Marine Science Chapters


Coral Reef Buildup and Breakdown

Both physical and biological factors cause the buildup and breakdown of coral reefs. It is the combination of these factors that results in the final form of the coral reef. Reefs may be almost any form - long and thin, circular, broken circles, or flat and wide. Each reef is unique.

A cay on a tropical coral reef
A cay on a tropical coral reef. (GA image)

Physical buildup of coral reefs is due to the piling up of eroded material on the reef. This usually results in a cay that may be somewhat permanent. Many coral atolls have relatively permanent cays that exist with tourist hotels and lovely gardens.

Coral reef
Coral reef. (SA image)

Biological buildup of coral reefs is primarily due to the ability of the coral animal to lay down its calcium carbonate corallite. This is usually in response to the sinking of the original crustal oceanic island. It is the coral's response to keeping its polyp in the upper sunlit waters (for its symbiotic zooxanthellae). Without this response, the coral animal would sink below the lighted waters required for its photosynthetic food producer, the zooxanthellae, and it would starve.

Wave battering a coral reef
A wave battering a coral reef. (GA image)

Physical breakdown of coral reefs is due to waves, winds, hurricanes, and currents. These may break apart the coral reef, forming grooves in the (once) circular reef. The higher areas, between the grooves, are known as buttresses and may actually grow together at the top producing arches in the 'buttress and groove' formations typical of coral reefs. Sometimes these physical processes erode the tops of reefs and they may be called 'table reefs' because the top of the reef is rather flat (like a table).

Coral bleaching
Coral bleaching. (SA image)

Biological breakdown of coral reefs may be due to the coral itself, coral eaters, or environmental factors. If the coral animal can not grow upwards fast enough to keep pace with the sinking island, the coral will eventually get to a depth where its zooxanthellae will not have enough light for photosynthesis and then the zooxanthellae will die and the coral will starve. When the zooxanthellae die or leave the coral, the coral tissues usually become pale - a condition on coral reefs called 'bleaching.' Bleaching events may be due to natural or man-made causes and are a concern worldwide as they are increasing in recent years. The main identified cause of bleaching is due to ocean warming events. Warmer water temperatures stress the corals and cause them to expel the zooxanthellae. However, there have been several other documented causes of coral bleaching, such as infection with a particular soil bacterium and unusual cold water events. Bleaching often leads to coral death. However, if limited in scope and duration, corals can re-establish their symbiotic relationship with zooxanthellae.

Parrot fish
Parrot fish are coral eaters. (SA image)

Parrot fish eat coral. These fish have a large number of different species worldwide in the tropics. They are well adapted to eat the coral with their large (and strong) front teeth.

The crown of thorns sea star eats coral. This species of sea star lives only in the tropical Pacific Ocean. Population explosions cause concern for many people who have establishments on tropical islands. The crown of thorns crawls along the surface of the coral, its stomach comes out of its mouth (on the underside of the sea star in the middle of the legs) and it digests the coral polyps right out of their corallites. The crown of thorns can leave a trail of empty white corallites along a reef overnight. The empty corallites may erode or they may be the next solid surface for another benthic (bottom dwelling) marine organism to settle upon and begin life. If the crown of thorns sea star kills too many coral heads, the entire reef (whether it is fringing, barrier or atoll) may be in jeopardy of eroding away and being unable to sustain growth.

The crown of thorns on a South Pacific coral reef
The crown of thorns sea star on a South Pacific coral reef. (SA image)

Periodic crown of thorns outbreaks may be natural. But some people think the outbreaks of the crown of thorns seastar in the tropical Pacific Ocean are due to human impacts. These human impacts include adding wastes to the water, adding sediment to the water with the construction of buildings, and over-collecting of the large triton snail (the main predator on the crown of thorns). Many scientists believe it is a natural cycle. From cores taken in the lagoons of coral reefs (both barrier reef lagoons and atoll lagoons) scientists have seen layers of spines from the crown of thorns sea star every hundred years or so. These spines, found in the center of the spiny protrusions on the top of the sea star, do not decompose when the crown of thorns dies. Care should always be taken in handling this species because these spines hurt (if you get poked by one) and leave a dead area of skin on human tissue.

Although each reef is unique there is a general zonation pattern (explained in the next lesson) that can be expected on each typical coral reef. Each zone may be expressed to a different measure depending on the history of reef buildup and breakdown.

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(Revised 17 Oct 2018 RMartin)
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