Coral reef coasts

Australia as a nation has the greatest area of coral reefs anywhere in the world. This includes the largest coral reef complex, the Great Barrier Reef, in addition to reef areas in the Torres Strait, the Coral Sea Territories, and central and northern Western Australia. Of these, the Great Barrier Reef in Queensland and the Ningaloo Fringing Reef in Western Australia are two of the more popular tourist destinations, which consequently require well-defined coastal management strategies. Apart from tourism, other pressures on Australian coral reefs include the effects of fishing, sediments and nutrients, and threats of oil spills. Specific threats to coral processes include 'elevated nutrients in the inner Great Barrier Reef; outbreaks of crown-of-thorns starfish in the outer central and northern Great Barrier and Tasman reefs; damage from the passage of tropical cyclones; and outbreaks of coral eating 'Drupella' snails in Ningaloo Reef, Western Australia' (Zann 1995, p. 14).

In discussing coral reef processes it is important to realise the dominant role of biological factors in reef development, and also the importance of understanding these processes for coastal management. As noted by Bennett, 'the Great Barrier Reef off the Queensland coast is the only considerable area of the globe, visible to the human eye, which is dominated by animals rather than plants' (Bennett 1974, p. 238). Coral reefs are complex ecosystems, which have a high diversity and productivity, an interdependence of animals and other organisms, and many symbiotic relationships. This high diversity of coral reefs is the reason why they are often referred to as the marine equivalent of the terrestrial rainforests.

Reef-building or hermatypic corals are distinguished by the presence of zooxanthellae, which are minute unicellular algae living within the coral tissue. These have a symbiotic relationship with the coral polyp by assisting it to lay down its skeleton (a corralite) through photosynthesis, taking up polyp wastes, and supplying the coral with about 98% of its food requirements. Over thousands of years the build-up of corallite forms a porous limestone that forms the basis for reef building, along with calcareous algae. There is a great variety in the hard reef-building corals; over 350 species have been identified. Some corals live as individuals, but most are in colonies. The variety of species, their life-forms, and their colour give the reef much of its tourist appeal. The colour, however, comes from the soft tissue; the skeleton, once bleached, appears as a white limestone.

Growth rates of corals are fastest in shallow, well-lit waters, and can be up to 20 mm/yr for massive corals and up to 100 mm/yr for branching corals. However, some species are soft corals which do not have the same role as reefbuilding corals. Coral reefs depend on other organisms, particularly algae, which are the primary energy source through photosynthesis. However there are also crustose corraline algae which are important reef-builders. In some places algae can form up to 80% of reef sediments, particularly the platey Halimeda. Boring algae are also important in reef breakdown.

The north-eastern Australian region is particularly rich in coral diversity and abundance by world standards. A major reason for this is the favourable environmental conditions for corals, which only grow in warm waters where the temperature in the coldest months is 20 to 26°C. The temperature range for coral growth is 16-36°C, so that Lord Howe Island, for example, supports some of the southernmost Pacific reefs. Corals can be almost self-sufficient in terms of nutrients provided there is no oversupply. They have an asexual reproduction phase involving major spawning events, which have been the subject of a number of recent scientific studies. Corals grow in the warm water down to a depth of 100 m, where they become light limited. Other factors restricting their growth are excessive freshwater and sediment deposition, so that they tend not to grow near areas of high river discharge.

There is great variation in growth rates, depending on the position of the reef surface relative to sea level, and vertical reef growth may be negligible on most reef surfaces that are exposed at low tide. Vertical reef growth will also be significantly altered once it comes directly under the influence of surface wave conditions, thus suffering modification and lateral accretion. Kinsey and Davies (1979) observed that most reef flats are between 100 and 400 m wide, and suggested that 400 m represents an equilibrium situation where the production of calcium carbonate balances the loss. Another important factor in reef growth is the role of corraline algae. On the Great Barrier Reef, the algal rim has been described as an elevated Lithothamnion ridge consisting of a hard, wave-resistant, stepped pavement of encrusting corralline algae, predominantly Lithothamnion and Porolithon.