Revision of European Brachysira species (Brachysiraceae, Bacillariophyta): I. The Brachysira microcephala - B. neoexilis enigma

The taxonomic identity of several similar Brachysira species in Europe is investigated based on a combination of the analysis of original type material, additional historic samples, and modern samples. Brachysira microcephala and B. neoexilis can be separated based on morphological and ecological differences. Brachysira microcephala shows a lanceolate valve outline lacking convex margins but with protracted, rostrate to non-protracted, acutely rounded apices. The species prefers oligotrophic, acidic conditions. Brachysira neoexilis possesses lanceolate valves with rounded, convex margins and distinctly protracted, capitate to subcapitate apices, and typically occurs in oligotrophic lakes and rivers with a pH ranging from slighty acid to cleary alkaline. In addition, the type materials of B. liliana and B. neglectissima, two species showing some morphological similarities with the other two species were reexamined. Both taxa have a high stria density, with individual striae not resolvable in LM and prefer calcium-bicarbonate, oligotrophic, alkaline lakes. All species are illustrated with both LM and SEM observations. Their ecological preferences are derived from the accompanying diatom flora in the different investigated samples and the literature.