Antarctic Westerly Winds Biome
457
are incomplete and complex, because these are interrelated and improperly understood
problems. To begin to understand them it is necessary to invoke the special characteristics
of this region discussed earlier in this chapter, and in Chapter 5.
Because it is here that subantarctic conditions are first encountered, it will perhaps
be useful to note a few common characteristics of the ecology of this region and of the
other antarctic provinces. I have found Trull et al. (2001) to be a particularly useful
source for such an evaluation. The Southern Ocean shows two principal zonal bands
of surface chlorophyll enhanced above background: the STCZ zone (discussed in the
previous section) and another that lies to the south of it. Comparison with maps of SLA
reveals that the southern zonal band corresponds both with the geographical position of
the PFZ and also with a zonal band of strong eddying in the ACC that matches the line
taken by the PF around the Southern Ocean. A zone of silica-rich sediment whose main
component is the diatom Fragilariopsis lies on the deep-sea floor, aligned below the PFZ.
Sediment traps and estimates of primary production suggest rates of <80 g C m
−2 y
−1 ,
or about twice as high as the Antarctic average and an order of magnitude higher than in
the oligotrophic zone of the Subantarctic Water Ring (Wefer and Fischer, 1991; Tréguer
and Jacques, 1992).
We are therefore confronted by the same relationship between vorticity and phytoplankton blooms as we were in the STCZ. Once again, observations confirm that
enhancement of chlorophyll biomass here is associated with mesoscale frontal dynamics.
Strass et al. (2002) review the mechanisms that are involved in this process, especially the
cross-frontal circulation related to baroclinic instability; confluence of surface water at
the edges of meanders concentrates phytoplankton blooms that have grown in response
to upwelling-supplied nutrients on the anticyclonic side of jets.
As shall be discussed later, these dynamic processes are imposed upon a general
background of the winter recharge of essential molecules, including dissolved iron, by
vertical transport into the APLR province from the subjacent and nutrient-rich UCDW.
During the growth season, N, P, and Fe can be recycled only where organic matter is
retained by a density discontinuity within the euphotic zone. Even in such places, of
course, Si is preferentially depleted by loss of very slowly dissolving diatom frustules to the
deeper layers. The findings of van Oijen (2004) concerning differential control of growth
by Fe limitation and light limitation in the PF have already been discussed in Chapter 5.
Here it is sufficient to remind ourselves of their finding that in the deep-mixing region
of the ACC to the south of the PF in late summer, phytoplankton cells are light-limited,
but in the shallower-mixing regime of the PF itself, they are Fe-limited. Nevertheless, the
PF carries a larger standing stock of diatoms than the ANTA zone to the south.
The regional evolution of satellite-observed surface chlorophyll in the SWR closely
tracks sun-angle, so that highest biomass occurs in midsummer, but local conditions
exhibit rapid variability. Chlorophyll biomass across the PFZ at 170
W was followed by
Abbott et al. (2000) at short time intervals throughout an austral summer. A few days
of calm weather in November initiated weak stratification, and the phytoplankton came
to photoadaptation with the resulting improved light environment within 1–2 days by
modification of their fluorescence/chlorophyll (F/C) ratio. Chlorophyll concentrations
rapidly increased, reaching maximum values (125 mg chl m
−3 ) in about 14 days, or well
before nutrient stress was revealed. A subsequent decrease of chlorophyll biomass was
almost as rapid, so that by the end of December a low level of biomass (025 mg m
−3 )
was reached and subsequently sustained. The initial rate of decrease, before the onset
of nutrient stress, was consistent with grazing rates of 02 d
−1 measured nearby. For the
remainder of the summer, steadily increasing F/C ratios indicated progressive nutrient
stress and grazing-balanced accumulation. Again, there is a close relationship between
vorticity and phytoplankton abundance in the frontal zones, and the mechanisms involved
in this process, especially cross-frontal circulation related to baroclinic instability, are now
457
are incomplete and complex, because these are interrelated and improperly understood
problems. To begin to understand them it is necessary to invoke the special characteristics
of this region discussed earlier in this chapter, and in Chapter 5.
Because it is here that subantarctic conditions are first encountered, it will perhaps
be useful to note a few common characteristics of the ecology of this region and of the
other antarctic provinces. I have found Trull et al. (2001) to be a particularly useful
source for such an evaluation. The Southern Ocean shows two principal zonal bands
of surface chlorophyll enhanced above background: the STCZ zone (discussed in the
previous section) and another that lies to the south of it. Comparison with maps of SLA
reveals that the southern zonal band corresponds both with the geographical position of
the PFZ and also with a zonal band of strong eddying in the ACC that matches the line
taken by the PF around the Southern Ocean. A zone of silica-rich sediment whose main
component is the diatom Fragilariopsis lies on the deep-sea floor, aligned below the PFZ.
Sediment traps and estimates of primary production suggest rates of <80 g C m
−2 y
−1 ,
or about twice as high as the Antarctic average and an order of magnitude higher than in
the oligotrophic zone of the Subantarctic Water Ring (Wefer and Fischer, 1991; Tréguer
and Jacques, 1992).
We are therefore confronted by the same relationship between vorticity and phytoplankton blooms as we were in the STCZ. Once again, observations confirm that
enhancement of chlorophyll biomass here is associated with mesoscale frontal dynamics.
Strass et al. (2002) review the mechanisms that are involved in this process, especially the
cross-frontal circulation related to baroclinic instability; confluence of surface water at
the edges of meanders concentrates phytoplankton blooms that have grown in response
to upwelling-supplied nutrients on the anticyclonic side of jets.
As shall be discussed later, these dynamic processes are imposed upon a general
background of the winter recharge of essential molecules, including dissolved iron, by
vertical transport into the APLR province from the subjacent and nutrient-rich UCDW.
During the growth season, N, P, and Fe can be recycled only where organic matter is
retained by a density discontinuity within the euphotic zone. Even in such places, of
course, Si is preferentially depleted by loss of very slowly dissolving diatom frustules to the
deeper layers. The findings of van Oijen (2004) concerning differential control of growth
by Fe limitation and light limitation in the PF have already been discussed in Chapter 5.
Here it is sufficient to remind ourselves of their finding that in the deep-mixing region
of the ACC to the south of the PF in late summer, phytoplankton cells are light-limited,
but in the shallower-mixing regime of the PF itself, they are Fe-limited. Nevertheless, the
PF carries a larger standing stock of diatoms than the ANTA zone to the south.
The regional evolution of satellite-observed surface chlorophyll in the SWR closely
tracks sun-angle, so that highest biomass occurs in midsummer, but local conditions
exhibit rapid variability. Chlorophyll biomass across the PFZ at 170
W was followed by
Abbott et al. (2000) at short time intervals throughout an austral summer. A few days
of calm weather in November initiated weak stratification, and the phytoplankton came
to photoadaptation with the resulting improved light environment within 1–2 days by
modification of their fluorescence/chlorophyll (F/C) ratio. Chlorophyll concentrations
rapidly increased, reaching maximum values (125 mg chl m
−3 ) in about 14 days, or well
before nutrient stress was revealed. A subsequent decrease of chlorophyll biomass was
almost as rapid, so that by the end of December a low level of biomass (025 mg m
−3 )
was reached and subsequently sustained. The initial rate of decrease, before the onset
of nutrient stress, was consistent with grazing rates of 02 d
−1 measured nearby. For the
remainder of the summer, steadily increasing F/C ratios indicated progressive nutrient
stress and grazing-balanced accumulation. Again, there is a close relationship between
vorticity and phytoplankton abundance in the frontal zones, and the mechanisms involved
in this process, especially cross-frontal circulation related to baroclinic instability, are now