2. Geomorphology of Coral Reefs
with Special Reference to the
Great Barrier Reef
D. Hopley
Coral reefs are found in shallow waters throughout the
tropics. Although corals contribute the largest amount
to reef-building (up to 10 kg CaCO 3 m
2 yr
1
) many other
organisms that secrete limestone-like forms of calcium
carbonate such as coralline algae, molluscs, bryozoans,
and the green alga Halimeda also play a part. Simultaneously, other plants and animals such as chitons, clionid
sponges, clams and parrotfish will be destroying the
reef through bioerosion at rates that can equal those of
production. The reef structure is thus the net result of
construction and removal, so that not everywhere that
corals grow do significant reefal structures develop.
However, corals remain the most important contributors to reef growth and have some strict environmental requirements, many of which are related to the
symbiotic zooxanthellae that require light for photosynthesis. Thus, corals in turbid waters may be limited
to a depth of only 8 m but in the clear waters of the
outer shelf of the GBR on Myrmidon Reef off Townsville
living corals have been observed to depths of 100 m
and Halimeda to 125 m.
Temperature is also important. The optimum temperature is about 26–27°C with an absolute range of
18–36°C; extremes being tolerable for short periods, for
5
example, at low tide in reef flat pools. Individual corals
appear to adapt to their ambient environmental conditions, generally also through the tolerance of their zooxanthellae. Temperature excursions, either higher or lower
from the normal range, even if withstood by the same
species elsewhere, may locally result in expulsion of the
zooxanthellae, coral bleaching and possible mortality.
Salinity affects most reef builders and optimal conditions of about 36‰ or about the level of the open
ocean are ideal, with the accepted range being between
30‰ and 40‰. Lower salinity produced by heavy rainfall or runoff can only be tolerated for short periods
and is generally the reason for the lack of reefs close to
the mouths of major rivers. As the freshwater floats
on the salt water, lowering of the low salinity surface
water onto a reef flat at low tide can be devastating.
Turbidity is also regarded as a major controlling factor on reef growth. Muddy substrata are unsuitable for
larval settlement and reefs are not found where fine
sediments have accumulated over time. Turbidity, the
result of suspension of sediment in the water column,
is more transient and surprisingly high levels may be
tolerated. The study of fringing reefs at Cape Tribulation and Magnetic Island have shown that much of the
with Special Reference to the
Great Barrier Reef
D. Hopley
Coral reefs are found in shallow waters throughout the
tropics. Although corals contribute the largest amount
to reef-building (up to 10 kg CaCO 3 m
2 yr
1
) many other
organisms that secrete limestone-like forms of calcium
carbonate such as coralline algae, molluscs, bryozoans,
and the green alga Halimeda also play a part. Simultaneously, other plants and animals such as chitons, clionid
sponges, clams and parrotfish will be destroying the
reef through bioerosion at rates that can equal those of
production. The reef structure is thus the net result of
construction and removal, so that not everywhere that
corals grow do significant reefal structures develop.
However, corals remain the most important contributors to reef growth and have some strict environmental requirements, many of which are related to the
symbiotic zooxanthellae that require light for photosynthesis. Thus, corals in turbid waters may be limited
to a depth of only 8 m but in the clear waters of the
outer shelf of the GBR on Myrmidon Reef off Townsville
living corals have been observed to depths of 100 m
and Halimeda to 125 m.
Temperature is also important. The optimum temperature is about 26–27°C with an absolute range of
18–36°C; extremes being tolerable for short periods, for
5
example, at low tide in reef flat pools. Individual corals
appear to adapt to their ambient environmental conditions, generally also through the tolerance of their zooxanthellae. Temperature excursions, either higher or lower
from the normal range, even if withstood by the same
species elsewhere, may locally result in expulsion of the
zooxanthellae, coral bleaching and possible mortality.
Salinity affects most reef builders and optimal conditions of about 36‰ or about the level of the open
ocean are ideal, with the accepted range being between
30‰ and 40‰. Lower salinity produced by heavy rainfall or runoff can only be tolerated for short periods
and is generally the reason for the lack of reefs close to
the mouths of major rivers. As the freshwater floats
on the salt water, lowering of the low salinity surface
water onto a reef flat at low tide can be devastating.
Turbidity is also regarded as a major controlling factor on reef growth. Muddy substrata are unsuitable for
larval settlement and reefs are not found where fine
sediments have accumulated over time. Turbidity, the
result of suspension of sediment in the water column,
is more transient and surprisingly high levels may be
tolerated. The study of fringing reefs at Cape Tribulation and Magnetic Island have shown that much of the