What are Coral Reefs?


Coral reefs are marine ridges or mounds, which have formed as a result of the deposition of calcium carbonate by living organisms, predominantly corals, but also a rich diversity of other organisms such as coralline algae and shellfish.

Figure 4. Distribution of coral, mangrove and seagrass diversity. Similar to corals, the region of greatest mangrove diversity is in Southeast Asia, particularly around the Indonesian Archipelago (Burke et al., 2001). There are three distinct areas of seagrass diversity in the Pacific region: the Indo-Pacific (areas around Indonesia, Malaysia, and Papua New Guinea), the seas around Japan, and southwest Australia (Spalding et al., 2002). This graphic illustrates the distribution and biodiversity (low, medium and high diversity) of corals, mangroves and seagrass in the world’s coastal and marine areas.

Coral reefs provide a unique habitat characterised by high diversity and density of life. They occur globally in two distinct marine environments; deep, cold water (3-14°C) coral reefs, and shallow, warm water (21-30°C) coral reefs in tropical latitudes.

To date cold-water corals have been identified in 41 countries worldwide, although their full extent is still not fully known (Freiwald et al., 2004). They are found at depths greater than 39m. The following descriptions relate to warmwater coral reefs only. Coral reefs support over a million animal and plant species and their economic value exceeds US $ 30 billion a year.

Warm-water coral reefs are found in circum-tropical shallow tropical waters along the shores of islands and continents. Corals consist of small polyps surrounded by tentacles. They feed through ingesting plankton, and also through the association with  symbiotic algae called zooxanthellae. Stony corals deposit calcium carbonate, which over time forms the geological reef structure. Many other invertebrates, vertebrates, and plants live in close association with the scleractinian corals, with tight resource coupling and recycling, allowing coral reefs to have extremely high biodiversity in nutrient poor waters, so much so that  they are referred to as ‘the Tropical Rainforests of the Oceans’. The table shows that corals have certain zones of tolerance to water temperature, salinity, UV radiation, opacity, and nutrient quantities. The extreme high diversity of coral reefs have led some erroneously to believe that they prefer nutrient rich environments, but, in fact, corals are extremely sensitive to silt and sewage at far lower concentrations than what is classified as hazardous to humans (Nyström et al. 2000). Hence, even minor pollution in apparently clear waters can severely impact coral reefs and their ability to support thousands of fish species and other marine life. Sea water quality and human impacts are particularly critical to coral reefs when they are exposed to other stressors or when they are recovering from storms or bleaching events (Burke et al., 2002; Wilkinson, 2002; Brown et al., 2006; UNEP, 2006).

Coral bleaching occurs when the corals are subjected to stress, and their tolerances are exceeded. When this occurs, the symbiotic algae are ejected and the corals lose their colour, and are white. One well documented cause of bleaching is increase of sea surface temperatures (SSTs). If SST rises for a period as little as 1°C higher than the usual average monthly maximum SST during the hottest months of the year, this can result in a bleaching event (Glynn, 1996).

Corals are beautiful living animals that are enjoyed by millions of snorklers and divers world wide, but they are also of vital importance to the whole coral reef ecosystem and for coastal fisheries. If corals die, the characteristic three dimensional structure of reefs that is essential to so many of the services provided, will be lost through natural physical and biological erosion as waves, storms, tsunamis, predators, and other factors affecting corals break it down to rubble.

Top 11 Coral Reef Facts
Number of states/territories with corals 109
Distribution 30°N - 30°S
Coverage 284 300km2
Number of reef building species described ~800
Temperature range 20-29°C
Salinity range 33-36%o
Depth range 0-100m
Age of living reefs 6 000-9 000 years
Nutrition Ssupended organic matter and photosythesis
Ecological importance -1 million associated animal and plant species
Economic importance Est>US$ 30 billion/year

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