Aulacoseira subarctica f. subborealis differs from A. subarctica var. subarctica by a number of morphological features (smaller linking spines without areolae on their base, shorter spine grooves, discus always areolated, shallower mantle and different 'Formwechsel', considerably finer structure, rimoportula on the inner side of the pseudoseptum), as well as its ecological preferences. It is therefore considered to represent a separate species: A. subborealis stat. nov. Characteristics that differ from those of the similar species, A. alpigena, include the non-spathulate linking spines, each one originating from two pervalvar ribs, more numerous areolae on the disci, smaller dimensions, finer areolation of the mantle and, in the LM, absence of more marked mantle areolae next to the collum. In the LM, confusion may further arise with A. laevissima, A. distans or A. nygaardii, which also show some resemblance to A. subborealis. Aulacoseira subborealis is widely distributed in rivers and lakes in W-Europe and Australia, and presumably occurs in New Zealand and North America as well, but has been reported only rarely because of confusion with other taxa. It appears to develop especially well in more alkaline and rather eutrophic, highly turbid fresh water. Yet, this planktonic species is unable to withstand extremely severe eutrophication or pollution.