432
Chapter 11: The Pacific Ocean
Regional Response of the Pelagic Ecosystem
The climatological sea surface chlorophyll archive reveals a strong overall seasonality. The
7-day and 30-day images of sea surface chlorophyll now available strikingly reveal that two
processes dominate the response of phytoplankton to physical forcing in this province.
First, during the late austral winter (April–June) the seasonal chlorophyll enhancement
of the Tasman Sea reaches its most northerly extent, involving the southwestern part of
the Coral Sea, beyond the Tasman Front, so that the Queensland shelf inshore of the
Great Barrier Reef shows some seasonal chlorophyll enhancement, although this remains
less strong than the chlorophyll biomass indicated for the reef itself. This phenomenon is
attributed by Gabric et al. (1990) to the effects of higher nutrient loading from the coast,
and wind mixing on the inner shelf, during the wet monsoon season.
It has long been understood that the winter-spring bloom of the Tasman Sea,
stronger toward the south and reaching highest surface biomass in the period September–
November, influences the western coast that is otherwise dominated by two processes
that cause biological enhancement: coastal upwelling cells and vorticity in the offshore
eddy field (Tranter et al., 1987; Rahmstorf, 1992). But, once again, the enhancement of
chlorophyll in features in the eddies is not in any way comparable with, for example, the
offshore eddies in the North Pacific; they may be distinguished from background in the
chlorophyll fields, but they are not prominent.
It is also clear from a general inspection of such images that the coastal upwelling
locations, discussed earlier, are not prominent features in the chlorophyll field. The bight
south of Ballina, including Evans Head (see earlier discussion), is perhaps the most
persistent, but I have seen no images in which it forms more than a very narrow coastal
strip in no way resembling chlorophyll plumes on classical upwelling coasts. But beyond
that, it is not only at the localities listed earlier that coastal enhancement of chlorophyll
is observed: satellite data for September 1999 suggest that a larger feature—though still
minor in the global sense—had developed in the bight north of Moreton Bay (27
S) and
also, although more diffuse, on the wider shelf south of Sandy Cape.
The various processes, discussed previously, that transport nutrients to the narrow
shelf of New South Wales result in regional enhancement, as discussed by Cresswell
(op. cit.), who found a strong DCM associated with a nutrient-rich coastal plume at
Sydney. The more general annual spring blooms that occur on this coast annually, and
are recorded in the SeaWiFS images, are due to short-lived diatom blooms that increase
regional column chlorophyll by a factor of 10 and evolve in a predictable sequence from
small chain-forming species to large centrics and finally to dinoflagellates (Hallegraef and
Jeffrey, 1993). This regional-scale bloom is not forced by the same mechanism as classical
spring blooms, but by the dynamic processes in the East Australian Current that lead to
nutrient enrichment and were discussed earlier.
Much attention has been given to the response of the pelagic ecosystem to the warmcore eddies, and their associated cyclonic crescents, that originate in the meanders of the
East Australian Current (e.g., Tranter et al., 1983). Elevated nitrate levels (15 g liter
−1
compared with 04 g liter
−1 ) and chlorophyll enhancement over background occur in
the cool water crescent on the margins of warm-core eddies; frontal curvature is, of course,
usually supposed to generate shear and to destabilize the interface between the eddy
and the surrounding water (Woods et al., 1977) and is often associated with biological
enhancement, up to higher trophic levels, such as tuna. The cool crescent studied by
Tranter et al. off Sydney supported very large concentrations of Calanoides carinatus of
300–400 mg m
−3 , an abundance similar to that occurring in coastal upwelling along the
New South Wales coastline.
The most prominent feature of enhanced chlorophyll, by far, on this coast is that
associated with the shelf break to the east of Bass Strait and along the Victoria coast as
far as the protruding feature of Cape Howe (38
S): here, the shelf-break front forms the
Chapter 11: The Pacific Ocean
Regional Response of the Pelagic Ecosystem
The climatological sea surface chlorophyll archive reveals a strong overall seasonality. The
7-day and 30-day images of sea surface chlorophyll now available strikingly reveal that two
processes dominate the response of phytoplankton to physical forcing in this province.
First, during the late austral winter (April–June) the seasonal chlorophyll enhancement
of the Tasman Sea reaches its most northerly extent, involving the southwestern part of
the Coral Sea, beyond the Tasman Front, so that the Queensland shelf inshore of the
Great Barrier Reef shows some seasonal chlorophyll enhancement, although this remains
less strong than the chlorophyll biomass indicated for the reef itself. This phenomenon is
attributed by Gabric et al. (1990) to the effects of higher nutrient loading from the coast,
and wind mixing on the inner shelf, during the wet monsoon season.
It has long been understood that the winter-spring bloom of the Tasman Sea,
stronger toward the south and reaching highest surface biomass in the period September–
November, influences the western coast that is otherwise dominated by two processes
that cause biological enhancement: coastal upwelling cells and vorticity in the offshore
eddy field (Tranter et al., 1987; Rahmstorf, 1992). But, once again, the enhancement of
chlorophyll in features in the eddies is not in any way comparable with, for example, the
offshore eddies in the North Pacific; they may be distinguished from background in the
chlorophyll fields, but they are not prominent.
It is also clear from a general inspection of such images that the coastal upwelling
locations, discussed earlier, are not prominent features in the chlorophyll field. The bight
south of Ballina, including Evans Head (see earlier discussion), is perhaps the most
persistent, but I have seen no images in which it forms more than a very narrow coastal
strip in no way resembling chlorophyll plumes on classical upwelling coasts. But beyond
that, it is not only at the localities listed earlier that coastal enhancement of chlorophyll
is observed: satellite data for September 1999 suggest that a larger feature—though still
minor in the global sense—had developed in the bight north of Moreton Bay (27
S) and
also, although more diffuse, on the wider shelf south of Sandy Cape.
The various processes, discussed previously, that transport nutrients to the narrow
shelf of New South Wales result in regional enhancement, as discussed by Cresswell
(op. cit.), who found a strong DCM associated with a nutrient-rich coastal plume at
Sydney. The more general annual spring blooms that occur on this coast annually, and
are recorded in the SeaWiFS images, are due to short-lived diatom blooms that increase
regional column chlorophyll by a factor of 10 and evolve in a predictable sequence from
small chain-forming species to large centrics and finally to dinoflagellates (Hallegraef and
Jeffrey, 1993). This regional-scale bloom is not forced by the same mechanism as classical
spring blooms, but by the dynamic processes in the East Australian Current that lead to
nutrient enrichment and were discussed earlier.
Much attention has been given to the response of the pelagic ecosystem to the warmcore eddies, and their associated cyclonic crescents, that originate in the meanders of the
East Australian Current (e.g., Tranter et al., 1983). Elevated nitrate levels (15 g liter
−1
compared with 04 g liter
−1 ) and chlorophyll enhancement over background occur in
the cool water crescent on the margins of warm-core eddies; frontal curvature is, of course,
usually supposed to generate shear and to destabilize the interface between the eddy
and the surrounding water (Woods et al., 1977) and is often associated with biological
enhancement, up to higher trophic levels, such as tuna. The cool crescent studied by
Tranter et al. off Sydney supported very large concentrations of Calanoides carinatus of
300–400 mg m
−3 , an abundance similar to that occurring in coastal upwelling along the
New South Wales coastline.
The most prominent feature of enhanced chlorophyll, by far, on this coast is that
associated with the shelf break to the east of Bass Strait and along the Victoria coast as
far as the protruding feature of Cape Howe (38
S): here, the shelf-break front forms the