The microscope analyzes for the evaluation of the taxonomic structure of the phytoplankton assembly and for the estimation of cell densities were performed according to the Utermöhl method (1958), which involves the use of the inverted optical microscope and sedimentation chambers. In this case, a known volume of the fixed sample, after manual homogenization for a few minutes, is placed in sedimentation chambers (generally 5 or 10 cc) for a time (variable depending on the height of the cylinder
of the chamber) necessary for all microscopic organisms in suspension to settle on the surface of the chamber's basal slide. Subsequently, based on the abundance of organisms present and the uniformity of the distribution on the bottom of the chamber, any dilutions are made or the cells of the individual species or algal forms present at magnifications 100x, 200x and 400x are counted. The counts are carried out on a statistically adequate number of optical fields, uniformly distributed according to predetermined transects and representative of the entire surface of the chamber.
The cell densities detected were subsequently referred to a liter of water. The recognition and classification of genera and species were carried out following different dichotomous keys.