When you go for a stroll along the beach at Cala Montgó or Riells, do you ever wonder about the piles of brown leaves washed up on the sand? They are the dead leaves of the underwater plant Posidonia oceanica, named after the Greek god of the sea, Poseidon, which are shed each winter. Left to remain on the beach the mounds of leaves can sustain a complex invertebrate food web, protect the shoreline from erosion, and deliver sand in the form of carbonate and silica shells.
As a true plant that evolved on land and returned to the sea millions of years ago, Posidonia oceanica has roots, stem, and leaves (between 20 and 80 cms in length), and is capable of bearing flowers and fruits with seeds (called “sea olives”). At the base of each plant is a rhizome, which is actually a modification of the stem. The hairy remains of old, degrading leaf sheaths found around the rhizomes can also be found as conspicuous balls of fibres washed onto the beaches, known as egagropili.
Dried Posidonia oceanica leaves were traditionally used in Mediterranean countries as packing material to transport fragile items of glassware and pottery, and also to ship fresh fish from the coast to cities. As parasites are less successful in Posidonia leaves than in straw, they were utilised in stables, as roof insulation, and as a filling material for pillows and mattresses to prevent respiratory infections. Further medicinal uses included the alleviation of skin diseases and leg pain caused by varicose veins.
Posidonia oceanica is unique to the Mediterranean sea where it grows in beds that cover between 25,000 and 50,000 km² of coastal areas, corresponding to 25% of the sea bottom at depths between 0 and 40 meters. Posidonia oceanica has been called “the lungs of the Mediterranean” due to the large amount of oxygen that it provides to coastal waters, producing oxygen at an average rate of 5110 liters/m²/per year. It also absorbs carbon dioxide, storing carbon at an average rate of 83 g C/m² per year and helping to alleviate the effects of climate change.
The plants of Posidonia oceanica grow very slowly, at a rate of only 1 to 6 centimeters per year, but over thousands of years they form meadows that support a wide variety of species. Gaining its energy from the process of photosynthesis, Posidonia oceanica needs transparent waters. For this reason, the presence of large, dense meadows is a clear sign of the quality of local waters.
In fact, Posidonia meadows are excellent indicators of environmental quality as they can only grow in clean unpolluted waters. Despite legislation* protecting this important species, it is threatened by pollution, coastal development, the presence of fish farms, bottom-trawling and boat anchoring. Studies show that 46% of the underwater meadows in the Mediterranean have experienced some reduction in range, density and/or coverage, and 20% have severely regressed since the 1970s. Of Spanish meadows, 60% are found to be declining at a rate of 5% per year.
A cartographic survey of the 595 km coast of Catalunya in 2002 found that the Tarragona area had 2483 hectares (ha) of Posidonia oceanica, the Barcelona region had 1275 ha, and the Girona coast only 302 ha. Near L’Escala the Posidonia oceanica beds in Cala Montgó and around Illa Mateua are responsible for attracting hundreds of scuba divers and snorkellers every summer, who come for the clear waters and the abundance of marine life associated with the meadows.
The leaves and rhizomes of Posidonia plants increase the surface available to sessile species and offer shelter to mobile species, sustaining a diverse community in which more than 700 different species have been identified. A complex community lives on the leaves of Posidonia oceanica, composed of large quantities of micro- and macro-algae, hydroids and briozoa. These provide food for a wide variety of molluscs, crustacea and fish.
Among the Echinoderms, Sea urchins are common. They are able to digest the tough lignin of Posidonia oceanica leaves . There are also delicate Feather stars (photo: Antedon Mediterranea) and Brittle stars. The most abundant Echinoderms are Sea cucumbers which play an important ecological role in filtering the sediment. Among them, Holothuria tubulosa predominates. Many species of worms (Polychaeta) are found living in the substrate within the Posidonia meadows and sponges are encountered around the rhizomes.
Posidonia beds are especially valuable as nursery grounds for several commercial species such as anchovy, sardinella and bream which find food and shelter in the meadows during their juvenile stage. The most common resident species are Gobies (Gobius spp.), Wrasse (Labrus merula, L. viridis, Symphodus spp., Coris julis), and Sea bream (Diplodus spp,). There are also species living within the leaf canopy, like the Seagrass clingfish (Opeatogenys gracilis), the Seagrass pipefish (Syngnathus typhle). European seahorsea (Hippocampus hippocampusand Hippocampus guttulatus)depend on the seagrass for shelter and camouflage, and are the subject of Gaye´s Seahorse Project, which she began in 2013.
Blotched picarel (Spicara maena) take on a brilliant irridescent blue colouration during the mating season, when they build and defend nests within the Posidonia beds. Damselfish (Chromis chromis) also lay eggs within the Posidonia beds, but it is their young that have an irridescent blue coloration, which changes to brown as they mature.
The Cow bream (Sarpa salpa) is the only species of fish able to digest Posidonia leaves thanks to a bacteria in the gut. Huge shoals graze on the seagrass meadows each summer, just like herds of cows!
Within the rhizome substrate, some sessile species are only found in healthy Posidonia oceanica beds. This is true of the the large fan mussel Pinna nobilis, which, due to their filter feeding habits, longevity and slow growth, are good indicators of water quality and mechanical stability within the meadows. Unfortunately most of the remaining large and ancient examples of Pinna Nobilis in Cala Montgó were destroyed during the severe storm on 26th December 2009 along with an estimated 25% of the Posidonia oceanica meadow. Pinna nobilis is a protected species, but like Posidonia it is also vulnerable to damage from the number of pleasure boats anchoring in Cala Montgó each summer.
Gaye Rosier has been carrying out research into Posidonia oceanica for the past 16 years. Since 2009 she has also been a voluntary coordinator for the Silmar Project researching two L’Escala research stations, located in Cala Montgó and near Illa Mateua. She scuba dives at these locations to monitor the health of the Posidonia oceanica beds and to gather data on indicator species that show how the ecosystem is coping with the impacts upon it. Volunteer divers from all over the world have been coming to L’Escala during spring and summer to help Gaye with this marine conservation activity.
“One of the most destructive things we see affecting the Posidonia meadows in Cala Montgó is pleasure boat anchoring. Posidonia plants grow very slowly and an anchor can destroy decades of growth in seconds” said Gaye.
The Cala Montgó transect now lies within the Parc Natural del Montgri, les Illes Medes i el Baix Ter and Gaynor and her team of volunteers will be assessing the improvements as protective regulations come into force, the most important of which should be a strict regulation of anchoring activity.
So the next time that you see piles of brown leaves on the beach, remember! It is thanks to the presence of Posidonia oceanica that Escalencs can benefit from the transparent waters and abundant marine life that attracts so many beach tourists to the lovely sea-side town of L’Escala.
*Posidonia oceanica meadows are protected at the European level, under the European Union’s Habitats Directive as a priority habitat (Dir. 92/43/CEE 21/05/92 and 97/62/CE 27/10/1997) and as a species (Bern Convention, Annex 1). Bottom-trawling is expressly forbidden on seagrass meadows (Fishing regulation 1626/94). At the national and regional levels, Posidonia oceanica meadows are protected in Spain (RD 7/12/1995, BOE nº310) and in Catalonia (Spain, Orden 91.210.098 DOGC nº 1479 12/08/1991), where all seagrass species are protected.
In the strictly protected area of Illas Medas, 54 moorings have been installed and free anchoring has been banned since 1994. The costs of installation and management are largely covered by a small fee of €3.5 per mooring user.