atmospheric CO 2 at the ocean–atmosphere interface today
(385 ppm in 2010) compared to preindustrial times
(280 ppm) that much more goes into solution, despite
the marginal solubility loss caused by increased sea temperature (Sabine et al., 2004). These authors report that
if CO 2 reaches double preindustrial levels (560 ppm),
there will be a 30% decrease in carbonate ion concentration and a 60% increase in hydrogen ion concentration.
There are likely to be differences among calcifiers in their
responses to decreasing carbonate ion concentrations,
reflecting differences in their carbonate mineralogy and
local environmental parameters such as temperature, light,
and available nutrients and in the mechanism of biomineralization (Feely et al., 2004). A review by Guinotte and
Fabry (2008) suggests that this would cause a 20–60%
reduction of calcification rates in tropical reef-building
corals, manifest in the skeleton in its two measurable
parameters: rate of linear extension (cm day
À1 ) and density (g cm
À3
) (Barnes and Chalker, 1990; Lough and
Barnes, 2000). Any loss of density would cause
a decrease in strength and greater vulnerability to storms
in corals on the reef (Massel, 1999). A reduction in linear
extension rate would mean that individual colonies would
take longer to reach size-related thresholds, such as sexual
maturity (Albright et al., 2008), escape from overgrowth
by other benthos, or vulnerability to dislodgment by storm
waves (Massel and Done, 1993; Madin and Connelly,
2006). Most recent field studies have shown a reduction
in linear extension and density coincident with rising temperature and atmospheric pCO 2 over recent decades (Cooper et al., 2008; De’ath et al., 2009; Tanzil et al., 2009).
A number of studies (noted in Albright et al., 2008) have
reported that reduced calcium carbonate saturation has
no negative effect on physiological processes other than
calcification (viz, tissue growth and photosynthesis) and
indeed may even augment them. Paradoxically, any
decline in reef-wide calcification caused by atmospheric
CO 2 will reduce aqueous CO 2 emanating from the reef
itself. Reefs as a whole, including their reef flats and
lagoons, tend to be sources of CO 2 , due both to calcification by reef communities and the decomposition of plant
matter produced on the reef (Suzuki and Kawahata, 1999).
Anthropogenic climate change may be precipitating the
sixth great coral reef extinction in 430 million years
(Veron, 2008). Reefs are confined to warm shallow waters
that are becoming too hot too often, bleaching and killing
coral populations on reefs scattered over vast areas of
ocean. As a result, recolonization of damaged areas will
be weakened, and even in those places where by chance
there is good recolonization, the next heatwave, hurricane,
flood, pest outbreak, or disease will cut off recovery of
high coral cover before it can be completed. Refuge
populations themselves are threatened. For shallow ones,
it may simply be a matter of time before they are killed
by a heatwave, and for deeper and cooler ones, before they
become victims of shallowing of the aragonite saturation
zone caused by ocean acidification (Feely et al., 2004).
This view of the future – predicated on the assumption
that environmental changes will be too great and are
occurring too fast for there to be any effective adaptation
in reef organisms (see Chapter Adaptation) – raises some
key questions about the viability of the processes and circumstances responsible for maintaining modern coral
reefs, i.e., environmental controls on coral growth.
Summary
Coral growth (individuals, populations, and communities)
responds to variations in their external environment that
are manifest over spatial scales spanning centimeters to
degrees of latitude. The specific microenvironment in
which a coral settles and in which it may spend anything
from years to centuries is determined by vagaries of currents and early survival. Corals and coral reefs exist within
a range of local settings within a region (e.g., oceanic vs.
continental shelf) and benthic communities and reefs further modify their own environments – dissipation and
refraction of waves; ponding of reef-top waters; removal
of aragonite precursors and addition of organic and inorganic detritus. As a result, there may be as much variability in the range of microenvironments existing within
regions and on individual reefs as there is across much
broader geographic areas. This existing variability in
microenvironments and the concomitant diversity in species and coral–zooxanthellae partnerships are extremely
important for the survival of reefs in a future with global
climate change, in which they are faced with rapid
changes in environment over all spatial scales.
Bibliography
Abelson, A., and Denny, M., 1997. Settlement of marine organisms
in flow. Annual Review of Ecology and Systematics, 28,
317–339.
Albright, R., Mason, B., and Langdon, C., 2008. Effect of aragonite
saturation state on settlement and post-settlement growth of
Porites astreoides larvae. Coral Reefs, 27, 485–490.
Anthony, K. R. N., 2000. Enhanced particle-feeding capacity of
corals on turbid reefs (Great Barrier Reef, Australia). Coral
Reefs, 19, 59–67.
Anthony, K. R. N., Ridd, P. V., Orpin, A. R., Larcombe, P., and
Lough, J., 2004. Temporal variation of light availability in
coastal benthic habitats: effects of clouds, turbidity, and tides.
Limnology and Oceanography, 49, 2201–2211.
Atkinson, M. J., and Bilger, R. W., 1992. Effects of water velocity
on phosphate uptake in coral reef-flat communities. Limnology
and Oceanography, 37, 273–279.
Baker, K. S., and Smith, R. C., 1982. Bio-optical classification and
model of natural waters. 2. Limnology and Oceanography, 27,
500–509.
Barnes, D. J., and Chalker, B. E., 1990. Calcification and photosynthesis in reef-building coral and algae. In Dubinsky, Z. (ed.),
Ecosystems of the World, Vol. 25: Coral Reefs. Amsterdam:
Elsevier, pp. 109–131.
Beer, T., 1997. Environmental Oceanography, 2nd edn. Boca
Raton, FL: CRC.
Berkelmans, R., and van Oppen, M. J. H., 2006. The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for
coral reefs in an era of climate change. Proceedings of the Royal
Society of London Series B, 273, 2305–2312, doi:10.1098/
rspb.2006.
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH
291
(385 ppm in 2010) compared to preindustrial times
(280 ppm) that much more goes into solution, despite
the marginal solubility loss caused by increased sea temperature (Sabine et al., 2004). These authors report that
if CO 2 reaches double preindustrial levels (560 ppm),
there will be a 30% decrease in carbonate ion concentration and a 60% increase in hydrogen ion concentration.
There are likely to be differences among calcifiers in their
responses to decreasing carbonate ion concentrations,
reflecting differences in their carbonate mineralogy and
local environmental parameters such as temperature, light,
and available nutrients and in the mechanism of biomineralization (Feely et al., 2004). A review by Guinotte and
Fabry (2008) suggests that this would cause a 20–60%
reduction of calcification rates in tropical reef-building
corals, manifest in the skeleton in its two measurable
parameters: rate of linear extension (cm day
À1 ) and density (g cm
À3
) (Barnes and Chalker, 1990; Lough and
Barnes, 2000). Any loss of density would cause
a decrease in strength and greater vulnerability to storms
in corals on the reef (Massel, 1999). A reduction in linear
extension rate would mean that individual colonies would
take longer to reach size-related thresholds, such as sexual
maturity (Albright et al., 2008), escape from overgrowth
by other benthos, or vulnerability to dislodgment by storm
waves (Massel and Done, 1993; Madin and Connelly,
2006). Most recent field studies have shown a reduction
in linear extension and density coincident with rising temperature and atmospheric pCO 2 over recent decades (Cooper et al., 2008; De’ath et al., 2009; Tanzil et al., 2009).
A number of studies (noted in Albright et al., 2008) have
reported that reduced calcium carbonate saturation has
no negative effect on physiological processes other than
calcification (viz, tissue growth and photosynthesis) and
indeed may even augment them. Paradoxically, any
decline in reef-wide calcification caused by atmospheric
CO 2 will reduce aqueous CO 2 emanating from the reef
itself. Reefs as a whole, including their reef flats and
lagoons, tend to be sources of CO 2 , due both to calcification by reef communities and the decomposition of plant
matter produced on the reef (Suzuki and Kawahata, 1999).
Anthropogenic climate change may be precipitating the
sixth great coral reef extinction in 430 million years
(Veron, 2008). Reefs are confined to warm shallow waters
that are becoming too hot too often, bleaching and killing
coral populations on reefs scattered over vast areas of
ocean. As a result, recolonization of damaged areas will
be weakened, and even in those places where by chance
there is good recolonization, the next heatwave, hurricane,
flood, pest outbreak, or disease will cut off recovery of
high coral cover before it can be completed. Refuge
populations themselves are threatened. For shallow ones,
it may simply be a matter of time before they are killed
by a heatwave, and for deeper and cooler ones, before they
become victims of shallowing of the aragonite saturation
zone caused by ocean acidification (Feely et al., 2004).
This view of the future – predicated on the assumption
that environmental changes will be too great and are
occurring too fast for there to be any effective adaptation
in reef organisms (see Chapter Adaptation) – raises some
key questions about the viability of the processes and circumstances responsible for maintaining modern coral
reefs, i.e., environmental controls on coral growth.
Summary
Coral growth (individuals, populations, and communities)
responds to variations in their external environment that
are manifest over spatial scales spanning centimeters to
degrees of latitude. The specific microenvironment in
which a coral settles and in which it may spend anything
from years to centuries is determined by vagaries of currents and early survival. Corals and coral reefs exist within
a range of local settings within a region (e.g., oceanic vs.
continental shelf) and benthic communities and reefs further modify their own environments – dissipation and
refraction of waves; ponding of reef-top waters; removal
of aragonite precursors and addition of organic and inorganic detritus. As a result, there may be as much variability in the range of microenvironments existing within
regions and on individual reefs as there is across much
broader geographic areas. This existing variability in
microenvironments and the concomitant diversity in species and coral–zooxanthellae partnerships are extremely
important for the survival of reefs in a future with global
climate change, in which they are faced with rapid
changes in environment over all spatial scales.
Bibliography
Abelson, A., and Denny, M., 1997. Settlement of marine organisms
in flow. Annual Review of Ecology and Systematics, 28,
317–339.
Albright, R., Mason, B., and Langdon, C., 2008. Effect of aragonite
saturation state on settlement and post-settlement growth of
Porites astreoides larvae. Coral Reefs, 27, 485–490.
Anthony, K. R. N., 2000. Enhanced particle-feeding capacity of
corals on turbid reefs (Great Barrier Reef, Australia). Coral
Reefs, 19, 59–67.
Anthony, K. R. N., Ridd, P. V., Orpin, A. R., Larcombe, P., and
Lough, J., 2004. Temporal variation of light availability in
coastal benthic habitats: effects of clouds, turbidity, and tides.
Limnology and Oceanography, 49, 2201–2211.
Atkinson, M. J., and Bilger, R. W., 1992. Effects of water velocity
on phosphate uptake in coral reef-flat communities. Limnology
and Oceanography, 37, 273–279.
Baker, K. S., and Smith, R. C., 1982. Bio-optical classification and
model of natural waters. 2. Limnology and Oceanography, 27,
500–509.
Barnes, D. J., and Chalker, B. E., 1990. Calcification and photosynthesis in reef-building coral and algae. In Dubinsky, Z. (ed.),
Ecosystems of the World, Vol. 25: Coral Reefs. Amsterdam:
Elsevier, pp. 109–131.
Beer, T., 1997. Environmental Oceanography, 2nd edn. Boca
Raton, FL: CRC.
Berkelmans, R., and van Oppen, M. J. H., 2006. The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for
coral reefs in an era of climate change. Proceedings of the Royal
Society of London Series B, 273, 2305–2312, doi:10.1098/
rspb.2006.
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH
291