WORLD ATLAS OF CORAL REEFS
scour many of the channels between the reefs. Within the
lagoons of individual reefs there are complex patterns of
channels and mid-reef coral ridges. To the south the Swain
Reefs form a second distinctive complex, dominated by
many much smaller and even more closely spaced reefs
where large numbers of small coral cays have developed.
Inshore of the main Swain and Pompey Complexes reef
development is limited, however there are some reefs
close to the mainland and associated with island groups
such as the Northumberland Islands and Percy Isles.
The Capricorn-Bunker group
South of the Swain Reefs Complex the contmental shelf
rapidly narrows again and the southernmost reefs of the
Great Barrier Reef, the Capricorn-Bunker group, lie a
little over 50 kilometers offshore. This is a relatively small
complex, well defined with steeply sloping reef edges and
deep inter-reefal waters. There are several well developed
coral cays, including One Tree Island and Heron Island,
which are among the best known reefs of the entire Great
Barrier Reef These reefs acmally traverse the tropic of
Capricorn and cooler waters are largely responsible for the
lower coral diversities found here.
Biodiversity
Levels of diversity are generally very high in the Great
Barrier Reef, with some 350 coral species, 1 500-2 000
species of fish, and over 4 000 species of mollusc. While
these large numbers may be partly a function of the fact
that this area has been intensively studied by scientists for
many years, it is further related to the vast area of reefs
as well as the great diversity of reef types and physical
conditions. Over such a large area it is impossible to generalize about coral cover. However, it should be noted that
cyclones and crown-of-thorns starfish clearly impact such
statistics. The central areas of the Great Barrier Reef are
the most affected by both of these phenomena, and many
reefs in these parts have relatively low levels of coral
cover when compared to reefs elsewhere in the world. This
may well be their natural state, which clearly points to the
caution which must be exercised when utilizing measures
of coral cover as an expression of reef health.
As might be expected with any reef system traversing
such a wide latitudinal range, there is a gradual diminution
of species diversity towards higher latitudes. While most of
the 350 coral species are recorded in the north, only about
244 species are recorded further south. Even more notable
are cross-shelf differences. Close to the mainland there are
high levels of nutrient inputs, sediments and freshwater,
while offshore such inputs diminish and conditions on
the outer reefs can be considered near oceanic, with low
levels of nutrients and clear waters. These differences have
led to considerable variation in the species assemblages
depending on their location on the continental shelf Such
differences are further maintained by the patterns of water
movement in the Great Barrier Reef generally north-south
with little cross-shelf transport.
As mentioned in Chapter 1. many corals reproduce
in a mass spawning event which takes place once a year.
While this is globally widespread, it was first observed and
is best documented on the Great Barrier Reef For a few
nights after a particular full moon in the late austral
spring (typically November) the majority of scleractinian
coral species, together with many other reef organisms
including sponges, holothurians, polychaetes and giant
clams, undergo a mass spawning event. This is highly
Left: To the south the coastai shelf of the Great Barrier Reef widens considerably around the vast complex of the Swain
and Pompey Reefs, before narrowing again around the small Capricorn group ISTS043-151-77, 19911. Right: The beaked
butterflyfish Chelmon rostratus is found across Southeast Asia and the Great Barrier Reef
scour many of the channels between the reefs. Within the
lagoons of individual reefs there are complex patterns of
channels and mid-reef coral ridges. To the south the Swain
Reefs form a second distinctive complex, dominated by
many much smaller and even more closely spaced reefs
where large numbers of small coral cays have developed.
Inshore of the main Swain and Pompey Complexes reef
development is limited, however there are some reefs
close to the mainland and associated with island groups
such as the Northumberland Islands and Percy Isles.
The Capricorn-Bunker group
South of the Swain Reefs Complex the contmental shelf
rapidly narrows again and the southernmost reefs of the
Great Barrier Reef, the Capricorn-Bunker group, lie a
little over 50 kilometers offshore. This is a relatively small
complex, well defined with steeply sloping reef edges and
deep inter-reefal waters. There are several well developed
coral cays, including One Tree Island and Heron Island,
which are among the best known reefs of the entire Great
Barrier Reef These reefs acmally traverse the tropic of
Capricorn and cooler waters are largely responsible for the
lower coral diversities found here.
Biodiversity
Levels of diversity are generally very high in the Great
Barrier Reef, with some 350 coral species, 1 500-2 000
species of fish, and over 4 000 species of mollusc. While
these large numbers may be partly a function of the fact
that this area has been intensively studied by scientists for
many years, it is further related to the vast area of reefs
as well as the great diversity of reef types and physical
conditions. Over such a large area it is impossible to generalize about coral cover. However, it should be noted that
cyclones and crown-of-thorns starfish clearly impact such
statistics. The central areas of the Great Barrier Reef are
the most affected by both of these phenomena, and many
reefs in these parts have relatively low levels of coral
cover when compared to reefs elsewhere in the world. This
may well be their natural state, which clearly points to the
caution which must be exercised when utilizing measures
of coral cover as an expression of reef health.
As might be expected with any reef system traversing
such a wide latitudinal range, there is a gradual diminution
of species diversity towards higher latitudes. While most of
the 350 coral species are recorded in the north, only about
244 species are recorded further south. Even more notable
are cross-shelf differences. Close to the mainland there are
high levels of nutrient inputs, sediments and freshwater,
while offshore such inputs diminish and conditions on
the outer reefs can be considered near oceanic, with low
levels of nutrients and clear waters. These differences have
led to considerable variation in the species assemblages
depending on their location on the continental shelf Such
differences are further maintained by the patterns of water
movement in the Great Barrier Reef generally north-south
with little cross-shelf transport.
As mentioned in Chapter 1. many corals reproduce
in a mass spawning event which takes place once a year.
While this is globally widespread, it was first observed and
is best documented on the Great Barrier Reef For a few
nights after a particular full moon in the late austral
spring (typically November) the majority of scleractinian
coral species, together with many other reef organisms
including sponges, holothurians, polychaetes and giant
clams, undergo a mass spawning event. This is highly
Left: To the south the coastai shelf of the Great Barrier Reef widens considerably around the vast complex of the Swain
and Pompey Reefs, before narrowing again around the small Capricorn group ISTS043-151-77, 19911. Right: The beaked
butterflyfish Chelmon rostratus is found across Southeast Asia and the Great Barrier Reef