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Posidonia meadows

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  1. Posidonia Oceanic Meadows
  2. You can protect the Meadows

Posidonia Oceanic Meadows Subir al contenido anterior

1. A marine plant

Posidonia oceanica is not an algae, but a marine plant. Of course, both are vegetables, capable of growing from water, mineral salts, dissolved CO2 and light, through photosynthesis, and both can reproduce in the sea. However, algae have a very simple and primitive structure, as compared with marine plants

Algae appeared in the sea about one thousand million years and they show a huge diversity of life forms. A few hundreds of million years ago some green algae started to colonize freshwater and shores. In this process they developed more and more adaptations to life on land.

They created compounds, such as lignin, as well as structural elements that allowed them to maintain upright in the air, many times less dense than water. Roots and vessels allowed them to pump water from underground and distribute it to the whole plant, and gamets and spores developed shields to prevent dehydration.

Mosses and ferns appeared first, followed by spermatophyte plants, which divide in gymnosperms (pine tree, yew tree, ginkgos….) and the angiosperms –flowering plants-such as magnoliae, olive trees, daisies, wheat, and others. 

At that time algae ruled the top of the oceans, but about 140 million years ago –in the era of the great dinosaurs- some species of angiosperms likely living in shores, turned back to marine life, conserving some terrestrial adaptations, like roots, rhizomes and flowers.

A marine PlantFig. 1. Scheme of plant evolution, from green algae to marine plants, with a long period on land. Picture extracted from the story «Els prats submarins de Posidonia oceanica» by Toni Llobet, Pere Renom and Javier Romero.

This evolutive process in plants is somewhat similar to that found among vertebrates, which evolved from fishes to land vertebrates; amphibians, reptiles, birds and mammals, and later, some groups, like extinct ictiosaurs among the reptiles, and the cetaceans (whales and dolphins) among mammals, adapted again to live all their life cycle in the sea.

Scheme of the vertebrate’s evolutive processFig. 2. Scheme of the vertebrate’s evolutive process, from primitive fishes to cetaceans. The picture is based on the story «Els prats submarins de Posidonia oceanica» by Toni Llobet, Pere Renom and Javier Romero.

Adaptation to marine life occurred independently in the 4 to 6 families of marine plants currenlty known 

Hence, marine plants form a paraphyletic group (i.e. with different origins) within the order Alismatales, a group that comprises both aquatic and terrestrial plants, belonging to the Class Monocotyledonae.

Marine plants, composed presently of about 60 species, are divided in 12 exclusively marine genera, all grouped in 4 families, and 2 genera containing both freshwater and marine plants  grouped in two other families (Fig. 3).

Phylogenetic tree showing the multiple origins of marine plants within the order AlistamatalesFig. 3. Phylogenetic tree showing the multiple origins of marine plants within the order Alistamatales, following the classification of the group APG III "http://www.mobot.org/mobot/research/apweb/"

The families in bold comprise genera including exclusively marine species. The families in blue comprise genera that have recently evolved some species adapted to live and reproduce entirely in the sea, but also contain other species that are not seagrasses. The names of the seagrass genera/species appear within brackets following to the corresponding family. The symbol “•” indicates branches with less than 50% of statistical consistence. The symbol “*” signal branches with 50-70% of consistence. The rest of branches present more than 70% of statistical support. For further information please consult Virtual Library: Click here

Marine plants inhabit coastal areas along all continent, except the Antarctic (Fig. 4).

Distribution of seagrasses following the «World Atlas of Seagrasses»Fig. 4. Distribution of seagrasses following the «World Atlas of Seagrasses». Source: seagrassnet 

Posidonia is one of the most ancient genera of seagrasses, already found in the cretacean (145 to 65 millions years ago) represented by the species Posidonia cretacea

2. Posidonia oceanica: 100% Mediterranean

Posidonia oceanica inhabits the Mediterranean sea, from Punta Chullera, in the border between Málaga and Cádiz, to the Dardanelles straits, which separate Asia and Europe. Moreover, an isolated population of P. oceanica has been discovered in the Mármara Sea, between the Mediterranean and the Black Sea. There is also a reference one Posidonia meadow in Gibraltar, that would have been destroyed with the airport extension works.

Posidonia oceanica is abundant in the Mediterranean Sea. It has been found in all its coastal countries, except in Lebanon, Palestine and Israel (Fig. 5), occupying between 2.5 and 4.5 million hectares, which is equivalent to almost 25% of all the Mediterranean seafloor between 0 and 50 metres depth.

Presence of Posidonia oceanica in the coastal countries of the Mediterranean.Fig. 5. Presence of Posidonia oceanica in the coastal countries of the Mediterranean.

In Spain, Posidonia oceanica is found in all the coastal provinces of the Mediterranan. Following Ruiz (2012), Posidonia oceanica meadows extend today in around 1226 km2 of the Spanish coast. 

Posidonia oceanica meadows experienced important declines in the last 40 years, particularly in Catalonia and Valencia, although the provinces of Tarragona, Barcelona and Alicante still host vast meadows. In Andalousia, P. oceanica meadows have also suffered an important regression in the province of Málaga, its Western distribution extreme.

The most extensive meadows are found in Balearic Island, followed by Alicante, Murcia and Almeria, being less abundant in Catalonia and the sea of Alboran. 

The Mediterranean peoples baptised it with many names. In Spain the common name is «alga de vidrieros» (glazier alga), which was widely used before Linnaeus invented the binomial nomenclature in the XVIII century. In those times, the dry leaves of this seagrass were used to pack glassware. In Almeria it is referred to as «lijo», and in Catalonia as «altina». In English is bas been recently baptised by “ Neptune grass”.

However, as the environmental value of this plant spreads, its scientific name (devoted to Poseidon, the Greek god of Sea, called Neptune by Romans) is more and more used, and nowadays many people know this species just as «Posidonia».

Interestingly, other species belonging to the genus Posidonia are only found near the Spain antipods, in Australia, where the climate is also temperate, but there is no other species of this genus between these two regions. What made that disjoint distribution possible?

The continents are in continuous movement, they move some centimetres per year. Therefore, in the Cretaceous, the Australian and Mediterranean coasts were much closer than today, with a common marine area: the Thetys Sea (Fig. 6).

rigin of the disjoint distribution of the genus Posidonia.Fig. 6. Origin of the disjoint distribution of the genus Posidonia. Green leaves represent the genus distribution in each geologic period. Picture extracted from the story «Els prats submarins de Posidonia oceanica» by Toni Llobet, Pere Renom and Javier Romero.

Over millions of years the coasts of the present Europea, African and Australian continents separated thousands of kilometres and hence, populations of Posidonia cretacea, the original species living in the Thetys Sea, dispersed and became isolated from each other

Each population evolved independently resulting in the current species. The intermediate populations extinguished

3. Description and Biology of Posidonia oceanica

Posidonia oceanica is one of the largest seagrasses of the world. It is perennial, with a woody rhizome ending in a shoot of green, long leaves with uniform edges and parallel nerves. Posidonia leaves can be longer than one meter (Fig. 7).

Description and Biology of Posidonia oceanicaFig. 7. Plant of Posidonia oceanica with one horizontal and two vertical rhizomes and one inflorescence. Detail of picture of a hermaphrodite flower with empty anthers, as well as one mature fruit, called «sea olive». Author: Jordi Corbera. Picture taken from the book «Praderas y Bosques Marinos de Andalucía».

The leaf petiole and blade are flat. Youngest leaves grow from the shoot center, the position of the apical meristem that, by division, produces alternated leaves to the right and the left sides.

The rhizome is covered by «scales», which actually consist of fallen leaves (Fig. 8). The rhizome elongates between consecutive leave shoots. This section of newly grown rhizome is called internode. A new root may grow from there, and is later lignified.


Description and Biology of Posidonia oceanicaFig. 8. Detail of a Posiodnia horizontal apex, coverd by scales, which are hardened petioles of ancient leaves. Author: Carlos M. Duarte.

As plant produces a permanent mark (node) in the rhizome joint with every leaf base, this allows resarchers to know retrospectively how the plant growth was in the past, just as the trunk rings of trees (Fig. 9)

Description and Biology of Posidonia oceanica Fig. 9. Nodes or leaf insertion marks in the plucked rhizome of a vertical shoot of P. oceanica. The internode nº 1 is the youngest, and the nº 9 is the eldest. Author: Elena Díaz Almela.

Some shoots have horizontal growth (called apices) and divide several times per year, colonizing the substrate. They may grow between 1 and 10 cm per year. Their internodes may be more than 1 centimeter long.

However, most rhizomes grow vertically, towards light, escaping from burial. The vertical rhizomes, which internodes measure 1 or 2 mm long (Fig. 8), generally do not grow more than 1 cm per year, except when they are submitted to intensive burial. Then they may grow up to 4 cm per year. 

The plant compensates its slow growth with highly long-standing rhizomes, sometimes older than 60 years. Taking this into account, as well as the extension of some clones, it seems that every whole individual may live for hundreds or even thousands of years. Some evidences indicate that Posidonia clones may live tens to hundreds thousands years, and in this case it could be considered the longest-lived species over the world. 

Sediment accumulation together with the slow vertical growth makes Posidonia meadows to rise from the sea-bottom across millenia, forming a kind of reef called «mat» (Fig. 10).

Description and Biology of Posidonia oceanicaFig.10. Formation of the Posidonia mat by the combination of sedimentation and vertical plant growth. Picture taken from the didactic guide Posidoniaen tus manos»

The vertical shoots remain interconnected by the rhizomes over several meters, interchanging nutrients and carbohydrates. When they are pulled off they have a chance to disperse, take roots and start growing as a new plant, thus colonizing new areas (Fig. 10).

Posidonia fig. 11. Vegetative fragment of Posidonia with 2 horizontal shoots that took roots spontaneously in the seafloor. Author: Elena Díaz Almela.

Although Posidonia oceanica mostly reproduce vegetatively, it is also able to have sexual reproduction. In certain years, in Autumn, the meadows blossom. Some shoots develop green inflorescences, composed of small spikes with hemaphrodite (with ovary and stamens) and masculine (only stamens) flowers (Fig. 12).

Posidonia Fig. 12. Posidonia oceanica inflorescence, showing the central spike totally open. One of the flowers is releasing pollen. Author: R. Graille.

The seagrass pollen is adapted to water and dispersed by currents. Posidonia’S pollen is long and flexible and adheres to the feminine stigma as soon as it contacts it (Fig. 13).

Posidonia Fig.13. filiform pollen grains from P. oceanicA, sticked to the stigma of an hermaphrodite flower. Author: Elena Díaz Almela.

Posidonia oceanica fruit contains a single seed and looks like an olive. That is why they are called «sea olives» (Fig.14)

Sea oliveFig.14. a) Sea olive, still attached to the plant,
Sea oliveFig.14. b) Dissected sea olive, showing the mature seed.

 When released from the infrutescence, fruits can drifts randomly for hours or even days (Fig. 15).

Sea oliveFig. 15. Exceptional cumulation of floating sea olives in Jun 2003 (Balearic Island). Author: Elvira Álvarez

 When the seed is mature and germinated, the fruit cracks and the seed falls to the seafloor. It takes root and grows, when the appropriate conditions are met (Fig. 16)

Seafloor Fig. 16. Posidonia seed on the seafloor. Author: Elena Díaz Almela

4. Ecology of Posidonia oceanica

Posidonia oceanica meadows develop on sandy and rocky substrates, and especially well on dead mat. They don’t grow properly on gravel or mud.

They require very clean and transparent waters, with salt concentrations ranging between 33 and 30 psu (Practical Salinity Unit). Therefore, they are not found close to river mouths

Posidonia oceanica meadows tolerate a wide range of temperatures, beween 10ºC and 29ºC.

Posidonia oceanica meadows develop between 0.3 and 50 meters depth. In the Andalusian coasts meadows rarely surpass 25 meters depth. The waves, currents and water transparency determine the shape and natural extension of the meadows, as well as their shallow and deep limits.

Seagrasses need at least 11% of the total sunlight reaching the water surface, which is a bit more than what most algae need to survive.
For this reason, they live at shallow depths and need transparent waters (Fig. 17).

Density and cover of seagrass meadows decreases exponentially with depth and its deep limit decreases exponentially with turbidity.

PosidoniaFig. 17. Posidonia oceanica growing in the seabeds of Eastern Almeri¿a (El Calo¿n, Villaricos). Source: Program for Sustainable Management of the Marine Environment. Department of Environment, Junta de Andaluci¿a

Nevertheless, seagrasses need less nutrients than algae (nitrogen, phosphorous, iron...) to make to fabricate their tissues. This gives them an advantage over algae and bacteria in nutrient-poor waters.

In addition, Posidonia store nutrients and reserve substances in their thick rhizomes, which provides them an additional advantage as it allows them to start growing at the end of winter, restricting the growth of their competitors. This is why they say that Posidonia control the carbon and nutrients cycles in meadows.

However, when marine waters and sediments receive excessive organic matter and nutrients, this balance is disturbed in a typical process of eutrophication:

Epiphytic algae and phytoplankton grow in excess, puting Posidonia leaves in shade. The richness and abundance of epiphytes stimulate grazing by herbivores, like sea urchins, which in extreme cases can defoliate an entire meadow.

In the sediment, bacteria also grow in excess, depleting the oxygen and producing toxic compounds such as sulphide (Fig. 18).

PosidoniaFig. 18. Scheme showing the process that produce seagrass decline by excessive supply of labile organic matter (OM)

Sedimentation of organic matter or nutrients increases seagrass shoot mortality: sedimentation rates higher than 2 grams of organic matter per square meter and day accelerate the meadow decline.

Seasonal changes of temperature, daylight length and storms have generated an annual cycle in Posidonia oceanica meadows: increase of water temperature and daylight length in Spring stimulate leaf growth.
Algae and epiphytic animals proliferated on them (Fig. 19).

Epyphitic Fig. 19. a) Epyphitic algae on a Posidonia leaf, Authors: (a) Juan Manuel Ruiz
Briozoans Fig. 19. b) Colony of briozoans (Electra posidoniae) and calcareous encrusting red alga. Authors: (b) Diego Moreno Lampreave.

In summer, Posidonia meadows have long leaves, highly colonized by algae and epiphytic animals (Fig. 20).

Posidonia meadow Fig. 20. Posidonia meadow in winter, with short leaves, and in the forefront, a feather worm (Sabella spallanzani). Author: Juan Carlos Calvi¿n.

Autum storms remove a large amount of leaves, like winds for terrestrial trees. Much of it accumulates in sea shores, forming cushions of leaf litter (Fig. 21)

Posidonia Leaf LitterFig. 21. Posidonia leaf litter stranded at Roquetas de Mar beach. Author: Diego Moreno Lampreave

In Winter and early Spring, meadows have short leaves with few epiphytes (Fig. 20)

It is said that Posidonia oceanica is an engineer or structural species, generator of the meadow habitat, in which a highly diverse community finds its place. The denser the meadow is, the higher the richness of the community becomes. Rocky and sandy patches provide additional complexity, further increasing flora and fauna diversity in the ecosystem.

Three levels can be distinguished:

Foliar stratum or canopy. Leaf community is especialized in this highly illuminated and unstable habitat, which is renewed every year (Fig. 22).

Animals Fig. 22. Some mobile animals in the community of leaves: a) the isopod crustacean Synichia hectic, with perfect camouflage and one of the few herbivores can digest the leaves of Posidonia; b) the sucker fish Opeatogenys gracilis, one of the smallest fish in Europe, also conceals very well among the leaves; c) the gastropod mollusk Tricolia speciosa, which feeds scraping epiphytic leaves; Authors: (a and c) Lampreave Diego Moreno, (b) Gaynor Rosier

Rhizome stratum (Fig. 23), the community varies depending on the substrate type (rocky, sandy or «mat»), is composed by species requiring low light intensity.

Animals Fig. 23. Some organisms living between rhizomes: a) sponge crab (Dromia personata), which is normally hidden under death leaves; Authors: (a) Diego Moreno Lampreave,
Red Alga Fig. 23. b) the red alga (Peyssonnelia squamaria), ifrequently found on Posidonia rhizomes. Authors: (b) Jose Carlos Moreno Robledo.

The hypogeum (underground) stratum, highly rich in thick «mats».

The community also varies with meadow depth. The species that can be observed also change between night and day, as in the night, many animals climb from rhizomes to leaves looking for food, while other species seek refuge from pelagic predators.

Posidonia meadows are highly productive: the plant produces almost 1 Kg of dry organic matter per square meter and year. Epiphytic algae may add up to 300 g of organic matter per square meter and year.

Leaf growth is responsible for the 90% of annual meadow production (75% from leaf-blades and 15% from petioles). The other 10% is due to rhizome and root growth.

Only a few species of herbivores are able to digest Posidonia leaves, rich in cellulose and tannins; but many feed on the epiphytes. Normally, less than ten percent of leaf production is grazed.

From 90 to 95% of the leaf-blade production, enters the food web through decomposers, either within the meadow or, after being exported through waves or currents, in other habitats, sea shorelike s (Fig. 24).

Seagrass primary productionFig. 24. Fate of seagrass primary production. Author: Jordi Corbera.. Extracted from the book “Praderas y bosques marinos de Andaluci¿a”.

The trophic web of Posidonia meadows is highly complex, and not completely elucidated. 70% of the meadow fauna are herbivores (Fig. 25). Most of them graze epiphytic algae, and predominate in the canopy, especially in summer.

Some typical herbivores in Posidonia meadowsFig. 25. Some typical herbivores in Posidonia meadows: a) Salema porgies (Sarpa salpa) grazing on P. oceanica leaves, b) Hole made by the isopod Synischia hectica, c) The sea urchin Paracentrothus lividus has grown massively, close to a fish farm, defoliating the surrounding meadow. Authors: (a) Ramo¿n Esteban, (b) A¿ngel Luque, (c) MedVeg project.

The other two levels (rhizome and hypogeum), are dominated by decomposer organisms, especially in Autumn and Winter (Fig. 26)

Some meadows descomposersFig. 26. Some meadow decomposers: a) bacteria, diatoms and fungi on the tissue of dead Posidonia leaf, observed by scanning electronic microscope; b) The sea cucumber filter sand and may digest particles of the partially decomposed leaves; c) Stained copepod observed through the optical microscope. It measures 0.25 mm. Copepods also feed on partially decomposed leaves. They proliferate in Autum, when dead Posidonia leaves are abundant. Authors: (a) Lepoint et al (2006), (b) Diego Moreno Lampreave, (c) Cristina Gambi.

Roots and rhizomes, particularly leaf petioles attached to rhizomes, are highly resistant to degradation and predominate within the mat material.

5. The submersed treasure: environmental and socioeconomic services.

Benefits for tourism: Posidonia meadows improve beach quality:

Posidonia meadows partially absorb wave intensity and make the suspended particles to settle, retaining them within the net. This increases water transparency and, at the same time, contributes to reduce coastal erosion (Fig. 27).

Scheme of processes mediated by Posidonia meadows that contribute to prevent beach erosion

leaf-litter cushions also protect beach from erosion, as waves and currents must first remove all the cumulated accumulated litter before the sand (Fig. 27).

In addition, shells from epiphytes composed of calcium carbonate or silica, are shredded but not degraded, and is transformed into a high quality sand. White sands around Formentera (Balearic islands), Aguamarga or El Plomo (in Cabo de Gata) constitute examples of the great benefits provided by Posidonia oceanica meadows (Fig. 28).

Dead Mollusc SHellsFig. 28. a) Dead mollusc shells carpet Posidonia meadows. In the forefront two live giant clams (Pinna nobilis), and some dead shells around them; b) Posidoni growing on white sands, mostly constituted of meadow shell debries. In the forefront a school of Myscidiacea,, a kind of crustaceans similar to shrimps. Authors: Diego Moreno and Carlos M. Duarte.

During the day, Posidonia meadows oxygenate seawater. Only the meadows from Cabo de Gata have a net production of 21 to 37 tonnes of oxygen per day in Springs and Summers.

Seagrass meadows are beautiful habitats, hosting a great diversity of life forms. They generate diving spots, because they enrich diversity and improve quality of surrounding ecosystems (Fig. 29).

Alphabuceo dive centerFig. 29. A) Author: Luis Reinoso, Alphabuceo dive center.

Alphabuceo dive centerFig. 29.b) author: Nieves Castan¿o, La isleta Dive center.

Posidonia meadows constitute the unique or preferred habitat for a number of protected and endangered species as the giant clam (Pinna nobilis) or the cushion star (Asterina pancerii) (Fig. 30).

Cushion StarFig. 30. Cushion star (Asterina pancerii). Source: Program for the Sustainable Management of the Marine Environment.

Source of fisheries. Many fishes of commercial interestest, as the cuttlefish, spawn in the Posidonia meadows and/or develop live in the meadow when they are juvenile (Fig. 31)

Some speciesFig. 31. Some species that reproduce or grow in Posidonia meadows: a) Eggs laid by different animal species over a Posidonia leaf; b) Cuttlefish’s egg on a leaf; c) School of juvenile fishes in a meadow sand patch; d) Juvenile scorpion fish hidden between Posidonia rhizomes. Authors: (a and c) Diego Moreno Lampreave, (b and d) Gaynor Rosier

Some species visit the meadow to hunt, as is the case of the murain eel (Muraena helena) and the octopus (Octopus vulgaris). The latter sometimes spawn in meadow nooks.

Breams are very abundant in the meadows, and the red mullet (Mullus surmuletus, Fig. 32) often digs in the meadow sand patches.

Some species of fishingFig. 32. Some species of fishing interest that hunt or reproduce in the meadows: a) Octopus, b) Red mullet, c) Breams and barracudas (in the background), d) Cuttlefish. Author: (a) Ramo¿n Esteban, (b y c) Diego Moreno Lampreave, (d) Gaynor Rosier.

Global services: long-term carbon sink

As the meadow rises, rhizomes with their petioles and roots attached, get trapped within the «mat». Moreover, larges amounts of organic matter from different origins get trapped within the meadow rhizomes.

This organic matter gets buried beneath the sediment in anoxic conditions and, as a result, undergoes a very slow mineralization process. Annual accumulation rate of refractory organic matter is estimated to range between 70 and 660 g of dry matter per square meter and year.

Extrapolating these local data to the total surface of Spanish seagrass meadows, we can estimate that they sequester around a million tones of refractory organic matter per year, value that is most probably, an underestimate.

One posidonia mat 1 to 4 meters thick may store between 40 and 160 kg of organic carbon per square meter, cumulated in approximately 6000 years (Fig. 33). This estimation gives an idea of the great importance of Posidonia meadows as long- term carbon sinks

Profile of PosidoniaFig. 33. Profile of a Posidonia oceanica «mat» around 2 meters thick showing the old rhizomes and roots. Source: Mateo et al (2011).

When a meadow disappears, the organic carbon cumulation stops. Moreover, if the mat is not covered by a thick sand layer, it can be eroded by stormy waves and other processes, thus the carbon stored over thousands of years is released back to the atmosphere in just a few years.

Meadows as environmental indicators

The presence of Posidonia meadows indicates good water and beach environmental quality, as they require clean waters and substrates.
The http://eur-lex.europa.eu/browse/summaries.html Water-framework Directive uses several parameters related to the health status of seagrass plants and meadows as indicators of coastal water quality.

In addition, rhizomes fix heavy metals and radioactive elements, so that the mat and rhizomes serve as an historical record of these pollution types. Specific methods to analyze those records already exist "http://www.lifeposidoniandalucia.es/es/detalle_noticia.aspx?id=111" (see the new from 15 November 2011)

Traditional uses of Posidonia oceanica:

  • The leaf litter of the Neptune grass has been widely used in the past, and some of its applications are still in use today or are being recovered:
  • For filling mattresses or as cattle beddings, as this litter efficiently repels bedbugs. Source: “The updated Dioscorides”, by Pio Font Quer.
  • For the high leaf content in tannins, they have antiseptic, appetite- enhancing and astringents properties. Source: “The updated Dioscorides”, by Pio Font Quer
    • In Corsica, Posidonia leaf litter was burnt on agricultural lands, in order to fertilize them with mineral salts (Fig. 34a). Source: Boudouresque et al. (2006)
    • In Formentera, the leaf litter was used as cattle bedding, and then, mixed with the animal excrements, it was dispersed on the fields, where it increased soil porosity and contributed to retain humidity. Nowadays, there is renewed interest in this particular use and it is even sold as compost in Greece (Fig. 34b).
  • As Posidonia leaf litter burns with difficulty, they have been used as construction material, to isolate roofs from heat and sound.
  • They have been used to pack glassware. That is why it was traditionally named “glass-worker alga”.
  • In periods of famine, farmer fed their cattle with this plant fresh, including the fruits.
  • Its high content in mineral salts and silica has recently been extracted to make cement, and promising results have been obtained.

The sister project  P.R.I.M.E. Life (LIFE09 ENV/IT/000061) is developing a model of management and sustainable use of the Posidonia leaf litter

Young Girl carrying a basket of Posidonia Fig. 34. a) Young girl carrying a basket of Posidonia leaves in Corsica. Source: “Ramoge” by Boudouresque et al. (2006). b) Bag of Posidonia compost marketed en Greece.

6. Threats

 6.1. Illegal trawling fishing

Trawling constitutes one of the most destructive fishing techniques existing today, because it damages almost all bottom types where it is practised. Even in the XVIII century, fishermen that used other methods complained about trawlers to their governors. In addition, trawling is an un-selective fishing practice, in which over 60% of the extracted biomass is discarded.

Trawl fishing is forbidden at depths shallower than 50 meters (Real Decreto 10/9/1999), and explicitly forbidden on Posidonia meadows (Real Decreto 7/12/1995)

Nevertheless, some trawling vessels do not observe the law and use their fishing gears at shallower depths (Fig. 35), even over seagrass meadows.

Trawler boat fishing too close to coast Fig. 35. Trawler boat fishing too close to coast and at depths much shallower than 50 meters. Source: Agency for Agricultural and Fisheries Management

Vessels often trawl directly over the Posidonia Meadows  (Fig. 36)

Trawling tracks on Posidonia meadowFig. 36. Trawling tracks on Posidonia meadow in the LIC Punta Entinas-Sabinar, detected by mean of a side-scan sonar. Source: Agency of Agricultural and Fisheries Management (Junta de Andaluci¿a).

A small trawling boat may remove between 100.000 and 360.000 shoots per hour if they trawl on a Posidonia meadow. Moreover, trawling in the seabed alters the sediment structure and promotes permanent sediment re-suspension and siltation, further delaying meadow recovery, which is already slow. Meadow loss is an irreversible process at the human life scale.

Illegal trawling is a “short-term gain, long-term pain” ill strategy, and it is primarily detrimental to all fishermen, specially those that observe the regulations, as meadow destruction leads to the total loss or strong reduction of many associated fishing species

To address this problem is a priority for the project Life+Posidonia Andaluci¿a, which deals with this situation through dialogue with fishers (actions D4 and C1), installing artificial reefs (actions A4, A5 and C3), as well as reinforcing the control using video-surveillance systems.        

6.2. Illegal dredging

Illegal dredging to extract sand is other important threat to Posidonia meadows. Sand is used for beach restoration, greenhouse agriculture, or construction works.

Too often, dredging is carried out at depths shallower than allowed or using improper methods in order to reduce. Occasionally, sand has been extracted even on the meadow, causing great damages.

Sometimes it is possible to observe, through satellites, those meadow damages (Fig. 37).

Satellite Photograph Fig. 37. Satellite photograph extracted from Google Earth, in front of the SCI Bajos de Roquetas. Lines produced by illegal dredging are observed. The depth meadow is seriously damaged as a consequence of illegal trawling and dredging.

6.3. Pollutant spills (waste water, fertilizers, chemical and thermal plants...)

Often, coastal population waste waters undergoe just a decantation treatment, and are then released to the sea. Therefore, discharged waters are rich in dissolved organic matter and nutrients and damage the environment by favouring eutrophication processes. In addition, they are fresh waters that may further further affect coastal vegetation.

Those spills, continuous or intermittent, are usually channelled through submarine pipes (emissaries) (Fig. 38), but sometimes their mouth is near a Posidonia meadow, thus poisoning the seabed and producing a fast decline in seagrass meadows or other coastal communities

Submarine emissaryFig. 38. a) Mouth of a submarine emissary releasing waste-water, b) waste pipe flowing directly on the beach, at Bajos de Roquetas, and damaging this Natural Monument. Source: Program for Sustainable Management of the Marine Environment, Department of Environment, Junta de Andaluci¿a).

Other kinds of spills, usually less frequent, are those from industrial plants. As an example we can indicate the chemical spills caused by the company Deretil, Villaricos (Fig. 39): in the 90s this plant released repeatedly phenols and other substances directly to the sea, causing the irreversible destruction of some Posidonia meadows at human life scale, along 2 Km of coast and up to 10 meters depth.

ImpactFig. 39. Extension of the impact of the spill from the Deretil plant over the Posidonia meadows until 1999. Meadow decline has been slowed down in the last 10 years, since the company installed a chemical filter. Map taken from Moreno et al (1999)..

The brine from the desalination plants as well as hot waters from thermal plant refrigeration systems need also to be controlled and correctly channelled.

Finally, another source of diffuse contamination comes from water run-off carrying excess of chemical fertilizer from agricultural lands. Thanks to the low rainfall registered in Almeri¿a, such diffuse contamination is not as intensive as it could be in intensive agricultural areas like El Ejido.  

6.4. Aquaculture.

The rests of food and faeces from fish-cages are rich in organic matter, nitrogen and phosphorus, thus producing an effect similar to city sewage waters. Moreover, the cages shadow may further affect to the seagrass and seaweeds at the bottom (Fig. 40).

 Aquaculture cages viewed over and below the water surfaceFig. 40. a) and b) Aquaculture cages viewed over and below the water surface, c) sea-breams growing in cages, d) some Posidonia shoots living near a fish-cage. Source: European project MedVeg. Author of the picture c): Fe¿lix Sa¿nchez

The impact of aquaculture cages on seagrass meadows is reduced by placing the cages in open waters at a good distance of sensitive ecosystems, as well as improving feeding efficiency and installing bio-filters around the cages (for example, cultivation of algae).

Fish-farm concessions are conditioned by local authorities to their engagement in performing a continuous environmental monitoring of the seabead around fish- cages.         

6.5. Coastal construction.

Nowadays, more than 70% of the global population lives in the coast. This implies a high pressure on the coastal environment.

Building construction, roads and seafronts often block natural sand inputs to the beach, thus promoting beach erosion, as well as seagrass meadow erosion.

A common practice is to replenish the beach with foreign sand. However, if this sand is too fine-grained turbidity and siltation increases, and this may promote meadow decline. Seagrass loss promotes further beach erosion, leading to a well- known vicious circle.<br />

Construction of ports, breakwaters and other structures invading the sea (Fig. 41) frequently destroy the surrounding ecosystems by direct burial and siltation. Moreover, these structures often alter permanently coastal currents and hydrodynamism, resulting in erosion or burial phenomena (depending on the particular case) in other coastal places.

Breakwater construction. SourceFig. 41. Breakwater construction. Source: Program for the Sustainable Management of the Marine Environment. Council for Environment (Junta de Andaluci¿a).

6.6. Invasive algae

The increase in maritime communications, opening of the Suez Channel1867, and global warming accelerate the arrival of exotic species to the Mediterranean, a process that is also occurring worldwide.

Around a ten percent of exotic species have an invasive behaviour, affecting the autochthonous ecosystems. In the Mediterranean coast, two tropical algae of the genus Caulerpa show such invasive dynamics.

 Caulerpa taxifolia (Fig. 42) was accidentally introduced from the Monaco aquarium. At the beginning, it showed a highly invasive growth and dispersion that was subsequently slowed down. It is believed that, due to their low genetic diversity, the populations of this species may have collapsed or are controlled by some pathogen.

Caulerpa taxifolia invading a Posidonia meadowFig. 42. Caulerpa taxifolia invading a Posidonia meadow. Author: Alexandre Meinesz.

Caulerpa racemosa (Fig. 43) arrived in the 90s trough the Suez Channel. It first colonized the south-eastern mediterranean coasts, and then moved counter-clock, first to the eastern mediterranean, and then towards the western mediterranean. It is currently present in all the Levantine coast of Spain, and reached the northern coast of Almeria in 2009. The continuous inoculation of C. racemosa trough the Suez Channel makes that populations of this species show a higher genetic diversity, and thus greater stability than C. taxifolia

C. racemosa has invaded many kinds of coastal ecosystems, such as rocky bottoms covered by algae, ma¿erl and corals. In the particular case of Posidonia, when the meadow is dense and healthy C. racemosa does not penetrate deep between the shoots and just invades the bare patches. But, when the meadow is not dense (for example if it is deep or has been previously damaged) this alga may invade the meadow and worsen its regression or alter its rhizome community

The invasive alga Caulerpa racemosaFig. 43. The invasive alga Caulerpa racemosa (green fronds with round branches), the autochthonous alga Caulerpa prolifera (green flat fronds) and a red, filamentous alga (left-top) invading a meadow that has been cleared by the impact of an aquaculture cage. Source: European project MedVeg.

Both invasive species of Caulerpa release toxic compounds (caulerpenins) against competitive and herbivore species and they are capable of heterotrophic metabolism (such as the animals), reducing oxygen in the water.

Another invasive alga is Lophocladia lallemandii (Fig. 44), a red alga also introduced through the Suez Chanel and that has caused shoot mortality at some Posidonia meadows in the Balearic Islands. It also affects the seaweed rockey communities. I has been recently (2012) noticed in Almerian coasts.

Lophocladia lallemandii entangled in Posidonia leaves in the Levantine coast of AlmeriaFig. 44. Lophocladia lallemandii entangled in Posidonia leaves in the Levantine coast of Almeria. Source: Andalousian program for sustainable management of the marine environment

Filamentous algae Acrothamnion preisii and Womersleyella setacea (Fig. 45) invade the rhizome stratum of Posidonia meadows and displace the rhizome encrusting algae, reducing the diversity of flora and fauna of the rhizomes. These species have not been detected in Andalusia yet.

Acrothamnion preisii completely covering a Posidonia oceanica rhizomeFig. 45. Acrothamnion preisii completely covering a Posidonia oceanica rhizome. Author: Enrique Ballesteros.

Invasive algae spread easily, entangled with fishing net or boat anchors, as well as with currents.

The project Life+Posidonia Andaluci¿a has started a strategy to control the advance of invasive algae. For further information go to action C4. If you want to know how to help fighting invasive algae visit the section (enlace a proteger) “You can protect the meadows”

6.7. Intensive free anchoring.

Every time a boat anchors on a Posidonia meadow it may uproot many shoots during the anchoring manoeuvre, as well as when lifting the anchor back (Fig. 46).

Anchor and chain over a Posidonia meadow. Source:Fig. 46. Anchor and chain over a Posidonia meadow. Source: Program for the Sustainable Management of the Marine Environment, Council for Environment (Andalusian Regional Government).

If the anchor doesn't grip properly, or when wind direction changes, causing the boat to revolve, the chain and/or the anchor drag the bottom, uprooting more shoots

A small boat may have a slight effect on a meadow, however many boats anchoring in the same bay, the small damages cumulate

Therefore, some bays and natural harbours undergo a serious pressure due to this problem. As Posidonia oceanica grows so slowly, if the meadow cannot recover from year to year it declines.

To afford this problem one of the actions previewed in the Project Life+Posidonia Andaluci¿a is to install 41 ecological moorings in 4 SCI included in the project. If you want to know the place where these mooring will be installed consult the action C2 and the section « You can protect the meadows».

You can protect the Meadows Subir al contenido anterior

1. SOS Posidonia endangered

In the last thirty years nearly half of the Mediterranean Posidonia meadows have disappeared or experiment a clear decline. This is due to different causes as organic and chemical pollution, illegal trawling and dredging, coastal construction, introduction of invasive species, massive boat anchoring and sea warming.


2. You decide

Per every hectare of meadow destroyed, we lose


3. We rely on you

3.1 Posidonia and you are in the same boat.

Commercial fishing is a traditional activity linked to the Mediterranean culture. You, as fisherman, are an important user and connoisseur of the marine environment. So you have a key role in protecting it. For this reason the Project LIFE+Posidonia Andalucía develops specific activities for (enlace volutarios) you and your family 

Your involvement is important, 8 basic ideas:

  1. Practice sustainable fishing, so that the sea will keep providing your livelihood. 
  2. Use fishing gear that does not damage the marine environment.
  3. Take care of the meadows. They have a key role in the reproduction, nursery and growing of commercial species. If Posidonia disappears, fishing resources will be reduced.
  4. Observe the restrictions. According to the Regulation CE n.º 1626/94 the trawling fishing is not allowed at depths shallower than 50 meters as well as over Posidonia oceanica meadow beds along the whole Mediterranean Sea.
  5. If a fellow fisherman does not respect those limitations, remember that he is not being fair with his fellows, because in order to get a short-time profit, he destroys a long-term resource for everybody, included himself
  6. Beware of the algae Caulerpa racemosa!! If you find it in the anchor or nets, do not throw it back to the sea, otherwise it will keep spreading (negatively affecting to the fishing resources, as very few species are able to feed on it). Throw it in a bin, or burn it. Take note of the geographic coordinates and give a warn to: exoticas.invasoras.cma@juntadeandalucia.es
  7. Do not throw wastes ¿either solid or liquid- to the sea. Take particular attention to those made of plastic or nylon, as they take centuries to be degraded. 
  8. Recover the lost fishing material or, if not possible, note the coordinates, which will make possible to recover it in the future.
  9. Get involved in programs about sustainable and responsible fishing.

3.2 Lift your anchor leaving no trace 

Did you know that..

Posidonia grows very slowly (1 to 6 cm per year). For this reason, when boat anchors remove Posidonia shoots, the plant takes months or years to recover. In highly crowd bays, the meadow cannot recover from year to year and declines. 

Your involvement is important, 10 basic ideas:

  1. When sailing, remember that using only the sails is cheaper and nicer than using the engine. Try to minimize your impact, the sea is sensitive to human activity
  2. When planning a sailing out:
    1. Before going into a marine protected area, check the specific regulations about anchoring at the chosen area. 
    2. Check if there are mooring buoys in the area: your mooring will be safer and easier, and you will reduce the risk of damaging the seabottom or spreading invasive species. Check the location of the mooring buoys that will be installed as part of the project actions from 31st December 2012. You can also check at sites like:"http://www.bloosee.com" 
  3. If there are no available mooring buoys and you need to drop the anchor, avoid dropping it over the meadows or rocks: choose a wide sand patch. 
  4. Make sure the gear is in neutral before dropping the anchor, as the current provide enough strength to fix the anchor. 
  5. Make sure that the anchor chain is long enough so that the anchor works properly and does not drag the bottom. 
  6. The next advices when lifting the anchor are useful: 
    1. Tie a line to the anchor -one end to the anchor cross and the other to a small buoy- so that you can recover the anchor pulling from that line (rather than from the chain).
    2. With or without that line, if you place the boat completely vertical over the anchor line ¿or just a little bit further- it will be much easier to recover the anchor and the impact on the bottom will be minimal.
  7. Don¿t pour toxic liquids such as oil, petrol, black waters, or any kind of wastes to the sea. Have a look to the MARPOL regulation applying to recreational vessels in the area (text in Spanish). 
  8. Don¿t forget to close the black water tank when the boat is anchored. If there is toilet on the boat it is mandatory to install this tank, which can only be discharged in the enabled installations in the port or at a distance of more than twelve (12) nautical miles from the coast.
  9. Beware of Caulerpa racemosa. This is an invasive algae that negatively affects to the autochthonous communities. If you find it in the anchor, do not throw it back to the sea. Throw it in a bin, or burn it. If possible, take note of the coordinates and a picture, and give a warn to: exoticas.invasoras.cma@juntadeandalucia.es
  10. Do the maintenance works always inside the port, try to use biodegradable products, and throw the wasting products in the facilities enabled to do so.

3.3. Dive without leaving a trace

Did you know that...

Posidonia oceanica is a flowering plant living in the Mediterranean. It forms extensive meadows which harbour more than 400 flora species and 1000 species of fauna, enriching the surrounding habitats. It also contributes to the water transparency and provides high quality white sand. Discover the meadow diversity.  

Your involvement is important, 10 basic ideas:

  1. Before dropping the anchor, make sure that you are not anchoring over Posidonia meadows.
  2. Whenever possible, use the ecological mooring buoys: your sailing will be safer and you will avoid damaging the Posidonia meadows. Check the location of the moorings that will be installed as part of the project actions from 31st December 2012.
  3. Take care of the meadow marine life: don¿t disturb the fauna or pick up any vegetal species. Keeping only your pictures is the best practice. 
  4. Train your diving skills, specially your buoyancy and kicking, in order to avoid damaging the marine organisms as well as stirring up the sediment. 
  5. Beware of the algae Caulerpa racemosa!!. If you find it during your dive do not pull it up. Take note of the coordinates, depth and approximate area, and take a picture, if possible. Please provide such information to: exoticas.invasoras.cma@juntadeandalucia.es Check your diving gear after the dive and if you find any piece of the algae throw the algae to a bin, or burn it. 
  6. Keep the sea bottom clean. Don¿t throw any waste. If you find any plastic, fishing weight, or batteries during your dive pick them off and throw them in the corresponding container. 
  7. Observe the diving restrictions in the chosen area. Check if you need any kind of permission to dive there.  
  8. Start acting: join -as a volunteer- to the network for monitoring the Posidonia meadows, POSIMED-Andalucía.
  9. If you manage a diving centre or club, incorporate sustainable diving strategies, and help in the early detection of invasive species

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