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Some of the factors that are thought to cause the bleaching are elevated sea temperatures, exposure to excessive irradiance and lowered salinity. However, when corals approach their upper thermal limits, even small additional doses of ultra-violet light or other sunlight spectra can cause them to bleach. In some parts of the world coral bleaching has occurred every three to four years since the late ´70s. The subsequent research has led to increased knowledge about the event but much is still unsubstantiated.

Bleaching and its Extent

Bleaching is often referred to as the whitening of corals. However, bleaching has now been observed in just about all marine organisms that host zooxanthellae.

Many marine invertebrates such as most species of hard and soft corals, sea anemones, zoanthids (related to hard corals), giant clams, some sponges, and foraminifera have a symbiotic relationship with types of algae known as zooxanthellae. The invertebrates host these photosynthetic algae within their tissues.

Bleaching usually occurs when environmental stress causes the host species to suffer a loss of zooxanthellae. However, it can also occur when the host retains the zooxanthella but the alga expels its brownish-green pigmentation.

Psychedelic colours can also result during partial bleaching. Although hosts commonly turn white when fully bleached, there are some host species that turn pink, yellow, purple, blue or iridescent green when partially bleached.

Of the invertebrate hosts, it is the corals and giant clams that appear to rely most heavily on zooxanthellae for the production of energy for metabolic processes. More than 90 per cent of their energy requirements, for some coral and giant clam species, are provided through the process of photosynthesis.

It is not known if invertebrates, such as coral, expel the zooxanthellae, or if the algae leave their host, says a Research Scientist from the Australian Institute of Marine Science (AIMS), Katharina Fabricius. 'We don't know what initiates bleaching. Most people think it is the animal host, but at this stage the data are insufficient,' Dr Fabricius said.

Reports on coral bleaching have attested to the fact that the 1997-98 event has been the most geographically extensive bleaching event scientifically recorded.

Sites that have been identified as being affected by the phenomenon are in Kenya, the Netherlands Antilles, Cayman Islands, Florida Keys, the Yucatan coast, Baja California, Galapagos Islands, French Polynesia, Christmas Island, Lord Howe Island and the Great Barrier Reef. Dozens of other sites have been affected also.

Surveys of the Great Barrier Reef, conducted by Ray Berkelmans (see What's Out There?), indicate that 88 per cent of inshore reefs from Gladstone to Cape York are bleached to some extent (25 per cent severely bleached) and around 28 per cent of mid-shelf reefs have been affected (5 per cent severely).

It is the inshore reefs that have been most susceptible during this year's event. Most bleaching occurs in the top few metres of the water column due to the temperatures being highest at the surface. Because most inshore reefs are in shallower waters than mid-shelf reefs, more of the inshore reef corals are affected.

Speculative Causes

Links between the Great Barrier Reef bleaching event and this year's El Niño Southern Oscillation (ENSO) event as well as to global warming have been drawn. However, to date there are no data available to support or reject these links.

Director of Research and Monitoring at the Great Barrier Reef Marine Park Authority Jon Brodie says regional weather patterns experienced in north Queensland this summer are the opposite to those expected during 'normal' El Niño years.

'Normally during an El Niño year the east coast of Australia experiences cooler waters and lowered rainfall. This year we experienced the reverse,' Mr Brodie said.

Mr Brodie also says while increased bleaching may be linked to global warming there is no conclusive evidence to prove this at present. Rises in sea water temperatures may be due to natural global climate or regional changes.

'We are at the peak of a warming phase in geological terms and sea temperatures are naturally still on the rise. Global climate change has occurred throughout the history of the earth,' Mr Brodie said.

'It is difficult to determine the difference between a natural sea temperature rise and an unnatural one.'

Although an agreement has not been reached on the cause of sea temperature rises, the general consensus appears to be that on the central Great Barrier Reef, in early 1998, a combined effect of high sea temperature and exposure to high irradiance caused widespread bleaching. In addition, lowered salinity caused extensive bleaching on inshore reefs.

In January, north Queensland was subject to floods that saw a deluge of fresh water run-off pour onto the inshore reefs between Townsville and Cooktown.

'These inshore reefs suffered up to 5 weeks of depressed seawater salinity due to flooding of major river systems. The low salinity level is likely to have exacerbated the severity of bleaching in this area,' Mr Brodie said.

On 13 January, salinities ranged from 19 to 26 parts per thousand (ppt) on the surface and 21 to 32 ppt at 3 metres depth (normal is 36 ppt). The water column gradually became better mixed but six weeks after the event, salinities were still depressed at around 33 ppt throughout Cleveland Bay.

In addition, three weeks after the floods an unusual hot spell occurred. This saw an exceedence in ocean maximum summer temperatures of 1-2°C, that is, from 30°C up to 31-32°C and the occasional peak of over 33°C.

'The mirror-calm seas which prevailed at the same time allowed unusually high transmission of light onto the reef and increased the exposure to irradiance. Therefore, corals became both heat and light stressed,' Mr Brodie said.

'Coral reef systems naturally live close to their thermal limit in summer. Therefore, it doesn't take much of a temperature rise to stress them and make them more susceptible to other factors that can contribute to bleaching.'

Different Tolerance Levels

Preliminary studies, such as those conducted by Hoegh-Guldberg and Salvat (1995), have shown that different species of coral show different tolerance levels.

The three hard coral genera looked at in this study were Acropora spp. (least tolerant), Pocillopora spp. (intermediately tolerant), and Porites spp. (most tolerant).

Results indicated those coral genera with fast growth rates and high metabolic rates, such as Acropora spp., are the most susceptible. The study also showed that Acropora spp. recovered less well, if at all, compared to 100 per cent recovery of Porites spp.

This would then suggest that mass bleaching has the potential to change the structure of coral communities, that is, tolerant genera or species may temporarily dominate.

Additionally, the study states that 'reef connectivity and larval supplies are also likely to play key roles in determining the extent to which particular reefs will recover from mass bleaching'.

Adaptation

The big question is whether or not corals and other marine invertebrates can adapt to the likely increases in sea water temperature predicted to occur due to global climate change.

'In the Red Sea and Persian Gulf corals exist in relatively high temperatures that average up to about 34°C in summer. This is 4°C higher than the average maximum summer temperatures normally experienced on the central Great Barrier Reef,' Mr Brodie said.

'Many of the coral species that live in these high temperatures are identical to those found on the Great Barrier Reef.

'The present reefs of the Red Sea, formed around the same time as the Great Barrier Reef, were colonised by coral larvae from the Indian Ocean. Therefore, the larvae had to adapt to the higher sea temperatures.

'Once the corals acclimatised to the average local maximum sea temperatures of 34°C there was no need for them to be any more robust or heat resistant than was required. So we now see that when the sea temperatures in these regions rise 1-2°C above the maximum average they too experience bleaching.'

The rate and the ways in which marine invertebrates may be able to adapt to bleaching episodes is not known. Preliminary studies do suggest though that the ecology of both corals and zooxanthellae and their interrelation needs to be studied further.

A study by Ware, Fautin and Buddemeier, 'Patterns of coral bleaching: modeling the adaptive bleaching hypothesis' (1996), suggests that bleaching is an adaptive mechanism that increases stress resistance.

They state that 'bleaching is not merely pathological, but is also adaptive, providing an opportunity for recombining hosts and algae to form symbioses better suited to altered circumstances.'

Dr Fabricius of AIMS has concerns about some assumptions expressed in the paper. She says the problem with their hypotheses is that there is no evidence that corals can simply take up more robust strains of zooxanthellae.

'Firstly, nobody has ever been able to show that corals can get infected later than in the very initial larval or, post-settlement, phases,' Dr Fabricius said.

'Secondly, zooxanthellae are asexual and thus may not be able to adapt rapidly. There obviously exist more temperature robust zooxanthellae, however they may have other disadvantages, otherwise corals would have selected for those already.'

While there may be uncertainty over the mechanism in the short-term, in the long-term Jon Brodie points to adaptation over longer periods. Some evidence suggests that corals and zooxanthellae have 'acclimatised' to the average temperatures of the local area.

'In the inshore areas of the southern Red Sea with the highest water temperatures, reefs are dominated by the species most tolerant to higher temperatures, for example Porites spp. and Siderastrea savignana, as well as encrusting coralline red algae,' he said.

The Future

An accurate picture of the level of recovery of affected corals from the 1998 mass bleaching event on the Great Barrier Reef could take at least six months. In 1982, coral bleaching started in early January but it took until September for the last vestiges of white coral to disappear.

Preliminary reports of some mid-shelf reefs indicate that the recovery rate of the bleached invertebrates is around 90 per cent. Surveys of inshore reefs are still being conducted and the results from these surveys should be available in a few months.

There are still no definitive answers as to why this mass bleaching event happened on such a large scale or what the short- and long-term consequences may be.

Many questions are being asked. For example, why do entire coral colonies bleach? Why do some corals have a mohawk or striped bleached appearance? Why do adjacent colonies of the same genus as those bleached, apparently subject to the same thermal environment, not bleach at all? Is bleaching a recent phenomenon? Are we in some way responsible for current episodes of bleaching?

As with the crown-of-thorns starfish phenomenon, the answers will not come easily and the research could go on for many years before theories that attempt to explain widespread bleaching are accepted.

Although the causes of the mass bleaching phenomenon, such as the rise in sea temperature (albeit, what causes that rise is questioned), are known, it is not yet known what the long-term consequences are. The answers to this question will not come easily and the debate will rage amongst scientists indefinitely.

Information provided by: http://www.gbrmpa.gov.au