Monitoring a seagrass bed at a seascape level: Overcoming challenges with technology

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.

Because most fish and invertebrates are hidden inside the canopy formed by Posidonia leaves that can reach up to 1 meter in length, they are very difficult to observe using traditional visual censuses. We used novel techniques like bioacoustics to characterise the associated benthic fauna. This method allows us to listen to the environment within hundreds of meters in all directions. Many marine invertebrate species produce sounds when eating, moving or communicating. We are then able to hear all these sounds as long as we have a highly sensitive hydrophone to record them and a skilful set of ears and software to interpret this biophony.

However, to evaluate the ecosystem and its services at a precise spatial scale, direct observation by divers is still a very accurate option. Since we are not aquatic species, it is hard to spend a lot of time underwater, moving fast, being located in this 3D environment or see far away. These limitations explain why monitoring at a seascape level can be difficult to achieve. Fortunately, thanks to innovative tools these challenges can be overcome. Indeed, using close circuit rebreather (CCR), dry suits and waterproof scooters, we were able to spend more time in the water and to move faster between each sampling point. Underwater navigation between sampling points was further facilitated by wireless geolocated waterproof tablets. We were pretty heavy underwater with this complete set of tools, but we still had space to carry our sampling materials. At each sampling point, we characterised the structural complexity of the habitat by 3D photogrammetry, made in situ samplings, evaluation of benthic communities and assessment of fish diversity that will be complimentary to the biodiversity assessed through bioacoustics.

During our first round of fieldwork this summer, we monitored a 3 km long seagrass bed with a strong gradient of health conditions due to wastewater flow. The first part of the mission included an adjustment period to master all the material and complete multiple sampling protocols in only a few minutes per sampling point. In the end, we obtained an excellent ecological overview of this large area. We are excited and impatient to sample the next station in the National Park of Port Cros and to analyse the data.

Text by Géraldine Pérez and Rémy Simide