The Great Barrier Reef
52
a tidal node, which causes very large amplitude tides
in the Broad Sound and Shoalwater Bay region and extreme tidal currents. The same processes also cause
very strong currents through the narrow passages between the Pompeys hard-line reefs offshore. Such currents scour away sediments, progressively depositing
them in less energetic areas.
In the far northern GBR, the out-of-phase tides of
the Coral and Timor Seas also cause very strong tidal
currents, with similar effects. In addition, tidal jets
flowing into passages between some ribbon reefs can
also pump nutrient rich Coral Sea water onto localised
areas of the outer shelf.
N BIOPHYSICAL RELATIONSHIPS AND
ASSEMBLAGE PATTERNS
The Seabed Biodiversity Project sought to examine the
relationships between the major driving factors outlined above and the biology observed by video and collected in epibenthic sled and scientific trawl samples at
almost 1400 sites. These relationships are important in
order to deliver integrated landscape maps of seabed
assemblages, which are needed for management in the
Marine Park. Such maps were produced by predicting
species distributions and assemblage patterns in areas
not sampled, on the basis of modelling their relationships with 28 more broadly available geo-physicochemical variables. While this surrogate approach has
limitations, it is the only feasible option given the vast
size of the GBR, which means that despite considerable
effort, sampling sites were relatively sparsely (average
~12 km apart) distributed over the shelf seabed.
The biophysical modelling indicated the major environmental factors that appeared to affect the distribution patterns of seabed habitats and assemblages in
the GBR, including: sediment grain size (particularly
the percentage of mud); force of water currents on the
seabed (benthic stress); chlorophyll and/or turbidity;
and, to a lesser extent, depth and some nutrients. While
the correlations between the 28 physical factors in the
GBR are complex and multidimensional, they have
been simplified to just two dimensions (Fig. 6.1), after
rescaling them in proportion to their importance for
biological patterns. The most common environments
emerge from a shallow (30–50 m) relic limestone
platform that dominates along much of the outer shelf.
The Lagoon also broadens to 100 km and deepens
(50–70 m) to the southeast, becoming the Capricorn
Channel with depths in excess of 100 m at its mouth.
At the very southern end of the GBR (23.0–24.5°S),
the Capricorn–Bunker Reefs rise from the outer areas
of an 80 km wide sandy shelf of 30–40 m depth.
Coastal influences
Along much of the coast, rivers export terrigenous sediments to inshore seas. After heavy rainfall, turbid flood
plumes carry suspended solids and nutrients along
the coastline and into the GBR Lagoon. Over the last
8000 years, this has resulted in a ~15 km wide inshore
deposit of muddy sediments, the Holocene Wedge, particularly between 12–21°S. Elsewhere, the inshore sediments are largely silica sands—especially at the southern
end, where the shelf is extensively covered by silica
sands from the Great Sandy Region further south.
On exposed coasts, wave action from trade winds
regularly turns over the sediments to depths of 20 m,
creating unstable habitat and redistributing finer particles to less exposed areas. Cyclones are particularly frequent in the open central GBR, and their disturbing
effects reach much greater depths, transporting fine particles north and inshore, leaving behind a thin veneer of
coarse particles over much of the shelf in this region.
Oceanic influences
In the Coral Sea, the westerly South Equatorial Current
bifurcates at the continental margin between 14–16°S,
to produce a northward off-shelf current and the southerly East Australia Current (EAC). The ribbon barrier
reefs in the north limit the exchange of oceanic water
onto the shallow shelf; here the trade winds drive transient northward flows of shelf water. The Pompeys
hard-line has a similar barrier effect in the south. In
contrast, the open reef matrix of the deeper central GBR
shelf allows episodic inflows of water from the EAC.
Occasionally, upwellings of cool nutrient rich water intrude up and across the shelf.
Oceanic tides exert a strong influence on parts of
the shelf. The topography of the southern GBR creates
52
a tidal node, which causes very large amplitude tides
in the Broad Sound and Shoalwater Bay region and extreme tidal currents. The same processes also cause
very strong currents through the narrow passages between the Pompeys hard-line reefs offshore. Such currents scour away sediments, progressively depositing
them in less energetic areas.
In the far northern GBR, the out-of-phase tides of
the Coral and Timor Seas also cause very strong tidal
currents, with similar effects. In addition, tidal jets
flowing into passages between some ribbon reefs can
also pump nutrient rich Coral Sea water onto localised
areas of the outer shelf.
N BIOPHYSICAL RELATIONSHIPS AND
ASSEMBLAGE PATTERNS
The Seabed Biodiversity Project sought to examine the
relationships between the major driving factors outlined above and the biology observed by video and collected in epibenthic sled and scientific trawl samples at
almost 1400 sites. These relationships are important in
order to deliver integrated landscape maps of seabed
assemblages, which are needed for management in the
Marine Park. Such maps were produced by predicting
species distributions and assemblage patterns in areas
not sampled, on the basis of modelling their relationships with 28 more broadly available geo-physicochemical variables. While this surrogate approach has
limitations, it is the only feasible option given the vast
size of the GBR, which means that despite considerable
effort, sampling sites were relatively sparsely (average
~12 km apart) distributed over the shelf seabed.
The biophysical modelling indicated the major environmental factors that appeared to affect the distribution patterns of seabed habitats and assemblages in
the GBR, including: sediment grain size (particularly
the percentage of mud); force of water currents on the
seabed (benthic stress); chlorophyll and/or turbidity;
and, to a lesser extent, depth and some nutrients. While
the correlations between the 28 physical factors in the
GBR are complex and multidimensional, they have
been simplified to just two dimensions (Fig. 6.1), after
rescaling them in proportion to their importance for
biological patterns. The most common environments
emerge from a shallow (30–50 m) relic limestone
platform that dominates along much of the outer shelf.
The Lagoon also broadens to 100 km and deepens
(50–70 m) to the southeast, becoming the Capricorn
Channel with depths in excess of 100 m at its mouth.
At the very southern end of the GBR (23.0–24.5°S),
the Capricorn–Bunker Reefs rise from the outer areas
of an 80 km wide sandy shelf of 30–40 m depth.
Coastal influences
Along much of the coast, rivers export terrigenous sediments to inshore seas. After heavy rainfall, turbid flood
plumes carry suspended solids and nutrients along
the coastline and into the GBR Lagoon. Over the last
8000 years, this has resulted in a ~15 km wide inshore
deposit of muddy sediments, the Holocene Wedge, particularly between 12–21°S. Elsewhere, the inshore sediments are largely silica sands—especially at the southern
end, where the shelf is extensively covered by silica
sands from the Great Sandy Region further south.
On exposed coasts, wave action from trade winds
regularly turns over the sediments to depths of 20 m,
creating unstable habitat and redistributing finer particles to less exposed areas. Cyclones are particularly frequent in the open central GBR, and their disturbing
effects reach much greater depths, transporting fine particles north and inshore, leaving behind a thin veneer of
coarse particles over much of the shelf in this region.
Oceanic influences
In the Coral Sea, the westerly South Equatorial Current
bifurcates at the continental margin between 14–16°S,
to produce a northward off-shelf current and the southerly East Australia Current (EAC). The ribbon barrier
reefs in the north limit the exchange of oceanic water
onto the shallow shelf; here the trade winds drive transient northward flows of shelf water. The Pompeys
hard-line has a similar barrier effect in the south. In
contrast, the open reef matrix of the deeper central GBR
shelf allows episodic inflows of water from the EAC.
Occasionally, upwellings of cool nutrient rich water intrude up and across the shelf.
Oceanic tides exert a strong influence on parts of
the shelf. The topography of the southern GBR creates