Great Barrier Reef
The Great Barrier Reef lies off the northeastern coast of Australia and is the largest structure ever made by living organisms including human beings, consisting of the skeletons of tiny coral polyps and hydrocorals bounded together by the soft remains of coralline algae and microorganisms.
The Great Barrier Reef is over 1,250 mi (2,000 km) long and is 80,000 mi2 (207,000 km2) in surface area, which is larger than the island of Great Britain. It snakes along the coast of the continent of Australia, roughly paralleling the coast of the State of Queensland, at distances ranging 10–100 mi (16–160 km) from the shore. The reef is so prominent a feature on Earth that it has been photographed from satellites. The reef is located on the continental shelf that forms the perimeter of the Australian landmass where the ocean water is warm and clear. At the edge of the continental shelf and the reef, the shelf becomes a range of steep cliffs that plunge to great depths with much colder water. The coral polyps require a temperature of at least 70°F (21°C), and the water temperature often reaches 100°F (38°C).
The tiny coral polyps began building their great reef in the Miocene Epoch that began 23.7 million years ago and ended 5.3 million years ago. The continental shelf has subsided almost continually since the Miocene Epoch. In response, the reef has grown upward with living additions in the shallow, warm water near the surface; live coral cannot survive below a depth of about 25 fathoms (150 ft, or 46 m) and also depend on the salt content in seawater. As the hydro-corals and polyps died and became cemented together by algae, the spaces between the skeletons were filled in by wave action that forced in other debris called infill to create a relatively solid mass at depth. The upper reaches of the reef are more open and are riddled with grottoes, canyons, caves, holes bored by mollusks, and many other cavities that provide natural homes and breeding grounds for thousands of other species of sea life. The Great Barrier Reef is, in reality, a string of 2,900 reefs, cays, inlets, 900 islands, lagoons, and shoals, some with beaches of sand made of pulverized coral.
The reef is the product of over 350 species of coral and red and green algae. The number of coral species in the northern section of the reef exceeds the number (65) of coral species found in the entire Atlantic Ocean. Polyps are the live organisms inside the coral, and most are less than 0.3 in (8 mm) in diameter. They feed at night by extending frond-like fingers to wave zooplankton toward their mouths. In 1981, marine biologists discovered that the coral polyps spawn at the same time on one or two nights in November. Their eggs and sperm form an orange and pink cloud that coats hundreds of square miles of the ocean surface. As the polyps attach to the reef, they secrete lime around themselves to build secure turrets or cups that protect the living organisms. The daisy- or feather-like polyps leave limestone skeletons when they die. The creation of a 1 in (2.5 cm) thick layer of coral takes five years.
The coral is a laboratory of the living and once-living; scientists have found that coral grows in bands that can be read much like the rings in trees or the icecaps in polar regions. By drilling cores 25 ft (7.6 m) down into the coral, 1,000 years of lifestyles among the coral can be interpreted from the density, skeleton size, band thickness, and chemical makeup of the formation. The drilling program also proved that the reef has died and revived at least a dozen times during its 25-million-year history, but it should be understood that this resiliency predated human activities. The reef as we know it is about 8,000 years old and rests on its ancestors. In the early 1990s, study of the coral cores has yielded data about temperature ranges, rainfall, and other climate changes; in fact, rainfall data for design of a dam were extracted from the wealth of information collected from analysis of the coral formation.
Animal life forms flourish on and along the reef, but plants are rare. The Great Barrier Reef has a distinctive purple fringe that is made of the coralline or encrusting algae Lithothamnion (also called stony seaweed), and the green algae Halimeda discodea that has a creeping form and excretes lime. The algae are microscopic and give the coral its many colors; this is a symbiotic relationship in which both partners, the coral and the algae, benefit. Scientists have found that variations in water temperature stress the coral causing them to evict the resident algae. The loss of color is called coral bleaching, and it may be indicative of global warming or other effects like El Niño.
This biodiversity makes the reef a unique ecosystem. Fish shelter in the reef's intricacies, find their food there, and spawn there. Other marine life experience the same benefits. The coastline is protected from waves and the battering of storms, so life on the shore also thrives.
El Niño and La Niña Phenomena; Greenhouse Gases and Greenhouse Effect; Tropical Cyclone
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