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CORAL reefs have withstood some severe batterings in the past few decades.
From predation by starfish on the Great Barrier Reef to the permeating chill
of an abnormally cold influx of water in the Florida Keys, modern reefs
have, by and large, overcome these and other challenges. The potential agents
of destruction have mostly been local, affecting small sections of reef
or confined to a particular region. A new, more extensive phenomenon – coral
bleaching – now threatens to destroy the delicate balance of reefs worldwide.

Corals become bleached when the algae that normally live in their tissues
are expelled. The brilliant colour of corals is due to the presence of these
algae, and their loss often leaves the animal pale or white. When the algae,
or zooxanthellae, are absent for long periods, the coral dies. Many reefs,
from Easter Island to the Great Barrier Reef and the coasts of Central America,
showed some bleaching during the 1980s. Reefs on the Pacific coast of Panama
showed widespread damage in 1983. Peter Glynn, of the University of Miami,
blames the El Nino of 1982 to 1983, which raised the temperature of the
waters around the reef above 29 Degree C. Bleaching was more widespread
in 1987 and 1988, particularly in the Caribbean, causing alarm among marine
scientists.

During a survey of reefs near the Cayman Islands, my colleagues at the
Natural Resources Laboratory, Grand Cayman, and I found just how much damage
bleaching can do to the ecology of a reef. The phenomenon became obvious
early in October 1987 and continued well into the next year.

Twenty-eight of the 43 species of stony corals on the reef were affected.
The worst hit were the mountainous star coral, Montastrea annularis, the
delicate staghorn coral, Acropora cervicornis, and a variety of leafy or
saucer-shaped forms belonging to the genus Agaricia. The only important
corals to escape this bout of bleaching were the massive elkhorn corals,
Acropora palmata. Several species of sponges that also harbour zooxanthellae
and some sting hydrocorals of the genus Millepora also suffered from bleaching.

Not all species showed symptoms of bleaching at the same time. The first
six species to succumb initially grew pale and mottled. Within 7 days of
the first sighting, more than half the colonies of these six species were
white. Within 2 weeks, another 22 species were bleached or showed some signs
of bleaching. The extent of bleaching varied with depth, the shallower the
coral, the worse the bleaching. No coral below 85 metres was affected.

Zooxanthellae can re-enter their hosts if conditions do not deteriorate
too badly. But the longer a coral remains bleached, the slimmer its chances
of recovery. On the Cayman reefs, most of those colonies that had recovered
or were improving by March 1988 had already begun to show signs of recovery
the previous December. Some corals recovered within 3 weeks; others took
2 or 3 months.

The most badly affected mountainous star coral, 90 per cent of whose
colonies were bleached, was also the slowest to recover. By April 1988,
only half the colonies had fully recovered. Those that had perished were
quickly overgrown by Intense bleaching, recovery and later relapse of a
mountainous star coral filamentous algae. To make matters worse, many of
the bleached colonies had trouble capturing prey, perhaps being deprived
of the nutrients normally produced by the algae, their tentacles could not
function properly. Only after they had fully recovered did they begin to
feed normally. During this period, the reef corals in the Caribbean did
not grow, probably because of the interruption to their feeding.

The cause of bleaching is unclear. Most biologists attribute it to stress.
But stress comes in many forms: unusually warm water, prologed darkness,
excessive exposure to ultraviolet radiation, a drop in salinity or severe
changes in the weather. Many studies have shown that algae desert their
coral hosts when the temperature exceeds 30 Degree C. Studies around the
Florida Keys, Hawaii and Enewetok Atoll have shown that corals exposed to
prolonged warming, above 30 Degree C, for several weeks or more, lost their
algae permanently and died. One theory for the departure of zooxanthellae
is that, under such stress, coral tissues fail to provide the algae with
enough nutrients; another is that the coral releases substances that are
harmful to the algae.

So far, however, no one has been able to explain worldwide episodes
of bleaching satisfactorily. Some scientists have suggested that the latest
outbreaks in the Caribbean might be the result of unusually high temperatures.
Others believe that a combination of high temperature, high levels of ultraviolet
radiation and a decrease in salinity might be responsible.

Some faint patterns are beginning to emerge with each new bleaching
event. A severe episode of bleaching of the reefs off Jamaica last October
shared many features with previous episodes in the Caribbean. Last year,
however, the bleaching was far more intense. Those species that suffered
during 1987 and 1988 were hit again, and colonies that escaped the last
bleaching succumbed this time. A check of the temperature at the surface
of the water showed that it was higher than 30 Degree C from August to October.
In the 1987 incident, the water was already this warm by July and stayed
warm until well into December. One puzzling aspect of the most recent bleaching
was that it was largely confined to Jamaican reefs and did not spread elsewhere
in the Caribbean.

The fossil record provides some clues to the danger posed by sudden
changes in temperature. Most of the organisms that build reefs, or built
reefs in the past, are tropical or subtropical organisms. During some of
the mass extinctions of the Ordovician period, some 430 million years ago,
and the Late Devonian, around 350 million years ago, many marine organisms
became extinct. Most of the extinctions were among the animals of tropical
seas, with the near destruction of reef communities. The probable cause
in both cases was a sudden cooling of global temperature.

If global cooling could create a crisis among fossil reefs, perhaps
global warming will also cause havoc in today’s tropics. Most corals and
their algal partners flourish within a narrow temperature range of 25 to
29 Degree C. Yet those areas where bleaching was intense were subject to
temperatures only slightly above normal, although they stayed high for several
months. If such a small shift in water temperature is indeed responsible
for bleaching, then even a small degree of global warming could deal coral
reefs a fatal blow.

While the jury is out on the role that changing climate will play in
the health of marine ecosystems, scientists and government agencies are
monitoring reefs, hoping that the next episode of bleaching will provide
a better idea of the cause.

Joe Ghiold is a research associate at the Western Australian Museum
and assistant professor of geology and geophysics at Louisiana State University,
Baton Rouge.