The posidonia (also known as Neptune grass), Posidonia oceanica, is a marine plant endemic to the Mediterranean Sea. It consists of creeping or erect stems typically buried in sediment, known as rhizomes. These rhizomes also have roots that can penetrate up to 70 cm beneath the sediment surface. The leaves grow throughout the year and have a lifespan of 5 to 8 months, occasionally up to 13 months.
Posidonia oceanica primarily propagates through vegetative reproduction, extending its rhizomes and growing from cuttings. It can also flower, with hermaphrodite flowers that produce fruits resembling olives capable of germination. Flowering occurs in the autumn every 5 to 10 years, and the fruits may wash ashore on beaches between May and July.
Posidonia oceanica has the remarkable ability to form extensive underwater meadows, creating vital habitats in the Mediterranean Sea. These meadows are key elements for the richness of Mediterranean coastal waters due to the surface covered (20 to 50% of the seabed between 0 and 50 m depth) and, above all, to the fundamental role they play on a biological level in maintaining coastal equilibrium and supporting economic activities.
- It is a breeding area, a nursery or a permanent habitat for many species. It is home to more than 400 plant species and several thousand animal species, thus making Posidonia meadows a biodiversity hotspot.
- It is an important vector of water oxygenation: 1 m2 of meadow can produce up to 14 litres of oxygen per day!
- It absorbs 0.6% of CO2 emissions from Mediterranean countries, with higher percentages locally (15 to 20% in Tunisia, nearly 15% in Corsica).
- It produces and exports biomass to neighbouring ecosystems and deeper into the sea.
- Finally, it protects from coastal erosion., by attenuating the force of waves and shielding the coastline when the leaves, which fall in autumn, wash ashore and form natural barriers. These barriers also create a rich habitat for numerous organisms on the beaches.
The RAMOGE Agreement published in 2006 In 2006, RAMOGE published a book on Posidonia oceanica entitled “Protection and conservation of Posidonia oceanica meadows”, which can be downloaded in French, Italian and English.
The Mediterranean Posidonia Network, established in 2019 and comprising around sixty members, including the RAMOGE Agreement, is currently working on updating this guide, taking into account recent developments and current issues.
This network brings together scientists, authorities, stakeholders, international environmental organizations, managers of Marine Protected Areas, and marine professionals to conduct studies on a Mediterranean scale and propose effective conservation measures.
The posidonia meadows are currently threatened by human activities, particularly boat anchoring. Each year, these meadows lose nearly 2% of their surface area. Due to the slow growth of posidonia (about 5 cm per year, with flowering occurring every 5 to 10 years), the destruction of these meadows can be devastating locally. Preservation measures, or failing that, restoration efforts, are therefore necessary.
For more information on the study commissioned by the RAMOGE Agreement and published in 2021, which addresses the impacts of cruising and yachting including the issue of posidonia, please visit the “Sustainable management of recreational boating and cruising” page.
You can also find the guidelines for active restoration of Posidonia oceanica developed by the Mediterranean Posidonia Network in 2024.
