In order to investigate the bacterial diversity in a number of rivers, reservoirs and lakes in northern and central Portugal during the winter of 200

The diversity within the genus Aphanizomenon (Nostocales, Cyanobacteria) is still controversial and more studies are needed to clarify its heterogeneity. Aphanizomenon strains isolated from the eutrophic shallow Vela Lake (Centralwestern Portugal) during bloom formation in summer months from 2001 to 2007 were characterized combining phenotypic and molecular approaches. The molecular affiliation of the strains was established through sequencing of the 16S rDNA, as well as nifH and hetR genes fragments, and inter-specific genetic variability was checked through repetitive sequences fingerprinting. The morphological characters were characteristic of the descriptions of Aph. issatschenkoi (Cuspidothrix issatschenkoi), Aphanizomenon gracile and Aph. aphanizomenoides. Molecular phylogenetic results corroborated the morphological identification and fingerprinting showed a persistent occurrence of Aph. gracile and Aph. aphanizomenoides strains over the years but also indicated that Aph. aphanizomenoides strains are not clonal within a same bloom. Partial nifH and hetR gene sequences of the cyanobacterial strains showed a high phylogenetic differentiation capacity, even at the strain level.