Gomphonema gracile Ehrenberg sensu stricto et sensu auct. (Bacillariophyceae): A taxonomic revision
Gomphonema gracile is the name of a huge cluster of more or less similar taxa and demes that are reported from nearly all regions worldwide. But the identity resp. type of this species is unknown up to present. In this paper G. gracile is lectotypified and it is shown that it differs from current concepts including G. naviculoides as one of the principal species. Therefore an investigation of the type-slide of G. naviculoides is presented together with descriptions and discussions of the most important species of this complex occurring especially in Central Europe (G. acidoclinatum, G. auritum, G. lippertii) including four new species (G. campodunense, G. disgracile, G. graciledictum, G. stagnorum). In addition, information on the distribution and morphology of the tropical G. guaraniarum and G. graciloides are given and there is a discussion of forms which have been called "lanceolatum" within this cluster of taxa. A new name (G. graciolatum) is proposed for G. lanceolatum sensu Kützing resp. Grunow.