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In nautical parlance, a reef is a rock, sandbar, or other feature beneath the surface of the water, but shallow enough to be a hazard to ships; see also shoal. Many reefs result from abiotic processes—deposition of sands, wave erosion planning down rock outcrops—but the best-known reefs are those of tropical waters developed through biotic processes dominated by corals and calcareous algae.

1 Biotic Reef Types

There are a number of biotic reef types, including oyster reefs , but the most massive and widely distributed are coral reefs, limited to tropical waters. Although corals are major contributors to the framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault from ocean waves are calcarous algae, especially, although not entirely, species of red algae.

1.1 Coral Reefs

These reefs take various forms described as apron reefs, fringing reefs, patch reefs, ribbon reefs, table reefs, barrier reefs, and atolls.

• 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

About 100,000 species living in and around coral reefs have so far been noted, although some scientists believe the real total may top 2 million. Currently 58 percent of the world's coral reefs are now endangered. Humans continue to represent the single biggest threat to coral reefs, in particular, land-based pollutionLachine Canal, in Montreal, is badly polluted Pollution is the release of harmful environmental contaminants, or the substances so released. Generally the process needs to result from human activity to be regarded as pollution. Even relatively benign prod and over-fishing are the biggest threats to the ecosystems.

During the 1998 El Nino 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 warmingGlobal warming is an increase over time of the average temperature of Earth's atmosphere and oceans. Global warming theories attempt to account for the documented rise in average global temperatures since the late 19th century and assess the extent to whi could negate some of this recovery in the future.