CORAL BLE ACHING
F7
What
What Is It?
BENCHMARK
Coral bleaching occurs when the coral's symbiotic
No mass mortality of corals (including
zooxanthellae (single-celled algae) are released from
disease outbreaks over six months
the original host coral due to stress (e.g., unusually
after
after the event).
high or low water temperatures, high or low salinities,
or excessive sedimentation). The coral loses its
TA R G E T
pigment (color) when the pigment-rich zooxanthellae
No increase in the frequency of
No
are expelled. Coral bleaching can be characterized
occurrence of large bleaching events
in terms of bleaching intensity (e.g., pale to fully
(last decade had three events in
bleached) and extent.
10 years in the MAR). This requires
Bleached tissue may appear white (translucent) or
some acclimatization or adaptation of
pale, but live polyp tissue can still be seen above
corals (or reef systems) to increasing
the skeleton. Bleaching is not always uniform; it may
temperatures.
result in a mottled appearance.
RED FL AG
Corals are highly sensitive to changes in water
Coral bleaching prevalence >10%
temperature, with increases of only 1 to 2°C having
(average % of corals exhibiting
potentially lethal effects. Temporary bleaching does
bleaching). Corals should be monitored
not always kill coral polyps; they often regain their
for about a year after bleaching to track
pigmentation after several weeks or a few months.
However, polyps will die if the stress lasts for an
subsequent effects on mortality, coral
extended period of time or is very severe.
cover, etc.
cover,
Mass bleaching events -- which are almost always
associated with elevated sea surface temperatures
Why Do We Measure It?
(SST), sometimes in combination with elevated light
levels (due to calm seas) -- were unknown before
Bleaching is an important signal of coral condition.
197923. Human-induced global warming is believed
We are most concerned with mass coral bleaching
events related to elevated SST, especially because the
to be responsible for recent increases in sea surface
intensity and frequency of such events have increased
temperature, with prediction models for the next 100
over the last decades and will likely continue to pose
years suggesting that the warming trend will continue
a threat.
and that bleaching events will become more frequent
and more extreme24.
Bleaching does not necessarily result in coral mortality.
Corals can recover, depending on the severity of the
Melanie McField / WWF
bleaching. Bleaching can have the following non-lethal
deleterious impacts: inhibition of the coral's ability to
recover from small-scale tissue damage, increase
in partial or total mortality, reduced reproductive
success, and increased susceptibility to disease and
other stresses.
Severe or prolonged bleaching may result in diminished
reef growth and the transformation of reef-building
communities to alternate, non-reef-building states.
How Do We Measure It?
How
Coral bleaching can be assessed in terms of degree
of bleaching (percent of bleached tissue within each
colony) and prevalence of bleaching (the percent
60