What is a reef?

Natural reefs are made of coral or rock.  Artificial reefs may be made of concrete, ship wrecks and any other man made structure.

Most coral reefs are found at latitudes of less than 25oC and in waters of 18oC or greater.  Warm waters are required for the skeletons of corals to grow and grow they do.  Most reefs are composed of live corals living on a foundation of dead coral.  In fact, many Pacific islands are made solely of coral that is usually resting on top of a sea mount of volcanic rock.  The Great Barrier Reef, in Australia, is the longest continuous series of reefs in the world and is made up of over 2500 individual reefs that rest on a continental shelf that is generally less than 60m deep; the reef is more than 2000 km long and in places is hundreds of kilometres wide.

There is a rich diversity of fishes and invertebrates on coral reefs. In fact, the diversity of organisms is often argued to be greater than any environment on the planet.  Species diversity of a wide range of tropical taxa is greatest in waters around Indonesia.

Rocky reefs are more typical of subtropical, temperate and sub-polar latitudes.  Rock type will often dictate the form of reefs (e.g, granite of Nova Scotia, sandstone of New South Wales Australia, and argillitic metamorphic rocks in many parts of New Zealand.  The reefs are generally covered with rich algal habitats.  The canopy forming algae are usually kelp plants, especially Macrocystis at high latitudes (=giant kelp, the world’s fastest growing plant) and small kelps such as Ecklonia, which is abundant on many reefs in the Southern Hemisphere.  Laminaria is the Northern Hemisphere equivalent.  Rocky reefs are often subjected to heavy seas that test necessarily robust assemblages of plants and animals.  There is great beauty on rocky as well as coral reefs.  On rocky reefs the algae move gracefully with the waves, caves and tunnels are often the most colourful areas with a rich diversity of sponges, sea squirts and anemones.  The fishes are beautiful also, although the diversity of fishes is less than most tropical reefs, but the abundance is often high.  On many reefs of the world, including near Sydney, the reef is treated to a seasonal influx of tropical fishes that die out with the chill of winter.

Major concerns or ‘wicked issues’ for reefs include

Coral reefs

Rocky reefs

Artificial reefs