Posidonia, the underwater treasure we can discover while snorkelling in the Medes Islands

One of the most valuable habitats in the Mediterranean is Posidonia oceanica, a marine plant endemic to this sea. Although it is often mistaken for algae, Posidonia is a marine flowering plant, meaning it has roots, stems, leaves, flowers and fruits.

Posidonia forms underwater meadows on coastal seabeds, from shallow areas down to around 35 or 40 metres deep. In very clear waters, it can even grow at much greater depths. These meadows create an underwater ecosystem of great ecological importance, capable of supporting a wide variety of marine fauna and flora.

Posidonia meadows are a true refuge for life. Many coastal fish species find protection, food and nursery areas there during the first stages of their lives. For this reason, Posidonia is essential for maintaining marine biodiversity and the balance of coastal ecosystems.

Its presence is also an excellent indicator of the environmental quality of the underwater coastline. Where well-preserved Posidonia meadows are found, the waters are usually cleaner, clearer and home to a healthier ecosystem.

Posidonia also plays a very important role in protecting the coast. Its leaves and rhizomes help stabilise the seabed, reduce coastal erosion and contribute to maintaining the quality of beaches. At the same time, these meadows capture and store CO₂, making them a natural ally in the fight against climate change.

Despite its importance, Posidonia is in decline in different areas of the Mediterranean. In recent decades, its surface area has decreased worryingly, especially in the western Mediterranean, due to several pressures mainly linked to human activity.

Among the main threats are illegal bottom trawling, coastal development, the construction of coastal infrastructure, dredging, marine pollution, declining water quality, the presence of invasive species and boats anchoring directly on the meadows.

It is also important to remember that mass tourism that does not respect the environment can worsen this situation, especially when it involves inappropriate anchoring, pressure on beaches or the generation of waste.

The importance of this habitat is so great that Posidonia meadows are recognised as a priority habitat for conservation under European regulations. This reinforces the need to protect them and to promote a more responsible relationship with the sea.

That is why, at Medaqua, we believe it is essential to raise awareness of Posidonia through our environmental education activities. Learning about this habitat helps students and visitors understand that the sea is a living, fragile ecosystem that is essential for the future of the Mediterranean.

During our activities in the Montgrí, Medes Islands and Baix Ter Natural Park, we promote good practices such as not touching or removing marine organisms, not leaving waste behind, respecting the seabed, always following the guides’ instructions and enjoying the sea responsibly.

Protecting Posidonia starts with getting to know it. Environmental education is a key tool to help new generations learn to love and care for the sea.

Webography

• The decline of Posidonia in the Mediterranean
• Information about Posidonia oceanica