Regno Plantae
Kingdom Tracheobionta (vascular plant)
Superdivision Spermatophyta (seed plant)
Division Magnoliophyta (flowering plant)
Class Liliopsida (monocotyledon)
Order Najadales
Family Posidoniaceae
Genus Posidonia
Species oceanica
Classification (L.) Delile
Common Name posidonia

Geographic Range endemic to Mediterranean.
Physical Characteristics it is a marine plant, endemic to the Mediterranean Sea; it builds thick meadows, from 1 to 40 meters deep. Green leaves, which may reach 1 meter in height, grow from the subterranean rhizome. Leaves are ribbon-like; the youngest ones are green, while the oldest turn brown. It is hermaphrodite; flowers are inflorescences protected by leaves. If the sedimentation process is not high, the rhizome develops horizontally, when the sedimentation rate is high the rhizome develops vertically. This vertical growth and the trapping of the sediments by the leaves of the Posidonia result in sea-bed raising. The interwoven rhizomes form, together with the trapped sediment, a kind of submarine terraces called "mattes", which may be even several decimetres high. It has green flowers and fruits which look like green olives (sea olives) but the spreading of the plant occurs essentially by stolons. In the basal part of the leaves the rhizome is covered by scales testifying the presence of fallen leaves. These scales are beaten by the waves and turned into spherical agglomerates called "aegagropyles", which can be found on the beaches.

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.

Habitat it is generally found in sandy and muddy sea-beds and at depth ranging from the surface to 40 meters according to the level of luminosity.

References Ecological overwiew of tunisian coasts
Ministero dell'Ambiente e della Tutela del Territorio - Difesa del mare
EUCC Coastal Guide
Web References http://www.geocities.com/RainForest/2238/
Source of the photo http://www.areamarinaprotetta5terre.com/

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