458
Chapter 12: The Southern Ocean
quite clear. One of the principal mechanisms for cell accumulation is the confluence of
surface water at the edges of meanders, thus serving to concentrate cells of phytoplankton
blooms that themselves are responses to upwelling-supplied nutrients on the anticyclonic
side of jets, especially where these exhibit sloping isopycnals (Strass et al., 2002). Highchlorophyll anomalies in the PFZ along a cyclonic bend in flow are likely to be associated
with nutrient flux and a sloping density surface; such enhancement is often sheared into
10- to 20-km-wide bands that are coherent at the scale of hundreds of kilometers.
Uptake of macronutrients in the developing bloom is co-regulated by levels of available
iron, so that iron limitation prevents complete drawdown of both silicate and nitrate.
The progressive excess during the summer of NO 3 over SiOH 4 —normally required
by diatoms in a 1:1 ratio—has generally been ascribed to the effect of the Si-pump
model, which allows no regeneration of Si in the euphotic zone. This model reflects
the fact that whereas NO 3 and PO 4 are regenerated in the upper kilometer of the water
column from sinking organic particles, SiO 3 is regenerated from sinking silica particles
only by solution. Because solution is a negative function of temperature, much silica in
the form of partially dissolved diatom frustules reaches the deep sea floor. But this is not
the only mechanism that produces a disproportionate drawdown of SiOH 4 relative to
NO 3 as the growth season advances; this imbalance is accentuated by the anomalously
high silica demand of diatoms under conditions of Fe stress. Southern Ocean JGOFS
experiments found that, in vitro, Fe addition decreased SiOH 4 :NO 3
− ratios by three
to five times over controls in high-Si water south of the nutrient gradient (Franck et al.,
2000, 2003).
Nutrient limitation of growth in the declining bloom is a complex interplay between
iron and silicate. The meridional gradient in surface nitrate in summer is from 1–3 M
just south of the SAF to 10–15 M at the PF itself, but the silicate gradient across the
SAF is stronger: to the north of this front, silicate is <1–15 M, whereas on the southern
side it is 40–60 M because of the influence of upwelled UCDW further to the south.
Deep convection of Subantarctic Mode Water (SAMW) having low concentrations of
silicate occurs in the SAZ in winter and of Antarctic Intermediate Water (AAIW) in the
PFZ. The descending SAMW water enters the main thermocline of the ocean and its Si
deficiency is conserved, spreading almost globally through the main thermocline and so
determining the nutrient environment of the global nutricline (Sarmiento et al., 2004),
excepting only in parts of the North Pacific. Thus the NO 3 /SiO 3 ratio of the nutrient
supply available to fuel algal blooms in much of the global ocean is consequent upon
phytoplankton/nutrient dynamics within the SAWR province. Sarmiento et al. suggest
that this may determine the surprisingly low production of diatoms, compared with other
phytoplankton taxa, in much of the present-day ocean.
As I noted in the introduction to this chapter, I have previously attributed the high
chlorophyll in the PFZ largely to relief from Fe limitation, as suggested by de Baar et al.
(1995). An Fe anomaly had been located in the PF at 43–50
S, reaching >5 nM in nearsurface water against a background of around 0.5 nM. Because this was associated with
an Al anomaly, de Baar et al. invoked a terrestrial input from the shallow topography
of the Scotia Arc. The most likely source, the Birdwood Bank, is far upstream, and
it is very unlikely that the discrete patch that was observed could be conserved so far
downstream in a dynamic, meandering frontal system. Similar results were obtained in
the PF downstream of the Kerguelen plateau at 70
E during the JGOFS/ANTARES 3
investigations; Fe td was very high in coastal water (∼10 nM), but complex mesoscale
eddying made it difficult to trace this water progressively to the east.
I suggest, therefore, that de Baar et al.’s interpretation of its source may be incorrect.
Nor do we need to suggest that the generally enhanced phytoplankton biomass in the
circumglobal PFZ must result from an upstream terrestrial Fe input. It will also be well
to remember, when considering sources of iron for surface water in the Southern Ocean
Chapter 12: The Southern Ocean
quite clear. One of the principal mechanisms for cell accumulation is the confluence of
surface water at the edges of meanders, thus serving to concentrate cells of phytoplankton
blooms that themselves are responses to upwelling-supplied nutrients on the anticyclonic
side of jets, especially where these exhibit sloping isopycnals (Strass et al., 2002). Highchlorophyll anomalies in the PFZ along a cyclonic bend in flow are likely to be associated
with nutrient flux and a sloping density surface; such enhancement is often sheared into
10- to 20-km-wide bands that are coherent at the scale of hundreds of kilometers.
Uptake of macronutrients in the developing bloom is co-regulated by levels of available
iron, so that iron limitation prevents complete drawdown of both silicate and nitrate.
The progressive excess during the summer of NO 3 over SiOH 4 —normally required
by diatoms in a 1:1 ratio—has generally been ascribed to the effect of the Si-pump
model, which allows no regeneration of Si in the euphotic zone. This model reflects
the fact that whereas NO 3 and PO 4 are regenerated in the upper kilometer of the water
column from sinking organic particles, SiO 3 is regenerated from sinking silica particles
only by solution. Because solution is a negative function of temperature, much silica in
the form of partially dissolved diatom frustules reaches the deep sea floor. But this is not
the only mechanism that produces a disproportionate drawdown of SiOH 4 relative to
NO 3 as the growth season advances; this imbalance is accentuated by the anomalously
high silica demand of diatoms under conditions of Fe stress. Southern Ocean JGOFS
experiments found that, in vitro, Fe addition decreased SiOH 4 :NO 3
− ratios by three
to five times over controls in high-Si water south of the nutrient gradient (Franck et al.,
2000, 2003).
Nutrient limitation of growth in the declining bloom is a complex interplay between
iron and silicate. The meridional gradient in surface nitrate in summer is from 1–3 M
just south of the SAF to 10–15 M at the PF itself, but the silicate gradient across the
SAF is stronger: to the north of this front, silicate is <1–15 M, whereas on the southern
side it is 40–60 M because of the influence of upwelled UCDW further to the south.
Deep convection of Subantarctic Mode Water (SAMW) having low concentrations of
silicate occurs in the SAZ in winter and of Antarctic Intermediate Water (AAIW) in the
PFZ. The descending SAMW water enters the main thermocline of the ocean and its Si
deficiency is conserved, spreading almost globally through the main thermocline and so
determining the nutrient environment of the global nutricline (Sarmiento et al., 2004),
excepting only in parts of the North Pacific. Thus the NO 3 /SiO 3 ratio of the nutrient
supply available to fuel algal blooms in much of the global ocean is consequent upon
phytoplankton/nutrient dynamics within the SAWR province. Sarmiento et al. suggest
that this may determine the surprisingly low production of diatoms, compared with other
phytoplankton taxa, in much of the present-day ocean.
As I noted in the introduction to this chapter, I have previously attributed the high
chlorophyll in the PFZ largely to relief from Fe limitation, as suggested by de Baar et al.
(1995). An Fe anomaly had been located in the PF at 43–50
S, reaching >5 nM in nearsurface water against a background of around 0.5 nM. Because this was associated with
an Al anomaly, de Baar et al. invoked a terrestrial input from the shallow topography
of the Scotia Arc. The most likely source, the Birdwood Bank, is far upstream, and
it is very unlikely that the discrete patch that was observed could be conserved so far
downstream in a dynamic, meandering frontal system. Similar results were obtained in
the PF downstream of the Kerguelen plateau at 70
E during the JGOFS/ANTARES 3
investigations; Fe td was very high in coastal water (∼10 nM), but complex mesoscale
eddying made it difficult to trace this water progressively to the east.
I suggest, therefore, that de Baar et al.’s interpretation of its source may be incorrect.
Nor do we need to suggest that the generally enhanced phytoplankton biomass in the
circumglobal PFZ must result from an upstream terrestrial Fe input. It will also be well
to remember, when considering sources of iron for surface water in the Southern Ocean