WORLD ATLAS OF CORAL REEFS
eastern Indonesia. Species numbers here outstrip any region
of the world, and species counts from individual bays or
islands typically outstrip total species counts from the
entire Caribbean. Some of this great diversity may in fact
be linked to the same period of glaciations which caused
destruction elsewhere. This region is believed to have
maintained somewhat benign conditions during this time,
allowing the survival of many species. Additionally, during
certain periods, species may only have survived in relict
populations restricted to small refugia. Their isolation,
exacerbated by changing sea levels, may have allowed the
independent evolution of populations and the formation
of new species. These would have repopulated the wider
region as conditions ameliorated. Further species may have
accumulated here from outside the region, driven by
patterns of ocean currents flowing westwards from the
islands of the Pacific Ocean.
A number of other historical and contemporary
factors are responsible for driving regional patterns in
biodiversity, notably the low diversity observed in the
Eastern Pacific. Brazilian and West African faunas, and
the sustained high diversity in the Red Sea and low
diversity in the Arabian Gulf These are considered more
fully in the regional chapters.
Patterns at finer scales
Moving in to study reef distribution at finer resolutions,
the discontinuous nature of coral reefs within countries or
along particular coastlines is highly apparent. Corals, and
the reefs they form, are highly sensitive to factors such
as salinity, sediments and nutrients. Where conditions are
inappropriate they do not occur More importantly in
recent times, where conditions change, the corals, and the
reefs themselves, may die.
Sediments and sedimentation
The initial growth of a coral is dependent on a larval animal
finding the right substrate on which to settle. Corals cannot
grow on fine muds or shifting sediments, and such
sediments are a common feature along many of the world's
coastlines. Where corals cannot settle and grow, reefs do
not form. This is at least part of the e.xplanation for the
absence of reefs close to large river mouths and along other
stretches of sediment-laden coastlines. Another influence of
sediments is that of turbidity - in areas with large amounts
of suspended sediments in the water column, the loss of
light further reduces or prevents coral growth.
Once established, corals can cope with limited
amounts of sediments settling upon them from the waters
above, actively removing sediments which smother their
tissues and block out the light. Similarly, once a reef is
formed it is often able to maintain a presence in areas of
otherwise shifting sediments. The reef structure lifts itself
above the sediments, and provides the hard substrate on
which new corals can grow. Reefs can also reduce the
influence of currents and waves which, in some areas,
are responsible for resuspending sediments that might
otherwise smother corals.
Where conditions of sediments and turbidity change
considerably this can lead to the rapid demise of corals
and the death of reefs. There is an energetic cost to a coral
in removing sediments which settle upon it. while the loss
of light associated with increasing turbidity greatly
reduces a coral's chances of survival.
Salinity
Corals are wholly marine organisms, unable to grow in
freshwater It is sometimes quite hard to distinguish the
effects of freshwater from the influence of the sediments,
typically also carried by streams and rivers. However, the
absence of corals from wide areas associated with major
rivers is at least in part related to the low salinities in
these areas.
Nutrients
The considerable biomass and wealth of diversity observed
on coral reefs around the world has led to a common
misconception among non-specialists that reefs may be
dependent on considerable inputs of nutrients. In fact reefs
are highly efficient at nutrient recycling, and are widespread in some of the most nutrient-poor parts of the
oceans. Where nutrient levels are higher, often close to
coastlines or areas of upwelling. reefs still survive, but in
very high nutrient situations other opportunistic species
Damsetfish and butterflyfish around a blue coral Heliopora coerulea.
eastern Indonesia. Species numbers here outstrip any region
of the world, and species counts from individual bays or
islands typically outstrip total species counts from the
entire Caribbean. Some of this great diversity may in fact
be linked to the same period of glaciations which caused
destruction elsewhere. This region is believed to have
maintained somewhat benign conditions during this time,
allowing the survival of many species. Additionally, during
certain periods, species may only have survived in relict
populations restricted to small refugia. Their isolation,
exacerbated by changing sea levels, may have allowed the
independent evolution of populations and the formation
of new species. These would have repopulated the wider
region as conditions ameliorated. Further species may have
accumulated here from outside the region, driven by
patterns of ocean currents flowing westwards from the
islands of the Pacific Ocean.
A number of other historical and contemporary
factors are responsible for driving regional patterns in
biodiversity, notably the low diversity observed in the
Eastern Pacific. Brazilian and West African faunas, and
the sustained high diversity in the Red Sea and low
diversity in the Arabian Gulf These are considered more
fully in the regional chapters.
Patterns at finer scales
Moving in to study reef distribution at finer resolutions,
the discontinuous nature of coral reefs within countries or
along particular coastlines is highly apparent. Corals, and
the reefs they form, are highly sensitive to factors such
as salinity, sediments and nutrients. Where conditions are
inappropriate they do not occur More importantly in
recent times, where conditions change, the corals, and the
reefs themselves, may die.
Sediments and sedimentation
The initial growth of a coral is dependent on a larval animal
finding the right substrate on which to settle. Corals cannot
grow on fine muds or shifting sediments, and such
sediments are a common feature along many of the world's
coastlines. Where corals cannot settle and grow, reefs do
not form. This is at least part of the e.xplanation for the
absence of reefs close to large river mouths and along other
stretches of sediment-laden coastlines. Another influence of
sediments is that of turbidity - in areas with large amounts
of suspended sediments in the water column, the loss of
light further reduces or prevents coral growth.
Once established, corals can cope with limited
amounts of sediments settling upon them from the waters
above, actively removing sediments which smother their
tissues and block out the light. Similarly, once a reef is
formed it is often able to maintain a presence in areas of
otherwise shifting sediments. The reef structure lifts itself
above the sediments, and provides the hard substrate on
which new corals can grow. Reefs can also reduce the
influence of currents and waves which, in some areas,
are responsible for resuspending sediments that might
otherwise smother corals.
Where conditions of sediments and turbidity change
considerably this can lead to the rapid demise of corals
and the death of reefs. There is an energetic cost to a coral
in removing sediments which settle upon it. while the loss
of light associated with increasing turbidity greatly
reduces a coral's chances of survival.
Salinity
Corals are wholly marine organisms, unable to grow in
freshwater It is sometimes quite hard to distinguish the
effects of freshwater from the influence of the sediments,
typically also carried by streams and rivers. However, the
absence of corals from wide areas associated with major
rivers is at least in part related to the low salinities in
these areas.
Nutrients
The considerable biomass and wealth of diversity observed
on coral reefs around the world has led to a common
misconception among non-specialists that reefs may be
dependent on considerable inputs of nutrients. In fact reefs
are highly efficient at nutrient recycling, and are widespread in some of the most nutrient-poor parts of the
oceans. Where nutrient levels are higher, often close to
coastlines or areas of upwelling. reefs still survive, but in
very high nutrient situations other opportunistic species
Damsetfish and butterflyfish around a blue coral Heliopora coerulea.