Coral Reefs, Atolls, and Reef Limestone

Low islands are called atolls if they consist of a ring of land or islets surrounding a shallow lagoon. Their land consists of broken pieces of coral and white sand. Many such low islands also have patches of fertile soil formed by slowly accumulated plant matter, airborne dust, and fertilizers applied by farmers. Kwajalein in the Marshall Islands is the largest atoll in the world. It consists of 90 islets on the rim of a lagoon of about 650 square miles (1,683 square kilometers). Many low islands are at risk from storms, tsunamis, and rising sea levels due to global warming. (See global warming.)

Some coral islands consist of reef limestone that was raised slowly over thousands or even millions of years. Most of Tonga’s islands are of this raised type, including the largest, Tongatapu, which covers some 100 square miles (260 square kilometers). Equally large is the raised island of Niue. The two islands have rather flat surfaces and reddish-brown soil.

Because the limestone and volcanic rock on some islands is porous, there is little or no surface water. Many people depend on rainwater collected in cisterns, as well as imported potable water. Rainwater seeps underground where it collects above heavier salt water in a so-called freshwater lens. The bigger the island, the thicker and larger the lens. Small islets perched on atoll rims have small, thin, and fragile lenses. Such lenses are easy to exhaust, in which case seawater flows up and replaces them—especially where town wells draw excessive water.

Coral Reef Ecosystem
A reef is composed of the skeletal remains of millions of tiny corals. A coral is a type of invertebrate, or animal without a backbone. Corals support their soft bodies within hollow “shells” of bonelike material made of calcium carbonate. When a coral dies, its shell remains, and other corals grow upon it. Over millions of years mounds of remains build up. They are bound together with simple organisms such as algae and bryozoans, tiny invertebrates whose name means “moss animals.”

Coral is also an important feature in the waters surrounding many islands. Nearly all the inhabited high islands of Oceania are surrounded by a type of coral reef called a fringing reef, except where the water is too cold.

The Great Barrier Reef was built upon a shallow fringe of continental shelf in waters warm enough for the coral to flourish. The variety of sea life on and around the Great Barrier Reef is extraordinary. Each species occupies a specific niche, or relationship to other organisms in the reef community. Besides corals, other forms of invertebrate reef life include anemones, worms, snails, lobsters, crayfish, prawns, jellyfish, and giant clams. Animals having backbones, the vertebrates, include a great number of sea birds and more than 1,500 species of saltwater fishes. Many of the small fishes have brilliant colors and unusual shapes. Plant life above the surface of the water is limited, with only about 30 to 40 species.