What is Posidonia

Posidonia oceanica is the most important endemic seagrass species of the Mediterranean Sea.

Posidonia oceanica forms extensive seagrass meadows around the Balearic Islands, known as underwater forests, with high biodiversity of species and their role in the ecosystem is of great importance.

Seagrass meadows provide shelter for many species; produce oxygen on a daily basis, removing carbon dioxide from the atmosphere and creating tons of biomass per year. Likewise, they act as barrier reef structure, which, although they are scarce today in the Mediterranean, have a high importance and longevity.

Seagrass meadows maintain the coastline and protect our beaches from erosion, in Winter, the accumulation of the leaves on the beach, called “banquettes”, stabilize the bottom and reduce the effect of the waves on the coast. In addition, the shells of the epiphytic calcareous organisms of the posidonia leaves are integrated into the accumulated substrate, generating whiter beaches.

One of the peculiarities of Posidonia is its tolerance induced by climate clange, acting as an excellent indicator of the quality of the waters. Since 1992 it has been considered a habitat of priority interest to the EU, and is currently regulated by different regulations.

However, despite Posidonia is being protected by different regulations, unfortunately it is also a threatened. The increase in activities in Balearic waters is endangering the conservation status of Posidonia through direct impacts such as anchors, uncontrolled anchoring of boats, as well as many other threats. Faced with this situation, the Balearic government approved Decree 25/2018 of July 27, aimed at the conservation of Posidonia oceanica, to make the development of human activities compatible with the protection of the species and its habitat.

Posidonea oceanica - que es

Life cycle

Posidonia oceanica colonizes new areas through sexual reproduction and vegetative reproduction, the second predominant over the first. How each reproductive strategy consists of is detailed below.


In summer, the meadows have the highest organic activity and they are filled with epiphytes and small microorganisms. During this process and with higher temperature on water, the plants acquire a whitish and brownish tone, especially in the shallower areas.


Flowering occurs in late autumn. The inflorescences develop in a few days. Grasslands, like deciduous forests, renew their leaves every year. This process, although it occurs from January to December, is more evident in the fall, since it is in this season when the storms tear off many leaves and accumulate the fallen remains on the beaches.

Posidonea oceanica - fruto - atlas posidonia


At this time is when the growth of the new leaves is slower, coinciding with the lowest temperatures of the year. Flowering continues and at the end of winter the first fruits can be seen


In Spring, the ripe fruit is released from the plant and floats adrift for a few days. The seeds are dispersed far, following the currents. After hours or even days, they open, releasing a single seed that falls to the seabed, where it germinates and creates a new plant.

Dune system

Posidonia is good for the growth of beaches and dune systems

At first glance, seagrass on beaches seems like an annoying impurity, but sand and seagrass on the Balearic Islands form an inseparable whole. Posidonia benefits the sand to grow and the dune systems are maintained, in good part, thanks to the Posidonia

Posidonia usually plays a triple role in terms of the formation of the beaches

  • Where Posidonia is placed is the place where a good part of the white sand is produced (which is formed by limestone shells of small organisms that live among the Posidonia).
  • The alguer can form a small underwater rise that forces the waves to break away from the beach and thus prevents their erosion.
  • The Posidonia reef forms a barrier that prevents the sand from going out to sea.

The dead seagrass on the beach also has an important protective function:

  • During Autumn and Winter, the sea removes the dead Posidonia leaves and accumulates them on the beaches. Thus, over the years, authentic compact walls are formed and they protect the sand from the big waves caused by storms.
  • On the other hand, the Posidonia gives organic matter in the dunes, necessary for the growth of the plant species that live on top and that prevent the winds from carrying the sand away
sistema dunar atlas posidonia

Other uses

For years, Posidonia has had different uses: medicinal, agricultural, for construction, etc. let’s see the best known:

Medicinal uses

  • Insecticide, bed bug repellent.
  • Relief of respiratory diseases, such as bronchitis or a sore throat.
  • Alternative treatment for acne.
  • Relief of leg pain and varicose veins.
  • Natural disinfectant. Wound healing

Agrarian uses:

  • Posidonia remains can be used as feed for ruminants.
  • Mattress for livestock.
  • As fertilizer for the fields.

Construction uses:

  • As a thermal insulator and biofuel

Others uses:

  • Packaging of fragile materials such as glass.
  • Packing and transport of paintings by painters.
  • Fuel (especially stems and roots).
  • Padding of mattresses and pillows.
  • Manufacture of paper

Posidonia conditions

In recent years, scientific studies aware that Posidonia meadows are being seriously damaged by human action. Marine pollution derived from outfalls and discharges, along with indiscriminate anchoring, are some of the factors that make grasslands are declining.

Likewise, the Balearic Islands receive a high number of tourists each year and each summer season they face the large influx of pleasure boats who want to enjoy their waters, therefore, among the pressures mentioned, it is worth highlighting the incorrect anchoring of vessels.

Uncontrolled anchoring

Impact on Posidonia meadow during the anchoring manoeuvre and lifting the anchor back: sometimes it is not easy for managers to see the seabed from the boat and distinguish the presence of Posidonia. Aspects such as the lack of light, the sunset or the turbidity of the waters, prevent the anchoring from being carried out in the correct areas, such as the sandbanks, without Posidonia.

Impact on Posidonia oceanica beds because the anchor chain is not in a vertical position: friction and movement of the chain gradually degrades the meadow.

 To solve this and other problems; In addition, to prevent the deterioration of Posidonia, the Balearic Government approved decree 25/2018 of July 27, which establishes a framework for the protection and conservation of Posidonia oceanica

In order to apply this Decree efficiently, it is necessary to know exactly where the Posidonia oceanica is.

For this, the surface of the seabed between 0 and 30 meters deep has been studied in order to have an Atlas of Posidonia that can serve as a useful tool.

Interesting facts about Posidonia

of the total Posidonia in Spain
Stock MtCO2
60000 F
Flux tCO2 yr