The bay of Sidi Raïs (northeastern coast of Tunisia) is home to a vast Posidonia oceanica (L.) Delile meadow containing a barrier reef inside which a shallow lagoon is individualized and a Cymodocea nodosa meadow has been installed. The particular conditions of this ecosystem, especially the presence of the invasive chlorophyte caulerpa racemosa, incited us to carry out a study that allows us to characterize this ecosystem.