While they only cover a small part of the seabed, the posidonia meadows constitute a complex ecosystem and play a fundamental role in the ecological balance of coastal areas. From ULg’s Underwater and Oceanographic Research Station (STARESO) in Calvi (Upper Corsica), bordering the headland of La Revellata, Sylvie Gobert continuously observes the state of the meadow, this “forest” that carpets the coastal seabed. Since the bay of Calvi is more or less free from pollution of anthropic origin, the data collected by Sylvie Gobert allows us to keep a sharp eye on the initial consequences of climate change on this essential yet fragile ecosystem.

Posidonia meadows belong to the group of marine phanerogams. These flowering plants develop under water along almost all the world’s coastlines. In the Mediterranean, the seabed is essentially colonised by the species Posidonia oceanica. It is this species that is present in the bay of Calvi and in the whole of the Mediterranean region, except for the estuaries, which are too saline and too turbid.

Marine phanerogams conquered coastal marine environments 100 million years ago, and only cover 10 % of coastal areas. The Mediterranean Posidonia oceanica meadows only established themselves 8,000 to 9,000 years ago, during the Holocene, the last time the sea levels rose in the Mediterranean. The surfaces colonised by marine phanerogams are restricted: in the Mediterranean, Posidonia oceanica covers between 25,000 and 50,000 km2, i.e. 1 to 2 % of the Mediterranean seabed, and the entire collection of phanerogams cover 0.15% of the oceans’ total surface area. And yet, these plants play a vital role.

The meadows form complex ecosystems that strongly contribute to the richness and balance of Mediterranean coastal waters. Obviously, they do not have any economic value in terms of food and their use as a medicinal plant, an insulator or for clothing, for instance, is reduced to a few local populations. On the other hand, as a terrestrial forest, they fully play their role in the carbon, nitrogen and phosphorous cycle, and do so far beyond their area of colonisation (beaches, abyssal zones, etc.).