Indian Ocean Coastal Biome
323
although tidal stream velocities have a fivefold amplitude range between spring and neap
tides, the water column is uniform closer to the coast than the 40-m depth contour.
Shelf sea fronts (and local eutrophication) are therefore not features of this otherwise
apparently suitable wide and flat shelf (Tranter and Leech, 1987) and despite the fact that
this is a region of strong potential internal tidal energy.
Further to the south, within the flow of the Leeuwin Current (both before and after its
turn to the east around Cape Leeuwin) the thermal signals from the mesoscale offshore
filaments and eddies are of similar magnitude to those observed seaward of eastern
boundary currents. Here, of course, it is not cool, upwelled water that it transported away
from the coast but rather warm, nutrient-poor water carrying low biomass of pelagic
biota. Thus, it is only in the vorticity of the offshore eddy field that nutrients will be
advected to the photic zone and where we can expect to observe algal blooms.
Though there is very little information on ecological seasons south of the Sahul shelf,
we have to assume that a temperate cycle is appropriate here. This may also be inferred
from the section at 110
E at relevant latitudes that, as noted previously, resembles the
seasonal cycle in the Sargasso Sea.
Very low productivity in summer (< 182 mgC m
−2 hr
−1 compared with almost
10 times that value in the Gulf of Carpentaria) was observed by Motoda (1978). They
believed this could be related to a regional lack of nutrient inputs—neither upwelling
nor river effluents appear to have any importance in this region.
At the shelf break of the Great Australian Bight, there is a subsurface front that is
capped by warm Leeuwin Current water in austral summer so that perhaps, at other times,
shelf-edge upwelling may occur here (Young et al., 1999). There is some evidence, too, of
the aggregation of nektonic organisms here, which may support observed aggregations of
southern bluefin tuna, but zooplankton biomass appears to be normally most abundant
in the neritic zone.
Based on a temperature criterion, the mixed-layer depth of the Great Australian Bight
deepens from < 50 m in summer to 100–150 m in winter. Such a seasonal range must
surely be associated with spring bloom and summer oligotrophic conditions perhaps
more like those of the North Atlantic model than those observed at the southern end
of the 110
E section. However, this remains only speculation. Indeed, the serial chlorophyll images make it absolutely clear that there is a major seasonal production cycle
here; from November until March, chlorophyll concentrations are very low except in
a narrow neritic zone, where the satellite sensors indicate of order 02–03 mg chl m
−3
compared with totally oligotrophic conditions offshore. However, during austral winter (April–September) the entire region supports significantly higher concentrations
(10–15 mg chl m
−3 ) from the coast south to the SSTC zone. At this season, the Great
Australian Bight, including the shelf regions, is indistinguishable from the circumpolar
bloom that lies just to the south of the continents.
Data collected as background for the Australian IIOE transect show that there are two
centers where mesozooplankton biomass is significantly higher than background, and
that these have a marked seasonal evolution. The highest observed biomass lies along
the southern coast of Java, within the upwelling region from 105
E to 120
E. This is
most strongly developed during the Southeast Monsoon (July–August) when biomass
>100 mg m
−3 is widely distributed south of Java and on the Sahul shelf at 18–20
S. The
Java zooplankton maximum (still >100 mg m
−3 ) is reduced to a linear, coastwise zone in
October–November while over the Sahul shelf biomass has fallen to 25% of that figure.
For the remainder of the year, only the Java region retains substantial mesozooplankton
biomass (>50 mg m
−3 ).
The pelagic community off Java is strongly differentiated from that along the Australian
shelf and also has greater endemism. The dominant copepod genera off southern Java
include the upwelling coast specialist Calanoides carinatus but they also comprise many
323
although tidal stream velocities have a fivefold amplitude range between spring and neap
tides, the water column is uniform closer to the coast than the 40-m depth contour.
Shelf sea fronts (and local eutrophication) are therefore not features of this otherwise
apparently suitable wide and flat shelf (Tranter and Leech, 1987) and despite the fact that
this is a region of strong potential internal tidal energy.
Further to the south, within the flow of the Leeuwin Current (both before and after its
turn to the east around Cape Leeuwin) the thermal signals from the mesoscale offshore
filaments and eddies are of similar magnitude to those observed seaward of eastern
boundary currents. Here, of course, it is not cool, upwelled water that it transported away
from the coast but rather warm, nutrient-poor water carrying low biomass of pelagic
biota. Thus, it is only in the vorticity of the offshore eddy field that nutrients will be
advected to the photic zone and where we can expect to observe algal blooms.
Though there is very little information on ecological seasons south of the Sahul shelf,
we have to assume that a temperate cycle is appropriate here. This may also be inferred
from the section at 110
E at relevant latitudes that, as noted previously, resembles the
seasonal cycle in the Sargasso Sea.
Very low productivity in summer (< 182 mgC m
−2 hr
−1 compared with almost
10 times that value in the Gulf of Carpentaria) was observed by Motoda (1978). They
believed this could be related to a regional lack of nutrient inputs—neither upwelling
nor river effluents appear to have any importance in this region.
At the shelf break of the Great Australian Bight, there is a subsurface front that is
capped by warm Leeuwin Current water in austral summer so that perhaps, at other times,
shelf-edge upwelling may occur here (Young et al., 1999). There is some evidence, too, of
the aggregation of nektonic organisms here, which may support observed aggregations of
southern bluefin tuna, but zooplankton biomass appears to be normally most abundant
in the neritic zone.
Based on a temperature criterion, the mixed-layer depth of the Great Australian Bight
deepens from < 50 m in summer to 100–150 m in winter. Such a seasonal range must
surely be associated with spring bloom and summer oligotrophic conditions perhaps
more like those of the North Atlantic model than those observed at the southern end
of the 110
E section. However, this remains only speculation. Indeed, the serial chlorophyll images make it absolutely clear that there is a major seasonal production cycle
here; from November until March, chlorophyll concentrations are very low except in
a narrow neritic zone, where the satellite sensors indicate of order 02–03 mg chl m
−3
compared with totally oligotrophic conditions offshore. However, during austral winter (April–September) the entire region supports significantly higher concentrations
(10–15 mg chl m
−3 ) from the coast south to the SSTC zone. At this season, the Great
Australian Bight, including the shelf regions, is indistinguishable from the circumpolar
bloom that lies just to the south of the continents.
Data collected as background for the Australian IIOE transect show that there are two
centers where mesozooplankton biomass is significantly higher than background, and
that these have a marked seasonal evolution. The highest observed biomass lies along
the southern coast of Java, within the upwelling region from 105
E to 120
E. This is
most strongly developed during the Southeast Monsoon (July–August) when biomass
>100 mg m
−3 is widely distributed south of Java and on the Sahul shelf at 18–20
S. The
Java zooplankton maximum (still >100 mg m
−3 ) is reduced to a linear, coastwise zone in
October–November while over the Sahul shelf biomass has fallen to 25% of that figure.
For the remainder of the year, only the Java region retains substantial mesozooplankton
biomass (>50 mg m
−3 ).
The pelagic community off Java is strongly differentiated from that along the Australian
shelf and also has greater endemism. The dominant copepod genera off southern Java
include the upwelling coast specialist Calanoides carinatus but they also comprise many