WORLD ATLAS OF CORAL REEFS
variations in other areas. Such adaptation is clearly seen
in the reefs of the Arabian Gulf (Chapter 9). Some observations of the 1998 event showed local-scale survival of
corals in a few areas of reef flats and lagoons even in the
most hard-hit regions. These areas are likely to be subject
to more extreme temperatures on a regular basis, from the
reduced water circulation and exposure to solar radiation
and/or cold conditions in these areas. It remains to be seen
whether larvae from these corals can recolonize reefs where
more sensitive corals have died, or whether there is indeed
sufficient genetic resilience within these species to adapt
to the continuing increases in temperatures predicted under
current models of climate change.
Further concerns compound the problems from rising
sea surface temperatures. Corals may be placed under
additional stress by the projected increases in concentrations of atmospheric carbon dioxide. It is believed that
the presence of aragonite in surface waters will be reduced
by such increases, and aragonite is an important mineral
component of the coral skeleton. Lower concentrations
will reduce calcification rates and skeletal strength, which
in turn may lead to reduced rates of reef growth and
weaker skeletal structures.
All reef development is the result of coral growth
out-pacing natural processes of erosion, from bioeroding
organisms and also physical processes such as storms.
Slower coral growth rates and weaker skeletal structures
may shift the balance of many reefs from that of gradually accreting structures to that of gradually eroding
structures, and this change will be further compounded
by increasing rates of sea-level rise. The impacts of this
will affect not only the coral reef ecosystems, but also
the long-term survival of island peoples and even of
entire nations.
Direct physical impacts
The threats described thus far are predominantly indirect in
their action, their impacts working by degrees from subtle
shifts to more extreme or longer-term impacts. Humans are
occasionally far more destructive to coral reefs. Dynamite
fishing has already been mentioned, and is an activity which
leads to the instantaneous destruction of small patches of
reef Similar damage is wrought at various scales through a
number of activities, as listed in Table 2.2.
Many of these activities are highly localized. But,
depending on their location, even small impacts may be
important. This is particularly true of popular tourist areas.
There have been a number of cases now where insurance
companies have been forced to compensate extremely
high sums per square meter of reef damaged where ships
have grounded in popular recreational areas. Likewise the
Coral disease
The first reports of disease affecting scleractinian
corals did not appear until the early 1970s when
it was quite shocking to read of rapid tissue degradation occurring in reef-buildmg corals. In all cases
there was loss of living tissue and. in many, complete
mortality of colonies occurred. Since then increasingly
frequent observations of coral diseases have been
reported. Diseases have been observed on 106 species
of coral lincluding some soft corals! on reefs in 5i
countries around the world. Coral diseases appear to
be particularly prevalent in the Caribbean.
Two patterns of disease were described at first,
both being characterized by a distinctive band, a few
millimeters to a few centimeters wide, of diseased
tissue separating healthy tissue from exposed dead
coral skeleton. Black band disease was observed
affecting a variety of massive Caribbean corals,
principally species of Montastrea and Diptoria,
whereas white band disease was noted on branching
acroporid corals.
These bands of infected tissue move across and
progressively destroy coral tissue at rates of several
millimeters per day.
Since the 1970s a plethora of conditions have been
described as new coral diseases, and many claims
have been made for the decline in coral reefs due to
disease. The exact causes of most of these conditions
remain unclear. Only two have been unquestionably
linked to a pathogen - aspergillosis la disease of
gorgonians in the Caribbean! and white plague
type II. Part of the problem is undoubtedly the difficulty of finding and identifying pathogens in marine
organisms, combined with the difficulty of linking any
such pathogens to specific conditions.
Despite the uncertainty of the causal agent, it is
commonly accepted that white band disease has
been a major contributor to the massive decline in
Caribbean acroporid corals. For example, palaeological studies in Belize documented a disappearance
of acroporid corals from 70 percent of the coral
canopy, and replacement by species of Agaricia,
between 1985 and 1996. While the reasons for the
variations in other areas. Such adaptation is clearly seen
in the reefs of the Arabian Gulf (Chapter 9). Some observations of the 1998 event showed local-scale survival of
corals in a few areas of reef flats and lagoons even in the
most hard-hit regions. These areas are likely to be subject
to more extreme temperatures on a regular basis, from the
reduced water circulation and exposure to solar radiation
and/or cold conditions in these areas. It remains to be seen
whether larvae from these corals can recolonize reefs where
more sensitive corals have died, or whether there is indeed
sufficient genetic resilience within these species to adapt
to the continuing increases in temperatures predicted under
current models of climate change.
Further concerns compound the problems from rising
sea surface temperatures. Corals may be placed under
additional stress by the projected increases in concentrations of atmospheric carbon dioxide. It is believed that
the presence of aragonite in surface waters will be reduced
by such increases, and aragonite is an important mineral
component of the coral skeleton. Lower concentrations
will reduce calcification rates and skeletal strength, which
in turn may lead to reduced rates of reef growth and
weaker skeletal structures.
All reef development is the result of coral growth
out-pacing natural processes of erosion, from bioeroding
organisms and also physical processes such as storms.
Slower coral growth rates and weaker skeletal structures
may shift the balance of many reefs from that of gradually accreting structures to that of gradually eroding
structures, and this change will be further compounded
by increasing rates of sea-level rise. The impacts of this
will affect not only the coral reef ecosystems, but also
the long-term survival of island peoples and even of
entire nations.
Direct physical impacts
The threats described thus far are predominantly indirect in
their action, their impacts working by degrees from subtle
shifts to more extreme or longer-term impacts. Humans are
occasionally far more destructive to coral reefs. Dynamite
fishing has already been mentioned, and is an activity which
leads to the instantaneous destruction of small patches of
reef Similar damage is wrought at various scales through a
number of activities, as listed in Table 2.2.
Many of these activities are highly localized. But,
depending on their location, even small impacts may be
important. This is particularly true of popular tourist areas.
There have been a number of cases now where insurance
companies have been forced to compensate extremely
high sums per square meter of reef damaged where ships
have grounded in popular recreational areas. Likewise the
Coral disease
The first reports of disease affecting scleractinian
corals did not appear until the early 1970s when
it was quite shocking to read of rapid tissue degradation occurring in reef-buildmg corals. In all cases
there was loss of living tissue and. in many, complete
mortality of colonies occurred. Since then increasingly
frequent observations of coral diseases have been
reported. Diseases have been observed on 106 species
of coral lincluding some soft corals! on reefs in 5i
countries around the world. Coral diseases appear to
be particularly prevalent in the Caribbean.
Two patterns of disease were described at first,
both being characterized by a distinctive band, a few
millimeters to a few centimeters wide, of diseased
tissue separating healthy tissue from exposed dead
coral skeleton. Black band disease was observed
affecting a variety of massive Caribbean corals,
principally species of Montastrea and Diptoria,
whereas white band disease was noted on branching
acroporid corals.
These bands of infected tissue move across and
progressively destroy coral tissue at rates of several
millimeters per day.
Since the 1970s a plethora of conditions have been
described as new coral diseases, and many claims
have been made for the decline in coral reefs due to
disease. The exact causes of most of these conditions
remain unclear. Only two have been unquestionably
linked to a pathogen - aspergillosis la disease of
gorgonians in the Caribbean! and white plague
type II. Part of the problem is undoubtedly the difficulty of finding and identifying pathogens in marine
organisms, combined with the difficulty of linking any
such pathogens to specific conditions.
Despite the uncertainty of the causal agent, it is
commonly accepted that white band disease has
been a major contributor to the massive decline in
Caribbean acroporid corals. For example, palaeological studies in Belize documented a disappearance
of acroporid corals from 70 percent of the coral
canopy, and replacement by species of Agaricia,
between 1985 and 1996. While the reasons for the