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examBoard: Cambridge
examType: IGCSE
lessonTitle: Coral Reef Global Distribution
Coral reefs are among the most diverse and valuable ecosystems on Earth. Often called the "rainforests of the sea," these underwater structures are built by tiny marine invertebrates called coral polyps that secrete calcium carbonate to form hard skeletons. Over time, these skeletons build up to create the massive, colourful structures we know as coral reefs.
Key Definitions:
Coral reefs form when coral polyps attach to rock or other hard surfaces in shallow ocean waters. The polyps extract calcium from seawater and combine it with carbon dioxide to create calcium carbonate (limestone) skeletons. As generations of coral polyps live and die, they leave behind their skeletons, which form the structure of the reef. This process is extremely slow - large reefs can take thousands of years to form.
Coral reefs have specific environmental requirements to thrive:
Coral reefs are found in tropical and subtropical waters around the world, primarily between 30°N and 30°S latitudes. This distribution is not random but follows specific patterns based on environmental conditions.
The Coral Triangle is the global centre of marine biodiversity, spanning Indonesia, Malaysia, the Philippines, Papua New Guinea, the Solomon Islands and Timor-Leste. This region contains 76% of all known coral species and supports more than 3,000 species of fish. It's often referred to as the "Amazon of the seas" due to its incredible biodiversity.
The Atlantic Ocean contains about 7% of the world's coral reefs. The most significant reef system is the Mesoamerican Barrier Reef, which stretches from Mexico to Honduras. The Caribbean Sea hosts numerous reef systems around islands like Jamaica, Cuba and the Bahamas.
The Indo-Pacific region contains the majority (about 75%) of the world's coral reefs. The Great Barrier Reef off Australia's coast is the largest reef system in the world. Other significant reefs are found in the Red Sea, around the Maldives and throughout Southeast Asia.
The Pacific Ocean hosts about 18% of global coral reefs. Major reef systems are found around Hawaii, Fiji, French Polynesia and many other island nations. The remote nature of many Pacific reefs has helped some remain relatively pristine compared to more accessible reefs.
Coral reefs form in different shapes and structures depending on their location and how they develop over time. There are three main types of coral reefs:
These reefs grow directly from the shoreline of islands or continents. They form borders along the shoreline and surrounding islands. Fringing reefs are the most common type of reef and can be found in the Caribbean, Red Sea and around many tropical islands.
Barrier reefs run parallel to the shoreline but are separated from land by a deeper lagoon. The Great Barrier Reef in Australia is the most famous example. These reefs can extend for hundreds of kilometres and form natural barriers against ocean waves.
Atolls are circular or oval-shaped reefs that surround a central lagoon. They form when islands surrounded by fringing reefs sink or erode away, while the reef continues to grow upward. Atolls are common in the Pacific and Indian Oceans, with the Maldives being a classic example.
The Great Barrier Reef off Australia's northeastern coast is the world's largest coral reef system, stretching over 2,300 kilometres. It consists of over 2,900 individual reefs and 900 islands. The reef supports incredible biodiversity with:
The global distribution of coral reefs is influenced by several key factors:
Corals rely on their symbiotic relationship with zooxanthellae algae, which need sunlight for photosynthesis. This is why coral reefs are typically found in shallow, clear waters where sunlight can penetrate. Water temperature is equally crucial - most reef-building corals thrive in water temperatures between 20-28°C, explaining their tropical and subtropical distribution.
Ocean currents influence coral reef distribution by affecting water temperature, nutrient availability and larval dispersal. Warm currents can extend the range of coral reefs beyond typical tropical zones. However, areas with cold-water upwelling (where deep, cold water rises to the surface) typically lack coral reefs, even if they're within tropical latitudes.
Despite their importance, coral reefs face numerous threats that are affecting their global distribution:
Rising ocean temperatures cause coral bleaching, where stressed corals expel their zooxanthellae and often die. Ocean acidification from increased CO₂ absorption makes it harder for corals to build their calcium carbonate skeletons. These climate-related threats are changing the global distribution of healthy coral reefs.
Agricultural runoff, sewage, plastic pollution and industrial waste harm coral reefs. Excess nutrients can cause algal blooms that smother reefs, while sediment from coastal development blocks sunlight. These human impacts are particularly severe near densely populated coastlines.
Overfishing disrupts reef ecosystems, while destructive fishing methods like dynamite fishing and cyanide fishing directly damage reefs. Coastal development, tourism impacts and coral harvesting further threaten reef systems globally.
Understanding the global distribution of coral reefs is crucial because these ecosystems provide vital services:
Though they cover less than 1% of the ocean floor, coral reefs support about 25% of all marine species. They act as nurseries for many fish species, including commercially important ones. Reefs also protect coastlines from storms and erosion, provide jobs through tourism and fishing and are sources of new medicines.
Scientists are working to map and monitor coral reef distribution globally. Conservation efforts include establishing marine protected areas, developing coral nurseries for reef restoration, reducing local pollution and addressing climate change. Understanding reef distribution helps target these efforts where they're most needed.
The Maldives, an island nation in the Indian Ocean, consists of 26 atolls and is heavily dependent on its coral reefs for tourism, fishing and coastal protection. After the 1998 El Niño event caused mass coral bleaching, killing about 90% of shallow water corals, the Maldivian government partnered with marine biologists to implement coral restoration projects. The Reefscapers project, started in 2005, uses "coral frames" - metal structures where fragments of resilient coral are attached to grow new colonies. Over 7,500 frames have been deployed, creating new reef habitats. Monitoring shows 80% survival rates of transplanted corals, with new fish communities establishing around the frames. This project demonstrates how human intervention can help restore coral reef distribution in areas affected by climate events.
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