Coral bleaching – will global warming kill the reefs?This topic is sponsored by the Australian Institute of Marine Science. Coral reefs are sensitive to environmental change. Recently, the frequency and distribution of coral bleaching have increased, and in 2002 the Great Barrier Reef experienced its worst case of coral bleaching on record. Key textImagine you are on a beautiful tropical island on the Great Barrier Reef. You put on your mask, fins and snorkel and jump into the water. But instead of the famous brightly coloured coral, brittle white branches spread as far as the eye can see. The coral is dead.As the world's oceans become warmer, this scenario is taking place more frequently, on the Great Barrier Reef and on coral reefs all over the world. Coral bleaching has never been more common or widespread, prompting the question: are rising sea temperatures killing the reefs? Corals are animals
The ancient Greeks mistakenly believed that corals were plants. Corals are actually animals, related to anemones and jellyfish. Corals consist of a limestone structure filled with thousands of small animals called polyps. Each polyp has a skeleton cup, tentacles with stinging cells, a mouth and a stomach. The tiny tentacles snatch at passing plankton for food, but for their main course, reef-building corals have devised a much more ingenious method to get fed. Algae called zooxanthellae live within each coral. In return for a safe sunny home, the zooxanthellae eat the nitrogen waste that the coral produces (nitrogen is very good for algal growth) and, like all plants, algae turn sunlight into sugars by the process of photosynthesis. The sugars produced by the zooxanthellae make up 98 per cent of the coral's food. So, without having to do any work at all, the coral is kept clean and well fed, and the zooxanthellae with their brilliant reds, oranges and browns give corals their colour. Importance of coral reefs
Coral reefs are important for many different reasons. Apart from protecting the shoreline from the damaging effects of the ocean, they provide habitats and shelter for many organisms and are the source of nitrogen and other nutrients essential for the food chain. This is why hundreds of thousands of marine species live in reefs. Many fisheries depend on the fish that spend the first part of their life in coral reefs, before making their way out to the open ocean. The Great Barrier Reef is especially important to the Australian economy and generates 1.5 billion dollars every year from fishing and tourism. In addition, the study of coral reefs can provide a history of past climates. Rising sea temperatures and coral bleaching Over the past one hundred years, the temperature of sea water in many tropical areas has been rising. For example, the Australian Institute of Marine Science has collated data showing that 2002 was the warmest year for water temperatures off northeast Australia since 1870. And there are predictions of a sea temperature rise of up to 1ºC within 50 years. Rising water temperatures block the photosynthetic reaction that converts carbon dioxide into sugar. This results in a build-up of products that poison the zooxanthellae. To save itself, the coral spits out the zooxanthellae and some of its own tissue, leaving the coral a bleached white. The bleached coral can recover, but only if cooler water temperatures return and the algae are able to grow again. Without the zooxanthellae, the coral slowly starves to death. Other causes of coral bleaching Apart from heat stress, other causes of coral bleaching may include:
Often coral reefs are exposed to a combination of these factors. Rate of coral bleaching Coral bleaching is a natural process. For thousands of years, fishermen have noticed mysterious whitening of the reefs. What is not natural is the rate at which coral bleaching is occurring. In 1998, the worst coral bleaching in 700 years struck the Great Barrier Reef, followed by an even worse bleaching only 4 years later. Massive areas of corals were affected all over the world. In Australia alone, the 2002 bleaching saw nearly 60 per cent of the reef suffer bleaching and, in the worst areas, 90 per cent of the coral was bleached. Can corals survive bleaching? In the past, rapid changes of climate have sometimes led to extinctions. The dinosaurs were wiped out 65 million years ago, and there have been several mass extinctions before and since. Does the coral have any hope? Australian scientists have found that corals contain a kind of sunblock called fluorescent pigments. These pigments form a kind of shield around the zooxanthellae and protect them from the harmful effects of sunlight at high temperatures. The pigments change the harmful UV and blue wavelengths in sunlight to softer, lower energy wavelengths like green and yellow.
Fluorescent corals survive bleaching events better than non-fluorescent corals, and every species of coral has some varieties that have fluorescent pigments. Over time, more fluorescent pigments may develop to protect the corals. Even though non-fluorescent varieties may die out, there will be some corals that have the fluorescent pigments and therefore may survive. If a channel of cold water constantly runs through the warm water, thus keeping the zooxanthellae cool, then those corals can survive. Another way in which corals can survive is just by being tougher than others, in the same way that some people are always healthy while other people are always sick. Protecting coral reefs Perhaps the best thing we can do is to protect the corals that do survive bleaching better than others. One way we can do this is by creating marine parks: areas protected in some way from souvenir hunters and activities such as fishing and boating. At the moment almost a third of the Great Barrier reef is protected from fishing. An important consideration is to locate marine parks in areas where the coral is resistant to bleaching. If the coral dies, eventually the fish and other marine species will die too.
The other thing we can do for the reef is to keep an eye on it. The reef has 2.4 million visitors every year, and these people go to places scientists can’t always get to. GBRMPA can use information from visitors about where they saw bleaching. And, just as importantly, where they didn’t see bleaching. This helps scientist to form an overall picture of which areas are vulnerable to bleaching. Though the reefs may not be doomed, there is definitely cause for concern. Some scientists have predicted that by 2030 massive and devastating coral bleaching events will occur every year. Other scientists believe that there is a future for the reefs, and that though the reefs may change, they will be there in some form or another. We can only hope that they are right.
Australasian Science May 2008, pages 34-35 Heat shock (by David Yellowlees and Bill Leggat) Examines gene expression of corals at high temperatures.
April 2008, page 10 Reef may shift south Describes research investigating the effect of global warming on location of reef species.
August 2007, page 43 Seeking clues to coral bleaching (by Katherine Munkres) Looks at the relationship between corals and algae in predicting the impact of climate change on the reefs.
June 2007, page 12 Coral disease linked to warming Confirms the relationship between a coral disease and rising ocean temperatures.
May 2007, page 12 Fat corals live longer Says that corals may find that being fat is the best way to survive global warming.
April 2007, page 13 Cold water corals chart heat Describes how cold water corals are providing a valuable insight into the temperature history of the waters off South Australia.
March 2007, pages 12 Coral stress without precedent Studies of fossilised coral reefs suggest that modern damage is worse than anything for thousands of years.
August 2006, page 8 Coral damage unprecedented Compares the species present in ancient and present day reefs.
May 2006, pages 23-24 Coral bleachings strike the world’s reefs (by Michael Perry) Describes the coral bleaching resulting from the hottest year on record.
March 2006, page 6 Corals kept cool in record heat Describes the ‘Shade Project’ to protect sections of the Great Barrier Reef from the effects of warming.
November/December 2004, pages 41-42 Algal shuffling helps corals cope with environmental change (by Madeline van Oppen, Angela Little and Bette Willis) Describes research into the symbiosis between reef-building corals and microscopic algae and how it helps corals acclimatise to environmental changes.
April 2004, pages 23-25 Time's almost up for the Great Barrier Reef (by Ove Hoegh-Guldberg and Hans Hoegh-Guldberg) Looks at the ecological and economic costs of coral bleaching on the Great Barrier Reef.
Ecos No. 124, 2005, page 35 A new bacterium may be associated with reef bleaching (by Steve Davidson) Describes complex interactions between a new temperature-sensitive bacterium, coral, algae and a coral-grazing marine worm.
No. 115, 2003, pages 37-41 Do sediments sully the reef? (by Steve Davidson) Investigates the effects of various land uses (eg, cattle and cane growing) on the corals of the Great Barrier Reef.
New Scientist 10 July 2008 Corals join frogs and toads as world's most endangered (by Catherine Brahic) Reports on the state of the world’s coral species.
13 September 2006 Most corals unable to adapt to warming oceans (by Roxanne Khamsi) Reports that less than one-quarter of coral species appear able to survive climate change by hosting new types of algae.
8 July 2006, pages 28-29 Intensive care heals damaged coral reefs (by Paul Marks) Reports on the damage to coral reefs caused by humans and the possibility of protecting them from further damage.
5 February 2005, page 17 Coral reefs create clouds to control the climate (by Alison George) Describes research showing that corals make a chemical that causes local cloud formation.
11 August 2004 Corals adapt to cope with global warming (by Jeff Hecht) Describes research suggesting that coral bleaching depends on the algal partner.
10 January 2004, pages 12-13 Sewage nutrients fuel coral disease (by Rachel Nowak) Discusses how nutrient-rich water, such as agricultural run-off and sewage, can destroy coral.
An introduction to coral, coral polyps and coral reefs.
The Great Barrier Reef: Designed to survive (built to last?) (Australian Academy of Science)
Transcript of a talk that looks at research into the resilience of corals to environmental change.
Australian Institute of Marine Science
Coral bleaching (Reef Futures, Australia)
Explains the causes of global climate change, the biology of coral bleaching, bleaching thresholds, coral adaptation to higher temperatures and recovery after bleaching events.
Global climate change and coral bleaching on the Great Barrier Reef (Queensland Department of Natural Resources and Mines, Australia)
This 2003 report includes background information about coral bleaching as well as possible scenarios for the future.
Australian Broadcasting Corporation (transcripts)
Coral bleaching: Current conditions report (Great Barrier Reef Marine Park Authority, Australia)
Summarises the current bleaching situation on the Barrier Reef.
electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Back to Basics: Electromagnetic radiation (Australian Academy of Science) and Electromagnetic Spectrum (NASA Goddard Space Flight Center, USA). fluorescent pigment. A pigment that absorbs light (electromagnetic radiation) at one wavelength and emits it at a different wavelength. Usually the emitted light has a lower energy than the light absorbed by the pigment. photosynthesis. The biochemical process in which green plants (and some microorganisms) use energy from light to synthesise carbohydrates from carbon dioxide and water. Photosynthesis can be shown as: CO2 + H2O + energy® [CH2O] + O2 plankton. Microscopic plants and animals that live near the surface of oceans and lakes. ultraviolet (UV). A form of electromagnetic radiation. UV radiation has shorter wavelengths than visible light and it therefore carries more energy. It is divided into three broad categories: A, B and C. UV-A has the longest wavelength and is the least damaging form, although sufficient exposure will cause sunburn. UV-B damages proteins in unprotected organisms and can cause cancer, while UV-C is extremely dangerous because it can cause mutations in DNA.
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This topic is sponsored by the Australian Institute of Marine Science.
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