468
Chapter 12: The Southern Ocean
correlation between computed salp/krill consumption and between-year variability in
satellite-observed sea-surface chlorophyll in summer. The balance between krill and salps
is not just a result of competition for food, because it has recently been shown that
salps may be a preferred food of adult krill (Kawaguchi and Takahashi, 1996). During
the period of routine observations, a trend to more frequent years with light ice cover
and lower krill densities has been detected; this trend can be extrapolated securely back
to 1947 when routine observations of relative winter ice cover were established. Using
net-tow data, the trend has recently been extrapolated back to 1926 by Atkinson et al.
(2004). The inferred progressive decrease in the abundance of krill during a period of 80
years is cause for concern for the direct fishery for E. superba of about 15 × 10
6 tons y
−1 ,
managed by the Commission for the Conservation of Antarctic Marine Living Resources,
and also for the marine mammals and birds dependent on krill for their livelihood. In
fact, very recent evidence suggests that this trend can be extrapolated back to the early
1930s in the records of whaling ships (de la Mare, 1997).
Synopsis
Case 1—Polar irradiance-mediated production peak—The low salinity mixed layer undergoes significant winter deepening to almost 150 m, while the photic depth remains shoaler,
so that the thermocline is never illuminated. Seasonal evolution of productivity rate is
symmetrical about a midsummer maximum that is significantly lower than in high boreal
latitudes. Rates of increase and decline of productivity are consistent and linear, suggesting that vernal productivity increase is more closely related to seasonal changes in the
light field than to mixed-layer depth. Chlorophyll biomass closely tracks the increase of P,
peaking (but only at 05 mg chl m
−3 at midsummer, with some renewal of accumulation
during March–May, until winter ice cover is again established. Consumption does not
balance production during summer bloom and is inferred to decline further from balance
when copepods descend to overwintering depths at the end of summer (March), this
reduction being mitigated by continued presence of krill until the ice cover is renewed
(Fig. 12.5).
Austral Polar Province (APLR)
Extent of the Province
This is the “real Antarctic” of oceanographers, whalers, early navigators, and 21st-century
tourists. It comprises the seasonally ice-covered sea from the coasts of Antarctica (including the peninsula) north to the Antarctic Divergence, and consequently much of this area
lies poleward of the Antarctic Circle and so has a period of total winter darkness. Because
of their ecological similarity to the rest of APLR, we should include in this province the
island arcs between the Antarctic Peninsula and South America (South Orkneys, South
Sandwich, and South Georgia) because their ecology is dominated by coastal and ice-edge
effects.
The Antarctic continental shelf is not extensive, and comprises only a few small patches
where water depth is <200 m, mostly near 30
E: elsewhere, water depth of 200–500 m is
found within a few miles of the coastline. Like that of Labrador, this deep continental
platform is a submerged continental shelf, its submergence caused by the weight of the
4-km-deep ice cap resting on the Antarctic continent. A subsurface thermal front of about
2
C at 200 m (the CWB already noted) occurs close to the continent: this, of course, is
not simply a typical shelf-edge front but rather a convergence of significance in the global
thermohaline circulation.
The unique characteristic of the coast of Antarctica must be, of course, that each of
the major shallow embayments on the coast, except the rather open Bellingshausen Sea,
Chapter 12: The Southern Ocean
correlation between computed salp/krill consumption and between-year variability in
satellite-observed sea-surface chlorophyll in summer. The balance between krill and salps
is not just a result of competition for food, because it has recently been shown that
salps may be a preferred food of adult krill (Kawaguchi and Takahashi, 1996). During
the period of routine observations, a trend to more frequent years with light ice cover
and lower krill densities has been detected; this trend can be extrapolated securely back
to 1947 when routine observations of relative winter ice cover were established. Using
net-tow data, the trend has recently been extrapolated back to 1926 by Atkinson et al.
(2004). The inferred progressive decrease in the abundance of krill during a period of 80
years is cause for concern for the direct fishery for E. superba of about 15 × 10
6 tons y
−1 ,
managed by the Commission for the Conservation of Antarctic Marine Living Resources,
and also for the marine mammals and birds dependent on krill for their livelihood. In
fact, very recent evidence suggests that this trend can be extrapolated back to the early
1930s in the records of whaling ships (de la Mare, 1997).
Synopsis
Case 1—Polar irradiance-mediated production peak—The low salinity mixed layer undergoes significant winter deepening to almost 150 m, while the photic depth remains shoaler,
so that the thermocline is never illuminated. Seasonal evolution of productivity rate is
symmetrical about a midsummer maximum that is significantly lower than in high boreal
latitudes. Rates of increase and decline of productivity are consistent and linear, suggesting that vernal productivity increase is more closely related to seasonal changes in the
light field than to mixed-layer depth. Chlorophyll biomass closely tracks the increase of P,
peaking (but only at 05 mg chl m
−3 at midsummer, with some renewal of accumulation
during March–May, until winter ice cover is again established. Consumption does not
balance production during summer bloom and is inferred to decline further from balance
when copepods descend to overwintering depths at the end of summer (March), this
reduction being mitigated by continued presence of krill until the ice cover is renewed
(Fig. 12.5).
Austral Polar Province (APLR)
Extent of the Province
This is the “real Antarctic” of oceanographers, whalers, early navigators, and 21st-century
tourists. It comprises the seasonally ice-covered sea from the coasts of Antarctica (including the peninsula) north to the Antarctic Divergence, and consequently much of this area
lies poleward of the Antarctic Circle and so has a period of total winter darkness. Because
of their ecological similarity to the rest of APLR, we should include in this province the
island arcs between the Antarctic Peninsula and South America (South Orkneys, South
Sandwich, and South Georgia) because their ecology is dominated by coastal and ice-edge
effects.
The Antarctic continental shelf is not extensive, and comprises only a few small patches
where water depth is <200 m, mostly near 30
E: elsewhere, water depth of 200–500 m is
found within a few miles of the coastline. Like that of Labrador, this deep continental
platform is a submerged continental shelf, its submergence caused by the weight of the
4-km-deep ice cap resting on the Antarctic continent. A subsurface thermal front of about
2
C at 200 m (the CWB already noted) occurs close to the continent: this, of course, is
not simply a typical shelf-edge front but rather a convergence of significance in the global
thermohaline circulation.
The unique characteristic of the coast of Antarctica must be, of course, that each of
the major shallow embayments on the coast, except the rather open Bellingshausen Sea,