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, which have 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 are of great socio-economic importance by preventing coastal erosion. This is achieved through their rhizomes and leaves; the network of roots and rhizomes stabilizes the sediments while the high density of 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.
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 rubrumand thevery 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, as trawling in the nearby area increases local sedimentation and affects the water turbidity. This in turn restricts the amount of light, preventing or limiting the algae from photosynthesizing which impedes their further growth. Fish farming produces a large amount of organic matter that settles on the seabed, altering the 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.
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.
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.
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.
- 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.
The conservation of Posidonia seagrass beds and coralligene reefs is a main priority of Archipelagos’ research work.
1.Mapping the sea
Produce detailed maps showing the location of Posidonia meadows and coralligenous reefs. From these maps, fishing grounds should be defined for large scale fishing gear in areas where the above habitats are not present, thus enabling the enforcement of the EU legislation protecting these unique habitats.
2.Collect biological data
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.
Monitoring of fishing activities and the promotion of sustainable fishing practices through the definition of fishing grounds for towed fishing gear (trawler, purse seiner, beach seiner), away from seagrass beds (as it is defined by the EU and national legislation).