Antarctic Westerly Winds Biome
451
There is, as I noted in Chapter 5, a general tendency for dynamic frontal processes
to establish near-surface stability in mixed water columns and so initiate accumulation
of plant biomass. Where density surface in fronts slope downward, as here, this may
provide sufficient stability to initiate and maintain a bloom (Marra, 1982). Cross-frontal
mixing of warm, nutrient-poor subtropical water and nutrient-rich subantarctic water
will induce thermal stability and a sufficient nutrient supply to establish a bloom. We
would anticipate that these processes would be least effective in austral winter when
deeper convective mixing will occur—and when irradiance will be minimal. A return to
net growth in spring in response to reduced mixing depth and increased irradiance is
entirely in accordance with Sverdrup.
Such a process was observed along a meridional section down 150
E during austral
summer 1983–83 that crossed the STF (Furuya et al., 1986). Here, subtropical water was
found to have penetrated (at 30–70 m) for about 16 km along an isopycnal surface down
into the subantarctic water mass that therefore lay both above and below it. A sharp,
shallow pycnocline resulted from the temperature inversion, the regional DCM at 75 m
being accompanied by an additional near-surface (0–20 m) chlorophyll layer above the
penetration. Phytoplankton of the subantarctic water mass (biomass was >90% diatom
carbon, principally Chaetoceros, Nitzschia, Thalassiothrix, Rhizosolenia) lay at these two
depths of high chlorophyll biomass, one above and the other below the subtropical
phytoplankton that comprised only about 50% diatom biomass. Once incorporated in the
front, the warm-water assemblage of unicellular cyanophytes and small flagellates were
never more than 15% of total autotrophic carbon. Silicate depletion is likely to occur in
these blooms and will result in an overall reduction in new production, and progressive
evolution of the algal assemblage to one dominated by dinoflagellates and microalgae.
Primary production was higher across the intrusion than either to north or to south.
This process occurred under very moderate wind stress and is unlikely to function in a
similar manner during the deeper convective mixing of austral winter.
A similar situation has been observed in the Indian Ocean sector, where Russian
investigations concluded that the strongest enhancement of productivity occurred at
intrusions of warm into cool water associated with the STCZ. The induction of phytoplankton blooms by eddy- and meander-induced stratification is perhaps a sufficient
general explanation of the existence of a strong chlorophyll field in the meandering STCZ.
Obviously, there are special regional characteristics in some parts of the STCZ; Gregg
and Conkright (2001) discuss decadal-scale changes in the retroflection to the east of the
Falkland plateau. This is a peculiarly complex region and it will bear some additional
discussion. Although the zonal currents of the Southern Ocean pass through the Drake
Passage without mingling, real continuity of frontal features may be weak. Many authors
suggest that the STF of the Pacific does not penetrate to the Atlantic, but rather that it
is formed again de novo where the ACC turns east across the Atlantic. This appears to
be supported by Color plate 13, representative of many such patterns. Arising from the
chlorophyll feature representing the summer bloom on the Falklands shelf and plateau
is a SW–NE linear feature of high chlorophyll that lies just outside the shelf edge as
far north as the River Plate. This linear bloom is unlikely to be an upwelling feature,
having no offshore plumes, and it lies in the jetlike Falklands Current, part of the ACC
flow. This cold water turns sharply back on itself at its confluence with the warm Brazil
Current and first flows southward, then turns east across the ocean. There, its zonal flow
lies to the south of the newly formed STF and STCZ of the South Atlantic. These features
may then be followed around the globe in TOPEX-Poseidon SLA maps and chlorophyll
images, right back to South America again.
A size-fractionated study of the autotrophic cells in the STCZ east of New Zealand
(Hall et al., 1999) found chlorophyll-a concentrations of up to 34 g liter
−1 , but only
as much as 02 g liter
−1 in the subantarctic water beyond the STF. At all seasons, the
451
There is, as I noted in Chapter 5, a general tendency for dynamic frontal processes
to establish near-surface stability in mixed water columns and so initiate accumulation
of plant biomass. Where density surface in fronts slope downward, as here, this may
provide sufficient stability to initiate and maintain a bloom (Marra, 1982). Cross-frontal
mixing of warm, nutrient-poor subtropical water and nutrient-rich subantarctic water
will induce thermal stability and a sufficient nutrient supply to establish a bloom. We
would anticipate that these processes would be least effective in austral winter when
deeper convective mixing will occur—and when irradiance will be minimal. A return to
net growth in spring in response to reduced mixing depth and increased irradiance is
entirely in accordance with Sverdrup.
Such a process was observed along a meridional section down 150
E during austral
summer 1983–83 that crossed the STF (Furuya et al., 1986). Here, subtropical water was
found to have penetrated (at 30–70 m) for about 16 km along an isopycnal surface down
into the subantarctic water mass that therefore lay both above and below it. A sharp,
shallow pycnocline resulted from the temperature inversion, the regional DCM at 75 m
being accompanied by an additional near-surface (0–20 m) chlorophyll layer above the
penetration. Phytoplankton of the subantarctic water mass (biomass was >90% diatom
carbon, principally Chaetoceros, Nitzschia, Thalassiothrix, Rhizosolenia) lay at these two
depths of high chlorophyll biomass, one above and the other below the subtropical
phytoplankton that comprised only about 50% diatom biomass. Once incorporated in the
front, the warm-water assemblage of unicellular cyanophytes and small flagellates were
never more than 15% of total autotrophic carbon. Silicate depletion is likely to occur in
these blooms and will result in an overall reduction in new production, and progressive
evolution of the algal assemblage to one dominated by dinoflagellates and microalgae.
Primary production was higher across the intrusion than either to north or to south.
This process occurred under very moderate wind stress and is unlikely to function in a
similar manner during the deeper convective mixing of austral winter.
A similar situation has been observed in the Indian Ocean sector, where Russian
investigations concluded that the strongest enhancement of productivity occurred at
intrusions of warm into cool water associated with the STCZ. The induction of phytoplankton blooms by eddy- and meander-induced stratification is perhaps a sufficient
general explanation of the existence of a strong chlorophyll field in the meandering STCZ.
Obviously, there are special regional characteristics in some parts of the STCZ; Gregg
and Conkright (2001) discuss decadal-scale changes in the retroflection to the east of the
Falkland plateau. This is a peculiarly complex region and it will bear some additional
discussion. Although the zonal currents of the Southern Ocean pass through the Drake
Passage without mingling, real continuity of frontal features may be weak. Many authors
suggest that the STF of the Pacific does not penetrate to the Atlantic, but rather that it
is formed again de novo where the ACC turns east across the Atlantic. This appears to
be supported by Color plate 13, representative of many such patterns. Arising from the
chlorophyll feature representing the summer bloom on the Falklands shelf and plateau
is a SW–NE linear feature of high chlorophyll that lies just outside the shelf edge as
far north as the River Plate. This linear bloom is unlikely to be an upwelling feature,
having no offshore plumes, and it lies in the jetlike Falklands Current, part of the ACC
flow. This cold water turns sharply back on itself at its confluence with the warm Brazil
Current and first flows southward, then turns east across the ocean. There, its zonal flow
lies to the south of the newly formed STF and STCZ of the South Atlantic. These features
may then be followed around the globe in TOPEX-Poseidon SLA maps and chlorophyll
images, right back to South America again.
A size-fractionated study of the autotrophic cells in the STCZ east of New Zealand
(Hall et al., 1999) found chlorophyll-a concentrations of up to 34 g liter
−1 , but only
as much as 02 g liter
−1 in the subantarctic water beyond the STF. At all seasons, the