Coral bleaching

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Dead hard corals
Healthy hard corals

With coral bleaching the bleaching of is stony corals pole hire referred, which can lead to the subsequent death of the coral.

Corals are living organisms. They belong to the cnidarians and live in symbiosis with photosynthetically active protozoa ( zooxanthellae ) on a layer of limestone that grows from year to year. If the zooxanthellae are repelled by the coral, the coral stock loses its splendor. This phenomenon can occur locally but also over a large area and is called coral bleaching. It mainly occurs when the water temperature is too high.

In addition to the rainforests , coral reefs are considered to be the most species-rich habitats in the world: around 60,000 different species have been discovered here to date. However, more than 400,000 species are suspected in the reefs. B. more than a quarter of all known marine fish. The size of all reefs is estimated at over 600,000 km². Coral reefs have existed for over 225 million years, making them one of the oldest known ecosystems in the world.

With their biodiversity, especially fish, and as a tourist diving and snorkeling attraction, they are a source of food and income for large parts of the people living nearby on the respective coasts ; by being able to absorb 70 to 90% of locally occurring wave energy, they protect the respective coasts from mechanical stress and destruction by waves.

In the course of man-made global warming with the associated permanent warming of the oceans , the frequency and intensity of bleaching events is increasing; Above all, the period of at least 10 years necessary for the reefs to recover is lost. Even with the meanwhile inevitable global warming of 1.5 degrees, the World Biodiversity Council IPBES expects that 70 to 90% of the corals will be lost, at two degrees it would be 99 percent: only adapted coral remnants could be retained.

Coral bleaching in connection with other stressors such as acidification of the oceans or the entry of pollutants or trace substances , including above all nitrogen or phosphorus, increases the mortality of corals worldwide: Large-scale coral bleaching can be the cause and visible indicator of massive coral death.

For many coral banks, a tipping point threatens to be exceeded, from which they are irretrievably lost.

Bleaching process

Hard corals live in symbiosis with zooxanthellae , which are also responsible for the color of the corals. The zooxanthellae live endosymbiotically in the cells of the coral polyps . Like a kind of kidney, they are able to eliminate pollutants in the cells of the polyps, but above all they carry out photosynthesis and supply the corals with glucose , glycerol and amino acids . Corals are also filter feeders , i. H. They filter microplankton from the sea water, but in the nutrient-poor warm tropical waters the supply of photosynthetically formed nutrients is essential for them, their share in the total nutrient supply can amount to up to 90% (→  reef paradox ).

This symbiotic relationship is sensitive to a number of stressors, including regional stressors such as various environmental toxins, excessive exposure to the sun or an abrupt change in the salinity of the seawater, and global stress, especially heat stress . The zooxanthellae begin when the temperature of the sea water is too high - in the case of the species Oculina patagonica the threshold is e.g. B. at 29 ° Celsius - to lose their ability to photosynthesize (→  photoinhibition ). This leads to oxidative stress , which is toxic to the host cells. As a result, the zooxanthellae are repelled by the corals, which also lose their color and "fade". After bleaching, some corals also look pastel-like blue, yellow or pink instead of chalk white, the cause of this being the proteins formed by the corals.

At what temperature threshold this disassociation of corals and zooxanthellae occurs depends u. a. on the species involved. Delicate, branched corals are particularly prone to higher temperatures, including Seriatopora , Stylophora , Pocillopora , Acropora, and Montipora . The more resistant massive genera include Porites or Goniopora . There is also a great variety of species among zooxanthellae; Zooxanthellae belonging to different clades are often found in the same coral species. More heat-tolerant zooxanthellae are found mainly in the very warm Persian Gulf , while more sensitive species are at home in the cooler neighboring Red Sea . The extent to which the type of zooxanthellae influences the nutrient supply, growth or heat resistance of the community is the subject of research.

Recent studies show that the availability of nutrients, especially nitrogen and phosphorus, also has an important influence on the symbiosis. In a reef near the Florida Keys , a major cause was human nitrogen input into the oceans, for example from washed-out manure or sewage. A disturbed phosphorus-nitrogen ratio probably made the corals vulnerable to thermal stress.

Corals cannot survive permanently without zooxanthellae. They die if the zooxanthellae do not return within a period of about eight weeks (the exact period is species-specific).

After being bleached, coral reefs may regenerate through repopulation. With certain corals this is possible within 10 to 15 years, with old reefs this process takes many decades. There must be no further coral bleaching or any other disturbance of the recovery phase. Factors such as water pollution , overfishing, and disease slow coral reef recovery.

causes

By the man-made global warming , it also comes to a permanent warming of the oceans worldwide. As a result, entire reefs can fall victim to coral bleaching and large areas can die. In addition, the increased water temperatures not only lead to coral bleaching, but also reduce the reproductive capacity of the surviving corals.

In the Pacific, for example, the El Niño weather phenomenon also creates exceptionally high water temperatures: the coral bleaching observed here in 2016 was the strongest ever recorded: 55% of the reefs were severely damaged, while the two previous bleaching in 1998 and 2002 had only 18% . In 2016, 93% of all reefs were affected. If the bleaching continues for a long time, the reefs die.

If the earth and the oceans continue to warm up, the frequency, intensity and duration of phases of unusually high ocean temperatures increase ( →  marine heatwaves ); the frequency of years with heat stress has increased significantly: 40 years ago, the probability of its occurrence was once every 25 to 30 years, now (2019) it is once every six years. As a result, the period between two coral bleaching is becoming too short for recovery in more and more regions. The frequency and intensity of bleaching events is increasing, although - possibly as a result of the death of particularly temperature-sensitive coral species - since the 2000s, bleaching has tended to only occur at a higher water temperature. If greenhouse gas concentrations continue to rise in the earth's atmosphere, all reefs worldwide could be affected by at least one bleaching per decade by the end of the century. If coral reefs are to continue to exist in the future, measures are necessary to rapidly limit global warming.

The continuing and increasing global acidification of the oceans, also in the course of man-made global warming and the increasing CO2 concentration in the atmosphere, makes it difficult for coral shells and skeletons to form limestone: Approx. 1/3 of the additional carbon dioxide contained in the earth's atmosphere is absorbed by the oceans, where carbonic acid is formed; the pH value of the sea ​​water has already fallen on average from 8.25 to 8.1, which equates to an increase of more than 25%. The average acidity of seawater threatens to drop to around 7.5 by 2100 if no drastic climate protection measures are taken.

According to the US maritime authority NOAA , around 4,000 to 6,000 tons of sunscreen products enter coral reefs every year : Their entry into the water on coastal bathing beaches , especially with the chemicals they contain, oxybenzone , octocrylene and parabens , which protect people from UV radiation and " Diving and snorkeling paradises "was identified as promoting coral bleaching; The Pacific island state of Palau banned sunscreens with the above-mentioned active ingredients even in small quantities from January 1, 2020 due to the potential for damage; In May 2018, the US state of Hawaii passed a sun milk ban from 2021. Octocrylene is found in four out of five European sun protection products.

Observed coral bleaching (selection)

Coral bleaching off Réunion ( Acropora , January 2006)

The phenomenon of coral bleaching is not new and was already observed in the 1970s: At that time, however, the event only occurred temporarily and locally, after heavy rainfall or during long-lasting low water . From 1997 bleaching events were observed for the first time worldwide for over a decade.

In 1998, particularly strong coral bleaching was observed during the El Niño phase in the Indian Ocean and the western Pacific ; Over a large area, the temperature of the water there was 1 to 3 ° C above average for months. In the Maldives, 98% of the corals bleached near the sea ​​surface during this period .

Further coral bleaching observed globally occurred in 2010 and 2015 to 2017; between 2014 and 2017, the longest and most extensive coral bleaching that has been observed to date occurred; coral reefs were affected worldwide.

A study by the Japanese Ministry of the Environment from January 2017 also reported 70.1% coral bleaching for the Sekiseishoku lagoon (largest coral reef in Japan , -> Iriomote Ishigaki National Park ) due to increased water temperatures.

Great Barrier Reef (Australia)

At coral bleaching in 1998 and 2002 in the world's largest coral reef, the Australian Great Barrier Reef ("Great Barrier Reef", the name also gives an indication of the importance of coral banks for coastal protection and as an obstacle to (commercial) shipping ), were about 50 up to 60% of the reefs there were affected, 18% were seriously damaged, about 5% died; Among the coral bleaching observed globally between 2014 and 2017, 93% of the reefs in the Great Barrier Reef were bleached and 55% were severely damaged. In 2020, the most extensive bleaching of the last five years was observed here: for the first time, all three areas of the reef, including the northern, central and now also large parts of the southern, were affected, while according to the Australian Meteorological Office, the area was the warmest month in terms of water surface temperature in February 2020 120 years behind me.

consequences

Dead coral limestone skeleton ("Bio- Erosion ", May 1974)

When corals die, biodiversity declines : entire ecosystems or food chains can collapse; Coral species can disappear and with them fish that depend on them as a source of food or need them as a place to raise their offspring.

Fishers lose their livelihoods and livelihoods; Diving tourism as a source of income disappears (the annual turnover of reef tourism is estimated at 9.6 billion dollars) as well as the protection of the respective coast from waves : the financial damage caused by floods and storms in affected coastal regions, e.g. B. doubles or triples if one meter of reef height is lost.

Possible recovery

Corals may regenerate after bleaching. With certain corals, which are particularly good at repopulating and growing rapidly, reefs can recover in a period of ten to fifteen years after bleaching; with old reefs this process takes many decades. During this period, however, there must be no further coral bleaching or other further disturbance of the recovery phase, which is judged to be an unrealistic assumption in view of the continuing global warming.

Countermeasures

Example of mineral deposition : Aragonite deposition in a water pipe (" Karlsbader Sprudelstein")

Attempts are being made to establish new corals on artificial coral reefs such as sunken ships, airplanes, car tires or steel structures: The artificial Osborne reef made from car tires in the USA has, however, developed into an ecological fiasco : The tires were torn from their anchoring and destroyed healthy reefs.

The artificial reefs of the Biorock technology , on the other hand, seem very promising: Here an attempt is made to create a substructure for artificial coral reefs by means of electrolytically triggered separation of minerals dissolved in the seawater on 3D metal grids with subsequent chemical aggregation . The licensed process was developed by the architect Wolf Hilbertz . With the scientist Tom Goreau , he founded the non-profit organization Global Coral Reef Alliance (" Global Coral Reef Alliance ") in 1990 .

The very committed Reef Ball Foundation should also be mentioned here: The organization was founded by Todd Barber after he had developed a patented process in 1993 with which reef balls (concrete structures) could be used for reef formation and for coastal protection.

Some states have established national parks in their coastal seas and banned all coral-damaging activities there. Healthy corals are also more resistant to temperature changes.

Of key importance is the rapid and comprehensive climate change worldwide with the achievement of 2015 in Paris at the World Climate Conference COP 21 adopted 1.5-degree target and the transition to climate neutrality in the industrialized countries and 2030 as well as the termination of illegal fishing in the affected zones like global overfishing .

literature

  • Madeleine JH van Oppen, Janice M. Lough: Coral Bleaching: Patterns, Processes, Causes and Consequences (=  Ecological Studies . Volume 233 ). Springer, 2018, ISBN 978-3-319-75392-8 .
  • TF Goreau, NI Goreau, TJ Goreau: Coral and Coral Reefs , in Biology of the Seas, 1991, Spektrum Akad. Verl., ISBN 3-89330-753-2
  • Sue Wells, Nick Hanna, The Greenpeace Book of Coral Reefs , CH Beck Verlag, ISBN 978-3-406-36797-7

Web links

Wiktionary: Coral bleaching  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ a b c d e f g If the corals die, the coasts die. Accessed April 12, 2020 (German).
  2. ↑ Terry P. Hughes et al. a .: Spatial and temporal patterns of mass bleaching of corals in the Anthropocene . In: Science . tape 359 , no. 6371 , January 5, 2018, p. 80–83 , doi : 10.1126 / science.aan8048 (English).
  3. ↑ Claudia Gack: Coral death . In: Spectrum Lexicon of Biology. Retrieved April 11, 2020 .
  4. ↑ a b c Terry P. Hughes et al.: Coral reefs in the Anthropocene . In: Nature . May 21, 2017, doi : 10.1038 / nature22901 .
  5. ^ Corals: Zooxanthellae ... What's That? In: NOAA Ocean Service Education. National Oceanic and Atmospheric Adminisration, accessed June 14, 2019 .
  6. ^ A b c d Paul Marshall, Heidi Schuttenberg: A Reef Manager's Guide to Coral Bleaching . 2006, ISBN 1-876945-40-0 , Chapter 4: A Review of the Causes and Consequences ( noaa.gov ).
  7. ↑ A. Kushmaro et al .: Effect of temperature on bleaching of the coral Oculina patagonica by Vibrio AK-1. : "Laboratory aquaria experiments demonstrated that Vibrio AK-1 caused rapid and extensive bleaching of Oculina patagonica at 29 ° C, slower and less complete bleaching at 25 ° C and 2O ° C, and no bleaching at 16 ° C." (on-line)
  8. ↑ a b c d e f Coral Bleaching and the Great Barrier Reef. (PDF) (No longer available online.) Great Barrier Reef Marine Park, archived from the original on June 8, 2011 ; accessed on February 14, 2016 .
  9. ↑ Claudio Richter: Coral reef ecosystem - Treasury of the seas . In: G. Hempel, K. Bischof, W. Hagen (Eds.): Fascination Marine Research . 2017, doi : 10.1007 / 978-3-662-49714-2_29 .
  10. ^ Luke A. Morris, Christian R. Voolstra, Kate M. Quigley, David G. Bourne, Line K. Bay: Nutrient Availability and Metabolism Affect the Stability of Coral-Symbiodiniaceae Symbioses . Review, Special Focus: Microbes in Biogeochemical Cycles during Climate Change. In: Trends in Microbiology . August 2019, doi : 10.1016 / j.tim.2019.03.004 .
  11. ^ Brian E. Lapointe, Rachel A. Brewton, Laura W. Herren, James W. Porter, Chuanmin Hu: Nitrogen enrichment, altered stoichiometry, and coral reef decline at Looe Key, Florida Keys, USA: a 3-decade study . In: Marine Biology . tape 166 , no. 8 , July 15, 2019, ISSN  1432-1793 , p. 108 , doi : 10.1007 / s00227-019-3538-9 .
  12. ↑ Great Barrier Reef: 93% of reefs hit by coral bleaching . In: The Guardian , April 19, 2016, accessed April 19, 2016.
  13. ↑ Terry P. Hughes et al .: Spatial and temporal patterns of mass bleaching of corals in the Anthropocene . In: Science . January 2018, doi : 10.1126 / science.aan8048 . See also the article: Warning signal from the ocean - coral bleaching follows at ever shorter intervals. In: Spiegel-Online. January 4, 2018, accessed March 29, 2019 .
  14. ↑ Terry P. Hughes et al: Global warming transforms coral reef assemblages . In: Nature . April 2018, doi : 10.1038 / s41586-018-0041-2 . See also: Volker Mrasek: Danger from heat waves - corals before collapse. Deutschlandfunk, April 19, 2018, accessed on March 7, 2019 .
  15. ↑ S. Sully et al. a .: A global analysis of coral bleaching over the past two decades . In: Nature Communications . No. 1264 , 2019, doi : 10.1038 / s41467-019-09238-2 .
  16. ^ PP Wong, IJ Losada, J.-P. Gattuso, J. Hinkel, A. Khattabi, KL McInnes, Y. Saito, A. Sallenger: Coastal systems and low-lying areas . In: CB Field et al. a. (Ed.): Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change . 2014, 5.4.2.4. Coral reefs.
  17. ↑ Terry P. Hughes et al .: Global warming and recurrent mass bleaching of corals . In: Nature . tape 543 , 2017, p. 373-377 , doi : 10.1038 / nature21707 .
  18. ↑ "Ocean acidification is the bad little brother of global warming" - AWI. Retrieved April 12, 2020 .
  19. ↑ a b Alina Schadwinkel: Sun creams bleach corals. In: The time . November 1, 2018, accessed April 11, 2020 .
  20. ↑ Mirabelle MP Tsui, James CW Lam, TY Ng, PO Ang, Margaret B. Murphy: Occurrence, Distribution, and Fate of Organic UV Filters in Coral Communities . In: Environmental Science & Technology . tape 51 , no. 8 , April 10, 2017, ISSN  0013-936X , p. 4182-4190 , doi : 10.1021 / acs.est.6b05211 .
  21. ↑ Chang-Beom Park, Jiyi Jang, Sanghun Kim, Young Jun Kim: Single- and mixture toxicity of three organic UV-filters, ethylhexyl methoxycinnamate, octocrylene, and avobenzone on Daphnia magna . In: Ecotoxicology and Environmental Safety . tape 137 , March 2017, ISSN  1090-2414 , p. 57-63 , doi : 10.1016 / j.ecoenv.2016.11.017 , PMID 27915143 .
  22. ↑ Alexandra Witze, Corals worldwide hit by bleaching . In: Nature (2015), doi: 10.1038 / nature.2015.18527 .
  23. ↑ Coral Bleaching During & Since the 2014-2017 Global Coral Bleaching Event - Status and an Appeal for Observations. NOAA Coral Reef Watch, March 19, 2018, accessed May 24, 2019 .
  24. ↑ a b Michael Slezak: Great Barrier Reef: 93% of reefs hit by coral bleaching . In: The Guardian . April 19, 2016, ISSN  0261-3077 ( theguardian.com [accessed April 10, 2020]).
  25. ↑ Great Barrier Reef - Horror at the true extent of coral bleaching. In: The world . March 25, 2016, accessed April 12, 2020 .
  26. ↑ Japan's largest coral reef is dying. In: Spiegel Online . Retrieved October 10, 2017 .
  27. ↑ a b Heavy bleaching on the coral reef. In: Badische Zeitung . April 11, 2020, accessed April 11, 2020 .
  28. ↑ Graham Readfearn: Great Barrier Reef's third mass bleaching in five years the most widespread yet. In: The Guardian . April 6, 2020, accessed on April 12, 2020 .
  29. ↑ NOAA Photo Library / National Undersea Research Program (NURP) Collection / Seascapes / Coral Reefs / nur03009. March 10, 2007, accessed April 11, 2020 .
  30. ↑ Terry P. Hughes et al .: Global warming and recurrent mass bleaching of corals . In: Nature . tape 543 , 2017, p. 373-377 , doi : 10.1038 / nature21707 .