Coral reefs

Coral reefs have always fascinated man. Their richness and diversity have provided a livelihood for coastal communities for thousands of years. Now their spectacularly colourful beauty can be seen by millions through television and increasing numbers of tourists visit reefs to scuba dive, snorkel or view the reef from glass bottomed boats.

The major part of most coral reefs lies below the low tide mark since corals are killed by prolonged exposure to air. So the coral reef is not truly a seashore habitat but is included here because it is such an important ecosystem. Many coral reefs do have a type of shore in the form of extensive 'reef flats' (see below) which are exposed at low tide. Such flats usually remain at least partially covered by a few inches of water. Only the hardier corals can thrive in these areas because although they rarely dry out completely, the temperature of the shallow water can rise dramatically when the tide is out and the corals will also be subjected to intense UV radiation. Coral reef animals such as shrimps, crabs and small fish will be subjected to increased predation from birds such as reef herons. Many reef flats are now being unwittingly damaged by tourists walking over the exposed corals and hunting for souvenirs. The reef flats also provide a source of food and materials for local people, and in many communities the women are sent out onto the reefs to collect clams, crustaceans, fish, seaweed and sea cucumbers. Valuable species such as Tridacna clams have become very scarce as both their flesh and shells can be sold.

The following brief account of coral reefs as a habitat is intended as an introduction to the subject. The geological history, formation, structure and functioning of coral reefs and their natural history are described in a number of well-illustrated recent texts to which the student should refer for more information and which are given in the references at the end of this chapter. Coral reefs in the context of climate change and global warming are referred to in Section 10.2.

Distribution

Coral reefs are found in tropical and sub-tropical areas where the average minimum temperature is not less than 20oC and so are found mainly between the Tropics of Cancer and Capricorn where water temperatures remain steady all year round. They are not able to withstand any great fluctuations in temperature and recent episodes of coral bleaching have been associated with rises in water temperature (see Section 10.2.3). The approximate worldwide distribution of coral reefs is shown in Figure 8.18, from which it can be seen that no reefs are found up the west coast of Africa, although this coast lies within the tropics. The reason for this is the occurrence of the Benguela current (see Figure 1.6) which carries cold water northwards from Antarctic waters. Wherever cold currents intrude into the tropics, corals will not thrive. Likewise, where significant amounts of fresh water flow into the sea from major rivers such as the Amazon, growth of coral reefs is also severely limited. Reefs cannot develop without a suitable hard substratum and so are not found in extensive areas of sediment.

Formation and structure

The physical basis of a coral reef is laid down by the corals themselves in their hard calcium carbonate skeletons. The deep underlying structure consists mostly of old corals that have been broken down and re-consolidated into calcite rock by many microbial and chemical processes. The living coral forms a layer on top

Figure 8.18 Worldwide distribution of coral reefs. Thick lines indicate where coral reefs are present.

of this. Massive amounts of material can be laid down over centuries and exploratory drilling through reefs in some areas has revealed coral rock deposits thousands of feet thick. Two boreholes drilled in the Enawetak Atoll in the Marshall Islands passed through about 1000 m (3280 ft) of coral rock before reaching volcanic rock. Living coral reefs need light (see below) and these deep deposits were formed in response to changes in land levels. Over geological time, the volcanic island with its surrounding reefs sank due to movements of the seafloor itself. As the island subsided, the corals maintained their upward growth in order to remain in the sunlit upper layers. When the land finally disappeared, a circular reef or atoll remained with a central lagoon inside. This theory of atoll formation was first put forward by Darwin and is still generally accepted.

Coral reefs are often classified according to the way in which they have been formed and where they are growing (Wood, 1983). Atolls have already been described. The shallow continental shelf provides a suitable base for the growth of many reefs known as fringing reefs. Fringing reefs grow close to the shore and as their name suggests, form a line along the coast or around an island or island group. Barrier reefs also form a line along the coast but further out on the continental edge between the waters of the continental shelf and the open sea. The Great Barrier Reef of Australia (Reader's Digest, 1984) is so large it can be seen from outer space and is known to have been in existence for at least 2 million years. Platform reefs and bank reefs occur away from land where irregularities in the continental shelf bring the sea-bed close enough to the surface for reef-building corals to grow. In the former, the reef is close to or breaks the surface whilst in the latter the reef top is up to 40 m below the surface.

Zonation

Within most coral reefs, a series of zones can be distinguished where bottom topography, depth (and therefore light), wave exposure and temperature vary. The patterns differ according to the type of reef and the location but the main divisions are illustrated in Figure 8.19. A typical fringing reef will have a shallow back reef or reef flat on the landward side, the seaward edge of which may be raised out of the water forming a reef crest. Beyond this, where the topography slopes downwards is the strongly growing fore reef. This is usually divided into the shallow fore reef (reef rim, reef front) and the deep fore reef (reef slope). The reef crest is exposed to the maximum wave action and here where the waves break, there is often extensive dead coral rock and rubble and sometimes extensive growths of calcareous algae (see below).

Corals living on the reef back are subjected to extreme conditions when the tide goes out. The corals may be exposed or if a shallow lagoon remains, then water temperatures can be very high. Consequently this area is often impoverished in terms of coral growth. However, areas of sand often support dense growths of

Back reef or Reef

Back reef or Reef

Figure 8.19 Main zones and coral habitats of a typical fringing reef.

seagrass and the back reef may be very rich in terms of invertebrate life. Molluscs, worms, crustaceans and echinoderms are often abundant. On the fore reef, coral growth is most vigorous below the depth at which wave pounding might cause damage but shallow enough for good light penetration and for a good supply of oxygen and nutrients brought in by water movements. At deeper depths, the species composition of the corals changes mostly in response to changing light intensity.

Apart from the corals themselves, many other organisms, especially red algae, help to build and consolidate the reef. A number of genera of red algae grow as heavily calcified crusts or nodules that help to bind the reef frame together. These algae are especially common on reefs exposed to considerable wave action, growing at the junction of the fore reef and back reef where the waves crash down. On some reefs their growth results in a broad algal ridge running parallel to the edge of the reef. Other material is added from the lime-containing skeletons of molluscs, echinoderms and crustaceans. Some reef fish, in particular parrotfish, eat the coral and excrete the hard material as sand. This can fill crevices and cracks and again adds to the strength of the reef.

Living coral

Reef-building stony corals are found only in well-lit, relatively shallow water (to about 50 m). Since corals are animals, this requirement for light puzzled biologists for many years. Microscopic studies of coral polyps finally revealed the answer. Coral polyps contain large numbers of microscopic dinoflagellate algae known as zooxanthellae in their tissues and it is the algae that require the light. Many details of this symbiotic relationship have still to be worked out but in simple terms the algae provide extra food for the corals, through photosynthesis, allowing them to build their massive skeletons, whilst the corals provide a safe haven for the algae and a supply of nutrients in the form of the corals' waste products. The corals also feed in a normal animal-like way by catching plankton with their tentacles. Many corals feed at night and retract their tentacles by day necessitating a night visit to see their true splendour. The vivid colours of corals are mostly imparted by their zooxanthellae and as the corals can expel their algal cells when under stress, the coral then appears white or bleached (see Section 10.2.3). Sometimes this results in the death of the coral but under the right conditions the zooxanthellae can be regained and the coral recovers.

Productivity and diversity

Apart from areas of back reef in shallow lagoons, algal growth on coral reefs is conspicuously absent and it requires careful searching to find the small clumps that are present. However, numerous algae are present and form a vital food source without which the reefs could not support the large numbers of fish that many do. A careful search of the reef around Sipadan Island in Sabah by one of the authors (Dipper), revealed at least 43 species on this relatively small reef (Wood, 1994). Part of the reason for the apparent lack of algae lies in the intensity with which some reefs may be grazed. Fish such as surgeonfish and parrotfish and also turtles eat the algal growths as rapidly as they grow. Thus although the seaweed is potentially abundant, it is never allowed to build up into large standing crops. Exclusion experiments have shown that on well-lit reefs as much as 1 to 5 kilograms of seaweed can grow per metre per year. At any one moment, however, there will be only a few grams present (Sheppard, 1983). Tough encrusting calcareous and coralline red algae and green algae with calcareous deposits such as Halimeda resist grazing and so may be more conspicuous.

Primary production in the reef ecosystem is carried out by benthic algae, suspended phytoplankton and especially by the zooxanthellae living in the coral tissues. It is therefore very difficult accurately to estimate primary productivity and few figures are available. However, those attempts that have been made suggest figures of around 1500 to 5000 gC m~2 yr_1. This is very high but most of the production is recycled particularly with respect to the corals and their zooxanthellae.

The diversity and complexity of the coral reef ecosystem is astonishing and rivalled only by that of tropical rain forests. Estimates suggest that a single reef may contain up to 3000 different species with more if the meiofauna and microfauna are included. The most diverse reefs with the highest number of coral genera are located in the Indo-Pacific where at least 500 reef-building species have been identified. This is thought to be due to the long evolutionary history, with reef ages measured in millions of years, stable environmental conditions and wide variety of suitable habitats present in this region. Atlantic reefs, in contrast, are 10 000 to 15 000 years old and support only around 60 reef-building corals. The diversity of the coral faunas is reflected in the diversity of the fish and invertebrates associated with the reef. There are around 2000 fish species in Indo-Pacific reef areas compared to 500 or so in Atlantic reef areas. This is further illustrated by a recent coral reef survey in which one of the authors (Dipper) took part. The reef around the tiny island of Sipadan off the coast of Sabah in Borneo was found to support 386 species of fish in spite of the fact that the reef is only a few kilometres in circumference (Wood, 1994).

Coral reefs are immensely important in terms of productivity and in their role as fixers of atmospheric carbon dioxide. We utilize them for food, profit and pleasure. Like the tropical rain forests, their inhabitants are a potential and as yet largely untapped source of an endless variety of medicines and drugs. They are also a wonder and delight, a place most of us would like to visit at some time in our lives. Unfortunately coral reefs are increasingly coming under threat from man's activities (see Section 10.2.3) and considerable effort will be needed to prevent the tragic loss and degradation of large areas of coral reef.

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