Coralligenous Algal Reefs: The Reefs of the Mediterranean

Coralligenous algal reefs (known in Greek as “tragàna”) are probably the Mediterranean’s best-kept secret, however, this has led to them being overlooked with regards to conservation. These biodiverse habitats are highly productive and hence are of great importance to the survival of the marine ecosystems; however, they are currently under threat from many different destructive fishing practices, mainly from trawling. Bottom trawling on these habitats is unfortunately legal in Greece today; as the areas covered with coralligene have not been mapped yet and therefore it is impossible to apply any distinctive measures for the sustainable management and conservation of this habitat, as is defined by EU legislation.

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 range from 520 to 7760 years old, and once destroyed, these reefs will require thousands of years to recover. Coralligenous reefs are formed from red algae, belonging to the family Corallinaceae, that produce a calcium carbonate skeleton that forms the basic structure of the reef. They are found in areas with dim light, and in the Aegean are more commonly found from depths of 70 - 90m but can reach depths up to 160m. In the western Mediterranean they are found in shallower waters.

Importance

Coralligenous reefs have been described as a biodiversity hotspot, only second in biodiversity in the Mediterranean to Posidonia oceanica seagrass beds. They are often a spectacular sight; the coralline alga is covered with large gorgonian fans, coral, and a diverse array of sponges and other organisms. It is estimated that there are approximately 1666 species; including 300 algae, 1200 invertebrates and over 100 fish species living on coralligenous assemblages. The reefs are highly important fishing grounds as well as an important habitat for some threatened and endangered species, including the commercially prized red coral Corallium rubrum. Some of the important endangered species that commonly inhabit coralligenous reefs include gorgonian fan species, a very rare black coral (Gerardia savaglia) and the Dusky Grouper (Epinephelus marginatus).

Threats

1. Fishing
Coralligenous reefs are at a great risk from several destructive fishing practices. The use of trawlers, dredges and similar equipment on these habitats, causes irreversible destruction of coralligene reefs; physically damaging and breaking them. Trawling also has indirect effects on coralligenous reefs, even if they do not directly destroy a part of a reef; by trawling in the nearby area, they increase local sedimentation and affect the water turbidity. This is detrimental to the assemblages as it restricts the amount of light, and so prevents or limits the algae from photosynthesizing and hence impedes on further growth.
2. Pollution
Scientists have concluded that pollution not only prevents the growth of coralligenous habitats by inhibiting the growth of coralline algae, but it also decreases the species diversity and the abundance of individuals.
3. Invasive Species
Coralligenous habitats are threatened by the introduction of algal species that have become invasive. The most threatening of these is Womersleyella setacea - a red algae that forms a 1-2mm thick blanket over the reef. This can have adverse consequences for 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.
4. Human Coastal Activities
Coralligenous reefs are at risk from changes in coastal land use that affect the amount of sedimentation and water turbidity, for example activities that involve deforestation, reclamation of land and other activities resulting in excessive deposition into the sea.
5. Large Scale Events
The increase in atmospheric CO2 from human activities is likely to result in the ocean absorbing high quantities of CO2. This would ultimately lead to a decrease in the ocean’s pH, a phenomenon known as ocean acidification. The change in the ocean’s pH is predicted to be greater than previously seen in the last 300 million years. This is particularly worrying for coralligenous algal reefs and coral reefs as the production of calcium carbonate by reef building organisms is inhibited by decreasing pH.
6. Scuba Diving
Coralligenous reefs that are accessible to sub aqua divers are at risk from physical damage; both from accidental damage and the removal of organisms such as the large gorgonian fans and the prized red coral. Fortunately, in the Aegean, as the majority of assemblages are in the 70-90m depth range, ensuring that they are out of the range of many recreational divers.

Conservation

Although it is illegal to trawl, dredge or use damaging nets on coralligenous reefs, very few of the coralligenous reefs have been mapped in Greek waters, so the EU protection given to this habitat is impossible to enforce until extensive mapping of the area that these habitats cover is undertaken. It is extremely important that the current protection status assigned to coralligene reefs is enforced and tightened, so a complete ban is applied on trawling and dredging, not only over the reefs but in the general area to prevent indirect damage by increased sedimentation.

Protection status
This unique, highly biodiverse habitat has been assigned important protection status to aid in its conservation, and is 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 large majority of coralligenous reefs is unknown and so these conventions are not being enforced.