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

  • Global warming and coral bleaching –  the seas are warming and becoming more acid, how do we deal with this?  Coral bleaching has affected reefs in all tropical oceans of the world.  It is argued by some that the frequency and intensity of bleaching is increasing as a result of global warming. With overpopulation and a related increase in carbon dioxide and other gases such as methane in the atmosphere this will get worse without an international commitment to reduce emissions.
  • Overfishing – a problem in tropical waters worldwide
  • Freshwater runoff and Eutrophication – an excess of nutrients in marine waters affect marine organisms.  In the tropics, waters that are clear and naturally low in nutrients,  nutrient pollution that may cause excessive growth of algae that kill corals and in turn can affect most organisms associated with reefs;
  • Crown-of-thorns starfish (COTS) – these voracious starfish consume live coral.  Outbreaks of COTS can decimate corals on a reef, but the reefs generally recover (in 5-10 years).  The outbreaks have most likely happened for millennia (though some argue that man’s activities have exacerbated outbreaks).  Resorts on the Great Barrier Reef often try to control numbers by killing starfish.
  • Coral disease – coral disease can be fatal to coral colonies or parts of colonies.  It has been poorly understood until recently and has often been confused with coral bleaching.
  • Pollutionthrough oil spills and diversity of pollutants through run off (e.g., fertilizers, industrial waste, sewage).
  • Developmentcovering reefs during land reclamation and increased sediment loads to reefs nearby.
  • Introduced species – fishes (e.g. snappers and groupers to Hawaii) have been introduced to some reefs and have greatly affected the local fauna, Further, exotic species have been introduced through ship’s ballast and by hitching rides on the sides of ships and smaller vessels.  Some of these organisms, such as the (‘Asian Mussel’) can have a great impact on the local fauna.

Rocky reefs

  • Climate changeclimate change may cause great changes in the distribution and survival of temperate plants and animals
  • Overfishing – as for coral reefs a problem worldwide
  • Pollutionthrough oil spills and a diversity of pollutants through run off (e.g., fertilizers, boat antifoul, industrial waste, sewage)
  • Developmentcovering reefs during land reclamation and increased sediment loads to reefs nearby
  • Introduced species – exotic species can be introduced through ship’s ballast and by hitching rides on the sides of ships and smaller vessels.  Some of these organisms can have a great impact on the local fauna.  Examples in temperate waters include introductions of the Pacific sea star (to Tasmania), crabs (San Francisco Harbor) and the Japanese kelp Undaria to New Zealand.
  • Protected Areas – On natural reefs all over the world a management option that popular if to protect reefs to varying degrees in so called ‘Marine Protected Areas’.

Artificial reefs

  • Positive points – enhance fish stocks, increase catch rates of fish,sustain populations of fishes that were not previously found in the area.
  • Negative points make fishes more vulnerable to fishers, justification for putting rubbish into the marine environment, change routes of migration, change location of fisheries and alter catch rates in areas where fishes were previously fished