Identification and counts of specimens were performed at the inverted microscope after sedimentation of variable volumes of seawater (1-100 ml), depending on cell concentration (Utermöhl, 1958), on two transepts representing ca. 1/30 of the whole bottom area of the sedimentation chamber at 400x magnification. Specimens were identified at the species level when possible, or at higher taxonomic levels. Taxonomic identification was performed according to the most relevant and updated taxonomic literature. For selected species, the identification was checked at the electron microscope.