This reduces the amount of light absorbed by coral, causing bleaching of coral. See also "sedimention" in anthropogenic disturbances.
Picture courtesy http://www.marinebiology.org
Threats to coral can be divided into two parts:
(2)Anthropogenic disturbances;
(1)Increase levels of ultraviolet radiation
Experiments have shown that an increase in UV light causes coral bleaching. UV light experienced by corals can increase in calm waters. The amount of mycosporine-like amino acids in a coral's tissues helps to determine how much UV it can withstand without bleaching (Gleason, 1993).
(2)Sedimentation
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This reduces the amount of light absorbed by coral, causing bleaching of coral. See also "sedimention" in anthropogenic disturbances. Picture courtesy http://www.marinebiology.org |
(3)Disease
All coral diseases have been found to be stress related, and anthropogenic stresses can increase a coral's susceptibility to these diseases. There are four identified
(iii)Bacterial infection;
(iv)Shut down reaction(Richmond 1993);
Also, corals are susceptible to tumors annd parasitic worms.
(i)The exact method of transmition of WBD is unknown. WBD, which is believed to be caused by a bacteria pathogen yet unknown, has much of the same effect on corals, leaving behind a white, lifeless coral skeleton. Gladfelter (1982) does not see WBD as being beneficial to reefs. He feels that WBD destroys the reef structure because the dead coral skeleton brought about by the algae is colonized by algae, invertebrates, gastropods, and boring cloinid sponges that work to weaken the coral skeletons making them more susceptible to breakage during storms. This situation is also probable for BBD.
(ii)The exact method by which the BBD is transmitted is unknown. Even though healthy corals may get BBD through contact with an infected coral, diseased corals are not aggregated naturally on the reef and can be separated by great distances. Thus, it is also suggested that BBD can be spread by currents through trichomes that come off infected colonies and land on other colonies (Edmunds 1991). Corals under stress are more likely to be infected, and BBD has a higher rate of infection in warmer water. Thus seasonal temperatures affect the spread of BBD, and also any anthropogenic warming of water temperatures may increase BBD. BBD was also found to be more abundant near anthropogenic disturbances. However, WBD has not been found to be related to anthropogenic disturbances (Gladfelter 1982).
(i)&(ii)
Diseases such as BBD and WBD actually kill coral tissue while advancing in a band around the coral and leaving the white coral skeleton behind. Edmunds (1991) stated that BBD, caused by cyanophyte Phormidium corallyticum, may have a role in maintaining coral diversity because it is most prevalent in coral species that form large colonies and provide a structural framework for the reef. When BBD kills part of these colonies, the skeleton is available to be colonized by other coral species recruits. However, after 25 months, there were not coral recruits among corals infected by BBD (Edmunds 1991).
(iii)Corals may be affected by diseases indirectly. Diseases that infect other reef inhabitants may affect ecology of corals. In 1993, a newly encountered pathogen was found to affect coralline algae in the Pacific Ocean (Littler and Littler 1995). This pathogen also leaves the coralline algae skeleton white as it progresses in a orange band, destroying the algae. Coralline algae help the coral reef community by cementing together sand, coral fragments, and other debris into a suitable hard substrate for the establishment of coral colonies and by absorbing wave energy in the outer reef rim that would otherwise erode the shoreline (Littler and Littler 1995). Another disease which caused a Caribbean-wide mortality in the reef dwelling sea urchin Diadema antillarum was also shown to indirectly affect coral reefs (Lessions 1988). The sea urchin functions as both a grazer on algae that can otherwise smother coral and as a bioeroder of corals as it feeds on them (Lessions 1988).
(4)Excess shade
This may be caused by sedimentation. See sedimentation
(5)Pollution
See also "pollution" in anthropogenic disturbances.
(6)Salinity changes
Corals tolerate a narrow temperature range between 25 degrees Celsius and 29 degrees Celsius depending on location. Corals bleach in response to prolonged temperature change and not due to rapidly fluctuating temperatures. Lab experiments show that corals bleach when water reaches a constant 32 degrees Celsius (Brown and Ogden, 1993). See also "increase in water temperature" in anthropogennic disturbances.
(8)Hurricanes and typhoons:
Hurricanes and typhoons generate large and powerful waves in the tropics, which cause large corals to break apart and scatter fragments about the reefs. After the storm, these slow growing corals might easily be overgrown by fast growing algae. In addition to this, these storms bring heavy rain, which increases runoff and sedimentation.
Earthquakes
Break apart coral reefs and generate huge waves. Similar destruction to typhoons and hurricanes.
Crown-of-thorns:
Crown-of-thorns, Acanthaster planci, is a large starfish which feeds on coral to eat the livinng tissue layer. This fast reproducing predator has had serious effects in many regions of the Pacific. The breakout of the predator has been linked to regions of increased development and eutrophication (Birkeland 1989).
Human expansion and development alter the landscape, and increases the amount of freshwater runoff. This terriginous runoff may carry large amounts of material which will harm coral, including:
(1)Pollution: Many pollutants such as petroleum products and insecticides will kill coral.In Caribbean, less than 10 percent of total domestic waste receives treatment before disposal and much reaches coastal waters, causing accelerated algal growth and eutrophication. Inadequate environmental standards in Egyptian and Saudi oil facilities, and the de-ballasting of ships in the Red Sea cause problems.
(2)Sedimentation:Large amounts of sediment from land-clearing areas decreases the amount of light reaching coral by either direct sedimentation onto the reef or an increase in the turbidity of the water due to eutrophication. This will cause the bleaching of coral.
(3)Increase in nutrients: Nutrients from agricultural areas or septic systems enhance the growth of other reef organisms such as sponges which may outcompete the corals for space on crowded reefs.
(4)Increase in water temperature: Water discharged from large power plants are extremely hot, and will alter the temperature of coastal waters. This change in temperature will kill coral. See "change in temperature".
(5)Over-harvest of resources from the sea: Due to overfishing, reef fish populations have been greatly reduced in some areas of the world. This has caused coral reef ecosystems to become unbalanced. Competitive organisms would overcompete the corals for space in reefs. Algae, for example, which were once controlled by large fish populations in coral reefs, has become dominant on reefs in many regions.
(6)Destructive fishing methods: Due to decreasing yields, fishermen have been forced to change their methods. One of these methods is fish traps with small mesh diameters. This would catch even small juvenile fish, thus greatly affecting the population of fish. In other areas, fishermen use poison or explosives. This not only kills all the fish in the affected areas, but also severely damage the corals in these areas.. Poisoning fish traps, and spear fishing have increased in direct proportion to an increase in human population. To date more than 40 countries are known to have problems with blast fishing on coral reefs, and more than 15 have reported cyanide fishing. Indonesia is example.
(7)Collection of coral: Corals are very popular decorations, so people collect pieces of corals from reefs or buy them from souvenier shops. These shops get their corals from commercial collectors which harvest corals from the most healthy coral reefs so they can earn the maximum amount of money.
Unplanned tourism. Too many hotels, inadequate waste water treatment, unregulated construction, collection of corals and ornamental species, spearfishing, walking on and anchoring in coral.
Construction activities such as dredging and filling also harm corals.
Human population increases. In Caribbean the islands have become overpopulated. Causes more building along shores and leads to overfishing. In Red sea oil realted industry and too many tourist facilities causing serious problem.