The temporal variability of diversity and cell abundance of the phytoplankton community was studied in the Beni-Zid reservoir using the Utermöhl method. A total of 54 taxa were identified, belonging to seven classes: Chlorophyceae (17 taxa), Bacillariophyceae (15 taxa), Cyanophyceae (14 taxa), Zygnematophyceae (4 taxa) Cryptophyceae (2 taxa), Dinophyceae (1 taxon), and Chrysophyceae (1 taxon). Throughout the study period, perennial species were Oscillatoria limnetica, Microcystis aeruginosa, Pinnularia major and Dinobryon sp. The highest cell densities were 585.104 Ind.l-1 recorded during June, with quantitative dominance of Cyanophyceae. The highest diversity and a quasi-homogeneous distribution of cells enumerated on the different inventoried classes were observed during the period from March to June; this is indicated by the maximum values of the evenness index of Pielou (J) and the Margalef index (Dm). The monthly assessment of the water trophic level by calculating the trophic state index (TSI), indicates an eutrophic state of the dam water. It confirms the tendency for blue-green algae to dominate the existing phytoplankton community. The Hierarchical Cluster Analysis (HCA) and the principal components analysis (PCA) both applied to the data of deviations calculated between the different TSIs, showed a strong correlation between the reduction of water transparency by planktonic microalgae cells and the limitation of its growth by the phosphorus element, this particularly concerns the period of the year from August to November. While nitrogen element limitation hardly occurred at any time of the year.
