The main objective of our work is to contribute to the knowledge of the diversity microscopic algae or microalgae from freshwater of Sidi M’Hamed Ben Taïba dam at Water Treatment Plant of Arib, their biological cycle, bioecological behavior in relation to environmental factors, the evolution of the structure and organization of phytoplankton populations. Continental aquatic environments are complex, fragile ecosystems governed by precise physicochemical laws. The microalgae that colonize them live in an unstable equilibrium and are totally dependent on them. The most tolerant genera adapt to the entry of an exogenous substance or to a modification of the physicochemical parameters. Microalgae are able to proliferate when environmental conditions are favorable to them. The role is essential within biogeochemical cycles and in the functioning of aquatic ecosystems since it is, among other things, at the base of trophic chains but the first biological community to respond to eutrophication.