The ecosystems of the Gulf of Annaba are under heavy anthropogenic pressure. The posidonia oceanica plant is considered one of the most important oxygen-producing ecosystems in the Mediterranean. In order to gain an overview of the north-eastern Algerian coast, monthly monitoring of physico-chemical parameters and a study of the seasonal variation of P. oceanica were carried out in the Gulf of Annaba at six stations at different depths (Cap de garde (S1), Ain Achir (S2), La caroube (S3), Lever de l'aurore (S4), Draouche (S5), and Cap rosa (S6)), during the year 2021. Physico-chemical parameters show variations in water temperature, with two main thermal periods: cold and warm. pH remains basic, varying between 8 and 9 throughout the year at the 6 stations, with high oxygenation in winter and more or less stable salinity. Nutrient levels vary according to season, with ammonium, nitrite and orthophosphate peaking in winter and nitrate peaking in summer, with peaks at stations S3 and S4. The most widespread epiphytic species in the Gulf of Annaba are Bryozoa (Electra posidonia), Hydrozoa (Diphasia attenuata and Amphisbetia operculata) and Molluscs (Balanus perforatus and Columbella rustica). The high percentage of epiphytic species is marked at stations S2, S3 and S4, where nutrient levels are at their highest compared with the other stations. Epiphytic biodiversity is linked to eutrophication, with eutrophied stations having the most epiphytes. Considerable effort is required to preserve the various localities on the Algerian east coast.