Posidonia oceanica

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Herbarium of Posidonia oceanica are the major Mediterranean ecosystem and play an important role in protecting against coastal erosion. It is in these herbarium that many organisms, animals and plants, find protection and food.

Geographic repartition
Posidonia oceanica  is an endemic species from the Mediterranean sea. It covers about 30% of the sea floor between 0 and 50 meters.

Life history traits
> Posidonia oceanica  has an terrestrial ancestor (returned to the sea there 100 Ma)  which it inherited flowers, fruits and roots. Flowering takes place in automn but not each year,  in an imprevisible way to avoid predators expecting the tasty fruits : sea olives.
P oceanica can reproduce in several ways : It can make sexual reproduction by selfing. The hermaphrodite flowers are fertilized by pollination hydrophilic. They lead to the growth of fruits that are marture in mai. When they are, they detached from the plant and float at the surface drifting with winds and currents. This way of reproduction allows Posidonia oceanica to colonize new regions and provides genetic diversity. This plant can also reproduce by cuttings and it has been observed that they can make pseudo-viviparity.

>The plants are organized in rhizomes and beams that can be orthotropic (perpendicular to the sea floor) or plagiotropic (parallel to the sea floor).

>  Posidonia oceanica fears freshening. Accordingly they are asbents from estuaries and outputs of groundwater zones. The optimal temperature for photosynthesis is 24°C and lethal temperatures are 10°C and 30°C.

Matte edification
Through the height growth of rhizomes in response to sediment deposition (trapped by the canopy), Posidonia built over the centuries a land called matte, formed by interlacing rhizomes and roots compacted by sediments. The matte can be several meters thick at a rate of one meter per century. This matte and leaves caused a diminution of currents in the herbarium. 

Herbarium types
Herbarium of plain : the most common  in the North Western Mediterranean Sea.
Herbarium of hill : where the depth is medium and the hydrodynamic strong.
Herbarium tabby : where the depth is low and is the warmer areas. Bands move parallel to themselves against the mainstream.
Herbarium in scales :  the walk back slope against the discharge current dominant.

Participating organisms and succession
Herbarium have a specific diversity very high : 20 to 25% of species of Mediterranean Sea live within it or can live within it. Herbarium are therefore a center of biodiversity.Over 50 characteristic species among which teleosts, molluscs, bryozoans, annelids, cnidarians, hydroids, echinoderms and rhodobiontes chromobiontes. Examples (from left to right) : Hippocampus guttulatus, Parastephanauge paxi, Asterina panceri and Opeatogenys gracilis.


A succession means a progressive series. For a herbarium of P. oceanica snaps into place, it is necessary that the following events take place :
After a few hours a bacterial film is deposited on the substrate. A few days/weeks after a turf of chromobiontes takes place. A few months/years after, chromobiontes Padina pavonica and Halopteris scoparia and the rhodobionte Jania rubens start to grow on the substrate. A few years/decades after rhodobionte Litophyllum and chromobionte Cystoseira also grow on the substrate. After a few decades if a cutting of P. oceanica is deposited on the substrate by the currents it will develop, grow and give birth to a herbarium with matte, 100 years after the filing of the biofilm.

This ecosystem is fragile and easily damaged by dredging and inks boats. it is important to preserve as best as possible.

2 Comments on “Posidonia oceanica

  1. Pingback: Cystoseira forests | Alexandra Caron

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