the leaves of less deep plants fall at the end of spring and heap up on the beaches. It needs a lot of light, therefore it can survive where there is a lot of light and good water transparency, but not in eutrophic environments. The increase in the water turbidity has resulted in a progressive loss of Posidonia, with a consequent decrease in the process of sediment trapping on the sea-beds and an increase in erosive strength on the sea-beds. One more factor damaging the posidonia beds is trawling. Posidonia is important for the marine ecosystem because it produces about 14 l/sqm oxygen a year and about 20 t/ha organic substances a year. It represents shelter, food, nursery for many sea organisms such as fishes, molluscs, crustaceans. In the 92/43/EEC Habitat Directive of the European Union Posidonia beds are considered as a priority habitat as they represent a very important ecosystem of the Mediterranean Sea for several reasons: 1) they are a nursery zone for their high primary productivity and supply of oxygen; 2) they support 25% of the flora and fauna of the Mediterranean region and represent an essential feeding area for sea turtles, waterfowl, cephalopodes, crustaceans, molluscs, fishes; 3) they are important for fishing and tourism; 4) they protect coasts from erosion: a loss of 1 meter of Posidonia bed may cause a shoreline regression of nearly 20 meters.
Progressive loss of this habitat in the Mediterranean is due to: 1) sand extraction and development of infrastructure, harbours and artificial beaches which have enhanced turbidity and have covered the beds with sand; 2) damming of rivers with a consequent change in the sedimentation in the littoral zone has led to either exposing or burying of habitat; 3) anchoring and trawling take away the sediments of the sea-bed and expose the rhizomes of the plants; 4) eutrophication, which causes algal blooming which compete with Posidonia; 5) Caulerpa taxifolia, a tropical alga introduced in the French Mediterranean in 1984, that is progressively overwhelming Posidonia beds.