Revision of the Nitzschia sigmacomplex (Bacillariophyta), a frequent cosmopolitan species in disguise with the description of two new species

ABSTRACTThe original material of four taxa in the Nitzschia sigmacomplex has been studied and revised based on light and scanning electron microscopy observations. Nitzschia sigma, in this paper lectotypified based on the original Kützing drawing and epitypified using original de Brébisson material from Courseulles (Calvados, France), is a large marine-brackish species with a distinct striation pattern and structure. Several of the studied taxa, originally considered as varieties of N. sigma, showed sufficient morphological differences to be raised to species level or be described as new species, due to nomenclatorial issues. Nitzschia rigidula(Grunow) stat. nov., a freshwater species, originally described as N. sigmavar. rigidulafrom the region around Brussels (Belgium) is the smallest of the new taxa. Analysis of another, former N. sigma-variety, proved to be based on an illegitimate, superfluous Kützing taxon, showed that two independent species were included under the same name, N. sigmavar. rigida, that are described in this paper as two new, brackish to freshwater species: Nitzschia neorigidasp. nov. and N. pararigidasp. nov. All species are morphologically characterised and illustrated and their ecological preferences based on the associated diatom flora in the samples, are better defined.