Coral Bleaching is the loss of colour from corals under stressful environmental conditions. While any stress can cause corals to bleach, high water temperature has been the major cause of coral bleaching events worldwide in recent decades . In 1998 and again in 2002, a major bleaching event in reef waters of the Great Barrier Reef raised concerns about the long-term health of the reef. With predictions of continued rising temperatures as a result of global warming, the future of tropical corals reefs is causing concern worldwide.
The corals that form the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called zooxanthellae that live within their tissues. Zooxanthellae give coral its particular coloration, depending on the clade living within the coral. Under stress, corals may expel their zooxantheallae, which leads to a lighter or completely white appearance, hence the term "bleached".
Once bleaching begins, corals tend to continue to bleach even if the stressor is removed. If the coral colony survives, it often requires weeks to months for the remaining symbiont population to reach a normal density . Following bleaching, corals may be recolonised by the same species of zooxanthellae, or by a different species. Different types of zooxanthellae respond differently to environmental conditions and may be more resistant to coral bleaching than other species. Some corals are known to host multiple clades of zooxanthellae within an individual coral. Ability to withstand stress and bleaching and ability to recover from a bleaching event varies greatly across coral species. Large massive corals, such as Porites lobata is able to withstand extreme temperature shocks, while fragile branching corals, such as Acropora spp. are far more susceptible to thermal stress following a bleaching event . Recent research has also shown that corals consistently exposed to low levels of stress may in fact be more resistant to bleaching. Factors that protect against mass coral bleaching are bleaching resistance, coral tolerance, reef recovery. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce the risk of bleaching. Also, the health and genetics of both the coral and its zooxanthellae can influence the risk of bleaching.
Coral Bleaching
Other reef creatures have symbiotic zooxanthellae, which they may also expel under stressful conditions. Bleaching stress is also exhibited by soft corals, giant Tridacna clams and some sponges.
The Great Barrier Reef along the northeast coast of Australia suffered two mass coral bleaching events in the summers of 1998 and 2002, and also in the southern GBR in 2006. While most reef areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed Based on IPCC 2007 assessment, coral reefs will be highly suspecptible to increase and more frequent bleaching events with the additional problem of acidification from increase carbon dioxides within the next twenty to thirty years.
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The Great Barrier Reef along the northeast coast of Australia suffered two mass coral bleaching events in the summers of 1998 and 2002, and also in the southern GBR in 2006. While most reef areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed Based on IPCC 2007 assessment, coral reefs will be highly suspecptible to increase and more frequent bleaching events with the additional problem of acidification from increase carbon dioxides within the next twenty to thirty years.
Ecological causes
As coral reef bleaching is a general response to stress, it can be induced by a variety of factors, alone or in combination. It is therefore difficult to unequivocally identify the causes for bleaching events. The following stressors have been implicated in coral reef bleaching events.
Temperature
Coral species live within a relatively narrow temperature margin, and anomalously low and high sea temperatures can induce coral bleaching. Bleaching events occur during sudden temperature drops accompanying intense upwelling episodes, (-3 º C to -5 º C for 5-10 days), seasonal cold-air outbreaks. Bleaching is much more frequently reported from elevated se water temperature. A small positive anomaly of 1-2 º C for 5-10 weeks during the summer season will usually induce bleaching.
Solar Irradiance
Bleaching during the summer months, during seasonal temperature and irradiance maxima often occurs disproportionately in shallow-living corals and on the exposed summits of colonies. Solar radiation has been suspected to play a role in coral bleaching. Both photosyntheticaly active radiation (PAR, 400-700nm) and ultraviolet radiation (UVR, 280-400nm) have been implicated in bleaching.
Subaerial Exposure
Sudden exposure of reef flat corals to the atmosphere during events such as extreme low tides, ENSO-related sea level drops or tectonic uplift can potentially induce bleaching. The consequent exposure to high or low temperatures, increased solar radiation, desiccation, and sea water dilution by heavy rains could all play a role in zooxanthellae loss, but could also very well lead to coral death.
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Fresh Water Dilution
Rapid dilution of reef waters from storm-generated precipitation and runoff has been demonstrated to cause coral reef bleaching. Generally, such bleaching events are rare and confined to relatively small, nearshore areas.
Inorganic Nutrients
Rather than causing coral reef bleaching, an increase in ambient elemental nutrient concentrations (e.g. ammonia and nitrate) actually increases zooxanthellae densities 2-3 times. Although eutrophication is not directly involved in zooxanthellae loss, it could cause secondary adverse affects such as lowering of coral resistance and greater susceptibility to diseases.
Xenobiotics
Zooxanthellae loss occurs during exposure of coral to elevated concentrations of various chemical contaminants, such as Cu, herbicides and oil. Because high concentrations of xenobiotics are required to induce zooxanthellae loss, bleaching from such sources is usually extremely localized and / or transitory .
Epizootics
Pathogen induced bleaching is different from other sorts of bleaching. Most coral diseases cause patchy or whole colony death and sloughing of soft tissues, resulting in a white skeleton (not to be confused with bleached corals). A few pathogens have been identified the cause translucent white tissues, a protozoan.
Major Threats
Each coral colony is assessed for the presence and intensity of bleaching. Bleaching has been found to inhibit the ability of corals to recover from small-scale tissue damage and may increase partial or total mortality of a colony. Coral bleaching is often transient in nature with corals regaining their pigmentation after several weeks or a few months. With severe or prolonged bleaching, corals may also experience reduced skeletal growth, decline in reproductive fitness or failure and inability to resist competition from algae or other invertebrates. In extreme cases, bleaching can overtime lead to a reduction in species diversity, cover and eventually, loss of reef framework.
Oil Spillage
Any oil spillage is a serious potential threat for marine biodiversity. Passenger and cargo ships dumping untreated waste into the sea around the islands and discharging waste oil cause severe pollution. Lakshadweep falls along the main route for oil tankers plying between the Middle East and South/South-east Asia. Furthermore, the reefs of Lakshadweep and Nicobar islands are considered to be the most polluted in the Indian ocean because the seas around them serve as major routes for oil tankers (Bakus, 1994).
A study conducted by the Cochin University of Science and Technology (CUST) on the ‘Coral reef ecosystem of Lakshadweep-a biogeochemical facsimile’ has found that 25% of the outer cells in the reefs were dead due to the prolonged trapping of sediment on the coral with a high concentration of toxic metals and other harmful chemical compounds on them (Source: Giji Marykulam, Indian Exp., 6/1/04). The effect of oil on corals, sea grasses and other marine resources needs further study.
A study conducted by the Cochin University of Science and Technology (CUST) on the ‘Coral reef ecosystem of Lakshadweep-a biogeochemical facsimile’ has found that 25% of the outer cells in the reefs were dead due to the prolonged trapping of sediment on the coral with a high concentration of toxic metals and other harmful chemical compounds on them (Source: Giji Marykulam, Indian Exp., 6/1/04). The effect of oil on corals, sea grasses and other marine resources needs further study.
Pollution
Coral reefs are the most threatened on account of pollution from the land and over exploitation of fishes – particularly bait fishes, corals and shells. Coral mining, dredging of navigational channels, destructive fishing practices, coastal development, souvenir collection and blast fishing are other reasons which lead to further problems of coastal erosion. Global factors like greenhouse effect, sea level rise and global warming may also be indirectly affecting the health of the lagoon. Dredging to deepen the lagoon for navigational purposes was carried out in almost all the atolls of Lakshadweep (Pillai, 1983 and 1996). The large number of boats and speed boats which traverse the lagoon also cause the same impact as dredging.
Sea erosion of the islands’ shoreline is aggravated by the removal of coral boulders for use in building materials and cement industry. Erosion also takes place by natural causes like wave action as well as due to destruction of coral reefs. The impacts of such activities need further study.
The use of chemical detergents (cleaning and washing) and the application of artificial fertilizers and chemicals cause leaching of excessive nutrients into the lagoon which results in eutrophication and cuts off sunlight and finally leads to the death of corals.
Plastic dumping is another hazard which kills sea turtles, crabs, shrimps and other crustaceans. Sites littered with plastic and linen trash is a major threat to these islands and strict laws need to be implemented to ban plastics, metals and batteries as waste/sewage disposal in the mainland is a serious issue which needs much attention. An incinerator is installed ‘only’ at Kavaratti for clearing the trash and needs to be distributed to the rest of the inhabited islands (Koya, S.I. Per. com.).
Sea erosion of the islands’ shoreline is aggravated by the removal of coral boulders for use in building materials and cement industry. Erosion also takes place by natural causes like wave action as well as due to destruction of coral reefs. The impacts of such activities need further study.
The use of chemical detergents (cleaning and washing) and the application of artificial fertilizers and chemicals cause leaching of excessive nutrients into the lagoon which results in eutrophication and cuts off sunlight and finally leads to the death of corals.
Plastic dumping is another hazard which kills sea turtles, crabs, shrimps and other crustaceans. Sites littered with plastic and linen trash is a major threat to these islands and strict laws need to be implemented to ban plastics, metals and batteries as waste/sewage disposal in the mainland is a serious issue which needs much attention. An incinerator is installed ‘only’ at Kavaratti for clearing the trash and needs to be distributed to the rest of the inhabited islands (Koya, S.I. Per. com.).
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