456
Chapter 12: The Southern Ocean
Southern Ocean, especially in the Indian and Pacific Ocean sectors, differing from regions
to north and south where winter mixing is much shallower. Here, Subantarctic Mode
Water (SAMW) is formed in winter, having nutrient characteristics imposed by biological
processes further south. SAMW is a pycnostad and passes down and northward into the
main thermocline of each ocean basin, which has consequences for the nutrient supply
in other parts of the oceans (see later discussion). Within deep mixed layers formed in
winter, near-surface stratification is induced as both sun angle and day length increase,
and by midsummer the seasonal mixed layer (9–12
C) deepens in this way to 60–75 m.
However, there is probably greater impermanence of stratification here than further to
the south in APLR, where winds are quieter.
The Fe profile in the SWR in summer is that of a typical nutrient so that, after winter
mixing, a ferricline is not observed. It is suggested that eastward jet flows, such as the
PF, may transport water that has been enhanced in Fe from shelf deposits in the Drake
Passage/Falkland Plateau region (Loscher et al., 1997b). High chlorophyll accumulation in
the Southern Ocean fronts have come to be attributed to “differences in iron availability”
between the frontal systems and other regions (e.g., de Baar et al., 1995). But it has never
been demonstrated that the iron-rich character of water in the frontal jet currents can
be sustained over great distances; in fact, the chemistry of iron in seawater suggests that
this will not occur. More realistic, perhaps, is the alternative view that most iron utilized
by phytoplankton in the Southern Ocean, including within frontal systems, is upwelled.
Indeed, as Fennel et al. (2003) comment:
In the contemporary Southern Ocean, deep mixing during winter and upwelling of iron-rich
subsurface waters are the main sources of dissolved iron in surface water. Aeolian deposition
plays only a minor role. Locally, significant amounts of iron can be added during ice melt,
i.e. in the Ross Sea, but in the SW Pacific sector this input is negligible.
This is the model followed here, and I specifically reject that of Smetacek et al. (2004),
who write: “Phytoplankton blooms are restricted in the modern Southern Ocean to
waters receiving an adequate supply of iron from landmasses and sediments. Evidence
is mounting that wind-blown dust is a major supply route of iron to the land-remote
Southern Ocean.”
There is a surface nitrate gradient across the SAZ, especially strong in summer that,
at this season, increases poleward from NO 3
− of 1–3 M just south of the SAF to
10–15 M in the PFZ. A similar gradient of SiOH 4 also occurs very strongly across
the SAF; spring-summer values to the north of the front are < 1–15 M but 40–60 M
on the southern side (Franck et al., 2003), although these are but discontinuities in the
general poleward progression of the surface nutrient gradient during summer. How the
different nitrate-silicate balance evolves, both spatially and seasonally, north of the PF
will be discussed in the following section.
Biological Response and Regional Ecology
The Polar Frontal Zone is a major discontinuity in the Southern Ocean, well expressed in
an evocative description (Deacon, 1984) of what it feels like to cross the convergence on
returning from the polar regions, because the air smells again as air at sea should smell,
but doesn’t in the high Antarctic. It is also a significant border in species distributions
and a major feeding zone of seabirds.
The principal observations of the planktonic ecosystem in this province that require
explanation are (i) that a zonal band of high chlorophyll biomass should lie around the
southern half of the province, (ii), that the seasonal surface chlorophyll cycle should
apparently be a simple function of sun angle, and (iii) that the phytoplankton should
be dominated everywhere by small cells. The explanations that can so far be offered
Chapter 12: The Southern Ocean
Southern Ocean, especially in the Indian and Pacific Ocean sectors, differing from regions
to north and south where winter mixing is much shallower. Here, Subantarctic Mode
Water (SAMW) is formed in winter, having nutrient characteristics imposed by biological
processes further south. SAMW is a pycnostad and passes down and northward into the
main thermocline of each ocean basin, which has consequences for the nutrient supply
in other parts of the oceans (see later discussion). Within deep mixed layers formed in
winter, near-surface stratification is induced as both sun angle and day length increase,
and by midsummer the seasonal mixed layer (9–12
C) deepens in this way to 60–75 m.
However, there is probably greater impermanence of stratification here than further to
the south in APLR, where winds are quieter.
The Fe profile in the SWR in summer is that of a typical nutrient so that, after winter
mixing, a ferricline is not observed. It is suggested that eastward jet flows, such as the
PF, may transport water that has been enhanced in Fe from shelf deposits in the Drake
Passage/Falkland Plateau region (Loscher et al., 1997b). High chlorophyll accumulation in
the Southern Ocean fronts have come to be attributed to “differences in iron availability”
between the frontal systems and other regions (e.g., de Baar et al., 1995). But it has never
been demonstrated that the iron-rich character of water in the frontal jet currents can
be sustained over great distances; in fact, the chemistry of iron in seawater suggests that
this will not occur. More realistic, perhaps, is the alternative view that most iron utilized
by phytoplankton in the Southern Ocean, including within frontal systems, is upwelled.
Indeed, as Fennel et al. (2003) comment:
In the contemporary Southern Ocean, deep mixing during winter and upwelling of iron-rich
subsurface waters are the main sources of dissolved iron in surface water. Aeolian deposition
plays only a minor role. Locally, significant amounts of iron can be added during ice melt,
i.e. in the Ross Sea, but in the SW Pacific sector this input is negligible.
This is the model followed here, and I specifically reject that of Smetacek et al. (2004),
who write: “Phytoplankton blooms are restricted in the modern Southern Ocean to
waters receiving an adequate supply of iron from landmasses and sediments. Evidence
is mounting that wind-blown dust is a major supply route of iron to the land-remote
Southern Ocean.”
There is a surface nitrate gradient across the SAZ, especially strong in summer that,
at this season, increases poleward from NO 3
− of 1–3 M just south of the SAF to
10–15 M in the PFZ. A similar gradient of SiOH 4 also occurs very strongly across
the SAF; spring-summer values to the north of the front are < 1–15 M but 40–60 M
on the southern side (Franck et al., 2003), although these are but discontinuities in the
general poleward progression of the surface nutrient gradient during summer. How the
different nitrate-silicate balance evolves, both spatially and seasonally, north of the PF
will be discussed in the following section.
Biological Response and Regional Ecology
The Polar Frontal Zone is a major discontinuity in the Southern Ocean, well expressed in
an evocative description (Deacon, 1984) of what it feels like to cross the convergence on
returning from the polar regions, because the air smells again as air at sea should smell,
but doesn’t in the high Antarctic. It is also a significant border in species distributions
and a major feeding zone of seabirds.
The principal observations of the planktonic ecosystem in this province that require
explanation are (i) that a zonal band of high chlorophyll biomass should lie around the
southern half of the province, (ii), that the seasonal surface chlorophyll cycle should
apparently be a simple function of sun angle, and (iii) that the phytoplankton should
be dominated everywhere by small cells. The explanations that can so far be offered