Posidonia form complete underwater ecosystems providing a crucial sanctuary and feeding ground for sea turtles. They enable a unique habitat to take form on the seafloor providing an array of nutrients and covering a total surface area of roughly 20.000 square nautical miles!

Posidonia oceanica is a slow growing seagrass found at depths of 5 to 35 meters along the Mediterranean coastline. It plays an important role in oxygenating and clarifying coastal waters, provides a habitat for a rich diversity of plants and animals, acts as a safe breeding-area for many species, and protects beaches from erosion.
These meadows also act as a "carbon sink" absorbing carbon dioxide from the atmosphere.

Photo: Archipelagos www.archipelago.gr

Due to the tough lignin that covers its cells, it is only grazed by animals that have special micro-organisms in their intestinal tract to help them digest it. One such animal is the endangered green sea turtle, Chelonia mydas.
Posidonia thrive at depths of 3-5 metres all the way to 30-40 metres in pristine conditions. They can be found on rocky as well as soft substrate, in temperatures between 15 and 20 degrees Celsius, and at stable salinity levels. Posidonia meadows are succeptible to even the slightest human activity.

Posidonia oceanica in the Mediterranean Sea. Photo by Manu San Félix. http://www.umces.edu/

The Posidonia Flower

Posidonia reproduce by vegetative reproduction (the breaking off of fragments and replanting itself) , or through the dispersal of seeds.

The fruit of the plant is extremely light. This enables the fruit of the plant to detatch itself and freely float to the surface where it will be taken away by waves through the current. Eventually, the fruit will dissintegrate, allowing the heavy seeds to sink to the seafloor and find a new home.


Pollution: A large proportion of waste produced by human activity ends up directly or indirectly in the sea. Polluted murky water reduces the quantity of light, inhibiting growth. Sewage and fertilizers cause an increase of particular biota further reducing neccessary oxygen concentrations.

Non-native seaweed species: Non-native seaweed species that tend to be more vigorous and taller than posidonia are taking over large proportions of subtidal coastal zones. Introduced species such as Caulerpa taxifolia are very fast-spreading and can colonize muddy and sandy sediment, cobbles and hard substrate. This rapid spread has resulted in the decay and regression of the important seagrass ecosystems.

Drag Fishing and Hauling Nets: These now-illegal methods of fishing damage the sea floor by pulling up Posidonia and stirring up sediments into the clear water blanketing the seabed from sunlight. To a lesser degree the casting of anchors has the same effect.

Dredging: Coast line activities such as dredging used to construct ports and piers increase the amount of particles in suspension as well.


Under the European Commission’s ’Habitat Directive’, Posidonia oceanica meadows are considered as priority Site of Community Importance (SIC). Some countries in the Mediterranean such as France, Italy, Greece and Tunisia have forbidden trawling less than three nautical miles from the coast, while Spain, Italy, Gulf of Tunis and Algeria forbid above the 50m isobath (contour lines showing sea floor). In practice this legislation is often not respected or enforced, in detriment to the seagrass survival.

For more information visit:

RAC/SPA's MedPosidonia Project: http://www.rac-spa.org


Cecherelli, C. and Cinelli, F. (1999) Effects of Posidonia oceania canopy on Caulerpa taxifolia size in a north-western Mediterranean bay. Journal of Experimental Marine Biology and Ecology. 240 (1999) 19-36.