Coral bleaching is the loss of intracellular endosymbionts (Symbiodinium, also known as zooxanthellae) through either expulsion or loss of algal pigmentation.1 The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with unicellular flagellate protozoa that are photosynthetic and live within their tissues. Zooxanthellae give coral its coloration, with the specific color depending on the particular clade. Under stress, corals may expel their zooxanthellae, which leads to a lighter or completely white appearance, hence the term "bleached".2
Bleaching occurs when the conditions necessary to sustain the coral's zooxanthellae cannot be maintained.3 Any environmental trigger that affects the coral's ability to supply the zooxanthellae with nutrients for photosynthesis (carbon dioxide, ammonium) will lead to expulsion.3 This process is a "downward spiral", whereby the coral's failure to prevent the division of zooxanthellae leads to ever-greater amounts of the photosynthesis-derived carbon to be diverted into the algae rather than the coral. This makes the energy balance required for the coral to continue sustaining its algae more fragile, and hence the coral loses the ability to maintain its parasitic control on its zooxanthellae.3
Physiologically the lipid composition of the symbiont thylakoid membrane affects their structural integrity when there is a change in temperature, which combined with increased nitric acid results in damage to Photosystem 2. As a result of accumulated oxidative stress and the damage to the thylakoid of chloroplasts there is a increase in degradation of the symbiosis and the symbionts will eventually abandon their host. Not only the change in temperature from the water increases the chances of bleaching there are other factors that play a role. Other factors that contribute to bleaching are increase in solar radiation (UV and visible light), regional weather conditions and for intertidal corals exposure to cold winds 4
Coral bleaching is a generalized stress response of corals and can be caused by a number of biotic and abiotic factors, including:
- increased (most commonly), or reduced water temperatures567
- starvation caused by a decline in zooplankton levels as a result of overfishing.8
- increased solar irradiance (photosynthetically active radiation and ultraviolet band light)9
- changes in water chemistry (in particular acidification)1011
- increased sedimentation (due to silt runoff)12
- bacterial infections13
- changes in salinity14
- low tide and exposure16
- cyanide fishing17
- elevated sea levels due to global warming (Watson)
While most of these triggers may result in localized bleaching events (tens to hundreds of kilometers), mass coral bleaching events occur at a regional or global scale and are triggered by periods of elevated thermal stress resulting from increased sea surface temperatures.11 The coral reefs that are more subject to continued bleaching threats are the ones located in warm and shallow water with low water flow. Physical factors that can prevent or reduce the severity of bleaching are available for the reefs located under conditions that include low light, cloud cover, high water flow and higher nutrient availability.4
Most evidence indicates that elevated temperature is the cause of mass bleaching events. Sixty major episodes of coral bleaching have occurred between 1979 and 1990,18 with the associated coral mortality affecting reefs in every part of the world. Correlative field studies have pointed to warmer-than normal conditions as being responsible for triggering mass bleaching events. These studies show a tight association between warmer-than-normal conditions (at least 1°C higher than the summer maximum) and the incidence of coral bleaching.
Factors that influence the outcome of a bleaching event include stress-resistance which reduces bleaching, tolerance to the absence of zooxanthellae, and how quickly new coral grows to replace the dead. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce bleaching incidence. Coral and zooxanthellae health and genetics also influence bleaching.19
Large coral colonies such as Porites are able to withstand extreme temperature shocks, while fragile branching corals such as table coral are far more susceptible to stress following a temperature change.20 Corals consistently exposed to low stress levels may be more resistant to bleaching.21
The US National Oceanic and Atmospheric Administration (NOAA) monitors for bleaching "hot spots", areas where sea surface temperature rises 1°C or more above the long-term monthly average. This system detected the worldwide 1998 bleaching event,2223 that corresponded to an El Niño. NOAA also uses a satellite with 50k resolution at night, which covers a large area and does not detect the maximum sea surface temperatures occurring usually around noon.citation needed
Increasing ocean acidification due to rises in carbon dioxide levels exacerbates the bleaching effects of thermal stress. Acidification affects the corals' ability to create calcareous skeletons, essential to their survival.24 A recent study from the Atkinson Center for a Sustainable Future found that with the combination of acidification and temperature rises, the levels of CO2 could become too high for coral to survive in as little as 50 years.24
Infectious bacteria of the species Vibrio shiloi are the bleaching agent of Oculina patagonica in the Mediterranean Sea, causing this effect by attacking the zooxanthellae.2526 V. shiloi is infectious only during warm periods. Elevated temperature increases the virulence of V. shiloi, which then become able to adhere to a beta-galactoside-containing receptor in the surface mucus of the host coral.2627 V. shiloi then penetrates the coral's epidermis, multiplies, and produces both heat-stable and heat-sensitive toxins, which affect zooxanthellae by inhibiting photosynthesis and causing lysis.
During the summer of 2003, coral reefs in the Mediterranean Sea appeared to gain resistance to the pathogen, and further infection was not observed.28 The main hypothesis for the emerged resistance is the presence of symbiotic communities of protective bacteria living in the corals. The bacterial species capable of lysing V. shiloi had not been identified as of 2011.
In the 2012-2040 period, coral reefs are expected to experience more frequent bleaching events. The Intergovernmental Panel on Climate Change (IPCC) sees this as the greatest threat to the world's reef systems.29303132
The Great Barrier Reef along the coast of Australia experienced bleaching events in 1980, 1982, 1992, 1994, 1998, 2002, and 2006.32 Some locations suffered severe damage, with up to 90% mortality.10 The most widespread and intense events occurred in the summers of 1998 and 2002, with 42% and 54% respectively of reefs bleached to some extent, and 18% strongly bleached.3334 However coral losses on the reef between 1995 and 2009 were largely offset by growth of new corals.35 An overall analysis of coral loss found that coral populations on the Great Barrier Reef had declined by 50.7% from 1985 to 2012, but with only about 10% of that decline attributable to bleaching, and the remaining 90% caused about equally by tropical cyclones and by predation by crown-of-thorns starfishes.36
The IPCC's moderate warming scenarios (B1 to A1T, 2°C by 2100, IPCC, 2007, Table SPM.3, p. 1337) forecast that corals on the Great Barrier Reef are very likely to regularly experience summer temperatures high enough to induce bleaching.33
Other coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles.38
Evidence from extensive research in the 1970s of thermal tolerance in Hawaiian corals and of oceanic warming led researchers in 1990 to predict mass occurrences of coral bleaching throughout Hawaii. Major bleaching occurred in 1996 and in 2002.39
According to Brian Skoloff of The Christian Science Monitor, "If the reefs vanished, experts say, hunger, poverty and political instability could ensue."40 Since countless sea life depends on the reefs for shelter and protection from predators, the extinction of the reefs would ultimately create a domino effect that would trickle down to the many human societies that depend on those fish for food and livelihood. There has been a 44% decline over the last 20 years in the Florida Keys, and up to 80% in the Caribbean alone.41
- Dove SG, Hoegh-Guldberg O (2006). "Coral bleaching can be caused by stress. The cell physiology of coral bleaching". In Ove Hoegh-Guldberg; Jonathan T. Phinney; William Skirving; Joanie Kleypas. Coral Reefs and Climate Change: Science and Management. [Washington]: American Geophysical Union. pp. 1–18. ISBN 0-87590-359-2.
- Hoegh-Guldberg O (1999). "Climate change, coral bleaching and the future of the world's coral reefs". Mar. Freshwater Res. 50 (8): 839–66. doi:10.1071/MF99078.
- Wooldridge, S. A. (2010). "Is the coral-algae symbiosis really 'mutually beneficial' for the partners?". BioEssays 32 (7): 615–625. doi:10.1002/bies.200900182. PMID 20517874.
- Baker, Andrew. Glynn, Peter. Riegl, Bernhard. 2008 Climate change and coral reef bleaching: An ecological assessment of long-term impacts, recovery trends and future outlook. Estuarine, Coastal and Shelf Science. 80: 435–471
- "REEF ‘AT RISK IN CLIMATE CHANGE’". Retrieved 12 July 2007.
- Anthony, K. 2007; Berkelmans
- Saxby, T.; Dennison, W. C.; Hoegh-Guldberg, O. (2003). "Photosynthetic responses of the coral Montipora digitata to cold temperature stress". Marine Ecology Progress Series 248: 85. doi:10.3354/meps248085.
- The Starving Ocean: Mass Coral Bleaching
- Fitts 2001
- Johnson, Johanna E; Marshall, Paul A (2007). Climate change and the Great Barrier Reef: a vulnerability assessment. Townsville, Qld.: Great Barrier Reef Marine Park Authority. ISBN 978-1-876945-61-9.
- Hoegh-Guldberg O, Mumby PJ, Hooten AJ, et al. (December 2007). "Coral reefs under rapid climate change and ocean acidification". Science 318 (5857): 1737–42. doi:10.1126/science.1152509. PMID 18079392.
- Rogers, S.R. (1990). "Responses of coral reefs and reef organisms to sedimentation". Marine Ecology Progress Series 62: 185–202. doi:10.3354/meps062185.
- Kushmaro, A., Rosenberg, E., Fine, M., Loya, Y. (1997). "l Beaching of the coral Oculina patagonica by Vibrio AK-1". Marine Ecology Progress Series 147: 159–165. doi:10.3354/meps147159.
- Hoegh-Guldberg, O. and Smith, G.J. (1989). "The effect of sudden changes in temperature, light and salinity on the population density and export of zooxanthellae from the reef corals Stylophora pistillata Esper and Seriatopora hystrix Dana". Journal of Experimental Marine Biology and Ecology 129 (3): 279–303. doi:10.1016/0022-0981(89)90109-3.
- Jones, R.J., Muller, J., Haynes, D., Schrieber, U., (2003). "Effects of herbicides diuron and atrazine on corals of the Great Barrier Reef, Australia". Marine Ecology Progress Series 251: 153–167. doi:10.3354/meps251153.
- Anthony, K.R.N. and Kerswell, A.P. (2007). "Coral mortality following extreme low tides and high solar radiation". Marine Ecology Progress Series. 151(5): 1623–1631.
- Jones, R.J. and Hoegh-Guldberg, O. (1999). Effects of cyanide on coral photosynthesis: implications for identifying the cause of coral bleaching and for assessing the environmental effects of cyanide fishing. Mar Ecol. Prog. Ser. 177: 83–91
- Huppert, Amit; Lewis Stone (September 1998). "Chaos in the Pacific's Coral Reef Bleaching Cycle". The American Naturalist 152 (3): 447–459.
- Marshall, Paul; Schuttenberg, Heidi (2006). A Reef Manager’s Guide to Coral Bleaching. Townsville, Australia: Great Barrier Reef Marine Park Authority,. ISBN 1-876945-40-0.
- Baird and Marshall 2002
- Gabriel D. Grinmsditch and Rodney V. Salm, Coral Reef Resilience and Resistance to Bleaching, "IUCN: The World Conservation Union", 2006
- "NOAA Hotspots".
- "Pro-opinion of NOAA Hotspots".
- Lang, Susan (13 December 2007). "Major international study warns global warming is destroying coral reefs and calls for 'drastic actions'". Cornell Chronicle. Retrieved 8 August 2011.
- Kushmaro, A.; Loya, Y.; Fine, M.; Rosenberg, E. (1996). "Bacterial infection and coral bleaching". Nature 380 (6573): 396. doi:10.1038/380396a0.
- Rosenberg E, Ben Haim Y (2002). "Microbial Diseases of Corals and Global Warming". Environ. Microbiol. 4 (6): 318–26. doi:10.1046/j.1462-2920.2002.00302.x. PMID 12071977.
- Sutherland KP, Porter J, Torres C (2004). "Disease and Immunity in Caribbean and Indo-pacific Zooxanthellate Corals". Marine Ecology Progress Series 266: 273–302. doi:10.3354/meps266273.
- Reshef L, Koren O, Loya Y, Zilber-Rosenberg I, Rosenberg E (December 2006). "The coral probiotic hypothesis". Environ. Microbiol. 8 (12): 2068–73. doi:10.1111/j.1462-2920.2006.01148.x. PMID 17107548.
- IPCC (2007). "Summary for policymakers". In Parry ML, Canziani OF, Palutikof JP, van der Linden PJ, Hanson CE. Climate Change 2007: impacts, adaptation and vulnerability: contribution of Working Group II to the fourth assessment report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press. pp. 7–22. ISBN 0-521-70597-5.
- Fischlin A, Midgley GF, Price JT, Leemans R, Gopal B, Turley C, Rounsevell MDA, Dube OP, Tarazona J, Velichko AA (2007). "Ch 4. Ecosystems, their properties, goods and services". In Parry ML, Canziani OF, Palutikof JP, van der Linden PJ, Hanson CE. Climate Change 2007: impacts, adaptation and vulnerability: contribution of Working Group II to the fourth assessment report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press. pp. 211–72. ISBN 0-521-70597-5.
- Nicholls RJ, Wong PP, Burkett V, Codignotto J, Hay J, McLean R, Ragoonaden S, Woodroffe CD (2007). "Ch 6. Coastal systems and low-lying areas". In Parry ML, Canziani OF, Palutikof JP, van der Linden PJ, Hanson CE. Climate Change 2007: impacts, adaptation and vulnerability: contribution of Working Group II to the fourth assessment report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press. pp. 315–57. ISBN 0-521-70597-5.
- Hennessy K, Fitzharris B, Bates BC, Harvey N, Howden M, Hughes L, Salinger J, Warrick R (2007). "Ch 11. Australia and New Zealand". In Parry ML, Canziani OF, Palutikof JP, van der Linden PJ, Hanson CE. Climate Change 2007: impacts, adaptation and vulnerability: contribution of Working Group II to the fourth assessment report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press. pp. 507–40. ISBN 0-521-70597-5.
- Done T, Whetton P, Jones R, Berkelmans R, Lough J, Skirving W, Wooldridge S (2003). Global Climate Change and Coral Bleaching on the Great Barrier Reef. Queensland Government Department of Natural Resources and Mines. ISBN 0-642-32220-1. Unknown parameter
- Berkelmans R, De'ath G, Kininmonth S, Skirving WJ (April 2004). "A comparison of the 1998 and 2002 coral bleaching events on the Great Barrier Reef: spatial correlation, patterns, and predictions". Coral Reefs 23 (1): 74–83. doi:10.1007/s00338-003-0353-y.
- De'ath, Glenn; Fabricius, Katharina E.; Sweatman, Hugh; Puotinen, Marji (October 1, 2012). "The 27–year decline of coral cover on the Great Barrier Reef and its causes". Proceedings of the National Academy of Sciences. Retrieved November 2, 2012.
- IPCC (2007). "Summary for policymakers". In Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL. Climate change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press. pp. 1–18..
- N. Middleton, Managing the Great Barrier Reef (Geography Review, January 2004)
- Hokiel, Paul J. "Climate Change and Hawaii’s Coral Reefs". Hawaii Coral Reef Monitoring and Assessment Program. US Fish and Wildlife Service.
- Brian Skoloff, , "The Christian Science Monitor", March 26, 2010
- PBS Newshour, "Endangered Coral Reefs Die as Ocean Temperatures Rise and Water Turns Acidic", "PBS Newshour", December 5, 2012
Watson, Megan E. "Coral Reefs." Encyclopedia of Environmental Issues. Rev. ed. Vol. 1. Pasadena: Salem Press, 2011. pp. 317–318. ISBN 978-1-58765-736-8
- Great Barrier Reef Marine Park Authority information on bleaching.
- ReefBase: a global information system on coral reefs.
- More details on coral bleaching, causes and effects.
- Travellers Impressions
- The Link between Overfishing and Mass Coral Bleaching
- Discussion on Overfishing and Coral Bleaching
- Social & Economic Costs of Coral Bleaching from "NOAA Socioeconomics" website initiative
- Microdocs: Coral bleaching
- Coral Bleaching at Maro Reef, September 2004
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