The type material of Gomphonema rosenstockianum LANGE–BERT. et E. REICHARDT from La Gomera (Canary Islands, Spain) and epilithic material of rivers from several European countries were examined using light and scanning electron microscopy in order to improve the knowledge on the taxonomical status of the G. rosenstockianum and G. tergestinum (GRUNOW) M. SCHMIDT species complex. Two other Gomphonema species, G. supertergestinum E. REICHARDT and G. angustius E. REICHARDT, recently described and belonging to the same group, are also presented in detail. After the analysis of several populations of G. rosenstockianum and G. tergestinum it was possible to define several reliable criteria to allow morphological differentiation of both species under LM and SEM as well. A geometric morphometric analysis clearly demonstrated the separation of these taxa considering valve outline, size and shape of the central area and position of the stigma. Apart from the morphological analysis, compilation of the results of this study has also revealed differences in ecological preferences of the two species. World and European distribution maps of the four Gomphonema taxa are presented.
