Global Coral Bleaching 1997-1998

John Ogden jogden at
Tue Oct 13 10:23:23 EDT 1998

The following is a scientific consensus statement developed within the
Council of the International Society for Reef Studies (ISRS) following our
meeting in Perpignan, France in early September.  It will be published in
the Society's newsletter REEF ENCOUNTER in December 1998.


International Society for Reef Studies

October 15, 1998

ISRS Statement on Global Coral Bleaching in 1997-1998

During 1997-98, reports of coral bleaching from all the major
tropical oceans of the world suggested that this time period had
seen the most geographically widespread bleaching ever recorded,
with some areas (eg. Singapore, and the Andaman Islands) witnessing
extensive bleaching for the first time in recent history.  Coral
bleaching has been described in at least 32 countries and island
nations in 1997-98; with reports from sites in the Pacific, Indian
Ocean, Red Sea, Persian Gulf, Mediterranean and Caribbean recording
widespread bleaching. The bleaching response represents a loss of
symbiotic algae and/or their pigments such that the coral may pale
in color to a varying extent, or turn starkly white.  Paling of
some coral species is an observed seasonal phenomenon in the
Pacific, Indian Ocean, and also the Caribbean.  Where bleaching is
seasonal, or less severe, the likelihood of full recovery of
pigmentation is high.  In the present bleaching episode the
response has been exceptionally severe with a large number of
corals turning completely white and subsequently dying.

Mass bleaching normally occurs when seasonally maximal sea-surface
temperatures (SST) are exceeded.  The likely triggers of bleaching
are elevated SST and solar radiation.  Research has indicated that
these factors act in combination, rather than alone.  Additional
causes of bleaching such as extreme low tides and reduced
salinities have also been implicated at some sites in 1998.

The occurrence of bleaching at many locations has been patchy with
more severe bleaching recorded in shallow waters than at deeper
offshore sites.  Not only hard and soft corals, but also sea
anemones, zoanthids, giant clams, foraminifera and many other
zooxanthellate invertebrates are affected by the loss of their
symbiotic algae. Corals can recover from bleaching but death may
result if environmental stressors are extreme and/or prolonged. In
the Indo-Pacific fast-growing, branching corals are more
susceptible to bleaching than slow-growing boulder corals, leading
to a high mortality in the former.  Recovery of boulder corals has
been frequently recorded in 1998, often within 1-2 months of
initial bleaching.  In the Caribbean, however, greater
bleaching-related mortality has been shown in boulder and
plate-like corals rather than in branching species, which had
already suffered extensive mortality from storms, diseases and
terrestrial run-off.  In the Indo-Pacific the susceptibility of
different corals to bleaching can significantly affect coral
community structure and diversity, depress the rate at which the
reef builds up, and reduce habitat availability for other reef
species.  Previous cases of bleaching-induced mortality from 1993
in the Pulau Seribu (Java Sea) and from 1996 in the Similan Islands
(eastern Indian Ocean) have provided examples of community change. 
At both sites shallow parts of the reef have been temporarily
transformed from being a mixture of branching and boulder corals to
areas in which virtually only the boulder corals survive.  During
the current 1998 bleaching, one reef on the Australian Great
Barrier Reef has been so severely affected that even many of the
robust boulder corals (one of them dated as over 700 years of age)
were badly damaged or died.  Complete recovery of reefs following
severe bleaching is dependent on growth and fragmentation of
remaining corals, and on recruitment from stocks in the area.
Evidence shows that restoration of the reef to its former state may
be slow or, if interrupted by man-made change, may even be halted

Links have been made between the widespread incidence of coral
bleaching in 1997/98 with one of the strongest El Ninos of this
century.  While past and present coral bleaching events in many
parts of the Pacific appear to be closely matched to El
Nino-induced seawater warming, the connections are not clear-cut
for all locations in the Pacific.  Nor can bleaching in Indian and
Atlantic waters be directly linked to only El Nino phenomena.
Factors responsible for elevated SSTs in the Indian Ocean, for
example, are likely to be the result of a complex product of El
Nino-related, monsoonal and local oceanographic factors which are
superimposed on interdecadal patterns in climate variability.

Links between the apparent changing nature and frequency of the El
Nino phenomena and global climate change have also been made but
are the subject of controversial debate among climatologists with
many suggesting that present patterns reflect the natural
variability of the system rather than the effects of greenhouse
gas-induced warming.  For some of the tropical oceans significant
increases in SST have been observed over the last 50 years. 
Concerns about the potential effect of global change on future
frequencies of severe bleaching events are based on the narrow
upper margin of environmental tolerance in corals.  While corals
display impressive acclimation processes to changes in some
environmental parameters, it is not known whether they are able to
adapt or acclimatize at rates which match the projected rates of
background seawater temperature increase. Should seawater
temperatures rise, either as a result of greenhouse gas emissions
or natural variability in the ocean/atmosphere system, then we
might expect the incidence and severity of coral bleaching to
increase yet further in the future with the possibility of
substantial changes to the coral reef community structure.

The 1997-98 episode of worldwide bleaching is a major cause for
concern.  Although sea temperatures have returned to normal in many
tropical areas of the world the full extent of bleaching-induced
mortality may not be fully apparent for several months yet.

********** End of ISRS Statement **********

The International Society for Reef Studies (ISRS), consisting of
over 750 members in over 50 countries, was founded in 1981 for the
purpose of promoting the production and dissemination of scientific
knowledge and understanding of coral reefs, both living and fossil. 
The ISRS publishes the scientific journal CORAL REEFS and holds
periodic meetings around the world. Further information as well as
membership details can be found at:

For Further Information Contact:

John C. Ogden, Ph.D., ISRS President
Florida Institute of Oceanography
830 First Street South
St. Petersburg, FL  33701 USA
Tel: 727-553-1100
Email: jogden at


Terence C. Done, Ph.D., ISRS President Elect
Australian Institute of Marine Science
PMB 3, Townsville MC
Queensland 4810
Tel: 61-77-534-344
Email: t.done at

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