Introduction
Corals grow vigorously and build reefs in shallow tropical
seas, due to the favorable environment, both “latitudecorrelated environmental factors” (Veron, 1995) and factors that are not related to latitude. The latitude-correlated
environmental factors are solar radiation, temperature, and
water chemistry (Kleypas et al., 1999), and those not
related to latitude include nature and depth of the substratum, wave climate, salinity, water clarity, nutritional properties of that water, and sedimentation regime.
Collectively, they affect growth rate, growth form, reproduction and longevity of individual corals, the trajectories
of abundance, size and age-frequency distribution and
turnover rates of populations, and the composition and
diversity of coral communities. These population
and community attributes in turn influence the frequency
and amounts of calcium carbonate skeletons that can be
delivered to, and accumulate in, reef-building units
(framework, boulders, blocks, rubble, and sand) – i.e.,
contribute to the growth of maintenance of a coral reef
and adjacent sedimentary deposits against disaggregating
forces of waves, currents, and gravity in shallow tropical
seas.
Just as important as the normal fair-weather environment in determining a coral community’s extent and structure are the legacies of their history of development,
disturbance, and recovery. Coral communities (see Coral
Reef, Definition) are all works in progress – outcomes of
the timing and makeup of successive settling cohorts of
coral propagules (larvae and fragments) and a long list of
species-specific differences among corals: their performance in competition with other benthic organisms trying
to occupy the same piece of substratum (corals, other sessile invertebrates, and algae); attractiveness or resistance
to predators and diseases; susceptibility to breakage or dislodgement by storms; propensity to collapse under their
own top-heavy weight and fall off the reef into inhospitable depths; propensity to be dislodged, swept, rolled, or
pushed from the area by waves and currents.
Coral: the animal–plant symbiosis
Underpinning the coral vigor that builds reefs in tropical
seas is the presence of hundreds of thousands of “zooxanthellae” (single-celled dinoflagellate algae in the genus
Symbiodinium) within every square centimeter of the
coral’s tissues (Fagoonee et al., 1999; Fitt et al., 2000).
Corals that host zooxanthellae are referred to as
“zooxanthellate” corals. Within its diurnally and seasonally variable external environment, the coral needs to
provide an internal environment that nurtures the zooxanthellae. Whereas corals generally live for decades to centuries, the zooxanthellae populations and coral tissue in
which they reside turn over on time scales of weeks to
months (Fitt et al., 2000). Under extreme environmental
stress, the coral’s internal environment can become inhospitable and zooxanthellae populations may crash and the
coral tissue get damaged, leading to injury or death
of entire coral colonies across vast areas (see Climate
Change and Coral Reefs).
Within coral’s favored environment, the symbiotic
arrangement between the coral “host” and its zooxanthella
“symbiont” provides such a hospitable intracellular environment for the symbionts that their photosynthesis produces a massive surplus of energy-rich compounds
(Gattuso et al., 1999). These are used by the coral polyps
to fuel their replication and to deposit their shared skeleton. The rate of calcification (see Definition above) in
zooxanthellate corals far exceeds that which is possible
in cnidarians that lack zooxanthellae: their presence drives
“light-enhanced” calcification (Goreau and Goreau,
1959). However, one decade into the twenty-first century,
the coral–zooxanthellae symbiosis, in existence since the
Tertiary and critical for the development of coral reefs
(Stanley and Swart, 1995), faces increasing incidence
and severity of both sublethal stresses and lethal extreme
events. Recent reviews of implications of global climate
change for corals and reefs include Done (1999);
Buddemeier et al. (2004); Chadwick-Furman (2006);
Kleypas and Langdon (2006); Guinotte and Fabry
(2008); and Veron (2008). See also below in this article
and the Chapter on Climate Change.
Environmental controls on global distribution
of corals
Today’s global coral distribution (Figure 1) is in part,
a legacy of earlier dispersal and establishment processes
(Veron, 1995) and in part, a reflection of habitability
constraints imposed by the present day environment,
notably, those affecting coral’s capacity to calcify.
Light-enhanced calcification requires a particular regime
of water chemistry, temperature, and solar radiation.
Globally, the coral regions of the world have been ranked
along a spectrum of environmental suitability for coral
calcification – from “optimal” to “marginal” (Kleypas
et al., 1999; Guinotte et al., 2003). Seas with an aragonite
saturation (O aragonite ) of >4.0 are “optimal” and >3.5 are
“adequate,” whereas <3.5 is only “marginal”; sea temperatures >18
C are warm enough to facilitate polyp replication, gametogenesis, and skeletogenesis; solar
radiation needs to be sufficient to sustain a dense population of zooxanthellae with a high production of photosynthate. For these reasons, corals are rare and reefs are
absent at latitudes >35
north or south of the equator
(Veron, 1995): these latitude’s low sun angles, short winter day lengths, low temperatures, and low aragonite saturation (Figure 1) all militate against vigorous coral
growth. The geographic or depth limit beyond which
coral growth is vigorous enough for reef growth has been
termed the “Darwin Point” (Grigg, 1982).
Chemistry
A location’s position on the “optimal to marginal” spectrum of Guinotte et al. (2003) reflects the ease or difficulty
282
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH
Corals grow vigorously and build reefs in shallow tropical
seas, due to the favorable environment, both “latitudecorrelated environmental factors” (Veron, 1995) and factors that are not related to latitude. The latitude-correlated
environmental factors are solar radiation, temperature, and
water chemistry (Kleypas et al., 1999), and those not
related to latitude include nature and depth of the substratum, wave climate, salinity, water clarity, nutritional properties of that water, and sedimentation regime.
Collectively, they affect growth rate, growth form, reproduction and longevity of individual corals, the trajectories
of abundance, size and age-frequency distribution and
turnover rates of populations, and the composition and
diversity of coral communities. These population
and community attributes in turn influence the frequency
and amounts of calcium carbonate skeletons that can be
delivered to, and accumulate in, reef-building units
(framework, boulders, blocks, rubble, and sand) – i.e.,
contribute to the growth of maintenance of a coral reef
and adjacent sedimentary deposits against disaggregating
forces of waves, currents, and gravity in shallow tropical
seas.
Just as important as the normal fair-weather environment in determining a coral community’s extent and structure are the legacies of their history of development,
disturbance, and recovery. Coral communities (see Coral
Reef, Definition) are all works in progress – outcomes of
the timing and makeup of successive settling cohorts of
coral propagules (larvae and fragments) and a long list of
species-specific differences among corals: their performance in competition with other benthic organisms trying
to occupy the same piece of substratum (corals, other sessile invertebrates, and algae); attractiveness or resistance
to predators and diseases; susceptibility to breakage or dislodgement by storms; propensity to collapse under their
own top-heavy weight and fall off the reef into inhospitable depths; propensity to be dislodged, swept, rolled, or
pushed from the area by waves and currents.
Coral: the animal–plant symbiosis
Underpinning the coral vigor that builds reefs in tropical
seas is the presence of hundreds of thousands of “zooxanthellae” (single-celled dinoflagellate algae in the genus
Symbiodinium) within every square centimeter of the
coral’s tissues (Fagoonee et al., 1999; Fitt et al., 2000).
Corals that host zooxanthellae are referred to as
“zooxanthellate” corals. Within its diurnally and seasonally variable external environment, the coral needs to
provide an internal environment that nurtures the zooxanthellae. Whereas corals generally live for decades to centuries, the zooxanthellae populations and coral tissue in
which they reside turn over on time scales of weeks to
months (Fitt et al., 2000). Under extreme environmental
stress, the coral’s internal environment can become inhospitable and zooxanthellae populations may crash and the
coral tissue get damaged, leading to injury or death
of entire coral colonies across vast areas (see Climate
Change and Coral Reefs).
Within coral’s favored environment, the symbiotic
arrangement between the coral “host” and its zooxanthella
“symbiont” provides such a hospitable intracellular environment for the symbionts that their photosynthesis produces a massive surplus of energy-rich compounds
(Gattuso et al., 1999). These are used by the coral polyps
to fuel their replication and to deposit their shared skeleton. The rate of calcification (see Definition above) in
zooxanthellate corals far exceeds that which is possible
in cnidarians that lack zooxanthellae: their presence drives
“light-enhanced” calcification (Goreau and Goreau,
1959). However, one decade into the twenty-first century,
the coral–zooxanthellae symbiosis, in existence since the
Tertiary and critical for the development of coral reefs
(Stanley and Swart, 1995), faces increasing incidence
and severity of both sublethal stresses and lethal extreme
events. Recent reviews of implications of global climate
change for corals and reefs include Done (1999);
Buddemeier et al. (2004); Chadwick-Furman (2006);
Kleypas and Langdon (2006); Guinotte and Fabry
(2008); and Veron (2008). See also below in this article
and the Chapter on Climate Change.
Environmental controls on global distribution
of corals
Today’s global coral distribution (Figure 1) is in part,
a legacy of earlier dispersal and establishment processes
(Veron, 1995) and in part, a reflection of habitability
constraints imposed by the present day environment,
notably, those affecting coral’s capacity to calcify.
Light-enhanced calcification requires a particular regime
of water chemistry, temperature, and solar radiation.
Globally, the coral regions of the world have been ranked
along a spectrum of environmental suitability for coral
calcification – from “optimal” to “marginal” (Kleypas
et al., 1999; Guinotte et al., 2003). Seas with an aragonite
saturation (O aragonite ) of >4.0 are “optimal” and >3.5 are
“adequate,” whereas <3.5 is only “marginal”; sea temperatures >18
C are warm enough to facilitate polyp replication, gametogenesis, and skeletogenesis; solar
radiation needs to be sufficient to sustain a dense population of zooxanthellae with a high production of photosynthate. For these reasons, corals are rare and reefs are
absent at latitudes >35
north or south of the equator
(Veron, 1995): these latitude’s low sun angles, short winter day lengths, low temperatures, and low aragonite saturation (Figure 1) all militate against vigorous coral
growth. The geographic or depth limit beyond which
coral growth is vigorous enough for reef growth has been
termed the “Darwin Point” (Grigg, 1982).
Chemistry
A location’s position on the “optimal to marginal” spectrum of Guinotte et al. (2003) reflects the ease or difficulty
282
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH