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The occurrence of plankton seasonal dynamics in Mediterranean shallow lakes is characterised by a marked interannual variability, which makes it difficult to establish reliable predictions on the dynamics and functioning of plankton in these ecosystems based on the Plankton Ecology Group (PEG) model. In the present paper we study the succession of the phytoplankton and zooplankton communities in the semi-arid Mediterranean Nabhana reservoir (Central Tunisia) and its relationships with environmental factors during the period from May 2005 to January 2006 in the deepest area of this oligo-mesotrophic reservoir. Water temperature was a keystone factors in the seasonal dynamics of zooplankton. Cyanobacteria dominated the phytoplankton community throughout the study year 2005-2006 (94-99%) and Microcystis aeruginosa contributed for 93% of the total cyanobacteria abundance. The zooplankton community was dominated by copepods (66%) and cladocerans (28%). The dominant species was Copidodiaptomus numidicus (66% of total zooplankton) followed by Diaphanosoma brachyurum (22% of the total zooplankton). In addition, our results showed that the peak of phytoplankton abundance (5.6 x10(6) cells l(-1), June) coincided with that of zooplankton abundance (2.1 x10(2) ind l(-1), June) contrary to the clear-water phase model. The cyanobacteria and copepods were responsible for these exceptional peaks that occurred in June, accounting respectively for 99% of the total phytoplankton and 82% of the total zooplankton. In the Nabhana reservoir, the trophic relationship between phytoplankton and zooplankton were implicated in the phytoplankton abundance and dynamics. The occurrence of cyanobacterial blooms is determined by biotic and abiotic factors. The dynamics between cyanobacteria and their grazers may emerge as a regulator of blooms.