1. A fringing reef forms around an island.
Step 2. The island sinks, and the fringing reef grows into a barrier reef.
Step 3. The island sinks below the surface, and only an atoll remains. (After Darwin, 1842)
Step 1. Limestone is exposed by geological forces.
Step 2. Rainfall erodes exposed limestone.
Step 3. Water level rises, and eroded limestone is colonized by coral. (After Purdy, 1974)
However, this still did not explain the mechanism for Darwin's subsistence of the island. We now know, for example, that the Ice Ages, by locking up much of the earth's water into ice, brought about large changes in the level of the earth's oceans. At the peak of the last Ice Age, 18,000 years ago, the sea level stood almost 130 meters lower than it does today. In some cases it has probably been the rising of the oceans and not the sinking of the island that has created atolls.
Karstic saucers. Darwin's is not the only theory of reef formation, and there are some areas where geological evidence does not accord well with his speculations. A newer explanation has been offered, called the karstic saucer theory. "Karst" is the name given to the formations caused by the action of rainwater on exposed limestone—caves, sinkholes and underground channels. (The name comes from the type region around the Dinaric Alps, near the Adriatic coast of Yugoslavia.)
The emperor angelfish, Pomacanthus imperator.
This theory proposes that an area of exposed limestone, acted upon by the weak carbonic acid produced by rainfall, would take the shapes Darwin's theory attributes to reef growth. When the water level subsequently rose, corals would colonize the already shaped and eroded limestone. (See illustration at left.)
One of the great appeals of this theory to scientists is that it can be tested in the laboratory. Weak acid applied to a flat-topped block of calcareous rock will tend to erode it into the shape of a saucer, the acid acting to a greater degree in the center than at the edges. It also better explains some of the structures, like blue holes, found in Caribbean reefs.
Products from the Sea
Indonesians have always been sea-farers, and for an archipelagic nation, the ocean is still its greatest resource. Fish provides the main source of protein to Indonesia's 204 million people. The waters off Indonesia are thought to be able to support a fishing industry of 5 million tons a year, with actual catches just 20 percent of this.
Commercially valuable sea products also provide some cash to people living on sandy islands with little or no resources, save perhaps copra from coconuts. Ujung Pandang, the capital of South Sulawesi, is the Indonesian leader in the export of sea products, shipping several thousand tons a year. These include pearl oysters, mother-of-pearl or Trochus shells, other shells and dried sea cucumbers. The sea cucumbers, or trepang, are used in Chinese soup. (See 'Trepang Fishing," page 27.)
Although collecting these animals provides necessary income to the islanders, over-harvesting by itinerant Bugis and Bajo fishermen has all but wiped out certain species in some areas. Particularly hard-hit are the giant clams (Tridacna). The meat is canned and then sold at considerable prices in East Asia. These clams used to grow in huge "fields" in the reefs of eastern Indonesia. You can now dive in the same areas and not see a single one.
Triton shells (Charonia tritonis), helmet conchs (Casts cornuta) and turban shells (Turbo marmoratus), which are sold as trinkets, have also disappeared from some areas.
Future of Coral Reefs
Although Indonesia has some of the most untouched coral reefs in the world, even in the remote parts of the archipelago, where industrialization has not yet reached, the reefs are not free of danger. According to officials of the World Wide Fund for Nature (WWF) in Irian Jaya, the Indonesian half of the island of New Guinea, it is predominantly the reefs, and not the great forests of that island, that are most at risk.
Like forests, reefs are subject to succession. A diverse, well-populated reef does not just spring from the sandy bottom. Once a reef is wiped out, unprotected wave action and current may prevent regrowth from taking place.
In the more developed areas of Indonesia, dredging of channels, harvesting of coral for construction materials, and filling of estuarial waters has had a devastating impact on the reefs. The Bay of Ambon, in the central Moluccas, once had a reef that moved naturalist Alfred Russel Wallace to write: "There is perhaps no spot in the world richer in marine productions, corals, shells and fishes, than the harbour of Amboyna." During the post-war building boom, the coral was dragged up for building material in Ambon town. Today, the bay is a wasteland.
Giant clams like this Tridacna gigas were once common on Indonesian reefs. But a market for the canned meat in Asia, and the use of giant clam shells in making terazzo in Surabaya, Java have decimated the population in many areas.
A gorgonian goby (Bryaninops sp.) on an antipatharian wire coral. This little animal is no more than 4 centimeters long. Many fishes rely on invetebrates for food, shelter, and protection, but few are as particular as the gorgonian gobies. Some species of Bryaninops live and lay their eggs on only a single species of gorgonian or antipatharian.
Also damaging is the continuing practice of fish bombing, in which small charges are thrown overboard to stun fish so they can be easily captured for market. (See 'The Biak Fish Bomb Industry," page 28). The bombs don't just kill the fish. They create lifeless craters in the reef, deserts where all the coral and the life it supported have been destroyed. In some places this practice has reduced all the nearshore reef to barren rubble. Fishing for aquarium specimens and groupers for the live seafood trade using sodium cyanide is just as damaging.
In the long run, however, the greatest damage to reefs will probably be a result of bad land use: poor farming practices, including overgrazing, public works projects that expose the thin tropical soil to erosion, and deforestation through timbering.
These practices increase runoff and erosion, loading rivers up with silt, which is then carried out to sea. Silt chokes off coral growth, and leads to eutrophication, a great increase in nutrients in the water. This, in turn, causes an algae bloom, which robs the water of oxygen and can form a lethal mat over the coral.
Divers in Indonesia also have a responsibility to keep the country's reefs, many of which truly are in pristine condition, in a continued state of health. This means taking no souvenirs, developing good diving habits so as not to break off or damage fragile corals, and not harassing animals. In some areas, careless placement of dive boat anchors has already caused damage.
In many areas the guides are not educated in reef conservation. I have been served fresh giant
