Antarctic Polar Biome
465
As the blooms propagate poleward, a seasonal succession ensues with the most mature
communities along the southern boundary of the PFZ and the youngest community near
the receding ice edge. The mature pelagic ecology of the open-water zone is characteristic
of a retention community in the sense of Peinert et al. (1989). Small cells dominate,
together with an appropriate population of heterotrophic protists and bacteria; nevertheless, diatoms may remain abundant for extended periods far from the receding ice edge.
This was observed during the AESOPS Process Study 1 voyage of 1997 along 107
W:
here, 2
north of the ice edge, a residual bloom characterized by a strong DCM was
dominated by bundle-forming Thalassiothrix and chains of Fragilariopsis. This bloom was
quite distinct from the Phaeocystis-dominated bloom at the ice edge, although biomass
of autotrophs was equivalent (Selph et al. 2001).
The JGOFS 1992 investigations confirmed that up to 90% of total chlorophyll in the
open-water post-bloom phytoplankton resides in pico- and nanophytoplankton, with
very small contributions from diatoms, dinoflagellates, prymnesiophytes and chlorophytes
(Peeken, 1997). Low chlorophyll values under such conditions may result in water of
exceptionally high clarity, and Secchi disc depths of >60 m have been recorded after the
open-water bloom has collapsed. The lack of permanence of such extremely oligotrophic
conditions in the Southern Ocean has been addressed earlier. The sedimentation rate
from the summer algal growth here is at least two orders of magnitude lower than at
the MIZ (Smetacek et al., 1990). The seasonally migrating MIZ is associated near-surface
stratification that we would expect to lead to the initiation of a bloom wherever irradiance
is sufficient. But in the nutrient regime south of the CWB, beyond the influence of
UCDW, Fe is always limiting until we reach the polar neritic regions further south. So the
spring breakup and retreat of the MIZ may not always result in strong diatom blooms.
Although highest chlorophyll biomass in this province usually occurs at the ice edge,
biomass at the receding MIZ bloom is not always significantly higher than in the adjacent
open-water bloom. The repeated ADEOS sections along 170
W showed that chlorophyll
biomass in open water at 65
S (far to the north of the MIZ) in late January was equivalent
to biomass at the ice edge in mid-December (Buesseler et al., 2003). Satellite images
showed a band of chlorophyll (05–30 mg m
−3 ) advancing southward with the ice front,
but with maximum chlorophyll always about 5–7
north of the 50% ice contour. During
austral spring and summer 1997–98, observations suggested a complex model of nutrient
supply and limitation (Hiscock et al., 2003), based on the upsloping of Fe-rich, nutrientrich Upper Circumpolar Deep Water (UCDW) to the surface between the PF and the
southern boundary of the ACC, south of which nutrient-poor LCDW surfaces. Silicate
is progressively depleted poleward by diatom growth (Nelson et al., 2001) and a strong
latitudinal gradient develops, the Si max shifting from 62–63
S near the PF in early spring
down to 66
S at the southern boundary of the ACC by midsummer.
The dynamics of the consumption of Antarctic blooms has become clearer in recent
years. The UK-JGOFS Southern Ocean studies showed that even here, in the APLR
province, we cannot ignore the protist microplankton, as has been done in the past.
Along the Bellingshausen Sea section mentioned previously, protistan microplankton
were sparse below continuous ice cover but increased in numbers in the open water, their
abundance being positively correlated with chlorophyll biomass. Where maximum numbers occurred (54 mg C or 17,000 organisms liter
−1 the microplankton was dominated by
oligotrich ciliates (Strombidium, Tontonia, and Lohmaniella) and heterotrophic dinoflagellates (Gyrodinium, Cochlodinium, Torodinium, and Pronoctiluca) of biomass equivalent
to about 25% of the phytoplankton. The feeding activity of these protists required from
3 to 40% of the phytoplankton biomass daily. Nano-protozoan biomass (especially heterotrophic dinoflagellates) was equivalent to about 70% of their total potential food
biomass—that is, of bacterial and nanophytoplankton cells (Becquevort, 1997). There is
heavy grazing pressure by small copepods (Smetacek et al., 1990), though this is likely
465
As the blooms propagate poleward, a seasonal succession ensues with the most mature
communities along the southern boundary of the PFZ and the youngest community near
the receding ice edge. The mature pelagic ecology of the open-water zone is characteristic
of a retention community in the sense of Peinert et al. (1989). Small cells dominate,
together with an appropriate population of heterotrophic protists and bacteria; nevertheless, diatoms may remain abundant for extended periods far from the receding ice edge.
This was observed during the AESOPS Process Study 1 voyage of 1997 along 107
W:
here, 2
north of the ice edge, a residual bloom characterized by a strong DCM was
dominated by bundle-forming Thalassiothrix and chains of Fragilariopsis. This bloom was
quite distinct from the Phaeocystis-dominated bloom at the ice edge, although biomass
of autotrophs was equivalent (Selph et al. 2001).
The JGOFS 1992 investigations confirmed that up to 90% of total chlorophyll in the
open-water post-bloom phytoplankton resides in pico- and nanophytoplankton, with
very small contributions from diatoms, dinoflagellates, prymnesiophytes and chlorophytes
(Peeken, 1997). Low chlorophyll values under such conditions may result in water of
exceptionally high clarity, and Secchi disc depths of >60 m have been recorded after the
open-water bloom has collapsed. The lack of permanence of such extremely oligotrophic
conditions in the Southern Ocean has been addressed earlier. The sedimentation rate
from the summer algal growth here is at least two orders of magnitude lower than at
the MIZ (Smetacek et al., 1990). The seasonally migrating MIZ is associated near-surface
stratification that we would expect to lead to the initiation of a bloom wherever irradiance
is sufficient. But in the nutrient regime south of the CWB, beyond the influence of
UCDW, Fe is always limiting until we reach the polar neritic regions further south. So the
spring breakup and retreat of the MIZ may not always result in strong diatom blooms.
Although highest chlorophyll biomass in this province usually occurs at the ice edge,
biomass at the receding MIZ bloom is not always significantly higher than in the adjacent
open-water bloom. The repeated ADEOS sections along 170
W showed that chlorophyll
biomass in open water at 65
S (far to the north of the MIZ) in late January was equivalent
to biomass at the ice edge in mid-December (Buesseler et al., 2003). Satellite images
showed a band of chlorophyll (05–30 mg m
−3 ) advancing southward with the ice front,
but with maximum chlorophyll always about 5–7
north of the 50% ice contour. During
austral spring and summer 1997–98, observations suggested a complex model of nutrient
supply and limitation (Hiscock et al., 2003), based on the upsloping of Fe-rich, nutrientrich Upper Circumpolar Deep Water (UCDW) to the surface between the PF and the
southern boundary of the ACC, south of which nutrient-poor LCDW surfaces. Silicate
is progressively depleted poleward by diatom growth (Nelson et al., 2001) and a strong
latitudinal gradient develops, the Si max shifting from 62–63
S near the PF in early spring
down to 66
S at the southern boundary of the ACC by midsummer.
The dynamics of the consumption of Antarctic blooms has become clearer in recent
years. The UK-JGOFS Southern Ocean studies showed that even here, in the APLR
province, we cannot ignore the protist microplankton, as has been done in the past.
Along the Bellingshausen Sea section mentioned previously, protistan microplankton
were sparse below continuous ice cover but increased in numbers in the open water, their
abundance being positively correlated with chlorophyll biomass. Where maximum numbers occurred (54 mg C or 17,000 organisms liter
−1 the microplankton was dominated by
oligotrich ciliates (Strombidium, Tontonia, and Lohmaniella) and heterotrophic dinoflagellates (Gyrodinium, Cochlodinium, Torodinium, and Pronoctiluca) of biomass equivalent
to about 25% of the phytoplankton. The feeding activity of these protists required from
3 to 40% of the phytoplankton biomass daily. Nano-protozoan biomass (especially heterotrophic dinoflagellates) was equivalent to about 70% of their total potential food
biomass—that is, of bacterial and nanophytoplankton cells (Becquevort, 1997). There is
heavy grazing pressure by small copepods (Smetacek et al., 1990), though this is likely