Definition and distribution of the coral reefs

 

Definition of coral reefs

• Reefs are concretions produced by living organisms. Some reefs are formed by calcareous algae (in South America and the Cap Verde Islands), others by molluscs (fused vermetid shells in Florida) or polychaeta (serpulid worms), but most of the reefs in the world are composed of hermatypic coral (cnidaria, especially scleractinozoa and hydroids) which contains microscopic symbiotic algae (zooxanthella). These complexes form the substrate on which the reef ecosystems are based (cf. mangroves).

• Coral reefs are solid, long-lasting structures. Their solidity is due to the fact that they are formed by the hard parts of the animal skeletons. Their longevity is due to their ability to resist shocks even after the death of organisms (cf. the high-rising primaeval reefs). The interstices are sealed and soldered by sediments and coral secretions.

• Coral reefs have a salient topography. They are based on the sea floor on solid rock foundations and some of them reach the ocean surface, while others are submerged and therefore constitute a serious risk to navigation.

• The coral reefs have developed considerably in many places: the largest coral reefs in the world include the Great Australian Barrier Reef, the New Caledonian reefs, which are more than  2000 km long, and the Mayotte reefs.

• Coral reefs have ecological effects on their surroundings. Their main effects are physical ones, since they protect the coastal shores from the impact of the sea swell and the waves. They also have chemical effects, since the organisms of which they consist use the nutrients available and produce detritus in exchange.

Generally speaking, coral reefs are to be found only in regions where the temperature is always above 18°C in winter.

Coral reefs can be viewed on various spatial scales.

• The role of the temperature: the optimum temperature for coral reefs is 22-28°C; the warm and cold sea currents to which they are exposed therefore play a decisive role. Reefs require a minimum temperature of 18°C, and therefore occur less frequently on the eastern Atlantic and  Pacific coasts than on the warmer western coasts. The threshold temperature ranges defining the conditions under which each species is able to survive probably also determine their dietary and reproductive habits.

• The effects of emersion: since coral complexes do not easily withstand emersion, they depend strongly on the tides. The various species differ in this respect, however: the Caribbean corals are rarely exposed to emersion, for example, and this factor might explain the structural differences observed between the Indo-Pacific and Caribbean reefs. Exceptionally low tides can also have catastrophic effects on the reefs.

• The effects of sedimentation: the considerable differences observed in the resistance of the coral species (which have been found to depend on the process of mucus secretion and on the movements of the cilia) may explain the occurrence of various local patterns. Few species are to be found in turbid regions, apart from some rare exceptions such as Fungia and Trachyphyllia

• The effects of the hydrodynamic conditions: the effects of this factor have been studied at various spatial scales, focusing on aspects such as the winds, currents and waves.

• The number of coral species observed decreases with the depth.

• The coral production rates have also been found to decrease with the depth

• The growth rates of the coral species decrease with the depth, due to the decreasing light levels, rather than to the decreasing oxygen supply and the decreasing temperature.

• The growth rates actually depend on the transparency of the water: the coral species continue to thrive up to irradiance levels of 30 à 40 %, reaching a maximum just below the surface of the water. At lower irradiance levels, some species such as Stylophora pistillata (in the Red Sea) can develop with only 1 % of the surface light; but in this case, only small colonies develop and coral reefs are never formed.

This is the part of the world with the largest number and the greatest variety of coral reefs. It can be divided into two sub-regions:

• The eastern sub-region, which includes the Red Sea, the western part of the Indian Ocean and the eastern part of the Indian Ocean.

• The western sub-region, which includes the central part of the tropical Pacific, Malaysia, the Philippines and Indonesia, where the coral reefs are extremely numerous and varied. The Great Australian Barrier Reef alone covers about 200 000 Km².

The western Pacific, with its many archipelagos, also contains many coral reefs, whereas the eastern Pacific does not have so many.

The coral reefs in the Atlantic amount to only 1/20th of the number to be found in the Indo-Pacific waters; they are not only cover a smaller area, but also show less diversity; not a single coral species is common to both of these parts of the world.

The Atlantic coral reefs are mainly located in the region around the  Bermudas and the Caribbean.

There exist a few coral complexes off the coasts of Brazil and West Africa, but these colonies are sparsely populated by only a small number of species.

Coral reefs were classified in terms of their morphology by Darwin, as follows: fringing reefs, barrier reefs,  flat reefs and atolls

Fringing reefs lie near emergent land. They are fairly narrow and recently formed. They can be separated form the coast by a navigable channel (which is sometimes incorrectly termed a " lagoon ").
Barrier reefs are broader and lie farther away from the coast. They are separated from the coast by a stretch of water which can be up to several miles wide and several tens of metres deep. Sandy islands covered with a characteristic pattern of vegetation have sometimes formed on top of a barrier reef. The coastline of these islands is broken by passes, which have occupied the beds of former rivers.
Flat reefs or shelf reefs are complexes which emerge from the open sea.
Atolls are large, ring-shaped reefs lying off the coast, with a lagoon in their middle. The emergent part of the reef is often covered with accumulated sediments and the most characteristic vegetation growing on these reefs consists of coconut trees.

The reefs of various kinds all have some similar local features due to the fact that similar  geomorphological factors are at work. There exists a particularly complex pattern of local zones on the atolls and barrier reefs; moving inland from the coast, one encounters a whole series of biotopes, each having a characteristic set of flora and fauna.

• The reef pediment forms a gentle slop in front of the reef, descending to depths of 30 to 50 m. It is covered with loose coral particles and debris.

• The outer slope, which is much steeper, is exposed to strong hydrodynamic forces. The upper part consists of crests and spurs, which alternate with gulfs strewn with boulders and debris. Coral and calcareous debrisare to be found in large quantities on this part of the reef.

• The bar of the reef is the narrow passage leading to the reef flats; the predominant organisms here tend to be algae classified as lithothamnia.

• The outer flats are more horizontal, although they too are exposed to strong  hydrodynamic forces; the coral is most abundant in this part of the reef.

• The inner flats were produced by falling reef detritus: in this part of the reef, debris and large boulders were detached from the outer slopes; this horizontal part can show features of various kinds (flats with a transversal  geological arrangement, compact structures formed by madreporarians,  scattered coral clumps, micro-atolls, fields of sea grasses, etc.); they can contain both compact coral formations and/or  sedimentary debris.

• The inner slope runs between the flats and the lagoon; this is a more sheltered slope containing fewer coral formations, and tends to consist mainly of coral debris which are coarse at first and gradually become finer.

• The lagoon is lined with loose sediments which follow a gradient, since the origin of their components  tends to be more terrestrial around the edges of the lagoon. Coral formations occur in the form of clumps and pinnacles; and there are also large slabs which are the remnants of former reefs.

• These various landscapes can also contain features such as fields of sea grasses, hillocks and madreporarian mounds.

• The passes which break into the reef make it possible for the lagoon waters to be renewed and are subject to strong currents (8 knots). They are inhabited by many pelagic species, and  a few benthic species (gorgones) cling to the substrates.

Like all other ecosystems, the reef ecosystems are based on complex food webs.

The producers mainly include benthic groups (mainly consisting of corals and their zooxanthella, sea grasses, macro-algae and the microphytobenthos) and pelagic groups such as the phytoplankton.

Herbivores are the primary consumers: these include several kinds of mollusca, sea urchins and fish.

The secondary consumers include filtering organisms polychaeta, echinodermata, etc., detritivores  such as crustaceans and holothuroidea, and carnivores such as fish and echinodermata  (Acantharia).

The structure of the reefs depends on antagonistic processes of construction and destruction.

Coral is formed by a polyp living in a rigid calcium carbonate (calcite or aragonite) shell or skeleton which it secretes itself. Along with its close relatives the jelly-fish, sea anemones, eel grass, millepores and other members of the cnidarian branch, they have the simplest mode of organization of all multicellular animals. This polyp has a cylindrical body and its mouth is surrounded by a ring of tentacles. In fact, apart form a few small differences, it is simply a miniature anemone.

Coral species can be either solitary (a single polyp inhabiting a single shell) or colonial. In the latter case, the shells, each of which contains a single polyp, fuse together, forming a colony of several thousand individuals which can occupy an area of several square metres. In this underwater condominium, the occupants benefit from several ecological advantages as far as their protection, nutrition, reproduction, genetic stability and respiratory requirements are concerned.

Hermatypic coral species are those which form reefs containing the unicellular algae called zooxanthella , which can have densities of up to 1 million cells per cm². These algae need to have light to be able to carry out the process of photosynthesis, using the carbonic gas dissolved in the sea water, nitrogen and mineral phosphorus to produce the organic matter required by the polyps. By causing a shift in the carbonate balance, they promote the precipitation of the calcium carbonate on which the coral skeleton is based, and thus induce the process of calcification.

The zooxanthella quickly eliminate the carbon anhydride produced by the coral metabolism, which would otherwise dissolve the calcium carbonate. The zooxanthella  thus enhance the calcareous production rate, which can be ten times greater than that which occurs in coral complexes devoid of algae.

The ectodermal cells in the basal region of the polyps produce extremely fine chitinous filaments which block up the empty spaces surrounding the polyps and the calcareous skeleton. Calcareous crystals develop in this region, in a solution which is hyper-saturated with calcium ions having a gelatinous consistency.

Reprocessing of the sediments

Coral complexes are being constantly reprocessed, especially in the sedimentary parts, which undergo a process of bioturbation due to the burrowing and/or feeding activities of psammivorous organisms such as the holothuroidea. The reprocessing of large quantities of superficial sediment contributes to the circulation of oxygen and nutrients.

 

Destruction by the cyclones

The coral reefs are right on the path of the oncoming cyclones. The most highly exposed parts are the reef pediments and the outer slopes, which have been severely damaged. Huge coral blocks are detached from the 0-30 m high complexes and roll down the slopes, destroying everything they encounter. More than 80 % of all the coral formed on the outer slopes of the atoll of Tikehau was destroyed by the cyclones which hit French Polynesia in 1983 (Harmelin-Vivien & Stoddart, 1985).

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