Revision of European <i>Brachysira</i> species (Brachysiraceae, Bacillariophyta): III. Species formerly included in the <i>Brachysira serians</i>-complex

This third paper revising the European <i>Brachysira</i> species deals with <i>Brachysira serians</i> and the closely related <i>B. brebissonii–intermedia</i> complex. The morphology of these species is characterised in greater detail than before based on a combination of analyses of original type material and additional historic and modern samples. All taxa in this complex are typically found in oligotrophic, acidic environments. The largest species in this complex are <i>B. serians</i> and <i>B. wygaschii</i>, which both possess a distinct Voigt discordance. Although they often co-occur, differences in their valve outline and dimensions are distinctive. The type material of <i>Brachysira brebissonii</i> is restudied and differs from a widespread use of this species name in Europe. As a result, <i>Anomoeoneis intermedia</i>, described from Denmark, is considered a more recent heterotypic synonym of <i>B. brebissonii</i>. Two new species are described to clarify the taxonomic identity of the <i>B. brebissonii–intermedia</i> species complex: <i>B. elisabethiana</i> Van de Vijver, C.E.Wetzel &amp; Ector, sp. nov. and <i>B. confusa</i> Van de Vijver, R.L.Albert, B.Kennedy &amp; Kusber, sp. nov. The type material of the morphologically similar <i>B. arctoborealis</i> from North America is studied for comparison. The ecological preferences of all the reported taxa are derived from the accompanying diatom flora in the investigated samples.