Coral Reefs

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Temps de lecture : 4 minutes  

barrière de coraux
Coral reefs make up — along with rain forest canopy ecosystems — the richest and most complex ecosystems on the planet. Even though they cover less than 0.25% of the aquatic environment, they could hold between 1 and 3 million species, only 10% of which have been identified. Close to 5,000 fish species — more than one quarter of all known marine fish — have been identified. On one square meter of coral reef, the number of animal and plant species is more than a hundred times greater than it is in the surrounding ocean.

Coral reefs can be found in more than a 100 countries and cover 112,000 to 4 million km2 depending on whether we consider reefs in the strict sense of the word or if we also include the lagoons they surround. The Great Barrier Reef (Australia) is the world’s largest coral reef system (350,000 km2). France has 55,000 km2 of them spread out over its 3 surrounding oceans, Indonesia has 51,020 km2, and the Philippines 25,000 km2.

These ecosystems protect the coasts from erosion caused by waves, serve as nurseries for young fish (particularly fringing reefs, which are the most threatened), and are a source of protein for local populations. They generate work and are the main source of support for small-scale fishing and tourism. In warm waters, 70% of coral reefs are threatened by climate change as well as direct effects of human activity: sewage and land runoff, tourist boats and diving, material removal and fishing practices.

The greatest concentrations of reefs occur in shallow tropical waters, especially the Pacific, South-East Asia and Indian Ocean. Nonetheless large reef systems have recently been discovered in deeper waters in many oceans, incuding the Arctic. These are particularly important as nursery grounds for fish.

Coral Reefs: Supporting the Economy

Approximately 500 million people, in more than 100 countries are thought to rely on tropical coral reefs for food and services. Of these, 30 million depend directly on coral reefs for food. According to the United Nations Environment Programme, (UNEP, report on the economic value and life saving function of coral reefs and mangroves, January 2006), reef fisheries generate between 15,000 and 150,000 dollars per km2 per year. The annual turnover in Southeast Asia is estimated at 2.5 billion dollars and 310 million in the Caribbean Islands. These results are due to the nature of the ecosystems themselves: coral reefs provide a multitude of shelters and spawning ground for many species. They are nursery areas for numerous fish species, which include fish for commercial use. Coral, which supports the ecosystem, is itself a source of food for many organisms.

Coral reefs also strongly contribute to tourism: 1 billion dollars of income per year on the Great Barrier Reef in Australia and 3 billion in the area of Key West, Florida.

Coral reefs provide many more services:

– Protecting coastal areas by serving as obstacles to storms, cyclones, tsunamis: coral reefs can potentially absorb 90% of the impact force of waves, protecting coasts and infrastructures.

– Reducing shoreline erosion: in Sri Lanka, one km2 of reef keeps 2,000 m3/year of coastal sediment from eroding.

– A source of molecules for the pharmaceutical industry: such as AZT used for the treatment of AIDS which was originally extracted from a coralline sponge from the Caribbean Islands.

According to the WWF, coral reefs, on the whole, could be contributing to approximately 30 billion dollars in the production of goods and services each year.

High Pressure on Corals

According to the “Status of Coral Reefs of the World” (2004) report by the Australian expert Clive Wilkinson, 20% of the world’s reefs have been destroyed and show no signs of recovery. According to the UNEP, 30% of coral reefs have either been destroyed or seriously damaged. The Caribbean and Southeast Asian areas have been damaged the most. The main causes are due to human activity:

– Thoughtless removal of fauna and coral material.

– Overfishing and destructive fishing practices. Dynamite fishing has severly damaged approximately 75% of Indonesian reefs.

– Soil erosion, which causes sediment deposits in reefs, increases turbidity and consequently decreases the light exposure needed by zooxanthellae, or even suffocates them completely.

– Ground water carrying pollutants such as sewage, pesticides, fertilisers and petroleum-derived products.

– Mass tourism which damages reefs through boat anchors, treading, damage by divers’ fins and …).

– Development such as marina and airport construction, embankment construction of coral and lagoon areas. 75% of coral reefs are located in areas with high development rates.

Global warming

The effects of global warming add to these factors. The increase in atmospheric CO2 concentration leads to seawater becoming less alkaline and consequently to a reduced ability of corals to make their coral skeletons. Massive coral bleaching is being caused by various stresses with increased water temperatures topping the list. There is a breakdown in the symbiotic relationship between corals and zooxanthellae (algae), which leave the polyps and lose their photosynthesis capability. Although bleached corals can recover, the frequency and severity of bleaching is leading to massive death of corals and the ecosystems they support. This has been the leading cause of reef degradation since the 1980sand is also linked to the increased frequency of events such as El Niño. According to Clive Wilkinson’s report, 16% of reefs were destroyed during the 1998 bleaching, mainly in the Indian Ocean and West Pacific. Since then, as much as 40% of the damaged reefs recovered quickly (Persian Gulf, East Africa, the Seychelles, the Maldives, Palau, Japan, and the Great Barrier Reef), particularly those in protected areas.

Coral reefs are divided in :

– Fringing reefs, which are either attached to the shore or located in very shallow water.

– Barrier reefs, which are wide and continuous as well as separated from the mainland by a lagoon and parallel to the shore.

– Atoll reefs, which are islands of coral whose support from an island or continent which encouraged the growth of the reef has disappeared either because it has sunk under the surface or because of erosion.

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