Antarctic Polar Biome
471
circulation. Along the southeast coast a persistent polynya is maintained by offshore
winds; this may be 800 km long even in winter, and in summer it expands into a mosaic
of shore leads and patches of open water that persist for several weeks. Persistent open
water also occurs in the much-studied Bransfield Sound and in the passages between the
islands of the South Orkneys.
The Ross Sea gyre is smaller and narrower than the Weddell gyre but has many of
the same properties; a wide area of sea ice continues to invest the eastern coast and
partly encloses the Ross Sea so that even in austral midsummer it is open to the ocean
only toward the northwest, off Cape Adare in Victoria Land. The Bellingshausen Sea is
hard to define, lying in the very open embayment west of the Antarctic Peninsula; most
is ice-covered even in summer, and the southern edge of eastward flow in the ACC is
unusually far south and lies quite close to the ice edge in summer (Read et al., 1995).
Biological Response and Regional Ecology
Because phytoplankton in this region always has a low photosynthetic performance as
well as a positive response to Fe fertilization, it is likely that here the cells are largely
Fe-limited. Nevertheless, it has been frequently suggested that, at least in the marginal
ice zone, a near-surface input of Fe may commonly occur with the summer flow of
meltwater from glacier mouths, from fleets of drifting icebergs having the same source,
and from freshwater deposited as snow on the Antarctic continent. This process has been
briefly discussed in Chapter 5 and it is, according to Smetacek et al. (2004), the principal
factor in forcing the development of seasonal phytoplankton blooms; but we should
remember that available Fe is only one among many relevant characteristics of the upper
water column in early summer, adjacent to clusters of icebergs or near the retreating ice
edge. Near-surface density stratification, which occurs in such regions and is so often
the principal factor in initiating high-latitude blooms, is certainly equally critical here as
elsewhere, as was suggested during the 1930s by the “Discovery” investigations.
Whatever the proximate factors may be in the neritic and MIZ regime, early summer
blooms are initiated close to the coast and in polynyas such as those of the Ross Sea;
these blooms may subsequently propagate seaward along with the effluent meltwater in
the surface layer. It should be noted that sea ice is a more complex substance than it
might appear to be at first sight. It is a two-phase material, containing seawater enriched
in nutrients and autotrophic biota; these are distributed in a complex manner. A “surface
slush community” extends down into crevices rather superficially, and within cavities in
the ice a “freeboard community” may develop in partially rotted ice, mixed with seawater freshened by ice melt. Finally, a “bottom community” may form in the columnar
or platelet ice that occurs below fast ice in spring or autumn, when light penetration is
sufficient. The start of freezing in the autumn causes nutrient-depleted water within the
ice to be replaced by nutrient-rich water from below the ice by “upwelling,” and fluid
motion in the ice cover can be driven by freezing-induced convection (Lytle and Ackley,
1996). Enrichment within second-year ice is due to brine rejection from the freezing
process at the ice surface (as air temperature drops in autumn) and fuels significant
blooms of algae.
Because seasonal ice is not very thick in these seas, there is often sufficient penetration
of light for the growth of algal “superblooms” on the underside of the continuous ice
cover, perhaps especially in the coastal regions of the Weddell Sea. These were investigated in very early spring (October and November) along the eastern coast of the
Weddell Sea in a feature that left a brown wake behind the ship as it passed through
continuous pack ice (Smetacek et al., 1992). This ice was only 20 cm thick but was
underlain by a low-salinity layer of nitrate-depleted water 1–5 m deep, rich in unconsolidated centimeter-scale ice platelets, where irradiance was significantly reduced. Within
471
circulation. Along the southeast coast a persistent polynya is maintained by offshore
winds; this may be 800 km long even in winter, and in summer it expands into a mosaic
of shore leads and patches of open water that persist for several weeks. Persistent open
water also occurs in the much-studied Bransfield Sound and in the passages between the
islands of the South Orkneys.
The Ross Sea gyre is smaller and narrower than the Weddell gyre but has many of
the same properties; a wide area of sea ice continues to invest the eastern coast and
partly encloses the Ross Sea so that even in austral midsummer it is open to the ocean
only toward the northwest, off Cape Adare in Victoria Land. The Bellingshausen Sea is
hard to define, lying in the very open embayment west of the Antarctic Peninsula; most
is ice-covered even in summer, and the southern edge of eastward flow in the ACC is
unusually far south and lies quite close to the ice edge in summer (Read et al., 1995).
Biological Response and Regional Ecology
Because phytoplankton in this region always has a low photosynthetic performance as
well as a positive response to Fe fertilization, it is likely that here the cells are largely
Fe-limited. Nevertheless, it has been frequently suggested that, at least in the marginal
ice zone, a near-surface input of Fe may commonly occur with the summer flow of
meltwater from glacier mouths, from fleets of drifting icebergs having the same source,
and from freshwater deposited as snow on the Antarctic continent. This process has been
briefly discussed in Chapter 5 and it is, according to Smetacek et al. (2004), the principal
factor in forcing the development of seasonal phytoplankton blooms; but we should
remember that available Fe is only one among many relevant characteristics of the upper
water column in early summer, adjacent to clusters of icebergs or near the retreating ice
edge. Near-surface density stratification, which occurs in such regions and is so often
the principal factor in initiating high-latitude blooms, is certainly equally critical here as
elsewhere, as was suggested during the 1930s by the “Discovery” investigations.
Whatever the proximate factors may be in the neritic and MIZ regime, early summer
blooms are initiated close to the coast and in polynyas such as those of the Ross Sea;
these blooms may subsequently propagate seaward along with the effluent meltwater in
the surface layer. It should be noted that sea ice is a more complex substance than it
might appear to be at first sight. It is a two-phase material, containing seawater enriched
in nutrients and autotrophic biota; these are distributed in a complex manner. A “surface
slush community” extends down into crevices rather superficially, and within cavities in
the ice a “freeboard community” may develop in partially rotted ice, mixed with seawater freshened by ice melt. Finally, a “bottom community” may form in the columnar
or platelet ice that occurs below fast ice in spring or autumn, when light penetration is
sufficient. The start of freezing in the autumn causes nutrient-depleted water within the
ice to be replaced by nutrient-rich water from below the ice by “upwelling,” and fluid
motion in the ice cover can be driven by freezing-induced convection (Lytle and Ackley,
1996). Enrichment within second-year ice is due to brine rejection from the freezing
process at the ice surface (as air temperature drops in autumn) and fuels significant
blooms of algae.
Because seasonal ice is not very thick in these seas, there is often sufficient penetration
of light for the growth of algal “superblooms” on the underside of the continuous ice
cover, perhaps especially in the coastal regions of the Weddell Sea. These were investigated in very early spring (October and November) along the eastern coast of the
Weddell Sea in a feature that left a brown wake behind the ship as it passed through
continuous pack ice (Smetacek et al., 1992). This ice was only 20 cm thick but was
underlain by a low-salinity layer of nitrate-depleted water 1–5 m deep, rich in unconsolidated centimeter-scale ice platelets, where irradiance was significantly reduced. Within