Posidonia oceanica beds (habitat 1120*)

In the marine SCI area, the Posidonia oceanica beds colonize most of the sea bottom, extending over a surface of nearly 2,000 hectares at up to 40 metres of depth (23% of total surface area). In general, Posidonia oceanica beds appear to be in good health, with a compact and continuous structure, but some areas where human presence is more significant, show signs of degradation with patchy coverage.

The characteristic plant is Posidonia oceanica, an endemic Mediterranean specie, which has roots, stems (known as rhizome), leaves, and is able to produce flowers and fruits. The rhizomes can grow vertically or horizontally and, together with the roots and sediments, make up a typical formation called ‘matte’. The leaves are ribbon shaped, intense green and reach up to 150 cm in length; they are arranged in groups of six to seven leaves. The flowers give way to fruits (similar to olives) that float away with the currents when they are ripe. When the fruits rot, the seeds are freed and, when the conditions are optimal, they give life to new plants.

In primo piano un fitto sciame di castagnole (Chromis chromis) che nuota sulla prateria.

A dense swarm of damselfish (Chromis chromis) swimming over the Posidonia oceanica bed

The beds play a key role in marine life, because they produce huge quantities of oxygen and are the source of food, directly or indirectly, for many organisms; they provide shelter for many animal and vegetable organisms, so much so that they are a key nursery for species that have great commercial relevance (fishing) and those that need protecting.

The beds strengthen the substrate, playing a key role in coastal protection; on one hand, they work as wave breakers, on the other hand the Posidonia dead leaves are transported by currents to the shore, where they make up formations known as ‘banquettes’ that can hold the sand.
Extremely sensitive to any environmental change, Posidonia oceanica is a good indicator of the health of our seas. The main causes of regression in seagrass beds are:

• Environmental pollution
• Building of ports, piers and dykes
• Anchoring
• Human activities such as trawling

Moreover, the problem of regression and/or degradation of the meadow becomes significantly important where the degraded spaces are occupied by some alien species, such as the alga Caurlerpa cylindracea, which prevent the plant to take back of its expansion range.

In the marine SCI area, human presence becomes intense during summer season and this exposes the Posidonia oceanica beds to several, often unintended, threats that may result in their degradation and fragmentation.

For example the anchoring on Posidonia oceanica represents one of the most widespread behaviors; the mechanical action of the anchor, in addition to tear parts of the plants, creates a favorable condition for the colonization of other species.