Reefs at Risk

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2. The living reef

 

Corals are colonial animals that produce a calcium carbonate (aragonite) skeleton beneath their film of living tissue. Reef-building or hermatypic corals contain within their tissues symbiotic algae, so that the colony actually functions as a plant-animal combination. A coral reef is the physical structure created by the growth of the reef community.

When a coral colony dies through storm damage, is broken by the action of living organisms, or is eaten by a parrotfish, the skeleton becomes the basic material forming the reef structure. Dead coral branches form the substrate on which new corals grow, while the fragments are cemented together by the action of coralline algae. The fragmented skeletons form the sand which contributes to reef growth by filling in the spaces between the larger fragments of dead coral skeletons. Continual deposition allows a reef to keep pace with rising sea-level by upward growth.

Coral species, coral communities and the reef structure differ widely in the growth rates. Among the species, branching and staghorn corals can add more than 10cm a year to their branches. Massive corals grow at about a tenth of that rate, or roughly 1Omm a year. As for vertical reef growth, in Mauritius it reaches as much as 1Omm a year, but no more than a few millimetres for some reefs in the Red Sea.

Coral reefs depend very much on the prevailing environmental conditions. Some reefs did not survive the rapid sea-level changes experienced during the ice ages. We find many dead reefs drowned in earlier periods, or stranded above present sea-level. But under the right conditions coral colonies can survive for centuries.

Although we think of reefs primarily in terms of corals, they are home to a myriad of other organisms, all of them important to the overall functioning of the community, and all of them sensitive to climate and environmental conditions. Coating the exposed sand grains of a coral lagoon are microscopic algae and bacteria grazed by molluscs, crustaceans, sea cucumbers, sea urchins and sediment-eating fish. "Turf" algae cover all bare surfaces and are grazed by large populations of fish when the tide is rising. Many of these animals provide food for fishers and gleaners of reefs.

Other organisms play an important role in building the reef by breaking down the calcium carbonate skeletons of larger organisms to produce sediments. Some organisms, like sponges, worms and molluscs, bore into the coral skeletons so that they become fragile and fracture in strong waves. Grazing fish and sea urchins at the surface produce large quantities of sediment.

A major role in the functioning and survival of coral reefs is played by the tiny plants and animals known as plankton (from the Greek for "floating"), which provide food for sedentary reef corals and other animals. The life cycle of many corals and other species, including fish, involves a larval planktonic stage, enabling them to disperse over long distances and between different reef areas.

Reefs are home to a myriad of organisms, all of them sensitive to prevailing climate and environmental conditions

A coral reef in Honduras

A coral reef in Honduras: because coral reefs survive only close to the interface between air and sea in warm waters, and often adjacent to land, changes in environmental conditions on land as well as in the atmosphere or sea are likely to have a major influence on reefs.

Photo: IUCN / Danny Elder.

Limits to abundance

The living matter produced by the plants and algae in a coral reef system, its gross primary productivity, is between 30 and 250 times as great as that of the open ocean. While the productivity of tropical oceans is very low (18-50 grammes of carbon per sq. metre in a year), coral reefs produce 1500-5000g. The reason for the higher productivity of reefs is that corals and coral communities recycle nutrients such as nitrate and phosphate which are in limited supply in open-ocean surface waters.

But it would be a mistake to assume that the high productivity of a coral reef provides an automatic surplus of potential food. The primary production - the amount of energy produced by photosynthesis - is very nearly balanced by the reef's whole consumption. Net productivity is often only 2-3% of the gross, and only slightly higher than the net productivity per unit area in the surrounding ocean water. The highly productive coral communities also occupy only a fraction of the surface of a coral reef system.

So the amount of organic matter that can be taken out of the reef whether by harvesting or other means without causing damage to the community remains severely limited. One calculation puts the amount that could be extracted on a sustainable basis each year at less than 50 grammes of carbon without causing per square metre (less than 50 tonnes per km2), an extremely small figure by agricultural standards. Modern rice-growing techniques can produce yields of. over 400 tonnes per km2.

The amount we can harvest from a reef damage is severely limited.

Brain coral

Brain coral at Curagao in the Netherlands Antilles.

Photo: WWF / Jeff Syb Esma.

Reef growth and the environment

Reef corals do best in shallow, warm, clear oceanic water. Therefore reefs are most abundant away from large land masses, which produce too much freshwater runoff and sediments. Coral reefs are found in warm sub-tropical or tropical oceans where the annual temperature range is 20-30OC. Nevertheless, reefs in the Florida Keys (USA) grow at 18OC, and temperatures above 33OC are tolerated by healthy coral communities in the northern Great Barrier Reef and the Persian Gulf. But when air and water temperatures in the Gulf fell to IOOC in 1968, almost all the inshore coral colonies died. Even small temperature increases above the normal local maximum temperature may result in coral bleaching, which occurs when the symbiotic algae are expelled by the polyps in response to stress. (see symbiotic algae and bleaching).

Though corals can be found to a depth of 100m, reef-building corals do not grow well below 20-30m because their symbiotic algae depend on sunlight for photosynthesis. Too much fresh water can kill corals: heavy run-off has wiped out shallow reefs off the north coast of Jamaica. So have hurricanes. But the reefs' ultimate chances of survival are determined by the fact that they often coexist with large human populations in the tropics. This could doom many to extinction.

Because coral reefs grow close to sea surface in warm waters, and often adjacent to land, changes in environmental conditions in the atmosphere, on land, or in the sea are all likely to have a marked influence on the reef ecosystem.

Changes in the atmosphere, on land, or in the sea all influence reef systems.

The coral polyp

Coral growth

The individual coral polyp is a hollow, cylindrical animal (1). The mouth is surrounded by tentacles armed with stinging cells for capturing plankton. During the day these tentacles are folded in the digestive sac (2).

Microscopic single-celled algae that give the coral its green, blue or brown colour are located in the tissue of the living coral (3). These symbiotic algae process the wastes produced by the polyps. They use the nitrates, phosphates and carbon dioxide produced in the polyp. Through photosynthesis they generate oxygen and organic compounds which the polyps themselves can use. They may also help the polyp lay down calcium carbonate.

In addition to their role as primary producers, the other algae coating much of the reef's surfaces also produce substantial amounts of calcium carbonate. Algae are so important to reef life that it has been suggested that "coral-algal reefs" is a more accurate term than coral reefs

 

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