Seagrass Meadows
& Coralligene Reefs

Posidonia Seagrass


Posidonia oceanica seagrass in the Mediterranean is one of the oldest living organisms on Earth, with a single clonal colony estimated to be over 100,000 years old. Posidonia seagrass beds are protected

habitats that play a fundamental role in the health and productivity of the Mediterranean marine ecosystems.

Posidonia meadows play a crucial role as breeding and nursery grounds for numerous species. Over 300 species of plants and 1.000 species of animals live within Posidonia meadows, including a large number of commercially important fish species. Apart from supporting productive marine ecosystems and fisheries, seagrass beds provide a great socio-economic service by preventing coastal erosion. The network of roots and rhizomes stabilizes the sediments while the high density of their leaves reduces wave energy. Also, much like land plants, Posidonia meadows make up a significant ocean carbon store, reducing the levels of carbon dioxide present in the atmosphere.

Coralligene reefs


Coralligenous reefs are biogenic assemblages formed by red algae, belonging to the family Corallinaceae, that produce a calcium carbonate skeleton which forms the basic structure of the reef. They are found within the Aegean Sea and the Western Mediterranean in areas of dim light, reaching depths of 150m. Coralligenous reefs (known in Greek as “tragàna”) are probably the best kept secret of the Mediterranean. Because of this lack of awareness, they have been overlooked with regards to conservation.

Coralligene reefs are highly productive, biodiverse habitats important to the survival of marine ecosystems. They are often a spectacular sight; the coralline alga is covered with large gorgonian fans, coral, and a wide array of sponges and other organisms. It is estimated that there are approximately 1,700 species, including 300 algae, 1,200 invertebrates and over 100 fish species living throughout the coralligenous reefs. Coralligene reefs are important fish nursing grounds and provide a vital habitat for threatened and endangered species, such as the commercially prized red coral Corallium rubrum and the very rare black coral (Gerardia savaglia). One of the most prized Mediterranean fish species, the dusky grouper (Epinephelus marginatus) also inhabits coralligene reefs.


The degradation of coralligenous algal reefs is a major concern, as they have a very slow growth rate (0.006 – 0.83 mm/year). Studies have shown that the age of the reefs ranges from 520 to 7,760 years and once destroyed, these reefs will require thousands of years to recover. Furthermore, since Posidonia oceanica has a very slow growth rate (<10cm/year), it is very vulnerable to anthropogenic impacts and it is difficult to recover once destroyed. In the Aegean Sea, the main factors of threat for both coralligenous reefs and seagrass meadows are:



Coralligenous reefs and seagrass meadows are at a great risk from several destructive fishing practices. The use of trawlers, dredges and similar equipment on these habitats causes irreversible destruction. Trawling also has indirect effects, since it increases local sedimentation and affects water turbidity. This in turn restricts the amount of light that reaches the seabed, limiting the algae's ability to photosynthesize, impeding further growth. Furthermore, fish farming produces a large amount of organic matter that settles on the seabed, altering its physiochemical characteristics, which can affect the growth of Posidonia oceanica.



Scientists have concluded that pollution not only prevents the growth of coralligenous habitats by inhibiting the growth of coralline algae, but decreases the species diversity and the abundance of individuals.

Caulerpa spread

3.Invasive species

Coralligenous reefs and seagrass meadows are threatened by the introduction of invasive algal species. The most threatening of these is Womersleyella setacea – a red algae which forms a 1-2 mm thick blanket over the corraligenous reefs. This can have adverse effects on the coralline algae as it can reduce or prevent photosynthesis. In addition, it can prevent larval settlement and inhibit the growth of other macro alga species. The damage caused by trawlers and uncontrolled anchorage gives the chance for opportunistic algae, such as the invasive green algae Caulerpa racemosa, to colonize new patches.


4.Human coastal activities

Coralligenous reefs and seagrass meadows are at risk because of changes in the coastal land use that affect the amount of sedimentation and water turbidity, resulting in excessive deposition into the sea. These activities include deforestation, reclamation of land and other coastal activities.


5.Ocean acidification

The increase of atmospheric CO2 from human activities is likely to result in the oceans absorbing high quantities of CO2, ultimately leading to a decrease in the oceanic pH - a phenomenon known as ocean acidification. This threatens coralligenous algal reefs because the production of calcium carbonate by reef building organisms is inhibited by the decreasing pH.


6.Scuba diving

Coralligenous reefs accessible to scuba divers are at risk of physical damage, both by accident and by the deliberate removal of organisms including the large gorgonian fans and the prized red coral. Fortunately in the Aegean, as the majority of assemblages are between depths of 70 and 90m, they are out of the range of many recreational divers.


poseidonia-snorkelingThese unique, highly biodiverse habitats have been assigned an important protection status to aid in their conservation and are protected under the following conventions:EU

  • Fishing Legislation EC 1967/2006 concerning management measures of the sustainable exploitation of fishery resources in the Mediterranean Sea: ‘Fishing with trawl nets, dredges, shore seines or similar nets above coralligenous habitats and mäerl beds shall be prohibited.’
  • Bern Convention
  • Mediterranean Red Book

Unfortunately, the location of the majority of coralligenous reefs and seagrass meadows has not been mapped, so the EU protection is impossible to be effectively implemented until extensive mapping of the habitats occurs. It is, however, vital for the above conventions to beenforced. A tightening of existing regulation is also necessary equating to a complete ban being applied on trawling and dredging, not only over the reefs, but in the general area to prevent any indirect damage.

Methods of Action:


1.Mapping the sea

Archipelagos produces detailed maps showing the location of Posidonia meadows and coralligenous reefs. From these maps, fishing grounds for large-scale fishing operations can be delineated where these delicate habitats are not present, thus enabling the enforcement of existing EU legislation designed to protect these unique habitats.

Marine_equipment (2)

2.Collect biological data

We collect biological data using an underwater digital cartographic camera system, providing:

  • Detailed information regarding the biodiversity and community composition of Posidonia meadows and coralligenous assemblages.
  • Evidence of damage and destruction.
  • Photographic and video footage of the seagrass beds and coralligene reefs to be used in public awareness campaigns.


3. Encourage Sustainable Fishing

We monitor fishing activities and promote sustainable practices by working to delineate away from seagrass beds fishing grounds for towed gear (trawler, purse seiner, beach seiner) as required by EU and national legislation.


4.Awareness campaigns

Archipelagos works to increase environmental awareness regarding Posidonia meadows and coralligenous reefs in order to strengthen public support for their protection as well as to educate professional and recreational fishermen so that they can limit the destructiveness of their fishing practices.