Seagrasses are marine flowering plants that form underwater meadows in shallow, sunlit coastal water. They are not seaweed: they have roots, veins, leaves, flowers, pollen and seeds, and their ancestors returned from land to the sea. Meadows provide habitat, stabilise sediment, improve water clarity and store carbon.
Posidonia oceanica is an iconic seagrass found only in the Mediterranean. Its long-lived meadows are central to regional biodiversity and coastal resilience.
How seagrass meadows support life
Leaves create shelter and feeding surfaces for small animals and algae. Roots and rhizomes form a complex habitat in the sediment. Juvenile fishes and invertebrates use meadows as nurseries, while sea turtles, dugongs and other grazers feed on seagrass in parts of the world.
Meadows connect with reefs, lagoons, salt marshes and mangroves. Animals move between them through their life cycles, so damage to one habitat can affect another.
Cleaner water and steadier shores
Dense leaves slow currents and trap suspended particles. Roots and rhizomes bind sediment, reducing resuspension and helping maintain clear water. Meadows also absorb dissolved nutrients, though excessive nutrient input can trigger algal growth that shades the plants.
By reducing wave energy and retaining seabed sediment, seagrass can contribute to coastal protection. Dead Posidonia leaves that accumulate on Mediterranean beaches form natural banquettes that reduce erosion and provide habitat; removing them for a perfectly bare tourist beach can also remove this service.
Seagrass as blue carbon
Seagrass stores carbon in leaves and roots and traps organic material in low-oxygen sediment. Much of the long-term stock is below ground. Dredging, anchoring or erosion can expose that sediment and release carbon accumulated over long periods.
Why Posidonia is special
Posidonia oceanica grows slowly and can form thick “matte” structures of roots, rhizomes and sediment. Meadows may persist for centuries or longer, but recovery from physical damage can be extremely slow. They provide habitat for fish and invertebrates, oxygenate water during photosynthesis and support the character of Mediterranean coasts.
Main threats
Anchors and moorings scar rhizomes; coastal construction and dredging bury or remove plants; pollution and algal blooms reduce light; trawling can damage the seabed; and marine heatwaves stress sensitive species. Invasive algae and shifting species interactions add pressure in some locations.
How to protect a meadow
Use mapped, seagrass-safe moorings and anchor only on permitted bare substrate. Improve wastewater and runoff control, protect water clarity, manage boating and fishing, and monitor heat impacts. Restoration is difficult and site-specific, so avoiding damage is the first choice.
Place these benefits in their climate context with our blue-carbon explainer and the restored Mediterranean Sea Decade overview.
Frequently asked questions
Is Posidonia a type of algae?
No. It is a flowering plant with roots, rhizomes, leaves, flowers and fruits.
Should dead leaves be removed from beaches?
They often protect sediment and support beach ecology. Any removal should follow local ecological guidance and avoid unnecessary clearance.