Reef

For other uses, see Reef (disambiguation).

A reef surrounding an islet

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.

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.

Part of a coral reef

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