In order to have an overview of the state of P. oceanica on the north-eastern Algerian coast, a monitoring of seasonal phenological parameters was carried out in the Gulf of Annaba in six stations at different depths (Cap de garde, Ain Achir, La caroube, Lever de l'aurore, Draouche, and Cap rosa), during the four seasons of the year 2021. The phenological study of P.oceanica shows that the number of leaves at the level of the clusters differs according to the stations and the seasons, we have highlighted a seasonal variation of the Posidonia meadow of this region. The results indicate that the leaf area calculated from the length and width of the leaves (intermediate and adult) depends significantly on the study season and the stations studied. For juvenile leaves, the study indicates that the phenological parameters of juvenile leaves are similar for the six stations and during the four seasons of study. The meadow of the Gulf of Annaba shows signs of disturbance, considering its weak vitality, especially in the stations Lever de l'aurore, Ain achir and La caroube. A considerable effort must be provided, in order to preserve the meadows with P.oceanica of the various localities of the Algerian littoral.