The Southern Ocean
447
been done in the Atlantic sector, where the frontal regions are unusually discrete and are
more widely separated than elsewhere in the Southern Ocean.
As has already been discussed in a general manner in Chapter 5, the Southern Ocean
presents us with an apparently simple question, posed by the observation that it habitually
exhibits regions of clear, blue water where phytoplankton biomass and primary production must take low values. Is this lack of plant growth caused by micronutrient deficiency,
given the characteristic excess availability of macronutrients, or is it due to Sverdrupian
dynamics appropriate to unusually deep wind-mixing and low solar irradiance? I have
had these questions in the front of my mind when writing each section of the present
chapter. I have also had in mind that the satellite chlorophyll fields now available suggest
that the widely held view that the Southern Ocean does not accumulate chlorophyll is
an overstatement: really, blooms are more widespread in this ocean than some accounts
would have us believe.
In the first edition of this book, I rather confidently asserted that the then proposed
partition was consistent with concepts concerning regional differences in the regulation
of algal growth by iron in the annular zones of the Southern Ocean (de Baar et al., 1995).
I stated that Fe-deficient water occurred south of the STF, Fe-replete water in the PFZ,
and Fe-limiting water in the ACC south of the PF. The Fe-replete water of the coastwise
East Wind Drift completed the logic.
However, as you will realize if you have read Chapter 5, I am no longer so confident
that this apparently simple model is useful because, for one thing, it is not Fe repletion
alone that induces strong blooms in the PFZ. It should be enough to note that average
chlorophyll biomass along 170
W does not correlate with Fe concentration in the 0–100 m
layer (Buesseler et al., 2003). Repeated cruises during austral spring and summer 1997–98
down this longitude by U.S. JGOFS ships revealed a more complex model of nutrient
supply and limitation (Hiscock et al., 2003) than I had in mind when writing earlier. The
Hiscock et al. model involves the upsloping of Fe-rich, nutrient-rich Upper Circumpolar
Deep Water (UCDW) to the surface between the PF and the southern boundary of the
ACC, south of which nutrient-poor LCDW comes to the surface. As austral summer
conditions move south along 170
W, and SiO 3 is progressively depleted poleward by
diatom growth (Nelson et al., 2001), a strong latitudinal gradient (Si max ) develops and
shifts from 62–63
S near the PF in early spring down to 66
S at the southern boundary
of the ACC by midsummer.
Evidently, the winter recharge of UCDW in the PFZ and to the south provides sufficient
Fe to support a diatom bloom that propagates south only to the southern boundary
of the ACC, where it encounters Fe-deficient LCDW. Productivity and phytoplankton
biomass are maximal within the Si max as this gradient shifts poleward. In this progression,
Hiscock et al. recognize three phases or zones of differential nutrient limitation that were
revealed by the repeated sections down 170
W:
(i) Already in spring, the SAZ (north of the PF, see above) is SiO 3 -limited and Fereplete
(ii) Between the PFZ and CWB the surface layers are SiO 3 -replete and Fe-limited in
spring, but SiO 3 -limited and Fe-replete in summer
(iii) Between the southern boundary of the ACC and the shelf edge around Antarctica,
conditions are consistently SiO 3 -replete, Fe-limited
You will have noted that nitrate is omitted from these comments because, in these
seas, it is always in excess of requirement for reasons discussed earlier in this chapter, and
in Chapter 5. During the open-water season, surface nitrate values of about 10–15 M
occur widely (Foster, 1984), although they are somewhat lower in the Indian than in
the other sectors of the Southern Ocean. Note also that light limitation is not discussed
but—once again if you have read Chapter 5—you will remember that in the deeply mixed
regime south of the PFZ, this limitation occurs in the second half of summer.
447
been done in the Atlantic sector, where the frontal regions are unusually discrete and are
more widely separated than elsewhere in the Southern Ocean.
As has already been discussed in a general manner in Chapter 5, the Southern Ocean
presents us with an apparently simple question, posed by the observation that it habitually
exhibits regions of clear, blue water where phytoplankton biomass and primary production must take low values. Is this lack of plant growth caused by micronutrient deficiency,
given the characteristic excess availability of macronutrients, or is it due to Sverdrupian
dynamics appropriate to unusually deep wind-mixing and low solar irradiance? I have
had these questions in the front of my mind when writing each section of the present
chapter. I have also had in mind that the satellite chlorophyll fields now available suggest
that the widely held view that the Southern Ocean does not accumulate chlorophyll is
an overstatement: really, blooms are more widespread in this ocean than some accounts
would have us believe.
In the first edition of this book, I rather confidently asserted that the then proposed
partition was consistent with concepts concerning regional differences in the regulation
of algal growth by iron in the annular zones of the Southern Ocean (de Baar et al., 1995).
I stated that Fe-deficient water occurred south of the STF, Fe-replete water in the PFZ,
and Fe-limiting water in the ACC south of the PF. The Fe-replete water of the coastwise
East Wind Drift completed the logic.
However, as you will realize if you have read Chapter 5, I am no longer so confident
that this apparently simple model is useful because, for one thing, it is not Fe repletion
alone that induces strong blooms in the PFZ. It should be enough to note that average
chlorophyll biomass along 170
W does not correlate with Fe concentration in the 0–100 m
layer (Buesseler et al., 2003). Repeated cruises during austral spring and summer 1997–98
down this longitude by U.S. JGOFS ships revealed a more complex model of nutrient
supply and limitation (Hiscock et al., 2003) than I had in mind when writing earlier. The
Hiscock et al. model involves the upsloping of Fe-rich, nutrient-rich Upper Circumpolar
Deep Water (UCDW) to the surface between the PF and the southern boundary of the
ACC, south of which nutrient-poor LCDW comes to the surface. As austral summer
conditions move south along 170
W, and SiO 3 is progressively depleted poleward by
diatom growth (Nelson et al., 2001), a strong latitudinal gradient (Si max ) develops and
shifts from 62–63
S near the PF in early spring down to 66
S at the southern boundary
of the ACC by midsummer.
Evidently, the winter recharge of UCDW in the PFZ and to the south provides sufficient
Fe to support a diatom bloom that propagates south only to the southern boundary
of the ACC, where it encounters Fe-deficient LCDW. Productivity and phytoplankton
biomass are maximal within the Si max as this gradient shifts poleward. In this progression,
Hiscock et al. recognize three phases or zones of differential nutrient limitation that were
revealed by the repeated sections down 170
W:
(i) Already in spring, the SAZ (north of the PF, see above) is SiO 3 -limited and Fereplete
(ii) Between the PFZ and CWB the surface layers are SiO 3 -replete and Fe-limited in
spring, but SiO 3 -limited and Fe-replete in summer
(iii) Between the southern boundary of the ACC and the shelf edge around Antarctica,
conditions are consistently SiO 3 -replete, Fe-limited
You will have noted that nitrate is omitted from these comments because, in these
seas, it is always in excess of requirement for reasons discussed earlier in this chapter, and
in Chapter 5. During the open-water season, surface nitrate values of about 10–15 M
occur widely (Foster, 1984), although they are somewhat lower in the Indian than in
the other sectors of the Southern Ocean. Note also that light limitation is not discussed
but—once again if you have read Chapter 5—you will remember that in the deeply mixed
regime south of the PFZ, this limitation occurs in the second half of summer.