Posidonia seagrass

Posidonia oceanica is the dominant seagrass of the Mediterranean and one of the most ecologically important marine plants on earth, forming dense meadows that shelter juvenile fish, filter sediment, produce oxygen, and lock away carbon in their slowly accumulating matte — leaf debris compacted over centuries. The meadows at Ile de Riou represent a relatively intact example of this habitat, which has declined by up to 30 percent across the Mediterranean since the 1970s due to anchoring, nutrient pollution, and warming.