This plant  as a subterranean stem called rhizome from which grows  green leaves which can reach 1m long.

The rhizome as an horizontal growth when  sedimentation is not important, and, vice-versa, a vertical growth when the sedimentation  is important;  this last  type of growing forms  vertical accumulation called  "matte".  Sediment trapping  by Posidonia leaves and the growing up of the matte result in raising the sea-bed , sometimes, nearly reach the surface of the water to form, in protected areas , a , to-day, rare  formation, called"barrier reef".
 
The Posidonia as green flowers and fruits who look like green olives (olives de mer), but the propagation of the plant is essentially done by natural cuttage.

At the basal part of leaves the rhizome is covered by scales, testimonies of the falled leaves. These scales are whip out by the currents and their trituration by waves gives out brownish balls called "aegagropyles" that finally are landing on the beaches  The leaves of the less deep plants fall at the end of summer and their accumulation on beaches built "laisses" and "banquettes" which can reach several meters high in the zones where the meadows are large and thick (photo:" laisses" on Monastir beach -eastern Tunisia).
 
 
 
By its botanical connection with the terrestrial plants, Posidonie is very demanding in light.  It can, thereby, lives only in a middle where the luminosity is strong, and, therefore, in a middle where the transparency of waters is  good,  so, it cannot extend in  an environment where eutrophication  is high. Consequently, Posidonia oceanica  can be listed as the main Mediterranean bioindicator of non polluted areas.

Posidonia meadows play, elsewhere,  important  roles

1. They are the largest  littoral source of  primary production (consequently of oxygen production) and biomass, and one of the most diverse marine communities  with more than 1000 associated species of algae, invertebrates and fishes.
2. When the meadows are thick, they control sediments flow by the way of their roots and decrease the strength of hydodyamic phenomena by the way of their leaves.

 

• The negative impact  of increased water turbidity which reduces light penetration at depth  is in the origin of the raising of the lower limit of the meadows.
• The coastal works  which cover large littoral areas  with polluted sediments  turn result  in the burial and death of the plants and are in the origin of the  downing of the upper limit of the meadows.

To safeguard the littoral environment of the Mediterranean, it is, therfore, imperative to preserve the Posidonia sea-grass meadows in the best state.
Photos: J. Zaouali