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temperature, corals can bleach (Figure 9.1). Bleaching occurs
when the corals lose their symbiotic single-celled algae, the zoo-
xanthellae, which photosynthesize and provide food to the cor-
als, and in turn receive protection and the nutrients needed for
photosynthesis. Bleached corals appear white. Zooxanthellae
are sensitive to stresses including temperature changes, and
when they die or leave, bleached corals are usually unable to
meet their energy requirements by filter feeding alone. In some
cases, zooxanthellae return and the coral will survive. Coral
death by bleaching and diseases due to increased heat and irra-
diation, as well as decreased calcification caused by ocean acidi-
fication (discussed later), are among the most important threats.
Since the 1980s, major bleaching events have increased around
the globe—for example, in 1998, 80% of the coral reefs in the
Indian Ocean bleached, causing 20% of them to die.
Reef recovery is thought to depend on arrival of larvae from
distant, interconnected reefs. Observations of relatively rapid
recovery of corals following a mass bleaching event suggests
Figure 9.1 Coral Bleaching (photo from NOAA)
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