Morphological variability within the Achnanthidium minutissimum species complex (Bacillariophyta): comparison between the type material of Achnanthes minutissima and related taxa, and new freshwater Achnanthidium species from Portugal - NOVAIS, MARIA HELENA; JÜTTNER, INGRID; VAN DE VIJVER, BART; MORAIS, MARIA MANUELA; HOFFMANN, LUCIEN; ECTOR, LUC (2015)