466
Chapter 12: The Southern Ocean
to remove no more than about 30% of the daily primary production: Salpa thompsoni,
on the other hand, is present in sufficient numbers to remove all of the daily production
(Dubischar and Bathman, 1997). It is not easy, here, to achieve a clear balance between
production and consumption.
Some studies suggest that only an insignificant part of the daily production during
diatom blooms is consumed by meso- and macro-zooplankton (e.g., 0.6% daily in the
Drake Passage and 8% in the Bellingshausen Sea), but in other situations, as in the
Ross Sea, it is suggested that larger herbivores are a major factor in constraining the
accumulation of cells. For example, in the Drake Passage, where phytoplankton biomass
is low (1 or 2g C m
−2 and copepods are numerous (about 08 g C m
−2 , the latter
consume 50% of daily primary production (Schnack et al., 1985). Whatever their relative
importance, the mesozooplankters involved in the reduction of diatoms blooms are
dominated by Calanus propinquus, Calanoides acutus, and Rhincalanus gigas, whereas
Metridia gerlachi is the principal diel migrant. All these come up from overwintering
depths to encounter the spring bloom, with C. acutus appearing a little later in the season
than the others. This species has a life cycle resembling that of the arctic calanoids,
though its life cycle appears to be completed within 1 year except where the summer
season of open water is shortest, as in the eastern Weddell Sea, where a small proportion
of the population takes 2 years to reach maturity (Atkinson et al., 1997). Though these
are the dominant species, it must be remembered that even here, in such high latitudes,
lists of dominants conceal the real diversity of the mesoplankton that consume both
phytoplankton and protists. In the western Weddell Sea in March, at the end of summer,
a total of 113 mesoplankton species, of which 31 species were encountered in the upper
50 m, were listed by Hopkins and Torres (1989).
Antarctic krill are the keystone organisms for the biomass buildup in this province
by baleen whales, crab-eater seals, and many seabirds that directly forage for it by a
variety of feeding techniques. It is hard to imagine a more efficient organism to harvest
the spring and under-ice blooms of this province and to transfer the harvest directly to
mammals and seabirds, so enabling them to establish the truly astonishing populations
that existed before whaling and other forms of exploitation were begun. Since salps
generally occur in warmer water than krill, Atkinson et al. (2004) suggest that—since the
western Atlantic sector of the Southern Ocean is one of the fastest-warming regions of
the globe—they may come progressively to replace krill as the major large herbivore of
the Southern Ocean: this, of course, would have very far-reaching consequences for the
entire ecosystem.
Together with salps, krill (Euphausia superba and S. thompson) are the principal
herbivores among the larger size classes of planktonic organisms; close to the coast,
and especially in shallow water as in the inner Weddell Sea, these species are replaced
largely by E. cristallophoria, whose life cycle is adapted to this environment. Krill occur
everywhere in this province, but significant abundance does not extend equatorward of
the PF; as Atkinson et al. (2004) showed, >50% of all krill present in the Southern
Ocean occur in the SW Atlantic sector of this circumpolar province. Thus, they are more
abundant than elsewhere by an order of magnitude eastward of a line joining the tip of
the peninsula to South Georgia, passing by way of the island arcs. Here, of course, the
Weddell Current (see earlier discussion), which is a component of the westward coastal
current, passes back along the island arc to rejoin the general eastward flow of the ACC,
carrying with it high concentrations of all planktonic biota. The consequent zone of high
krill abundance may be traced eastward over deep water to about 30
W. Intermittent
patches of high abundance may be encountered around the remainder of the southern
part of the ACC, though none are so important as that in the Weddell Current. Despite
exaggerated claims concerning their fishery potential, krill do represent an enormous
Chapter 12: The Southern Ocean
to remove no more than about 30% of the daily primary production: Salpa thompsoni,
on the other hand, is present in sufficient numbers to remove all of the daily production
(Dubischar and Bathman, 1997). It is not easy, here, to achieve a clear balance between
production and consumption.
Some studies suggest that only an insignificant part of the daily production during
diatom blooms is consumed by meso- and macro-zooplankton (e.g., 0.6% daily in the
Drake Passage and 8% in the Bellingshausen Sea), but in other situations, as in the
Ross Sea, it is suggested that larger herbivores are a major factor in constraining the
accumulation of cells. For example, in the Drake Passage, where phytoplankton biomass
is low (1 or 2g C m
−2 and copepods are numerous (about 08 g C m
−2 , the latter
consume 50% of daily primary production (Schnack et al., 1985). Whatever their relative
importance, the mesozooplankters involved in the reduction of diatoms blooms are
dominated by Calanus propinquus, Calanoides acutus, and Rhincalanus gigas, whereas
Metridia gerlachi is the principal diel migrant. All these come up from overwintering
depths to encounter the spring bloom, with C. acutus appearing a little later in the season
than the others. This species has a life cycle resembling that of the arctic calanoids,
though its life cycle appears to be completed within 1 year except where the summer
season of open water is shortest, as in the eastern Weddell Sea, where a small proportion
of the population takes 2 years to reach maturity (Atkinson et al., 1997). Though these
are the dominant species, it must be remembered that even here, in such high latitudes,
lists of dominants conceal the real diversity of the mesoplankton that consume both
phytoplankton and protists. In the western Weddell Sea in March, at the end of summer,
a total of 113 mesoplankton species, of which 31 species were encountered in the upper
50 m, were listed by Hopkins and Torres (1989).
Antarctic krill are the keystone organisms for the biomass buildup in this province
by baleen whales, crab-eater seals, and many seabirds that directly forage for it by a
variety of feeding techniques. It is hard to imagine a more efficient organism to harvest
the spring and under-ice blooms of this province and to transfer the harvest directly to
mammals and seabirds, so enabling them to establish the truly astonishing populations
that existed before whaling and other forms of exploitation were begun. Since salps
generally occur in warmer water than krill, Atkinson et al. (2004) suggest that—since the
western Atlantic sector of the Southern Ocean is one of the fastest-warming regions of
the globe—they may come progressively to replace krill as the major large herbivore of
the Southern Ocean: this, of course, would have very far-reaching consequences for the
entire ecosystem.
Together with salps, krill (Euphausia superba and S. thompson) are the principal
herbivores among the larger size classes of planktonic organisms; close to the coast,
and especially in shallow water as in the inner Weddell Sea, these species are replaced
largely by E. cristallophoria, whose life cycle is adapted to this environment. Krill occur
everywhere in this province, but significant abundance does not extend equatorward of
the PF; as Atkinson et al. (2004) showed, >50% of all krill present in the Southern
Ocean occur in the SW Atlantic sector of this circumpolar province. Thus, they are more
abundant than elsewhere by an order of magnitude eastward of a line joining the tip of
the peninsula to South Georgia, passing by way of the island arcs. Here, of course, the
Weddell Current (see earlier discussion), which is a component of the westward coastal
current, passes back along the island arc to rejoin the general eastward flow of the ACC,
carrying with it high concentrations of all planktonic biota. The consequent zone of high
krill abundance may be traced eastward over deep water to about 30
W. Intermittent
patches of high abundance may be encountered around the remainder of the southern
part of the ACC, though none are so important as that in the Weddell Current. Despite
exaggerated claims concerning their fishery potential, krill do represent an enormous