Antarctic Polar Biome
467
biomass, although some regions have an enigmatic low abundance; the Ross Sea, for
example, despite the very high concentrations of diatoms there, has rather few.
Krill occur in small to very large swarms that resemble those of clupeid fish, though
fish shoals are more uniform in terms of size of individuals, swimming speeds, and
maturity stage. Swarms may be aggregated into “superswarms” comprising more than
a million tons of biomass spread over several hundred square kilometers, though these
are mainly associated with coastal chlorophyll enrichment. During normal swimming,
krill are oriented with their dorsal side uppermost, but when food is encountered and
filter feeding begins, individuals may be oriented in any direction. Feeding is most active
in darkness and swarms undergo diel vertical migration between the surface and about
200 m. One reason for the success of these organisms is their ability to feed on a very
wide range of food particles from nanoflagellates to crustacean nauplii. Since krill have
such a patchy distribution, attempts to balance their consumption against production
in the blooms, or their browsing on under-ice algae, have been unsatisfactory on a
regional basis. Nevertheless, it is calculated that krill consume up to 60–80% of daily
production where they are abundant. The consumption of diatom blooms by krill can
be very spectacular: a summer bloom in the Weddell Sea was grazed down in 20 hours
from 4.0 to 05–10 mg chl m
−3 , this remnant apparently representing the unconsumed
nanophytoplankton component of the original bloom.
Euphausia superba has a life strategy that is more like that of a fish than of a planktonic
crustacean, being long-lived (5–8 years), during which period reproduction occurs two
or three times, usually late in the summer. Eggs of krill sink rapidly (out of the way of the
grazing activities of their myriad parental stock) to as much as 1000 m prior to hatching.
The nauplii and young larvae rise progressively toward the surface so that E. superba is
typically much more abundant over deep than shallow water. The two species of krill exist
in a dynamic balance with the relative abundance of one being associated with relative
scarcity of the other (Loeb et al., 1997). At Elephant Island, near the tip of the Antarctic
Peninsula, this balance is determined each spring by the extent of sea-ice cover during
the previous winter. The reestablishment of seasonal ice cover is progressive, starting with
the establishment of frazil ice, honeycombed with cavities that occlude only when the
frazil consolidates into pack ice (Smetacek et al., 1990). The lower surface and cavities
of frazil and lower surface of pack ice concentrate diatom cells that are readily “grazed”
by krill that accumulate for this purpose in and around the rugosities and cavities on
the lower surface of sea ice. During the remnant of the sunlit season and in the early
spring, sufficient irradiance penetrates the ice to maintain a diatom population here.
Leads in pack ice rapidly freeze over, and the undersurface of this new ice supports fresh
growth of algae. Dense concentrations of krill are supported during the winter within the
under-ice habitat, as many as 10–30 ind m
−2 having been observed within the interstices.
Atkinson et al. suggest that strong recruitment to the summer population depends on
ice conditions during the previous winter: extensive ice cover is associated with generous
recruitment.
Large salp populations occur after winters of small ice coverage followed by an early
open-sea spring bloom which then permits an early and rapid increase of the overwintering salp population. Krill have population dynamics resembling those of fish so that their
stock size is determined by reproductive success the previous year. Heavy recruitment
of first-year krill to the population occurs 1 year after a heavy ice winter that promotes
early spawning by the adult population and, incidentally, inhibits a spring outburst of
salps. Two consecutive heavy-ice years enhance the effect. In the seas around Antarctica,
the consumption of phytoplankton production by both salps and krill is reported to be
significantly lower than to the north of the Continental Water Boundary.
Salps are reported to consume only 19% of daily primary production in years of
high abundance (e.g., 1994) and < 1% in poor salp years (e.g., 1995), but there is a
467
biomass, although some regions have an enigmatic low abundance; the Ross Sea, for
example, despite the very high concentrations of diatoms there, has rather few.
Krill occur in small to very large swarms that resemble those of clupeid fish, though
fish shoals are more uniform in terms of size of individuals, swimming speeds, and
maturity stage. Swarms may be aggregated into “superswarms” comprising more than
a million tons of biomass spread over several hundred square kilometers, though these
are mainly associated with coastal chlorophyll enrichment. During normal swimming,
krill are oriented with their dorsal side uppermost, but when food is encountered and
filter feeding begins, individuals may be oriented in any direction. Feeding is most active
in darkness and swarms undergo diel vertical migration between the surface and about
200 m. One reason for the success of these organisms is their ability to feed on a very
wide range of food particles from nanoflagellates to crustacean nauplii. Since krill have
such a patchy distribution, attempts to balance their consumption against production
in the blooms, or their browsing on under-ice algae, have been unsatisfactory on a
regional basis. Nevertheless, it is calculated that krill consume up to 60–80% of daily
production where they are abundant. The consumption of diatom blooms by krill can
be very spectacular: a summer bloom in the Weddell Sea was grazed down in 20 hours
from 4.0 to 05–10 mg chl m
−3 , this remnant apparently representing the unconsumed
nanophytoplankton component of the original bloom.
Euphausia superba has a life strategy that is more like that of a fish than of a planktonic
crustacean, being long-lived (5–8 years), during which period reproduction occurs two
or three times, usually late in the summer. Eggs of krill sink rapidly (out of the way of the
grazing activities of their myriad parental stock) to as much as 1000 m prior to hatching.
The nauplii and young larvae rise progressively toward the surface so that E. superba is
typically much more abundant over deep than shallow water. The two species of krill exist
in a dynamic balance with the relative abundance of one being associated with relative
scarcity of the other (Loeb et al., 1997). At Elephant Island, near the tip of the Antarctic
Peninsula, this balance is determined each spring by the extent of sea-ice cover during
the previous winter. The reestablishment of seasonal ice cover is progressive, starting with
the establishment of frazil ice, honeycombed with cavities that occlude only when the
frazil consolidates into pack ice (Smetacek et al., 1990). The lower surface and cavities
of frazil and lower surface of pack ice concentrate diatom cells that are readily “grazed”
by krill that accumulate for this purpose in and around the rugosities and cavities on
the lower surface of sea ice. During the remnant of the sunlit season and in the early
spring, sufficient irradiance penetrates the ice to maintain a diatom population here.
Leads in pack ice rapidly freeze over, and the undersurface of this new ice supports fresh
growth of algae. Dense concentrations of krill are supported during the winter within the
under-ice habitat, as many as 10–30 ind m
−2 having been observed within the interstices.
Atkinson et al. suggest that strong recruitment to the summer population depends on
ice conditions during the previous winter: extensive ice cover is associated with generous
recruitment.
Large salp populations occur after winters of small ice coverage followed by an early
open-sea spring bloom which then permits an early and rapid increase of the overwintering salp population. Krill have population dynamics resembling those of fish so that their
stock size is determined by reproductive success the previous year. Heavy recruitment
of first-year krill to the population occurs 1 year after a heavy ice winter that promotes
early spawning by the adult population and, incidentally, inhibits a spring outburst of
salps. Two consecutive heavy-ice years enhance the effect. In the seas around Antarctica,
the consumption of phytoplankton production by both salps and krill is reported to be
significantly lower than to the north of the Continental Water Boundary.
Salps are reported to consume only 19% of daily primary production in years of
high abundance (e.g., 1994) and < 1% in poor salp years (e.g., 1995), but there is a