belonging to the whole world, not national or local; at home in all countries or places”. The term therefore implies global distribution, that is, the taxon is found on all continents. But the term is often used without evidence that the taxon is present on all continents.

Lange-Bertalot, as reported by Kociolek and Spaulding (2000), estimates that there are 150–200 taxa that “occur across great ecological and geographic space that has been influenced by urbanization”. It is true that taxa with broad ecological amplitudes tend to have widespread distributions, but we often don’t know if these taxa occur on all continents. Unless we do know this, diatomists should refrain from using the term cosmopolitan and use more precise terminology, e.g., “widespread in the Northern Hemisphere” or “widespread in temperate regions”, assuming the writer has the distribution records to support these statements.

Similarly, for clarity, the term endemic should not be used without qualification. Appropriate modifiers for endemic might include local, regional, and continental, e.g., North American endemic. For example, the diatom species Cymbella janischii (A. Schmidt) De Toni is endemic to the American Northwest region (Bahls 2007). Endemics can occur at all taxonomic levels, but diatom endemics at the genus level or higher are uncommon. An example is the diatom genus Gomphocymbella O. Müller, which is endemic to Africa (Kociolek and Spaulding 2000).

There is also some confusion with and misuse of the term “native” as it applies to diatoms. For example, Potapova and Charles (2004) define native diatom species as those “generally confined to the Americas; known only from America, or common in America but found extremely rarely elsewhere”. What they describe here are actually species that are endemic to the Americas, as well as being native to the Americas. Native means occurring before European settlement or before intercontinental travel and commerce became common and widespread. Documented cases of freshwater diatoms introduced from other continents since European settlement are not common (e.g., Edlund et al. 2000, Spaulding et al. 2010, Reid et al. 2012, Kilroy and Bothwell 2011). If one accepts evolution and vicariance or “historical biogeography” as the prevailing mechanisms that explain diatom distributions (as opposed to unrestricted dispersal and “ecological biogeography”), then one must assume that all taxa are native, unless there is evidence to the contrary of their recent introduction.