The Great Barrier Reef
6
turbidity is caused by resuspension of sediments during rough weather and, as long as the sediment does
not settle on the reef and is kept moving by wave action, turbidity levels of more than 100 mg L
1 can be
withstood. Further, corals from inshore environments
are far more tolerant of high suspended sediment levels than exactly the same species on mid and outer
shelves. Great diversity can be maintained on these
reefs, for example, 141 species from 50 genera of hard
corals have been recorded from the Cape Tribulation
reefs. Thus, although the total amount of sediment input to GBR waters has increased by more than three
times in the last 150 years this is insignificant compared
to the billions of tonnes delivered to the nearshore zone
and resuspended by wave and current activity over the
last 6000 years. Only where new sediment is reaching
existing reefs does it have detrimental effects. For example, the fringing reef on High Island, south of Cairns,
extends down to 20 m depth and new sediment settling
on the lower part of this reef is causing damage as it is
below the level of wave action and cannot be resuspended. Increased nutrients may also be associated with
increased sediment yield, further adding to reef decline.
N GEOMORPHOLOGICAL ZONATION
Coral reefs generally display a strong zonational pattern parallel to the reef front. Ecological zones are
clearly related to these geomorphological zones and
the higher the energy conditions, the stronger and more
distinct the zonation:
L The windward coral-covered reef front is generally
steeper in the high energy areas such as the ribbon
reefs. Near the top of the reef front may be a distinctive ‘spur and groove’ zone of coral covered buttresses and intervening channels within which
sediment may move.
L The reef crest commences with a living coral zone
just below the low tide level. Then follows an intertidal algal pavement, the exact composition of which
varies but on the GBR is rarely similar to the prominent coralline algal ridge of mid oceanic atolls. Coralline algae are present, but this zone is dominated
by turf algae, which hold together sediment such as
coral shingle. On this surface there may be large reef
blocks up to 4 m in diameter, torn from the reef front
in cyclones or by tsunamis.
L The highest part of the reef is a rubble zone of coral
shingle and larger fragments, forming a shingle
rampart. It is formed of material from the reef front
and deposited by waves as they lose energy passing
over the reef flat.
L The aligned coral zone forms the rearmost part of
the reef flat, with coral colonies growing in lines perpendicular to the reef front and separated by narrow
sandy channels.
L The sheltered back-reef area, often in the form of
a sand slope, contains the most fragile branching
colonies.
Within this zonation are distinctive changes to coral
colonial morphology (Fig. 2.1). Light, wave energy, and
emersion are the major controls on both ecomorphology and diversity. At greatest depth on the reef front,
light is the most limiting control and corals may adopt
a globose or plate-like shape. Higher up, in the optimal
photic zone, but below wave base, colonies can be intricately branching, but moving up into the wave zone
stronger colonial structures dominate with encrusting
forms on the reef crest.
On the reef flat a variety of forms may be found,
with exposure the limiting factor. The upper limit to
coral growth is about low tide level but varies with species and with local environmental conditions related,
for example, to wave action. However, in some instances the low tide level may be controlled by moating
behind shingle ramparts or algal ridges and under
these conditions, the same level is achieved on every
low tide. Corals respond by growing to these levels and
form distinctive microatolls: flat discs usually circular
and up to half a metre thick. Both head and branching
corals form microatolls, though the majority are head
corals, the most common being Porites sp. They can be
up to 10 m in diameter, and, as their upper surface is
determined by the moated low tide level, any change
in this level, for example as the result of a cyclone lowering a shingle rampart, will produce a change to the
surface morphology. Annual growth rings can determine the age of these environmental modifications.
6
turbidity is caused by resuspension of sediments during rough weather and, as long as the sediment does
not settle on the reef and is kept moving by wave action, turbidity levels of more than 100 mg L
1 can be
withstood. Further, corals from inshore environments
are far more tolerant of high suspended sediment levels than exactly the same species on mid and outer
shelves. Great diversity can be maintained on these
reefs, for example, 141 species from 50 genera of hard
corals have been recorded from the Cape Tribulation
reefs. Thus, although the total amount of sediment input to GBR waters has increased by more than three
times in the last 150 years this is insignificant compared
to the billions of tonnes delivered to the nearshore zone
and resuspended by wave and current activity over the
last 6000 years. Only where new sediment is reaching
existing reefs does it have detrimental effects. For example, the fringing reef on High Island, south of Cairns,
extends down to 20 m depth and new sediment settling
on the lower part of this reef is causing damage as it is
below the level of wave action and cannot be resuspended. Increased nutrients may also be associated with
increased sediment yield, further adding to reef decline.
N GEOMORPHOLOGICAL ZONATION
Coral reefs generally display a strong zonational pattern parallel to the reef front. Ecological zones are
clearly related to these geomorphological zones and
the higher the energy conditions, the stronger and more
distinct the zonation:
L The windward coral-covered reef front is generally
steeper in the high energy areas such as the ribbon
reefs. Near the top of the reef front may be a distinctive ‘spur and groove’ zone of coral covered buttresses and intervening channels within which
sediment may move.
L The reef crest commences with a living coral zone
just below the low tide level. Then follows an intertidal algal pavement, the exact composition of which
varies but on the GBR is rarely similar to the prominent coralline algal ridge of mid oceanic atolls. Coralline algae are present, but this zone is dominated
by turf algae, which hold together sediment such as
coral shingle. On this surface there may be large reef
blocks up to 4 m in diameter, torn from the reef front
in cyclones or by tsunamis.
L The highest part of the reef is a rubble zone of coral
shingle and larger fragments, forming a shingle
rampart. It is formed of material from the reef front
and deposited by waves as they lose energy passing
over the reef flat.
L The aligned coral zone forms the rearmost part of
the reef flat, with coral colonies growing in lines perpendicular to the reef front and separated by narrow
sandy channels.
L The sheltered back-reef area, often in the form of
a sand slope, contains the most fragile branching
colonies.
Within this zonation are distinctive changes to coral
colonial morphology (Fig. 2.1). Light, wave energy, and
emersion are the major controls on both ecomorphology and diversity. At greatest depth on the reef front,
light is the most limiting control and corals may adopt
a globose or plate-like shape. Higher up, in the optimal
photic zone, but below wave base, colonies can be intricately branching, but moving up into the wave zone
stronger colonial structures dominate with encrusting
forms on the reef crest.
On the reef flat a variety of forms may be found,
with exposure the limiting factor. The upper limit to
coral growth is about low tide level but varies with species and with local environmental conditions related,
for example, to wave action. However, in some instances the low tide level may be controlled by moating
behind shingle ramparts or algal ridges and under
these conditions, the same level is achieved on every
low tide. Corals respond by growing to these levels and
form distinctive microatolls: flat discs usually circular
and up to half a metre thick. Both head and branching
corals form microatolls, though the majority are head
corals, the most common being Porites sp. They can be
up to 10 m in diameter, and, as their upper surface is
determined by the moated low tide level, any change
in this level, for example as the result of a cyclone lowering a shingle rampart, will produce a change to the
surface morphology. Annual growth rings can determine the age of these environmental modifications.