Coral reef

Coral animals come in three varieties: hard, soft and sea fans. Coral reefs take two main forms: shallow water reefs and deep water reefs. Deep water reefs have only recently been discovered, often located in dark, cold water and growing very slowly in the absence of light. The rest of this article is concerned with the hard corals that build shallow water tropical reefs.

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 calcareous algae, especially, although not entirely, species of red algae.

Water temperature of 20 to 28°C (68 F.) 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 reef-building corals live in these 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 photosynthesise, but they live in a symbiotic relationship with types of microscopic algae that photosynthesise for them. Because of this, coral reefs also grow much faster in clear water, which absorbs less light.

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

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. It is up to three miles wide and about 20 miles long (5 km wide and 30 km long), and located 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 to increase surface area exposed to sunlight.

Threats to Reefs

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

During the 1998 and 2004 El Niño weather phenomenons, 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. However, Ben McNeil of the University of New South Wales hypothesises 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. However, growth in some reefs due to global warming is expected to be offset by declines in other reefs, due to the comfortable temperature range for a coral being close to the temperature at which they bleach.

Famous Coral Reefs

• Great Barrier Reef – Queensland, Australia
• Barrier Reef of Belize, second largest in the world – Belize, Central America

External links

• The Coral Reef Ecology Home Page from the University of the Virgin Islands. Information on anatomy, types, zooxanthellae, feeding, reproduction, diseases, bleaching, threats, Caribbean corals, animals associated with coral reefs
• USGS Coral Reefs website
• Photo of coral reef from German Wikipedia
• Deepest U.S. reef found, a January 2005 Associated Press article (via CNN)
• Pulley Ridge website from the USGS
• http://www.reef.crc.org.au/ CRC Reef Research Centre]