Kenna Eco Diving has led research into Posidonia oceanica (Neptune grass) ecosystem on the Costa Brava for the past seven years. With the assistance of Eco Dive volunteers from the UK, Ireland, Belgium, Netherlands, Sweden, Germany, Spain, and the USA we have monitored the seasonal impacts on this most productive yet endangered habitat and gathered data on the species that live and breed within the seagrass meadows.
Posidonia oceanica (Neptune grass) is a species that grows in a narrow coastal strip, at depths of up to 40 meters, only in the Mediterranean sea. Left undisturbed seagrass beds form into meadows that play an important biological and ecological role. They are vital in producing oxygen and organic material and they stabilise the continually shifting sand bars, serving as a good system of protection for maintaining the beaches and coves on our coasts. They sustain many animal species that utilise them as a site for breeding, feeding and shelter.
It is a priority habitat in the EU Habitats Directive. Posidonia beds are one of 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).
- They 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 1m of Posidonia bed may lead to a shoreline regression of nearly 20m.
Posidonia beds are not rare, but 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 have led to either exposing or burying of habitat.
- Trawling and anchoring are especially destuctive to exposed rhizomes.
- Eutrophication, augmenting algal blooming.
- Sewage and industrial waste discharge cause a complete loss of the habitat locally.
- Caulerpa taxifolia (a tropical alga accidentally introduced in the French Mediterranean in 1984) that 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.
Posidonia oceanica is one of the few phanerogamous plants (with roots and flowers) which grow on the sea floor. Posidonia meadows are formed very slowly by many individual plants which spread via the elongation of their rhizomes (roots) at a rate of about one centimeter per year. The long flat leaves one centimetre wide and up to a metre long, provide cover for many animals which live hidden among the leaves and a whole throng of little algae and animals which become attached to the leaves.
Posidonia contains a lot of cellulose and for this reason is eaten by species such as sea-urchins (Paracentrotus lividus), violet sea-urchins (Sphaerechinus granularis) and salemas (Sarpa salpa). The sediments which accumulate in the Posidonia beds are richer in organic material and nutritive salts than the sand banks and a multitude of suspension-feeding animals such as feather-stars, ascidia, sponges, hydrozoa, and worms, as well as sediment-feeding animals such as sea cucumbers, and brittle stars, which in turn serve as food for the carnivores which include crabs, fish, octopus, and starfish.
Each Autumn the outer leaves of the seagrass are ripped out by storms and in winter new leaves begin to sprout from the rhizomes, growing green and vigorous during Spring. To the extent that they keep growing small organisms become attached and by the end of summer the leaves appear white and heavy with the weight of these incrustations.
Although Posidonia oceanica can flower, under the right conditions, and bear fruit (called “sea olives”), it is rare for the seed to survive and settle in a suitable place to form a new plant. Hence Posidonia propagates slowly and is unable to recover from mechanical disturbances, such as trawler fishing and anchor damage. Trawler fishing in Catalonia is banned over Posidonia beds. However, anchoring is only prohibited within marine reserves at present.