Posidonia Beds

The beds of Posidonia oceanica, an endemic species of the Mediterranean Sea, occur in linear fringes in most of the infralittoral, between 0.2 and 40 m depth. It is a priority habitat in the EU Habitats Directive. Posidonia beds are not very extensive (e.g. France has ca. 115 000 ha), but it is the most important ecosystem of the Mediterranean Sea for a number of reasons:

they are the nurseries of the sea (high primary productivity and supply of oxygen)
their support 25% of the region’s flora and fauna and provide essential feeding grounds for sea turtles, waterfowl, cephalopodes, crustaceans, shellfish and finfish
they are of great economic importance for fisheries and tourism
they protect against coastal erosion; a loss of 1 m of Posidonia bed may cause a shoreline regression of nearly 20 m.

Posidonia beds are not rare (France alone has 115 000 hectares) they have suffered a progressive and irreversible regression throughout the Mediterranean due to:

Sand extraction and development of infrastructure, harbours and artificial beaches, enhancing turbidity and covering the beds with sand.
Damming of rivers. Changes in sedimentation in the littoral zone has led to either exposing or burying of habitat.
Trawling and anchoring are especiallly destructive to exposed rhizomes.
Eutrophication, augmenting algal blooming. Sewage and industrial waste discharge cause a complete loss of the habitat locally.
Caulerpa taxifolia (an tropical alga introduced in the French Mediterranean in 1984) is progressively overwhelming Posidonia beds.

The situation in the Western Mediterranean is most serious. Shoot density is rapidly decreasing, up to 50% over a few decades. Besides, increased turbidity and pollution have resulted into a squeeze of the beds; in various places living beds have withdrawn between 10 and 20 m depth. Dead beds occur abundantly, even in waters which have already been protected for 35 years. For the French mainland coast habitat loss is estimated 10-15%; but taking into account the decrease of shoot density the overall decline of the resource will be between 30 and 40%. This is probably a good estimate for most Western Mediterranean coastlines, although the situation around the islands and in the Eastern Mediterranean is better.

Published in ECNC Facts + Figures 1998-1999, Nov. 1998