It is a plant species that has lived in the sea for over one hundred million years, so it is not an alga. And although its name refers to the ocean, it is widespread only in the Mediterranean, where it is present everywhere in the coastal strip.

Posidonia oceanica is a marine plant, which flowers, produces fruit and reproduces by seed, forming submerged meadows, which can extend for tens of hectares on any type of seabed, whether sandy, rocky or even detrital. Being a plant, it needs light for photosynthesis, so it is present up to a depth of about 40 meters and then needs water that is as clear as possible and free from suspended sediments.

Yet, compared to the minimum natural conditions needed for meadows to remain healthy, Posidonia guarantees a much more significant contribution, even decisive for the health of the sea. This is why the habitat it creates is considered a priority for the Mediterranean and deserving of particular, specific protection wherever it is found. It is no coincidence that the Sites of Community Interest and the Special Areas of Conservation, protected at European level by the “Habitats” Directive, correspond to the meadows and points of diffusion of Posidonia, whose protection is also the main purpose of the Marine Protected Areas established so far in Italy.

The plant has ribbon-like leaves, which can reach up to a meter in length and are gathered in tufts, a rhizomatous stem and roots that keep it anchored to the substrate on which it grows. Flowering occurs in autumn with a green inflorescence that, after pollination, produces fruits called “sea olives” for their resemblance to land olives. The fruits remain on the plant for six months, before detaching and rising to the surface where they float. When the fruits rot, they release seeds, which sink to the bottom and, in favorable conditions, give rise to new plants.The growth of the plant occurs through the rhizomes, which expand horizontally, thus determining the conquest of new space and the enlargement of the meadow. But the growth of the rhizomes can also occur vertically, thus creating the so-called matte, a peculiarity of Posidonia and its habitat. The matte is an intertwined stratification of dry rhizomes and roots that also traps the sand on the seabed. This creates a plant structure in continuous, very slow growth (it is calculated one meter per century) on which the living plants vegetate. The presence of these platforms in the sea, near the coast, represents a precious barrier to protect the coastlines from the erosion of the wave motion. An effect to which the accumulations of dry Posidonia leaves that the sea deposits on the ground during the winter also contribute. The dead leaves, together with the rhizomes torn from the bottom, once washed ashore, form the so-called banquettes which, if not removed, stratify and consolidate, creating plant structures capable of also representing a barrier on the ground against beach erosion.

Among the many functions of Posidonia, the nursery function performed by the meadows is fundamental for biodiversity, because many animal species find refuge within it and reproduce there, allowing their young to grow safely. The importance of the Posidonia habitat lies precisely in offering favorable conditions for the life of many, various animal and plant species. Among these, various epiphytes such as diatoms and algae, including encrusting ones, live right on the leaves of Posidonia and are food for many of the animal species, mostly herbivorous, that populate the meadow, including polychaetes, molluscs and crustaceans.

Both sessile species, that is, anchored to the seabed, specifically to the matte, and the first of all is the Pinna nobilis, and vagile animals, that is, those like fish that move in the meadow safe from their natural predators, live in the meadow. Therefore, Posidonia is necessary for the repopulation of fish species of interest for fishing. Overall, the biodiversity heritage linked to Posidonia includes 400 plant species and approximately 1000 animal species.

Prairies are the marine equivalent of large forests in terms of oxygen production: one square meter can produce 10 to 15 liters of oxygen every day. And, on the other hand, it also has a remarkable capacity to capture and retain CO2, since matte holds half of the carbon buried in marine sediments worldwide. On both fronts, it is a defense against climate change.

The regression of Posidonia, which is recorded almost everywhere, with loss or reduction of its habitat is caused by marine pollution, trawling and wild anchoring, which tear the plants from the bottom, desertifying the seabed, due to the difficulty of recolonization by the plant and the slowness of any possible regrowth.