Most of the Mediterranean zooplankton groups are present in the area from the Protozoans up to Protochordates and Fish larvae. The zooplakton constitutes the secondary producer of organic substance, the phytoplankton being the primary producer. The zooplankton community, as that of the Levantine Basin present a high biodiversitry with low biomass and showing pronounced seasonal variations in quality and quantity of the zooplankton. The maximum development of zooplankton is observed in May June, just following the spring phytoplankton bloom. In winter, zooplankton is poor in quantity but very diversified corresponding with the mixing and turn over of water layers leading to the vertical homothermal conditions. During summer, the stratification of the water masses in the layer 35-75 m creating the thermocline may lead to the impoverishment of zooplankton in quantity and quality. The majority of zooplankton organisms display short biological life cycle and some of them showing more than one annual generation. Several species that migrated through the Suez Canal, or invaded the Lebanese waters are of Indo-Pacific and/or Erithrean origin, called Lessepsians. Many of them have established permanent populations competing with local and native species.
Many zooplankton species display vertical migrations due to hydrologic, physiologic and trophic factors.Usually the herbivorous zooplankton stay in the euphotic zone (100-0 m ) rich in phytoplankton, whereas the carnivorous species live in the deeper seawater. However the diurnal vertical migration practiced by many species has a physiological and ecological reasons, namely the action of the light, the nutrition and reproduction.
Generally, the LevantineBasin, including the Lebanese seawater is a zone show low primary productivity rate , inducing poor zooplankton and thus little fisheries rentability.