Marine phytoplankton include all pelagic microalgae that float in the water and represent the primary producers of organic substances and oxygen in the pelagic environment. Because of photosynthetic pigments stored in the chloroplasts of cells and depending to the natural light energy penetrating in the seawater, these algae can produce photosynthesis and synthesize basic organic substances to living marine organisms. Marine algae are highly diversified in the world ocean. During the last 40 years of survey in the Lebanese seawaters and the Levantine Basin, we identified and described about 400 species, belonging to 85 genera, including 230 dinoflagellates and 156 diatoms, 10 Silicoflagellates and 4 Ebriidae, many of them are introduced or migrated from Red Sea and Indian Ocean into the Mediterranean, through the Suez Canal pathway. These species show monthly and seasonal qualitative and quantitative distributions, with spring flowering (bloom) marking the maximum of standing crop reaching sometimes one million of cells/l-1 corresponding to about 1 mg of chlorophyll-a /m-3 of seawater. Geographic distribution as well as vertical and seasonal variations of populations depend on several hydro-climatic and physical-chemical factors. Chemical nutrients such as phosphates, nitrates and silicates, constitute determinant factors for the sustainable development of microalgae. On the other hand, the water temperature, salinity and water mass movement are major factors in the spatio-temporal distribution of the species. During winter (December-March) the upwelling and seawater mass mixing create homothermal conditions suitable for the development of cells to reach spring bloom. Inversely, during the summer hot season (June-October), the warming surface seawater and the heavy évaporation intensity, generate the stratification of water layers accompanied with sharp thermocline in the layer 100-0 m. These hydrological conditions, in addition to the shortage of nutrients concentration and dissolved oxygen may create impoverishment in the quality and quantity of the plankton community. Furthermore, this annual cycle is regular from year to year showing little fluctuations. The Levantine Basin, including the Lebanese sector constitute oligotrophic water body, the poorest in the entire Mediterranean. This oligotrophy induce impoverishment in marine resources and thus a low fishery production, despite of high marine biodiversity.