Part of a coral reef.

A coral reef is a type of biotic reef developing in tropical waters. Although corals are major contributors to the overall framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault by ocean waves are calcarous algae, especially, although not entirely, species of red algae.

A water temperature of 20 to 28°C is needed for growth of the coral reef. Coral reefs are found in all oceans of the world, generally between the Tropic of Cancer and the Tropic of Capricorn, because the reef-building corals are living in this waters. Reef-building corals are found mainly in the photic zone (<50m), where the sunlight reaches the ground and offers the corals enough energy. The corals themselves do not do photosynthesis, but they live in a symbiotic relationship with a type of microscopic algae, that does the photosynthesis for them. Because of this, coral reefs also grow much faster in clear water, where the light is less absorbed by the ocean water.

Such reefs take a variety of forms, defined as follows:

• Apron reef — short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore
• Fringing reef — reef extending directly out from a shoreline, and more or less following the trend of the shore.
• Barrier reef — reef separated from a mainland or island shore by a lagoon; see Great Barrier Reef.
• Patch reef — an isolated, often circular reef, usually within a lagoon or embayment
• Ribbon reef — long, narrow, somewhat winding reef, usually associated with an atoll lagoon.
• Table reef — isolated reef, approaching an atoll type, but without a lagoon
• Atoll reef — a more or less circular or continuous barrier reef surrounding a lagoon without a central island; see atoll

Biology

Humans continue to represent the single biggest threat to coral reefs, in particular, land-based pollution and over-fishing are the biggest threats to the ecosystems.

During the 1998 El Niño weather phenomenon, in which sea surface temperatures rose well above normal, many tropical coral reefs were bleached or killed. Some recovery has been noted in more remote locations, but global warming could negate some of this recovery in the future. But Ben McNeil of the University of New South Wales notes that reefs are not in decline, and may exceed pre-industrial levels by as much as 35 percent by 2100, especially because of the positive influence of global warming. On the other hand, however, certain types of coral are sensitive to the sun's rays and increased temperatures, which means that positive growth in some reefs due to global warming is expected to be offset by declines in other reefs.

In December 2004, United States Geological Survey (USGS) researchers announced the confirmation of the discovery of the deepest coral reef ever found in the United States. The reef is in the Pulley Ridge area, a north-south-trending drowned barrier island, more than 60 miles (100 km) long, approximately 40 miles (70 km) west of Dry Tortugas National Park. The reef is up to three miles wide and about 20 miles long (4.8 km wide and 32 km long). The reef is at a depth that ranges from 200 to 250 feet (60 to 80 m). Unlike most coral reefs, which tend to grow vertically, Pulley Ridge coral grows flat, an adaptation to the limited penetration of light at that depth.

External link

• USGS Coral Reefs website (http://coralreefs.wr.usgs.gov/)
• Photo of coral reef from German Wikipedia
• Deepest U.S. reef found (http://www.cnn.com/2005/TECH/science/01/02/deepest.reef.ap/index.html), a January 2005 Associated Press article (via CNN)
• Pulley Ridge website (http://coastal.er.usgs.gov/pulley-ridge/) from the USGS