shallower coral communities must be in order to have adequate light. This is of less consequence today than in past
geological intervals when waters were warm enough for
reef-building to occur at high latitudes, even as far as the
Arctic and Antarctic circles.
Temperature
Temperature, in synergy to some extent with light, sets
limits on the latitudinal spread of corals throughout the
world. A different temperature constrains the spread of
reefs. The difference between these two – constraints on
corals and on reefs – has created havoc in palaeoclimatic
reconstructions of past reef environments as well as studies of reef growth because it is so widely misinterpreted.
Low-temperature limits to reef growth
It has been known for decades that reefs do not form where
the ocean temperature regularly goes below 18
C for
intervals of weeks to months. Reef geologists concerned
with the history of reefs refer to this well-established fact,
yet in so-doing they often assume that lower temperatures
kill corals. This is seldom the case.
As noted above, reef-building allows entire ecosystems
to exist, a process that can only happen if rampant growth
of macroalgae is held in check (Crossland, 1988). This
requires a great deal of uninterrupted energy, which is
why reef-building corals are so dependent on symbiotic
algae. We have also seen that this symbiosis requires
exposure to sunlight, which means living in shallow water.
Around 18
C corals are able to produce calcium carbonate
fast enough to fulfil their guild role as producers of building materials. They are able to do this not by growing
faster than algae, but by creating three-dimensional habitats where herbivores, especially fish, can control algae
for them. At lower temperatures, algae usually get the
upper hand; however, the corals themselves are not
affected by temperatures lower than 18
C. This is best
seen along the Ryukyu Islands of Japan where the southern islands have extensive reefs, yet further north the sea
temperature progressively decreases until it reaches the
critical 18
C point. It is here that reef development fails.
The corals, however, do not: nearly half of all coral species
regularly tolerate prolonged exposure to 14
C (Veron and
Minchin, 1992). A few tolerate 12
C although seldom less
(azooxanthellate corals excepted).
High-temperature limits to reef and coral growth
Low- and high-temperature limits do not mirror each other.
Oceans can cool until they freeze, yet they cannot warm
much beyond the peak temperatures we see today (around
31
C). This is because evaporation holds the upper limit
in check, at least it does for extensive areas of ocean.
Smaller bodies of water are less constrained, thus, reef
lagoons can get at least 5
C warmer than this. Nevertheless,
high temperature per se has little direct negative effect
on corals. The warmer the water the faster most metabolic
processes become and the faster calcification could become
if it were not for its effect on zooxanthellae. Faster metabolic rates for zooxanthellae mean faster photosynthesis,
which in turn can result in oxygen being produced at rates
where it becomes toxic. Corals are forced to expel their
increasingly poisonous zooxanthellae and “bleach” in
response to temperature and light acting in concert.
High-temperature limits of coral growth and reef
growth are approximately the same as they are both linked
to the upper limit of the ocean. This link is an evolutionary
one and appears to have always existed for there is no
interval in geological time where high temperature has
excluded reefs from equatorial regions.
Substrate, turbulence, and mechanical effects
Substrate type and water clarity are always closely linked,
especially when depth and turbulence are factored in.
White calcareous sand, although typically coarse-grained,
is light and therefore readily moved around by wave
action, in which case it is capable of burying corals if
suspended in sufficient quantity. However, it is clay from
rivers that adversely affects corals, for not only does it
attenuate light, but it also requires cleaning, a costly activity in terms of metabolic energy.
Substrate is also of paramount importance to settling
larvae, for these will not settle on sand of any sort, or on
substrates that are coated with bacterial slime, as it commonly develops on reefs that have been degraded.
One very obvious effect of turbulence on coral skeleton
formation is that wave action produces dense skeletons.
Corals in a high-energy environment grow dense skeletons, whereas those in protected areas have light, brittle
skeletons. This is partly because of the differences in species that occupy these habitats, yet even within the same
species this effect is pronounced.
Water quality
The term “water quality” is commonly used in connection
with the health of the marine environment. Water quality
that is good for particular coral reefs or coral communities
is assumed to have tolerable levels of sediments and nutrients and environmental contaminants.
Salinity is an aspect of water quality that has not been
adequately studied. Corals appear to be sufficiently tolerant of high salinity that lethal levels seldom, if ever, occur
naturally. The opposite commonly applies to low salinities, for these play a large role in creating areas where
there is little or no coral or reef growth.
There are other environmental controls on reef-building
hidden in water chemistry that may not overtly limit reef
distribution today but which may have been important in
the geological past and are destined to become so in the
near future. Oceans are normally so well buffered that
chemical changes are infinitesimally slow, providing
plenty of time for organisms to evolve adaptations to any
alteration. However, sometimes the rate of change exceeds
physical or biological thresholds and cannot be tolerated
by any except the most specialised organisms. This can
CORALS: BIOLOGY, SKELETAL DEPOSITION, AND REEF-BUILDING
279
geological intervals when waters were warm enough for
reef-building to occur at high latitudes, even as far as the
Arctic and Antarctic circles.
Temperature
Temperature, in synergy to some extent with light, sets
limits on the latitudinal spread of corals throughout the
world. A different temperature constrains the spread of
reefs. The difference between these two – constraints on
corals and on reefs – has created havoc in palaeoclimatic
reconstructions of past reef environments as well as studies of reef growth because it is so widely misinterpreted.
Low-temperature limits to reef growth
It has been known for decades that reefs do not form where
the ocean temperature regularly goes below 18
C for
intervals of weeks to months. Reef geologists concerned
with the history of reefs refer to this well-established fact,
yet in so-doing they often assume that lower temperatures
kill corals. This is seldom the case.
As noted above, reef-building allows entire ecosystems
to exist, a process that can only happen if rampant growth
of macroalgae is held in check (Crossland, 1988). This
requires a great deal of uninterrupted energy, which is
why reef-building corals are so dependent on symbiotic
algae. We have also seen that this symbiosis requires
exposure to sunlight, which means living in shallow water.
Around 18
C corals are able to produce calcium carbonate
fast enough to fulfil their guild role as producers of building materials. They are able to do this not by growing
faster than algae, but by creating three-dimensional habitats where herbivores, especially fish, can control algae
for them. At lower temperatures, algae usually get the
upper hand; however, the corals themselves are not
affected by temperatures lower than 18
C. This is best
seen along the Ryukyu Islands of Japan where the southern islands have extensive reefs, yet further north the sea
temperature progressively decreases until it reaches the
critical 18
C point. It is here that reef development fails.
The corals, however, do not: nearly half of all coral species
regularly tolerate prolonged exposure to 14
C (Veron and
Minchin, 1992). A few tolerate 12
C although seldom less
(azooxanthellate corals excepted).
High-temperature limits to reef and coral growth
Low- and high-temperature limits do not mirror each other.
Oceans can cool until they freeze, yet they cannot warm
much beyond the peak temperatures we see today (around
31
C). This is because evaporation holds the upper limit
in check, at least it does for extensive areas of ocean.
Smaller bodies of water are less constrained, thus, reef
lagoons can get at least 5
C warmer than this. Nevertheless,
high temperature per se has little direct negative effect
on corals. The warmer the water the faster most metabolic
processes become and the faster calcification could become
if it were not for its effect on zooxanthellae. Faster metabolic rates for zooxanthellae mean faster photosynthesis,
which in turn can result in oxygen being produced at rates
where it becomes toxic. Corals are forced to expel their
increasingly poisonous zooxanthellae and “bleach” in
response to temperature and light acting in concert.
High-temperature limits of coral growth and reef
growth are approximately the same as they are both linked
to the upper limit of the ocean. This link is an evolutionary
one and appears to have always existed for there is no
interval in geological time where high temperature has
excluded reefs from equatorial regions.
Substrate, turbulence, and mechanical effects
Substrate type and water clarity are always closely linked,
especially when depth and turbulence are factored in.
White calcareous sand, although typically coarse-grained,
is light and therefore readily moved around by wave
action, in which case it is capable of burying corals if
suspended in sufficient quantity. However, it is clay from
rivers that adversely affects corals, for not only does it
attenuate light, but it also requires cleaning, a costly activity in terms of metabolic energy.
Substrate is also of paramount importance to settling
larvae, for these will not settle on sand of any sort, or on
substrates that are coated with bacterial slime, as it commonly develops on reefs that have been degraded.
One very obvious effect of turbulence on coral skeleton
formation is that wave action produces dense skeletons.
Corals in a high-energy environment grow dense skeletons, whereas those in protected areas have light, brittle
skeletons. This is partly because of the differences in species that occupy these habitats, yet even within the same
species this effect is pronounced.
Water quality
The term “water quality” is commonly used in connection
with the health of the marine environment. Water quality
that is good for particular coral reefs or coral communities
is assumed to have tolerable levels of sediments and nutrients and environmental contaminants.
Salinity is an aspect of water quality that has not been
adequately studied. Corals appear to be sufficiently tolerant of high salinity that lethal levels seldom, if ever, occur
naturally. The opposite commonly applies to low salinities, for these play a large role in creating areas where
there is little or no coral or reef growth.
There are other environmental controls on reef-building
hidden in water chemistry that may not overtly limit reef
distribution today but which may have been important in
the geological past and are destined to become so in the
near future. Oceans are normally so well buffered that
chemical changes are infinitesimally slow, providing
plenty of time for organisms to evolve adaptations to any
alteration. However, sometimes the rate of change exceeds
physical or biological thresholds and cannot be tolerated
by any except the most specialised organisms. This can
CORALS: BIOLOGY, SKELETAL DEPOSITION, AND REEF-BUILDING
279