Fringing reef

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A fringing reef is a coral reef that grows very close to a shore. It can have a shallow backreef lagoon or none at all. If the reef grows directly from the beach, the reef flat nearly reaches the sand. If it sits a bit offshore, there can be a backreef with water and nutrients.

Fringing reefs are common in the tropics, where water is warm (about 18–30 C or 64–86 F). They are the most common reef type and are found in places like the Philippines, Indonesia, Timor-Leste, western Australia, the Caribbean, East Africa, and the Red Sea. The Ningaloo Reef in Western Australia is the largest fringing reef, about 260 kilometers long.

How fringing reefs differ from barrier reefs: fringing reefs are close to shore and usually have little or no deep lagoon behind them. Barrier reefs lie farther from land and have a deeper, more developed lagoon.

A fringing reef has two main parts: the reef flat and the reef slope. The reef flat is the shallow, shoreward part of the reef and is often exposed at low tide. It is close to land and can be damaged by runoff and sediments, so few corals live there. The reef slope forms the outer edge toward the open ocean. It is steeper and deeper, where there is less sediment and more healthy coral growth. The top of the slope, called the reef crest, gets lots of sunlight and wave energy, which helps corals grow fastest. The bottom of the slope gets less light and grows more slowly.

Sea level changes are a key factor in reef growth. If sea level rises slowly, fringing reefs can keep up. If sea level rises quickly, they may “drown” or fall behind.

Fringing reefs face several threats. Destructive fishing methods, overfishing, and damage from tourists (boating, diving, fishing, touching corals, anchors) harm reefs. Pollution from sewage and runoff adds nutrients that cause too much algae and blocks sunlight. Erosion, mining, and deforestation increase sediments that smother corals. Climate change raises ocean temperatures and causes coral bleaching, which can kill reefs.

In the backreef area, the species mix is usually simpler and more affected by nutrients and pollution. Near the reef crest, corals and other life are more diverse and abundant. Fringing reefs are an important and widespread feature of tropical coasts around the world.