temperatures can cause expulsion of zoothanthellae, loss
of photosynthetic pigments, and subsequent decline of
physiological performance, including calcification (Porter
et al., 1989), and in severe cases, it can cause death of the
coral (Loya et al., 2001). Beyond the latitudinal margins of
coral and reef distribution, cold water events,
superimposed on low light and aragonite saturation, are
sufficiently frequent to cease to be anomalies and to be
considered part of a normal seasonal variability that
excludes phototrophic corals from establishing and growing, even on suitable substrata. For that reason, the corals
on deep cold water reefs lack zooxanthellae, rely entirely
on particulate food, and grow very slowly (Roberts et al.,
2006).
Environmental variability within regions
Within the broad bounds set by water chemistry, temperature, and solar radiation (see above), environmental factors affecting coral growth also vary greatly over scales
of kilometers to tens of kilometers: nature and depth of
the substratum, temperature (Figure 2c), wave climate,
salinity, water clarity, nutritional properties of that water,
and sedimentation regime. Clear waters have traditionally
been considered “optimal” for coral growth, and naturally
turbid water reefs “marginal,” but for the purposes of the
present account, they are viewed as “just different” (Perry
and Larcombe, 2003).
Hydrodynamics, light, and sediments
Either too little or too much water motion can limit coral
growth and survival. Whereas in moderation, water
motion facilitates gas and nutrient exchange through
the coral’s surface tissues (Atkinson and Bilger, 1992),
strong waves and currents cause physical destruction
(see Hydrodynamics in Coral Reef Systems; Tropical
Cyclone/Hurricane), and insufficient flow limits gas and
nutrient exchange to the detriment of coral health and survival. Too little flow, which exposes corals to potentially
fatal physiological stress, can occur over large areas of
the ocean in doldrum conditions. Corals in enclosed reef
flats and lagoons are particularly vulnerable to high temperatures, flat glassy waters, and prolonged periods of
calm weather. Low- or no-flow conditions thicken the diffusive boundary layer around corals, reduce gas exchange,
and make it hypoxic, inducing anaerobiosis in the coral.
Moreover, intense light beams condensed by surface ripples can exacerbate the stress caused by high temperatures
and low flow, causing corals to bleach at such times
(Nakamura and van Woesik, 2001).
Within those habitats where water is in motion, hydrodynamic variables (wave climate, currents, and residence
time – see Hydrodynamics in Coral Reef Systems) are
important determinants of where corals can grow and what
morphological form they adopt. In oceanic settings, reefs
and islands offer a broad range of environments for corals.
Shallow reefs that are exposed to the full force of ocean
swells and breaking waves do not necessarily support
significant coral growth. On the outer Great Barrier Reef,
for example, corals do build physically robust, waveresistant structures in the surf break area (Done, 1982).
But adjacent areas of the reef are often occupied by coral
assemblages consisting of mainly small, short-lived
corals, whose longevity is severely limited by recurrent
breakage, scour, and abrasion. The latter structure
and ephemeral dynamics are comparable to those
described for some shallow reef slopes on Oahu, Hawaii,
where the turnover of corals is so great that the coral veneer
is rarely thicker than a single living colony (Grigg, 1998).
The contrast of wave-beaten reefs with those of highly
enclosed, shallow reef lagoons and embayments is striking. These latter habitats are often occupied by dense
and extensive populations of corals whose shapes, sizes
(often large), and longevity (often very great) are
extremely variable from place to place, their specific composition dictated by the composition of the regional species pool, founder events, and environmental factors
other than waves, such as light, sediments, and currents.
On continental shelves, there are greater complexities
of environmental pattern and process, as exemplified in
the Great Barrier Reef (GBR), whose 2,900 reefs are distributed across a seafloor that slopes from coastal beaches
and headlands to a depth of $80m at the shelf edge
50–100 km offshore. On most days of the year, big, long
period waves generated by distant meteorological drivers
surge through interreef passages and break on the outermost reefs. They dissipate strong hydrodynamic forces
in the reef’s surf zones and across their tops (Hearn,
1999), and, in conjunction with tidal flows, set up powerful turbulent jets and eddies around their flanks (Wolanski
and Hamner, 1988). Depending on reef size, shape, orientation, weather, and state of the tide, different places along
the leeward sides of these outer reefs may be completely
calm, or they may be impacted by waves from the opposite
direction, generated by local winds. These wind waves,
which are of smaller wave height and shorter period than
the ocean swells, intersect and combine with those oceanic
swells that penetrate to behind the outer reefs and then
lose height, velocity, and power as they are intercepted
by mid-shelf reefs. As they travel into shallow coastal
waters, further energy is lost to bottom friction, coarse
sediments are rolled along the bottom, and fine sediments
are resuspended and mixed through the water column,
making it turbid (suspended sediment concentrations of
10–100 mg l
À1
– Larcombe and Carter, 2004). When
the winds subside and tides slacken, the coastal waters
clear, as suspended sediments fall to the bottom.
Nutrients
Corals need fixed nitrogen (NH
4+ , NO
2À , NO
3À ) and
phosphate (PO 4
3À
) to synthesize proteins and nuclear
material for cell maintenance, growth, and reproduction,
and they are well adapted to exploit them in the low concentrations of oligotrophic surface waters in the oceans.
However, excess nutrients are detrimental to coral growth
286
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH
of photosynthetic pigments, and subsequent decline of
physiological performance, including calcification (Porter
et al., 1989), and in severe cases, it can cause death of the
coral (Loya et al., 2001). Beyond the latitudinal margins of
coral and reef distribution, cold water events,
superimposed on low light and aragonite saturation, are
sufficiently frequent to cease to be anomalies and to be
considered part of a normal seasonal variability that
excludes phototrophic corals from establishing and growing, even on suitable substrata. For that reason, the corals
on deep cold water reefs lack zooxanthellae, rely entirely
on particulate food, and grow very slowly (Roberts et al.,
2006).
Environmental variability within regions
Within the broad bounds set by water chemistry, temperature, and solar radiation (see above), environmental factors affecting coral growth also vary greatly over scales
of kilometers to tens of kilometers: nature and depth of
the substratum, temperature (Figure 2c), wave climate,
salinity, water clarity, nutritional properties of that water,
and sedimentation regime. Clear waters have traditionally
been considered “optimal” for coral growth, and naturally
turbid water reefs “marginal,” but for the purposes of the
present account, they are viewed as “just different” (Perry
and Larcombe, 2003).
Hydrodynamics, light, and sediments
Either too little or too much water motion can limit coral
growth and survival. Whereas in moderation, water
motion facilitates gas and nutrient exchange through
the coral’s surface tissues (Atkinson and Bilger, 1992),
strong waves and currents cause physical destruction
(see Hydrodynamics in Coral Reef Systems; Tropical
Cyclone/Hurricane), and insufficient flow limits gas and
nutrient exchange to the detriment of coral health and survival. Too little flow, which exposes corals to potentially
fatal physiological stress, can occur over large areas of
the ocean in doldrum conditions. Corals in enclosed reef
flats and lagoons are particularly vulnerable to high temperatures, flat glassy waters, and prolonged periods of
calm weather. Low- or no-flow conditions thicken the diffusive boundary layer around corals, reduce gas exchange,
and make it hypoxic, inducing anaerobiosis in the coral.
Moreover, intense light beams condensed by surface ripples can exacerbate the stress caused by high temperatures
and low flow, causing corals to bleach at such times
(Nakamura and van Woesik, 2001).
Within those habitats where water is in motion, hydrodynamic variables (wave climate, currents, and residence
time – see Hydrodynamics in Coral Reef Systems) are
important determinants of where corals can grow and what
morphological form they adopt. In oceanic settings, reefs
and islands offer a broad range of environments for corals.
Shallow reefs that are exposed to the full force of ocean
swells and breaking waves do not necessarily support
significant coral growth. On the outer Great Barrier Reef,
for example, corals do build physically robust, waveresistant structures in the surf break area (Done, 1982).
But adjacent areas of the reef are often occupied by coral
assemblages consisting of mainly small, short-lived
corals, whose longevity is severely limited by recurrent
breakage, scour, and abrasion. The latter structure
and ephemeral dynamics are comparable to those
described for some shallow reef slopes on Oahu, Hawaii,
where the turnover of corals is so great that the coral veneer
is rarely thicker than a single living colony (Grigg, 1998).
The contrast of wave-beaten reefs with those of highly
enclosed, shallow reef lagoons and embayments is striking. These latter habitats are often occupied by dense
and extensive populations of corals whose shapes, sizes
(often large), and longevity (often very great) are
extremely variable from place to place, their specific composition dictated by the composition of the regional species pool, founder events, and environmental factors
other than waves, such as light, sediments, and currents.
On continental shelves, there are greater complexities
of environmental pattern and process, as exemplified in
the Great Barrier Reef (GBR), whose 2,900 reefs are distributed across a seafloor that slopes from coastal beaches
and headlands to a depth of $80m at the shelf edge
50–100 km offshore. On most days of the year, big, long
period waves generated by distant meteorological drivers
surge through interreef passages and break on the outermost reefs. They dissipate strong hydrodynamic forces
in the reef’s surf zones and across their tops (Hearn,
1999), and, in conjunction with tidal flows, set up powerful turbulent jets and eddies around their flanks (Wolanski
and Hamner, 1988). Depending on reef size, shape, orientation, weather, and state of the tide, different places along
the leeward sides of these outer reefs may be completely
calm, or they may be impacted by waves from the opposite
direction, generated by local winds. These wind waves,
which are of smaller wave height and shorter period than
the ocean swells, intersect and combine with those oceanic
swells that penetrate to behind the outer reefs and then
lose height, velocity, and power as they are intercepted
by mid-shelf reefs. As they travel into shallow coastal
waters, further energy is lost to bottom friction, coarse
sediments are rolled along the bottom, and fine sediments
are resuspended and mixed through the water column,
making it turbid (suspended sediment concentrations of
10–100 mg l
À1
– Larcombe and Carter, 2004). When
the winds subside and tides slacken, the coastal waters
clear, as suspended sediments fall to the bottom.
Nutrients
Corals need fixed nitrogen (NH
4+ , NO
2À , NO
3À ) and
phosphate (PO 4
3À
) to synthesize proteins and nuclear
material for cell maintenance, growth, and reproduction,
and they are well adapted to exploit them in the low concentrations of oligotrophic surface waters in the oceans.
However, excess nutrients are detrimental to coral growth
286
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH