8 – Calcification, Erosion and the Establishment of the Framework of Coral Reefs
83
water column. In contrast, at sites out in the Coral Sea
where water is clear (Fig. 8.3D), substrata are heavily
bored with endolithic algae, encouraging high rates of
grazing by scarids. Boring communities vary between
inshore and offshore sites with deposit feeding polychaete species dominant at inshore sites and filter and
surface deposit feeders at offshore sites. Boring sponges
are most abundant at inshore sites and boring bivalves
at offshore sites. Net rates of erosion vary between sites
and the relative importance of the components of erosion change markedly along the cross-shelf transect,
supporting the data on the distribution, abundance
and species composition of scarids across a similar
transect (Fig. 8.4).
N TIPPING POINT: HUMAN INFLUENCES ON
CALCIFICATION AND EROSION
As human populations have expanded in the coastal
areas of tropical and subtropical oceans their influence
on the environment that surrounds coral reefs has increased dramatically. Changes to the nutrient and sediment concentration of the waters surrounding corals
have impacted the growth and calcification of a wide
range of organisms. In the last two decades, these local
impacts have been joined by global factors such as global warming and ocean acidification (see Chapters 9
and 10). Together, local and global factors have decreased the growth in calcification of reefs while at the
same time probably increasing the rate of dissolution
and/or bioerosion. These changes are complex and interactive, and have far-reaching consequences for both
natural ecosystems and the human societies that depend on them.
The impact of coral bleaching, crown-of-thorns starfish (see Chapters 5 and 26) and a wide array of other
factors has decreased the proportion of reefs covered
by living coral, which normally maintains the carbonate reef substrates against infestation by bioeroders.
The loss of corals has in turn provided an increase in
supply of suitable substrata for bioerosion, so rates will
increase across the reef after a bleaching event. These
rates will either remain high or decline to prebleaching
levels, depending on other factors such as water quality
and supply of coral recruits. Clearly, if the growth and
survival of coral reefs is to continue to decline under
the rapid changes in global climate that are projected
for this century, then there will be an increasing proportion of reefs that will be no longer growing and will
be in net erosion. How fast a reef matrix can disappear
is probably dependent on a number of factors. Some
studies have suggested that accumulated calcium carbonate structures typical of many reefs can disappear
quite quickly.
An additional problem has arisen from the buildup
of carbon dioxide in the atmosphere as explained in
Chapter 10. In this particular case, roughly 40% of the
carbon dioxide that has entered the atmosphere has
been absorbed by the ocean. In the ocean, carbon dioxide reacts with water to create a dilute acid called carbonic acid. This acid releases protons which, combined
with carbonate, convert it to bicarbonate. The net effect
is that the carbonate ion concentration has been declining and will decline further as carbon dioxide builds
up in the atmosphere. Decreasing carbonate ion concentrations will decrease the ease with which calcification can occur and will increase the tendency for
calcium carbonate crystals to dissolve. This has many
people concerned about whether projected increases in
carbon dioxide in the atmosphere will tip the balance
of coral reefs away from the accumulation of calcium
carbonate and towards the erosion of this important resource. The implications that reefs are eroding are
highly significant and may involve a loss of the threedimensional structure of reefs. This structure is important habitat for many thousands of species worldwide
as well as being the ‘front line’ defence along coastlines
throughout the world. The prospect of reef barriers disappearing as they erode in a warm and acidic sea may
mean increased exposure of other ecosystems such as
mangroves and seagrasses, which generally shelter behind the reef crests from the full force of ocean waves.
These changes in wave energy, especially when combined with sea level rise, could have dire implications
for the extensive human infrastructure that often lines
tropical coastline, and which also shelters behind these
crucial reef barriers.
In summary, the factors controlling rates and agents
of bioerosion are complex and interrelated as are those
controlling reef growth, and superimposed on these are
83
water column. In contrast, at sites out in the Coral Sea
where water is clear (Fig. 8.3D), substrata are heavily
bored with endolithic algae, encouraging high rates of
grazing by scarids. Boring communities vary between
inshore and offshore sites with deposit feeding polychaete species dominant at inshore sites and filter and
surface deposit feeders at offshore sites. Boring sponges
are most abundant at inshore sites and boring bivalves
at offshore sites. Net rates of erosion vary between sites
and the relative importance of the components of erosion change markedly along the cross-shelf transect,
supporting the data on the distribution, abundance
and species composition of scarids across a similar
transect (Fig. 8.4).
N TIPPING POINT: HUMAN INFLUENCES ON
CALCIFICATION AND EROSION
As human populations have expanded in the coastal
areas of tropical and subtropical oceans their influence
on the environment that surrounds coral reefs has increased dramatically. Changes to the nutrient and sediment concentration of the waters surrounding corals
have impacted the growth and calcification of a wide
range of organisms. In the last two decades, these local
impacts have been joined by global factors such as global warming and ocean acidification (see Chapters 9
and 10). Together, local and global factors have decreased the growth in calcification of reefs while at the
same time probably increasing the rate of dissolution
and/or bioerosion. These changes are complex and interactive, and have far-reaching consequences for both
natural ecosystems and the human societies that depend on them.
The impact of coral bleaching, crown-of-thorns starfish (see Chapters 5 and 26) and a wide array of other
factors has decreased the proportion of reefs covered
by living coral, which normally maintains the carbonate reef substrates against infestation by bioeroders.
The loss of corals has in turn provided an increase in
supply of suitable substrata for bioerosion, so rates will
increase across the reef after a bleaching event. These
rates will either remain high or decline to prebleaching
levels, depending on other factors such as water quality
and supply of coral recruits. Clearly, if the growth and
survival of coral reefs is to continue to decline under
the rapid changes in global climate that are projected
for this century, then there will be an increasing proportion of reefs that will be no longer growing and will
be in net erosion. How fast a reef matrix can disappear
is probably dependent on a number of factors. Some
studies have suggested that accumulated calcium carbonate structures typical of many reefs can disappear
quite quickly.
An additional problem has arisen from the buildup
of carbon dioxide in the atmosphere as explained in
Chapter 10. In this particular case, roughly 40% of the
carbon dioxide that has entered the atmosphere has
been absorbed by the ocean. In the ocean, carbon dioxide reacts with water to create a dilute acid called carbonic acid. This acid releases protons which, combined
with carbonate, convert it to bicarbonate. The net effect
is that the carbonate ion concentration has been declining and will decline further as carbon dioxide builds
up in the atmosphere. Decreasing carbonate ion concentrations will decrease the ease with which calcification can occur and will increase the tendency for
calcium carbonate crystals to dissolve. This has many
people concerned about whether projected increases in
carbon dioxide in the atmosphere will tip the balance
of coral reefs away from the accumulation of calcium
carbonate and towards the erosion of this important resource. The implications that reefs are eroding are
highly significant and may involve a loss of the threedimensional structure of reefs. This structure is important habitat for many thousands of species worldwide
as well as being the ‘front line’ defence along coastlines
throughout the world. The prospect of reef barriers disappearing as they erode in a warm and acidic sea may
mean increased exposure of other ecosystems such as
mangroves and seagrasses, which generally shelter behind the reef crests from the full force of ocean waves.
These changes in wave energy, especially when combined with sea level rise, could have dire implications
for the extensive human infrastructure that often lines
tropical coastline, and which also shelters behind these
crucial reef barriers.
In summary, the factors controlling rates and agents
of bioerosion are complex and interrelated as are those
controlling reef growth, and superimposed on these are