The Great Barrier Reef
60
reefs. Coral reefs range from poorly developed reefs
that fringe continental islands and the Australian coastline to extensive carbonate barrier reefs. At latitudes
from 22–30°S, extensive coral reefs form but fail to
accumulate limestone (i.e. reef erosion exceeds calcification) and are referred to as non- carbonate reef systems (see Chapter 2).
Central to the existence of coral reefs are reefbuilding corals (Order Scleractinia, Class Anthozoa,
Phylum Cnidaria, see Chapter 20). Reef-building corals
are often referred to as the framework builders of coral
reefs, but they are not the only calcifiers on coral reefs.
Others organisms such as giant clams, foraminifera and
calcareous algae contribute substantial quantities of calcium carbonate to reef structures and sediment. Calcareous algae (see Chapter 15) play a particularly important
role in reef frameworks, some by coating and ‘gluing’
the coral framework together, and others by building
thick edifices of their own. The framework provided by
corals and other marine calcifiers forms the threedimensional structure into which hundreds of thousands
of species of animal, plant, fungi and bacteria live.
Several authors have explored the conditions under
which coral reefs thrive globally. Explanations have frequently focused on the fact that coral reefs form in warm
seas, leading to the ‘rule of thumb’ that coral reefs are
limited to waters that do not decrease below 18°C in the
winter. This principal is often incorrectly perceived as
the ultimate limit to the development of coral reefs that
ignores the many other variables that also change at
higher latitudes. While reefs are adapted to their local
temperature regime, the amount of light and the concentration of carbonate ions are at least as important in
limiting reef development. An exhaustive study of the
environmental factors associated with coral reefs using
data from close to 1000 coral reef locations found that
light availability and the concentration of carbonate
ions (that is ultimately determined by temperature) are
as potentially important as temperature in defining
where limestone coral reefs are found (Table 7.1).
The conditions that are associated with the distribution of coral reefs vary across spatial and temporal
scales. Variability across the year can be substantial,
while diurnal variability in temperature is usually
small (except in areas such as shallow intertidal reef
crests). The seasonal variability of conditions becomes
important on coral reefs at high latitudes where extremes in both winter (that can be too cold and dark)
and summer (too hot and bright) can cause stress on
coral reef organisms. At these sites in Australia, interannual variability such as that associated with the El
Niño cycle along the east coast of Australia can play a
large influence on coral reefs through warm (e.g. 2002)
and colder years (e.g. 2003). In coming decades, due to
rising background sea temperatures, warmer years can
exceed the tolerance of symbiotic organisms giving rise
to mass coral bleaching and associated mortality events
(see Chapter 10).
The global distribution of carbonate and noncarbonate coral reefs is strongly (and perhaps not surprisingly) correlated with the concentration of carbonate
ions, which is ultimately determined by ocean temperature, salinity, and factors such as the atmospheric carbon dioxide concentration (see Chapter 10). The
concentration of carbonate ions is highest at the equator and decreases at high latitudes due to the effect
of sea temperature on the solubility of CO 2 (see
Chapter 10). Coral reefs do not exist at carbonate concentrations below 200 μmol kg
1 ; this has significance
in terms of the problem of ocean acidification for coral
reefs as discussed in Chapter 10.
The last variable that is a major determinant of where
coral reefs are found is light. Because of the dependence
of primary production and calcification on light, coral
reefs are limited to clear tropical and subtropical waters
where depths are less than 100 m. Both light quantity
and quality (wavelength) are important, driving the primary step of photosynthesis of the symbionts within
corals and many other photosynthetic organisms. Coral
reefs only form where the average irradiance is at least
250 μmol m
2 s
1 (roughly 10% of surface irradiances in
tropical and sub-tropical regions).
Several variables affect the light available for coral
reefs. Light enters the outer atmosphere of the Earth
(Fig. 7.1) and is selectively filtered such that some
wavelengths (ultraviolet, infrared) are largely removed
by the ozone layer and water vapour (e.g. clouds). The
penetration of Photosynthetically Active Radiation
(PAR, 400–700 nm) is also reduced by dust and clouds.
Then, at the surface of the ocean, more light is reflected,
60
reefs. Coral reefs range from poorly developed reefs
that fringe continental islands and the Australian coastline to extensive carbonate barrier reefs. At latitudes
from 22–30°S, extensive coral reefs form but fail to
accumulate limestone (i.e. reef erosion exceeds calcification) and are referred to as non- carbonate reef systems (see Chapter 2).
Central to the existence of coral reefs are reefbuilding corals (Order Scleractinia, Class Anthozoa,
Phylum Cnidaria, see Chapter 20). Reef-building corals
are often referred to as the framework builders of coral
reefs, but they are not the only calcifiers on coral reefs.
Others organisms such as giant clams, foraminifera and
calcareous algae contribute substantial quantities of calcium carbonate to reef structures and sediment. Calcareous algae (see Chapter 15) play a particularly important
role in reef frameworks, some by coating and ‘gluing’
the coral framework together, and others by building
thick edifices of their own. The framework provided by
corals and other marine calcifiers forms the threedimensional structure into which hundreds of thousands
of species of animal, plant, fungi and bacteria live.
Several authors have explored the conditions under
which coral reefs thrive globally. Explanations have frequently focused on the fact that coral reefs form in warm
seas, leading to the ‘rule of thumb’ that coral reefs are
limited to waters that do not decrease below 18°C in the
winter. This principal is often incorrectly perceived as
the ultimate limit to the development of coral reefs that
ignores the many other variables that also change at
higher latitudes. While reefs are adapted to their local
temperature regime, the amount of light and the concentration of carbonate ions are at least as important in
limiting reef development. An exhaustive study of the
environmental factors associated with coral reefs using
data from close to 1000 coral reef locations found that
light availability and the concentration of carbonate
ions (that is ultimately determined by temperature) are
as potentially important as temperature in defining
where limestone coral reefs are found (Table 7.1).
The conditions that are associated with the distribution of coral reefs vary across spatial and temporal
scales. Variability across the year can be substantial,
while diurnal variability in temperature is usually
small (except in areas such as shallow intertidal reef
crests). The seasonal variability of conditions becomes
important on coral reefs at high latitudes where extremes in both winter (that can be too cold and dark)
and summer (too hot and bright) can cause stress on
coral reef organisms. At these sites in Australia, interannual variability such as that associated with the El
Niño cycle along the east coast of Australia can play a
large influence on coral reefs through warm (e.g. 2002)
and colder years (e.g. 2003). In coming decades, due to
rising background sea temperatures, warmer years can
exceed the tolerance of symbiotic organisms giving rise
to mass coral bleaching and associated mortality events
(see Chapter 10).
The global distribution of carbonate and noncarbonate coral reefs is strongly (and perhaps not surprisingly) correlated with the concentration of carbonate
ions, which is ultimately determined by ocean temperature, salinity, and factors such as the atmospheric carbon dioxide concentration (see Chapter 10). The
concentration of carbonate ions is highest at the equator and decreases at high latitudes due to the effect
of sea temperature on the solubility of CO 2 (see
Chapter 10). Coral reefs do not exist at carbonate concentrations below 200 μmol kg
1 ; this has significance
in terms of the problem of ocean acidification for coral
reefs as discussed in Chapter 10.
The last variable that is a major determinant of where
coral reefs are found is light. Because of the dependence
of primary production and calcification on light, coral
reefs are limited to clear tropical and subtropical waters
where depths are less than 100 m. Both light quantity
and quality (wavelength) are important, driving the primary step of photosynthesis of the symbionts within
corals and many other photosynthetic organisms. Coral
reefs only form where the average irradiance is at least
250 μmol m
2 s
1 (roughly 10% of surface irradiances in
tropical and sub-tropical regions).
Several variables affect the light available for coral
reefs. Light enters the outer atmosphere of the Earth
(Fig. 7.1) and is selectively filtered such that some
wavelengths (ultraviolet, infrared) are largely removed
by the ozone layer and water vapour (e.g. clouds). The
penetration of Photosynthetically Active Radiation
(PAR, 400–700 nm) is also reduced by dust and clouds.
Then, at the surface of the ocean, more light is reflected,