10 – The Future of Coral Reefs in a Rapidly Changing World
101
the months following an event if the stress involved is
mild and short-lived. But mortality (up to 100% of corals
over large areas of coral reef) occurs following intense
and long-lasting stress. This was seen in many parts of
the world in 1998, in which approximately 16% of corals
that were surveyed prior to the global cycle of bleaching
were estimated to have died by the end of 1998. This
particular figure is an average and conceals the fact that
in some oceans, for example, the Western Indian Ocean,
up to 46% of corals may have died.
Coral reefs in Australia have bleached repeatedly
over the past 30 years. Mass bleaching events occurred in
Australia in 1983, 1987, 1991, 1998, 2002 and in 2006, with
large sections of the GBR bleached in each case. Mortality rates on the GBR have been relatively low compared
to coral reefs elsewhere because conditions have been
relatively unchanged there so far. By contrast, a very
warm core of water sat above Scott Reef in the northwest waters of Australia for several months in 1998, resulting in an almost total bleaching and mortality of
corals down to 30 m and 95% of reef-building corals dying in the months that followed. Recent reports indicate
that recovery of these reefs has been very slow, primarily
because recruitment to these remote reefs is difficult and
rare. Coral disease, driven by pathogenic bacteria in addition to coral bleaching, is on the rise and may be connected to warmer than normal conditions. While coral
disease affects less than 5% of the population, the incidence of diseases such as ‘white syndrome’ and other
diseases (Fig. 10.4) are on the increase. While coral disease is currently not considered a major threat to coral
reefs in Australia and many parts of the Pacific, recent
experiences in the Caribbean, where coral disease decimated populations of Acropora corals, suggest that understanding and monitoring coral disease is important.
The strong relationship between coral bleaching and
sea temperature provides an opportunity to explore
how changes in sea temperature in the future might affect the incidence of mass coral bleaching. Past studies
have revealed that corals in a region have particular
thermal ‘thresholds’ for bleaching. These thresholds for
triggering bleaching are reliable to the point that they
predict which regions will experience coral bleaching
based on their sea-surface temperatures measured from
satellites. If these thresholds are compared to projections
of future sea temperatures trends (produced by the
Global Circulation Models, see above), it is possible to
estimate how the frequency and intensity of mass coral
bleaching and mortality will change over time. These
changes show that an increase of 2°C over pre-industrial
temperatures in the average sea temperature in tropical
and subtropical Australia (expected as a result of a doubling of CO 2 ) will lead to annual bleaching and a major
escalation in the number of mass mortality events.
The risk of mass bleaching has been examined for
the GBR and revealed that the return time of severe
mass coral bleaching in their models for low to moderate changes in the climate increased to the point
where the ability of reefs to recover is severely compromised. Deterioration of coral populations is likely
in most of the scenarios examined.
N OCEAN ACIDIFICATION
The increase in atmospheric CO 2 presents a second major
change for reef-building corals and other marine calcifiers.
Nearly half of the CO 2 that enters the atmosphere is absorbed by the ocean, where it reacts with water to form
carbonic acid (Equation 1). Carbonic acid dissociates into
bicarbonate and a proton (Equation 2). The protons released
by the entry of CO 2 into seawater then react with carbonate
ions to form additional bicarbonate ions (Equation 3). The
problem is that relatively small rises of CO 2 in the atmosphere at the pH range of seawater will cause a large decrease in the concentration of carbonate ions.
(1) CO 2 H 2 O m H 2 CO 3
(2) H 2 CO 3 m HCO 3
+ H
(3) CO 3
2 H
m HCO 3
The calcification rate of a range of marine organisms
as diverse as microalgae (coccolithophores), molluscs
(e.g. clams, pteropods) and corals is strongly dependent
on the concentration of carbonate ions in seawater. Naturally, the concentration is highest in the warmer tropical regions due to the reduced solubility of CO 2 in warm
v. cold water (i.e. less CO 2 dissolving into the ocean
means more carbonate ions). The pH of the ocean has
already decreased by 0.1 pH unit with the concentration
of carbonate ions decreasing as much as 30 Mmol kg
1
.
Corals were among the first organisms identified as
101
the months following an event if the stress involved is
mild and short-lived. But mortality (up to 100% of corals
over large areas of coral reef) occurs following intense
and long-lasting stress. This was seen in many parts of
the world in 1998, in which approximately 16% of corals
that were surveyed prior to the global cycle of bleaching
were estimated to have died by the end of 1998. This
particular figure is an average and conceals the fact that
in some oceans, for example, the Western Indian Ocean,
up to 46% of corals may have died.
Coral reefs in Australia have bleached repeatedly
over the past 30 years. Mass bleaching events occurred in
Australia in 1983, 1987, 1991, 1998, 2002 and in 2006, with
large sections of the GBR bleached in each case. Mortality rates on the GBR have been relatively low compared
to coral reefs elsewhere because conditions have been
relatively unchanged there so far. By contrast, a very
warm core of water sat above Scott Reef in the northwest waters of Australia for several months in 1998, resulting in an almost total bleaching and mortality of
corals down to 30 m and 95% of reef-building corals dying in the months that followed. Recent reports indicate
that recovery of these reefs has been very slow, primarily
because recruitment to these remote reefs is difficult and
rare. Coral disease, driven by pathogenic bacteria in addition to coral bleaching, is on the rise and may be connected to warmer than normal conditions. While coral
disease affects less than 5% of the population, the incidence of diseases such as ‘white syndrome’ and other
diseases (Fig. 10.4) are on the increase. While coral disease is currently not considered a major threat to coral
reefs in Australia and many parts of the Pacific, recent
experiences in the Caribbean, where coral disease decimated populations of Acropora corals, suggest that understanding and monitoring coral disease is important.
The strong relationship between coral bleaching and
sea temperature provides an opportunity to explore
how changes in sea temperature in the future might affect the incidence of mass coral bleaching. Past studies
have revealed that corals in a region have particular
thermal ‘thresholds’ for bleaching. These thresholds for
triggering bleaching are reliable to the point that they
predict which regions will experience coral bleaching
based on their sea-surface temperatures measured from
satellites. If these thresholds are compared to projections
of future sea temperatures trends (produced by the
Global Circulation Models, see above), it is possible to
estimate how the frequency and intensity of mass coral
bleaching and mortality will change over time. These
changes show that an increase of 2°C over pre-industrial
temperatures in the average sea temperature in tropical
and subtropical Australia (expected as a result of a doubling of CO 2 ) will lead to annual bleaching and a major
escalation in the number of mass mortality events.
The risk of mass bleaching has been examined for
the GBR and revealed that the return time of severe
mass coral bleaching in their models for low to moderate changes in the climate increased to the point
where the ability of reefs to recover is severely compromised. Deterioration of coral populations is likely
in most of the scenarios examined.
N OCEAN ACIDIFICATION
The increase in atmospheric CO 2 presents a second major
change for reef-building corals and other marine calcifiers.
Nearly half of the CO 2 that enters the atmosphere is absorbed by the ocean, where it reacts with water to form
carbonic acid (Equation 1). Carbonic acid dissociates into
bicarbonate and a proton (Equation 2). The protons released
by the entry of CO 2 into seawater then react with carbonate
ions to form additional bicarbonate ions (Equation 3). The
problem is that relatively small rises of CO 2 in the atmosphere at the pH range of seawater will cause a large decrease in the concentration of carbonate ions.
(1) CO 2 H 2 O m H 2 CO 3
(2) H 2 CO 3 m HCO 3
+ H
(3) CO 3
2 H
m HCO 3
The calcification rate of a range of marine organisms
as diverse as microalgae (coccolithophores), molluscs
(e.g. clams, pteropods) and corals is strongly dependent
on the concentration of carbonate ions in seawater. Naturally, the concentration is highest in the warmer tropical regions due to the reduced solubility of CO 2 in warm
v. cold water (i.e. less CO 2 dissolving into the ocean
means more carbonate ions). The pH of the ocean has
already decreased by 0.1 pH unit with the concentration
of carbonate ions decreasing as much as 30 Mmol kg
1
.
Corals were among the first organisms identified as