Posidonia oceanica seagrass beds are the most important marine ecosystems along the relatively shallow waters of the Mediterranean coast. These meadows can be found from the shore up to 30-35 m of depth and spread along a considerable distance. The French cartographic database of Posidonia distribution and health status is quite complete. However, the monitoring of this ecosystem is often done under the same conditions, i.e., in the centre of the meadow at an average depth of 15 m. Yet, to study biodiversity patterns and assess the services provided by this ecosystem at the seascape level, we need to monitor seagrass beds at various points from the upper to the lower depth limit, including the interfaces with other habitats. While monitoring meadows is easily achieved on land, it becomes very challenging underwater. According to the choice of the study area, such monitoring allows to study the global health status of Posidonia ecosystem under different levels of human pressures. Our fieldwork last month (July 2021) at Cap Sicié in the South of France aimed at monitoring this ecosystem using novel approaches to overcome those challenges. This specific site of Cap Sicié presents a strong gradient of habitat quality from healthy to degraded which offers the advantage to explore the ecological responses in terms of biodiversity and produced ecosystem services.
