4 – Oceanography
33
world have been recorded during La Nina and El Niño,
but the biggest bleaching events have been recorded
during El Niño conditions (1998 and 2002). Patches of
water that generally exceed 31°C stagnate on the GBR
and when they remain in areas for too long the symbiotic algae of corals (zooxanthellae) are lost and in severe circumstances the coral will bleach and die.
N TIDES
Tides have a great influence on currents within the GBR
lagoon. The tides flow on and off the shelf, resulting in
a considerable east-west movement of water. Maximum amplitudes of tides, by area, on the GBR range
from 2.5 m to 7 m. The tides are generally semidiurnal
(twice per day) and spring tides generate greatest currents (cf. neap tides). In regions that include the southern GBR and northern regions, tidal currents of 1 m s
–1
are generated through channels between coral reefs.
Numerical models of particle transport indicate that
particles can travel about 15 km westward on incoming
tides and 15 km eastward on outgoing tides. Tide and
bathymetry are critical components of oceanographic
models.
The interaction of tidal currents with the EAC depends on the region on the reef and distance across the
shelf. Where the EAC enters the GBR in the central region and forms the lagoonal current that flows south,
particles move on an east-west axis with the tide and
there is some movement to the south as a result of the
EAC. The influence of the CSCC (flowing north) on
inter-reefal waters of the northern GBR is lower and the
residence time of water masses tends to be greater in
this area, therefore, the effect of tide generally dominates here.
N TOPOGRAPHY
Topography has a great influence on currents of the GBR
and over 2500 reefs generate great complexities in flow.
Complex bathymetry and channels generate water jets,
eddies and convergence zones such as thermal fronts and
internal waves. All of these features have a great influence
on the transport and aggregation of plankton, connectivity
BOX 4.2 CONSEQUENCES OF GLOBAL WARMING ON GBR
OCEANOGRAPHY
Global warming will cause changes in the physical and chemical oceanography of the
GBR as well as related biological change. There is great speculation on exactly what will
happen, but predictions include the following: global warming is likely to cause increases in the frequency and duration of bleaching events on the GBR through warm
water intrusions. Storm frequency and intensity may also increase and this will have a
direct physical affect on reefs. Increased freshwater and nutrient input from cyclones
would facilitate phase shifts from coral-dominated to algal-dominated inshore reefs and
the nature of the pelagic environment would change as would its suitability to the survival of larvae. For example, the survival of crown-of-thorns starfish (COTS) larvae may
increase, causing greater frequency of COTS outbreaks. Other forms of plankton, however, may struggle with major changes in the abundance and species richness of the
plankton. An increase in atmospheric CO 2 will lower the pH of the sea so making it more
acidic (0.3–0.5 of a pH unit by 2100). This is critical for corals, shellfish and some plankton (e.g. coccolithophores) because a reduction in pH affects carbonate concentration
ions, making it more difficult to precipitate calcium carbonate.
33
world have been recorded during La Nina and El Niño,
but the biggest bleaching events have been recorded
during El Niño conditions (1998 and 2002). Patches of
water that generally exceed 31°C stagnate on the GBR
and when they remain in areas for too long the symbiotic algae of corals (zooxanthellae) are lost and in severe circumstances the coral will bleach and die.
N TIDES
Tides have a great influence on currents within the GBR
lagoon. The tides flow on and off the shelf, resulting in
a considerable east-west movement of water. Maximum amplitudes of tides, by area, on the GBR range
from 2.5 m to 7 m. The tides are generally semidiurnal
(twice per day) and spring tides generate greatest currents (cf. neap tides). In regions that include the southern GBR and northern regions, tidal currents of 1 m s
–1
are generated through channels between coral reefs.
Numerical models of particle transport indicate that
particles can travel about 15 km westward on incoming
tides and 15 km eastward on outgoing tides. Tide and
bathymetry are critical components of oceanographic
models.
The interaction of tidal currents with the EAC depends on the region on the reef and distance across the
shelf. Where the EAC enters the GBR in the central region and forms the lagoonal current that flows south,
particles move on an east-west axis with the tide and
there is some movement to the south as a result of the
EAC. The influence of the CSCC (flowing north) on
inter-reefal waters of the northern GBR is lower and the
residence time of water masses tends to be greater in
this area, therefore, the effect of tide generally dominates here.
N TOPOGRAPHY
Topography has a great influence on currents of the GBR
and over 2500 reefs generate great complexities in flow.
Complex bathymetry and channels generate water jets,
eddies and convergence zones such as thermal fronts and
internal waves. All of these features have a great influence
on the transport and aggregation of plankton, connectivity
BOX 4.2 CONSEQUENCES OF GLOBAL WARMING ON GBR
OCEANOGRAPHY
Global warming will cause changes in the physical and chemical oceanography of the
GBR as well as related biological change. There is great speculation on exactly what will
happen, but predictions include the following: global warming is likely to cause increases in the frequency and duration of bleaching events on the GBR through warm
water intrusions. Storm frequency and intensity may also increase and this will have a
direct physical affect on reefs. Increased freshwater and nutrient input from cyclones
would facilitate phase shifts from coral-dominated to algal-dominated inshore reefs and
the nature of the pelagic environment would change as would its suitability to the survival of larvae. For example, the survival of crown-of-thorns starfish (COTS) larvae may
increase, causing greater frequency of COTS outbreaks. Other forms of plankton, however, may struggle with major changes in the abundance and species richness of the
plankton. An increase in atmospheric CO 2 will lower the pH of the sea so making it more
acidic (0.3–0.5 of a pH unit by 2100). This is critical for corals, shellfish and some plankton (e.g. coccolithophores) because a reduction in pH affects carbonate concentration
ions, making it more difficult to precipitate calcium carbonate.