regimes of temperature, oxygenation, salinity, and turbidity
have become inimical for coral growth (Macintyre, 2007).
Massive structure
A coral reef is “massive” (relative to the size of a human
observer) in terms of both its basal area (usually hectares
and upward) and its thickness (a nonliving basement usually meters to tens of meters thick and the living corals and
diverse other sessile biota attached to it – usually <1 m to
a few meters tall). Coral covered substrata of smaller
dimensions than these could still legitimately be referred
to as “coral reefs,” but so long as they were in close vicinity to a larger structure, they would generally be referred to
as “patch reefs,” and considered to be a physically discrete
part of the larger unit: e.g., the patch reefs at the back of
such and such a reef.
A generalized picture of the “classic” reef-building processes based on James and Macintyre (1985) and Kan
et al. (1997) is as follows: large corals of diverse and irregular shapes remain in place after their death, forming
roofed-over cavities (robust branching forms such as
Acropora palmata – James and Macintyre, 1985) or stacks
of head and branching corals (Kan et al., 1997) – the
whole becoming a “framework” that becomes inhabited
by smaller attached calcareous benthos. Encrusting organisms (calcareous algae) grow over dead surfaces, aid in
stabilizing the structure, and provide environmental cues
that promote settlement of new corals (Harrington et al.,
2004). Assuming there is sufficient water above
(Figure 1; see entry Accommodation Space), another layer
of corals can thereby settle, grow, die, become incorporated into the reef framework, and thus incrementally
thicken the reef structure (Pichon, 1974; Kan et al.,
1997). In stormy weather (Figure 1c), weakly attached or
branching corals are broken, dislodged, and deposited in
reef cavities and skeletal conglomerations on the reef
slope or adjacent sea floor, allowing the reef to extend laterally by “stepping out over its own debris” (Blanchon and
Jones, 1997). The latter process may be a result of settlement of coral larvae on the debris, or regeneration of still
viable coral fragments among the rubble.
Another equally important process is infilling. The
developing framework can be buried by or partially filled
with sediment, with grain sizes from fine silts to coarse rubble, either before or after significant cementation has taken
place. The types of corals, types of sediments, stage of
cementation, and time and rate of burial produce distinctive
layers in the sedimentary profiles of the reef (Shinn et al.,
1977) that are reflected in distinctive limestone rocks in fossilized reefs (James and Macintyre, 1985; e.g., see entries
Rudstone, Bafflestone, Framestone, Floatstone).
Wave resistant structure
All coral reef definitions specify a structure that is
highly resistant to wave impacts. Strong swells and
waves do sometimes damage framework and cause
catastrophic losses to reef biota (see entry Tropical
Cyclone/Hurricane). However, the change (in terms of
tonnes of reef debris created and moved) is generally
insignificant compared to mass and volume of reef
framework that remain intact and in place. Moreover,
the dissipation of wave energy across reef flats, the
refraction of waves around reefs, and the flushing of
sand through canyons by return flows are highly instrumental in determining the shape of reefs (Blanchon and
Jones, 1997). Such a wave on reef interaction promotes
the retention of reef-derived blocks, rubble, and finer
sediments within the existing reef footprint, and thus
the reef’s propensity to “step out.”
Associated sediments
Coral reefs generate massive amounts of carbonate sediments. Fine sediments (sands and silts) are derived from
both bioerosion of large metazoans such as corals, and
the post-mortem disintegration other calcifiers, notably
foraminifera, small molluscs, and diverse segmented calcareous organisms including coralline algae and crinoids
(James and Macintyre, 1985). Coarse sediments (boulders, blocks, rubble, shingle) are moved by storms, currents and gravity (see above and entries Tropical
Cyclone/Hurricane, Hydrodynamics of Coral Reef Systems). They fall into lower energy deposition sites, including crevices within the reef, talus beds at the base of reef
slopes, reef-flat boulder ramparts, sheltered back-reef
lagoons and aprons, and gutters. In reefs far from land,
most if not all sediments are generated by the reef. However, on some coastal reefs, sediments of terrestrial origin
can dominate infilling sediments and the near-reef sedimentary apron (Kleypas et al., 2001; Larcombe and Carter, 2004). These imported sediments may include
(1) sediments from the present-day land mass (via surface
runoff, dust or ash) and/or (2) sediments from nowsubmerged mid-Holocene coastal low lands, re-suspended
and swept into and around reefs by currents and waves
(Larcombe and Carter, 2004).
Substantially built by successive generations of
corals and other reef builders
Over scales of decades to millennia, reef growth is dependent on ecological resilience in the resident stony coral
populations and other reef builders – their capacity to
recover dense populations following those necessary
physical disturbances that transform some of them into
debris. In a well functioning reef, “other reef builders” perform a number of direct and indirect “bioconstructional
roles” in addition to the primary framework building role
of stony corals and encrusting coralline algae (Done
et al., 1996): “secondary framework builders” such as
byrozoans and bivalve molluscs and tube worms add
small scale topographic complexity to the framework;
microbes condition substrata for coral settlement and contribute to wave-resistance through submarine lithification;
“non-framework reef builders” such as foraminifera, erect
coralline algae (especially the genus Halimeda) and molluscs contribute greatly to reef sediments; “sediment
CORAL REEF, DEFINITION
265
have become inimical for coral growth (Macintyre, 2007).
Massive structure
A coral reef is “massive” (relative to the size of a human
observer) in terms of both its basal area (usually hectares
and upward) and its thickness (a nonliving basement usually meters to tens of meters thick and the living corals and
diverse other sessile biota attached to it – usually <1 m to
a few meters tall). Coral covered substrata of smaller
dimensions than these could still legitimately be referred
to as “coral reefs,” but so long as they were in close vicinity to a larger structure, they would generally be referred to
as “patch reefs,” and considered to be a physically discrete
part of the larger unit: e.g., the patch reefs at the back of
such and such a reef.
A generalized picture of the “classic” reef-building processes based on James and Macintyre (1985) and Kan
et al. (1997) is as follows: large corals of diverse and irregular shapes remain in place after their death, forming
roofed-over cavities (robust branching forms such as
Acropora palmata – James and Macintyre, 1985) or stacks
of head and branching corals (Kan et al., 1997) – the
whole becoming a “framework” that becomes inhabited
by smaller attached calcareous benthos. Encrusting organisms (calcareous algae) grow over dead surfaces, aid in
stabilizing the structure, and provide environmental cues
that promote settlement of new corals (Harrington et al.,
2004). Assuming there is sufficient water above
(Figure 1; see entry Accommodation Space), another layer
of corals can thereby settle, grow, die, become incorporated into the reef framework, and thus incrementally
thicken the reef structure (Pichon, 1974; Kan et al.,
1997). In stormy weather (Figure 1c), weakly attached or
branching corals are broken, dislodged, and deposited in
reef cavities and skeletal conglomerations on the reef
slope or adjacent sea floor, allowing the reef to extend laterally by “stepping out over its own debris” (Blanchon and
Jones, 1997). The latter process may be a result of settlement of coral larvae on the debris, or regeneration of still
viable coral fragments among the rubble.
Another equally important process is infilling. The
developing framework can be buried by or partially filled
with sediment, with grain sizes from fine silts to coarse rubble, either before or after significant cementation has taken
place. The types of corals, types of sediments, stage of
cementation, and time and rate of burial produce distinctive
layers in the sedimentary profiles of the reef (Shinn et al.,
1977) that are reflected in distinctive limestone rocks in fossilized reefs (James and Macintyre, 1985; e.g., see entries
Rudstone, Bafflestone, Framestone, Floatstone).
Wave resistant structure
All coral reef definitions specify a structure that is
highly resistant to wave impacts. Strong swells and
waves do sometimes damage framework and cause
catastrophic losses to reef biota (see entry Tropical
Cyclone/Hurricane). However, the change (in terms of
tonnes of reef debris created and moved) is generally
insignificant compared to mass and volume of reef
framework that remain intact and in place. Moreover,
the dissipation of wave energy across reef flats, the
refraction of waves around reefs, and the flushing of
sand through canyons by return flows are highly instrumental in determining the shape of reefs (Blanchon and
Jones, 1997). Such a wave on reef interaction promotes
the retention of reef-derived blocks, rubble, and finer
sediments within the existing reef footprint, and thus
the reef’s propensity to “step out.”
Associated sediments
Coral reefs generate massive amounts of carbonate sediments. Fine sediments (sands and silts) are derived from
both bioerosion of large metazoans such as corals, and
the post-mortem disintegration other calcifiers, notably
foraminifera, small molluscs, and diverse segmented calcareous organisms including coralline algae and crinoids
(James and Macintyre, 1985). Coarse sediments (boulders, blocks, rubble, shingle) are moved by storms, currents and gravity (see above and entries Tropical
Cyclone/Hurricane, Hydrodynamics of Coral Reef Systems). They fall into lower energy deposition sites, including crevices within the reef, talus beds at the base of reef
slopes, reef-flat boulder ramparts, sheltered back-reef
lagoons and aprons, and gutters. In reefs far from land,
most if not all sediments are generated by the reef. However, on some coastal reefs, sediments of terrestrial origin
can dominate infilling sediments and the near-reef sedimentary apron (Kleypas et al., 2001; Larcombe and Carter, 2004). These imported sediments may include
(1) sediments from the present-day land mass (via surface
runoff, dust or ash) and/or (2) sediments from nowsubmerged mid-Holocene coastal low lands, re-suspended
and swept into and around reefs by currents and waves
(Larcombe and Carter, 2004).
Substantially built by successive generations of
corals and other reef builders
Over scales of decades to millennia, reef growth is dependent on ecological resilience in the resident stony coral
populations and other reef builders – their capacity to
recover dense populations following those necessary
physical disturbances that transform some of them into
debris. In a well functioning reef, “other reef builders” perform a number of direct and indirect “bioconstructional
roles” in addition to the primary framework building role
of stony corals and encrusting coralline algae (Done
et al., 1996): “secondary framework builders” such as
byrozoans and bivalve molluscs and tube worms add
small scale topographic complexity to the framework;
microbes condition substrata for coral settlement and contribute to wave-resistance through submarine lithification;
“non-framework reef builders” such as foraminifera, erect
coralline algae (especially the genus Halimeda) and molluscs contribute greatly to reef sediments; “sediment
CORAL REEF, DEFINITION
265