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Posidonia Oceanica Seagrass Meadows Under Threat

Posidonia Oceanica Seagrass - courtesy of Gaynor Rosier
Octopus in Posidonia Oceanica Seagrass - courtesy of Gaynor Rosier
Cuttlefish in Posidonia Oceanica Seagrass - courtesy of Gaynor Rosier

Posidonia oceanica, named after the ruler of the sea, Poseidon, and commonly called Neptune grass, is a seagrass which plays an important role in the Mediterranean ecosystem. 120 million years ago P. oceanica covered the coastal plains of an ocean that straddled the equator. Drifting over millennia with the tectonic plates, today P. oceanica occurs only in the Med and around the southern coasts of Australia.

As well as providing an important habitat for a great variety of marine species, seagrasses play a role in protecting our planet from the increasing build up of carbon dioxide. They act as a "carbon sink", like land-based plants, absorbing carbon dioxide from the atmosphere and thereby helping to slow down the effects of global warming.

Just like the rainforests, seagrass meadows are threatened by mans' insatiable need to expand, build and consume. They are increasingly being destroyed directly, by trawler-fishing, fish-farming, the building of new marinas and seaside complexes, and pleasure-boat anchoring. However, the greatest threat comes from land-based sources of pollution and especially nutrient loading into the sea.

Nutrient loading has been found to be responsible for the deterioration of seagrasses on a national and regional scale. Nitrogen and phosphorus find their way into the sea from urban sewage outlets, industrial outlets, run off from agricultural areas, and atmospheric deposition from agriculture and the burning of fossil fuels. Nitrogen and phosphorus are the most important nutrients for regulating the growth of planktonic algae, and hence water transparency and light conditions for seagrasses. Nutrients also stimulate the growth of algae living on seagrass leaves, causing additional shading and further diminishing the plants' ability to carry out the process of photosynthesis by which it lives. Studies over the past 10 years measuring P. oceanica cover and shoot density along the coasts have shown an overall decline.

Posidonia oceanica lives in a narrow coastal strip, normally on beds of soft sediment, at depths of between 5 and 40 to 50 metres, depending on water clarity. It is a true plant, in that it can bear flowers and disperse seeds. However, flowering is quite rare with less than one flower found per 10 square metres per year. An exception to this was the summer of 2003, when the rate of flowering off the coasts of France and Spain increased dramatically, possibly due to raised seawater temperatures.

P. oceanica is a large, long-living but very slow-growing seagrass. Its shoots, which are able to live for at least 30 years, are produced at a slow rate from rhizomes which grow horizontally by only 1 to 6 centimetres each year. Over centuries the rhizomes form mats which rise up into reefs that help to trap sediment and mediate the motion of waves, thus clarifying the water and protecting beaches from erosion. Dead leaves are shed in the autumn and can be seen washed up on beaches.

Dried P. oceanica leaves were traditionally used to stuff mattresses and pillows (apparently deterring bed bugs), to feed cattle, to provide packing material, and even to thatch roofs. Once viewed as beach litter, the importance of this biodegradable material in the ecosystem is now being recognised and several local authorities along the Spanish coasts hold annual information drives about preserving this seagrass and to explain their new policy of leaving areas of beach in their natural state.

In Catalunya P. oceanica has been listed as a protected species since 1991. Efforts have been made to prevent physical damage caused by trawler-fishing by placing artificial reefs, consisting of spiked concrete blocks, along certain stretches of the coast, and also by mounting a coastal watch to prevent illegal fishing. The Department for the Environment has also created a treatment plan for drain water purification and their boat can often be seen around the coast taking water samples.

Cow bream in Posidonia Oceanica Seagrass - courtesy of Gaynor Rosier
Painted comber in Posidonia Oceanica Seagrass - courtesy of Gaynor Rosier
Egg case on Posidonia Oceanica Seagrass - courtesy of Gaynor Rosier
Painted Wrasse in Posidonia Oceanica Seagrass - courtesy of Gaynor Rosier

Due to the tough lignin that covers its cells, Posidonia oceanica is only grazed by animals that have special micro-organisms in the gut to help them digest it. One such animal is the endangered green sea turtle, Chelonia mydas, another is the plentiful sea urchin Paracentrotus lividus. A recent study by Tomas et al* of Barcelona University found that urchins have a relatively minor impact on the seagrass whilst grazing by the fish Sarpa salpa can outstrip the plants' leaf production. Sarpa salpa, or cow bream, form large schools in seagrass meadows during summer causing mowed patches in which the biomass can be reduced by as much as 50%.

As seawater temperatures rise during summer the leaves of P. oceanica are colonised by microscopic encrusting algae, colonial sponges, bryozoa, and hydrozoa causing a greyish-brown appearance and limiting the absorption of sunlight needed for photosynthesis. However, these too provide a food source for sea slugs (Aplysia) and nudibranchs that can be found on the leaves where they also deposit their eggs.

Along the coast of Catalunya P. oceanica provides a breeding habitat and a home for many species of fish such as the silvery breams, colourful wrasses and neon-blue juvenile damselfish. Solitary combers are actively territorial in protecting their own patch of seagrass. Solitary scorpion fish remain very still, well camouflaged on the benthos, whilst waiting for unwary prey to draw within range of their lightening fast extendable mouths. Sand gobies, Tompot blennies, clingfish, pipefish and seahorses (Hippocampus ramulosus) are other residents of the P. oceanica fish community.

The fan mussel, Pinna nobilis, is the largest bivalve mollusc in the Mediterranean and is a protected species still occasionally seen in relatively undisturbed Posidonia meadows. Once plundered for the transparency of its shell, ancient examples reach up to 80 centimetres in length and stand majestically almost as tall as the seagrass itself.

Echinoderms are an important part of the benthonic fauna within P. oceanica meadows. As well as sea urchins, starfish and sea cucumbers are common, and brittle stars and delicate feather stars can sometimes be spotted between the rhizomes. Many hermit crabs are found here, living within discarded shells. Cuttlefish and octopus are frequently seen around the edges of seagrass beds, fascinating divers with their ability to adopt the colours and shade patterns of their surroundings.

At KennaEcodiving we dedicate our dive-time each spring and summer to studying P. oceanica and its associated species along the coast of Catalunya between L'Escala and Estartit. Beginning with a pilot study in 2004, we have developed a project with the aim of annually surveying certain areas of Posidonia in order to assess its current status, to detect changes, and to help forecast future status. We record data on species living within the P. oceanica habitat in order to measure diversity, and also photograph them. To achieve our aims we rely on the help of many dedicated divers. Each year we recruit volunteer divers from all over the world who want to learn more about Posidonia oceanica and wish to contribute some of their free time to conserving the rich biodiversity within it. Marine biology students, underwater photographers, and divers who simply share our passion for studying and preserving this marine habitat are welcome to join us throughout the season from April to October.

Ecodive Volunteer - courtesy of Gaynor Rosier

The Mediterranean has a very rich biodiversity, with a total of 10,000 to 12,000 recorded marine species, and new species still being recorded. The coastal zones supporting marine phanerogams like Posidonia oceanica are particularly productive. Despite their important function, the creation of marine reserves and protected areas are often insufficient as many of the negative impacts upon habitats such as P. oceanica are not of local origin.

Research is ongoing to understand the role of P. oceanica within the ecosystem and how best to control the man-made pressures upon its survival. It is one of the marine habitats to receive attention in the European Environment Agency's 2006 Report entitled Priority Issues in the Mediterranean Environment in which it is cited as "a key species for the Mediterranean region".

The European Environment Agency advocates the development of coordinated plans at an international level throughout the Mediterranean to protect important habitats such as P. oceanica through integrated environmental management. "The number one priority in environmental management in the Mediterranean region is to develop the necessary environmental legislation and to enforce it."

Initiatives to control and eliminate pollution in the Mediterranean area:

LBS protocol: Protocol related to the protection of the Mediterranean Sea from Land-based Sources of Pollution adopted on 17 May 1980, entered into force on 17 June 1983 and was amended on 7 March 1996. The amended version is not yet in force.

SAP: the Strategic Action Programme, adopted in 1977, is an action-oriented initiative under MAP/MEDPOL that identifies the priority target categories of substances and activities to be eliminated or controlled by the Mediterranean countries. This is to be achieved through a planned timetable for the implementation of specific measures and interventions. The SAP/MED is the basis for the implementation of the LBS Protocol by the Mediterranean countries over the next 25 years, starting in the year 2001.

NDA: National Diagnostic Analysis is the first step in the preparation of National Action Plan (NAP) to address LBS of pollution. It is an integrated analysis of the main issues related to LBS in coastal areas, including their environmental impacts.

* Reference: Seasonal and small-scale spatial variability of herbivory pressure on the temperate seagrass Posidonia oceanica. F. Thomas, X. Turon, J. Romero. Published October 11 2005 by Inter-Research in MEPS Vol.301.

Find out more and help with this issue:

This article was kindly written for us by Gaynor Rosier of KennaEcoDiving who help to monitor the oceans flora and fauna. To find our more about how they are helping and how you can join in please visit their web site:


We welcome any discussion about what is written here. If you have any comments or views on this article, please send them through!