Coral reefs are the largest living structures on earth, covering around 600,000 km2 of the ocean floor. This actually only equates to 0.17% of the total ocean surface, but they support more species per unit area than any other marine environment. The US Coral Reef Task Force estimates that 25-30% of all marine species rely on coral reefs for all or part of their lives, including fish, echinoderms, sponges and molluscs. They are among the most complex and beautiful habitats on Earth.
They are often referred to as “rainforests of the sea,” as they have a wide diversity and abundance of both plant and animal species which rely on the reef for food and shelter. Corals are a collection of individual animals called polyps that live within a hard exoskeleton made from calcium carbonate. They are sessile animals that do not move except when they are juveniles and are looking for a space to attach and inhabit. The exoskeleton is formed from limestone (calcium carbonate) deposits from the remains of tiny marine organisms. This builds up over time as the organisms that form their living structures multiply, spread and then die; creating reefs from their skeletons. Corals grow best in tropical seas, at temperatures around 20-30°C, and require shallow, clear, sunlit waters as they have a symbiotic relationship (a mutually beneficial relationship between two species) with single-celled organisms called zooxanthellae. This is algae that provide food for the coral via photosynthesis and in return the coral offers protection and a home for the algae. This algae affects the colour of the coral depending on how deep it is in the water; the deeper the coral, the less light reaches it so the bluer the coral is in colour.
There are two groups of the stony corals (Scleratinia) or hard corals. Colonial corals are found in clear, shallow warm waters and are the world’s primary reef builders, and then there are the cold-water corals, or solitary corals, that inhabit Polar Regions in deep water below the photic zone (also known as the sunlit zone). The photic zone is the depth of water that is exposed to sufficient sunlight to allow photosynthesis to occur and extends from the surface down to a depth where light intensity falls to one percent of that at the surface, usually around 200 metres in the open ocean. Around 90% of all marine life lives in the photic zone.
There are three main types of reefs and they were formed after the last glacial period when melting ice caused the sea level to rise and flood the continental shelves. They are:
- Fringing reefs, which are attached to a shore and found near land.
- Barrier reefs, which used to be fringing reefs but have slowly grown away from the mainland over time and have been separated by an island or deep lagoon.
- Atoll reefs, which have formed around small islands that have eventually sunk due to the rising sea levels, creating round or horseshoe shaped reefs.
For many years now, coral reefs have been degrading in size due to several problems; mostly those caused by humans. One of the main problems noted with corals is bleaching. This is where the coral loses its colour due to loss of the symbiotic algae or the pigments in the coral. Scientists believe that this is linked to increases of sea temperature, but it is also thought to be due to other factors, such as disease, shade, UV radiation, sedimentation, pollution and salinity changes. As with most natural systems under stress, the coral could survive the bleaching process if the cause was a temporary one and was removed after a short period of time. The zooxanthellae algae would return to the coral and be able to photosynthesise if conditions returned to normal. However, if the polluting continues, eventually the coral would die without the nutrients provided by the algae. Pollution is also observed to be detrimental to coral reefs in other ways than just bleaching alone, especially in the form of sewage, chemicals, fertilisers and pesticide run-off from land. For example, if the water around the coral gains an increase of nutrients (especially nitrates and phosphates from agricultural run-off) eutrophication will be stimulated. This is where algae in the water benefits from the extra nutrients and undergoes rapid growth creating algal blooms, causing little or no sunlight to reach the coral for the zooxanthellae to photosynthesise.
Overfishing is an indirect factor that causes major problems for coral reefs. Many fish eat algae or aquatic plants, of which many can be found amongst coral reefs. If an area near coral is overfished, then this can lead to a reduction in herbivory and cause an increase in algae and other species as they are not being predated upon, causing the problem of little or no light reaching the coral. An example of this is the crown-of-thorns starfish which is a ferocious carnivorous predator of coral polyps. Adult crown-of-thorns can consume up to 6 square metres (65 sq ft) of living coral reef per year. Reduced numbers of their natural predators (puffer fish, triton and shrimp), due to over fishing, leads to population explosions, which explains how outbreaks can cause damage to large reef areas in relatively short periods. In addition, harmful fishing practices have been linked to coral decline, such as using poisons and explosives to kill off fish, as well as bottom trawling which can be extremely damaging to the coral.
Along with human causes, natural events have also been shown to damage the corals, especially hurricanes. These disasters generate powerful waves that damage coral, along with mud and sediment intake from land that can block out the sunlight, causing corals to die in the process.
The importance of coral reefs and why we should care
Coral reefs are a vast ecosystem rich in marine animals and plants. They provide shelter for a variety of species and in doing so support many complex food webs. For example, rays are known to feed on plankton found on coral reefs. If these habitat areas are destroyed then many of the species it supports will also end up in decline. Due to the diverse ecosystem of corals, fisheries indirectly depend upon reefs for providing food for the species they fish. Around one third of coral-feeding fish completely rely on coral for their diet, meaning that without the corals they would not be able to survive.
Another reason we should care about coral reefs is that they provide all four ecosystem services (supporting, provisioning, regulating and cultural) that are important to the environment and to the economy, and are vital for our survival. Ecosystem services are defined as ‘benefits people obtain from ecosystems.’ One of the regulating services is that they are carbon dioxide stores and fix about 0.05% of carbon in the ocean. Carbon dioxide is one of the gases known to cause global warming, so without the coral stores more CO2 would exist in the atmosphere, causing average global temperatures to increase. Other regulating services include water purification and the prevention of land erosion. The reefs act as natural barriers and slow down the waves before they reach land.
The reefs also provide the sand found on tropical beaches that attracts tourists and helps with the local economy. This is a cultural service that we often take for granted. Some of the money from tourism goes back into the local area and can be used to help conserve species and habitats.
The solution: what we can do
The largest coral reefs are now protected areas, for example, the Great Barrier Reef was designated as a national park in 1975 and a world heritage site in 1981. Many have also been allocated different zones for activities to occur in, such as snorkelling and fishing, so that the most vulnerable areas are protected but people can still visit and carry out other activities in other areas of the reef. There are also organisations, such as the Environmental Protection Agency (EPA) and the Coral Reef Alliance that aim to protect and conserve coral reefs.
On a more localised level, although most people do not live near a coral reef to help the habitat directly, there are many things we can do to help reduce the negative impacts upon them. Any items of coral, especially jewellery, should be avoided, and coral friendly businesses and organisations, such as those mentioned above, should be supported. We should only eat sustainably caught seafood and we could raise more awareness about the valuable services they provide. Those lucky enough to see the coral in its natural environment should take care not to damage or endanger the corals’ survival. Coral can be incredibly fragile and snorkelers should take care not to break or remove parts as souvenirs. Just a centimetre of coral usually takes up to a year to grow.
Another very important conservation tool is the education of coral reef issues to the wider public and the government. Education is fundamental in creating new patterns of behaviour in individuals, groups and society. The greater the amount of people that know about these current issues the easier it will be to protect these habitats and get the local governments involved to provide extra protection for them.