8 – Calcification, Erosion and the Establishment of the Framework of Coral Reefs
75
to be crucial in the highly dynamic coral reef ecosystem, where competition and predation can be high.
The mechanism by which organisms produce
calcium carbonate skeletons has yet to be conclusively
determined. Within the water column of tropical and
subtropical oceans that is saturated with respect to both
calcium and carbonate ions, there appears to be three
possible ways in which a high rate of precipitation can
be fostered. The first depends on bulk metabolic energy
to concentrate calcium and carbonate ions within confined spaces and subsequently to cause a rapid precipitation of aragonite or calcite. The second is that the
symbionts assist by removing so-called ‘crystal poisons’
such as phosphate that otherwise retard the formation of
crystals. The third is the production of specialised proteins that are often referred to as ‘skeletal matrix proteins’. These particular proteins tend to be highly anionic
(covered in negative charges) and contain regions that
are associated with enzymes such as carbonic anhydrase
(that catalyses the rapid conversion of carbon dioxide to
bicarbonate and protons, a reaction that occurs rather
slowly in the absence of a catalyst). Although there is
some debate over which is more important, there is good
evidence that all of these processes may play roles of differing importance within the variety of organisms that
calcify within coral reef environments.
There are a number of ways calcium carbonate deposition is measured. These are outlined in Box 8.1. The
rates of calcification on coral reefs can be extremely high
in equatorial or low latitude areas of the planet. On a
more regional scale, the deposition of calcium carbonate varies with the presence or absence of rivers, where
high nutrients and sedimentation may slow the deposition of calcium carbonate. In this respect, inshore coral
reefs on the GBR do not deposit calcium carbonate as
fast as those reefs that are in more offshore positions.
Again, this is a consequence of changes in factors such
as light, temperature and nutrients. At the scale of a
reef, calcium carbonate deposition can be quite dynamic
and will vary between the slope, crest and back-reef
areas as discussed already in Chapter 7. Coral reefs are
also dynamic in geological time frames, with the shape
of deposited calcium carbonate varying over time in response to prevailing winds and currents. These aspects
of reef construction are discussed in Chapter 2.
The skeletons of calcifying organisms build up and
construct the accumulated calcium carbonate debris
that constitutes the solid component of the framework
of coral reefs. Calcifiers on coral reefs can have different
roles, for example the massive and branching structures provided by corals require the activities of encrusting red algae to essentially glue them into a
consolidated framework (Fig. 8.1A). The rate of calcification normally greatly exceeds the rate of erosion on
carbonate coral reefs. Estimates of calcification suggest
that rates vary from 1–2 m per century while rates of
reef growth are about 1–2 m per millennium. Based on
these rough figures, this would suggest that rates of
calcification are between 3–10 times higher than the
rate at which calcium carbonate is removed by physical
and biological erosion. As we will see later in this chapter, the balance between the two forces (calcification
versus erosion) is critical to understanding the impacts
of global change, such as ocean acidification.
PHYSICAL AND BIOLOGICAL EROSION
The removal of calcium carbonate from coral reefs (erosion) is a key process on coral reefs that involves a
number of elements including dissolution, physical
breakage and the activities of a number of so-called
bioeroders. These elements are intertwined and it is
difficult to separate them. They are a feature of recent
as well as fossil coral reefs.
Wave action erodes the reef slowly over time by
physical action and chemical dissolution of the reef
substratum. During storms, however, this rate will increase and large boulders may be dislodged. As they
roll down the reef slope they may physically remove
many more coral colonies (Fig. 8.1B, C). These forces
can have significant impacts on the shape of coral reefs
(e.g. spur and groove formations such as those seen on
Wistari Reef (Fig. 8.1D)). Depending on the wave energy and the relative hardness of coral skeletons (that
itself is affected by chemical and biological factors), the
impacts of storms can be substantial. After a cyclone
has passed through an area it appears as if the living
veneer of the reef has just been peeled off and shed.
Biological erosion consists of the loss of reef substratum by boring and by grazing. A suite of organisms
75
to be crucial in the highly dynamic coral reef ecosystem, where competition and predation can be high.
The mechanism by which organisms produce
calcium carbonate skeletons has yet to be conclusively
determined. Within the water column of tropical and
subtropical oceans that is saturated with respect to both
calcium and carbonate ions, there appears to be three
possible ways in which a high rate of precipitation can
be fostered. The first depends on bulk metabolic energy
to concentrate calcium and carbonate ions within confined spaces and subsequently to cause a rapid precipitation of aragonite or calcite. The second is that the
symbionts assist by removing so-called ‘crystal poisons’
such as phosphate that otherwise retard the formation of
crystals. The third is the production of specialised proteins that are often referred to as ‘skeletal matrix proteins’. These particular proteins tend to be highly anionic
(covered in negative charges) and contain regions that
are associated with enzymes such as carbonic anhydrase
(that catalyses the rapid conversion of carbon dioxide to
bicarbonate and protons, a reaction that occurs rather
slowly in the absence of a catalyst). Although there is
some debate over which is more important, there is good
evidence that all of these processes may play roles of differing importance within the variety of organisms that
calcify within coral reef environments.
There are a number of ways calcium carbonate deposition is measured. These are outlined in Box 8.1. The
rates of calcification on coral reefs can be extremely high
in equatorial or low latitude areas of the planet. On a
more regional scale, the deposition of calcium carbonate varies with the presence or absence of rivers, where
high nutrients and sedimentation may slow the deposition of calcium carbonate. In this respect, inshore coral
reefs on the GBR do not deposit calcium carbonate as
fast as those reefs that are in more offshore positions.
Again, this is a consequence of changes in factors such
as light, temperature and nutrients. At the scale of a
reef, calcium carbonate deposition can be quite dynamic
and will vary between the slope, crest and back-reef
areas as discussed already in Chapter 7. Coral reefs are
also dynamic in geological time frames, with the shape
of deposited calcium carbonate varying over time in response to prevailing winds and currents. These aspects
of reef construction are discussed in Chapter 2.
The skeletons of calcifying organisms build up and
construct the accumulated calcium carbonate debris
that constitutes the solid component of the framework
of coral reefs. Calcifiers on coral reefs can have different
roles, for example the massive and branching structures provided by corals require the activities of encrusting red algae to essentially glue them into a
consolidated framework (Fig. 8.1A). The rate of calcification normally greatly exceeds the rate of erosion on
carbonate coral reefs. Estimates of calcification suggest
that rates vary from 1–2 m per century while rates of
reef growth are about 1–2 m per millennium. Based on
these rough figures, this would suggest that rates of
calcification are between 3–10 times higher than the
rate at which calcium carbonate is removed by physical
and biological erosion. As we will see later in this chapter, the balance between the two forces (calcification
versus erosion) is critical to understanding the impacts
of global change, such as ocean acidification.
PHYSICAL AND BIOLOGICAL EROSION
The removal of calcium carbonate from coral reefs (erosion) is a key process on coral reefs that involves a
number of elements including dissolution, physical
breakage and the activities of a number of so-called
bioeroders. These elements are intertwined and it is
difficult to separate them. They are a feature of recent
as well as fossil coral reefs.
Wave action erodes the reef slowly over time by
physical action and chemical dissolution of the reef
substratum. During storms, however, this rate will increase and large boulders may be dislodged. As they
roll down the reef slope they may physically remove
many more coral colonies (Fig. 8.1B, C). These forces
can have significant impacts on the shape of coral reefs
(e.g. spur and groove formations such as those seen on
Wistari Reef (Fig. 8.1D)). Depending on the wave energy and the relative hardness of coral skeletons (that
itself is affected by chemical and biological factors), the
impacts of storms can be substantial. After a cyclone
has passed through an area it appears as if the living
veneer of the reef has just been peeled off and shed.
Biological erosion consists of the loss of reef substratum by boring and by grazing. A suite of organisms