Antarctic Westerly Winds Biome
461
Dubischar et al. (2002) showed that the large calanoids are—as it were—just the tip of the
copepod iceberg. In the PFZ of the Atlantic sector, dense populations of small copepods
occur within the frontal system, although diversity is not high: a few dominant species
take abundances five to ten times higher than the “common” species. Concentrations of
up to 50000 ind m
−3 , mostly copepodites (and mostly those of Oithona similis, O. frigida,
and Ctenocalanus citer) were recorded in obliquely hauled nets. Even higher concentrations must occur at some depths, since profiles of copepod abundance are rarely
uniform. The abundance of small copepods in the PFZ is therefore comparable with
the greatest abundances reported for productive coastal ecosystems. Retention in the
warm, downwelling side of meanders along the frontal jet assists in maintaining such
concentrations, but the fivefold increase in numbers of small calanoids and cyclopoids
in austral spring suggest that these may play an important role in the controlling diatom
accumulation. Although the daily carbon requirement of the <2-mm copepods exceeds
the 750–950 mg C m
−2 d
−1 typical of such blooms, at least Oithona is known to feed
largely on fecal pellets of larger zooplankters. Whatever they consume in these seas, we
cannot neglect the role that small copepods must play in regulating Southern Ocean
plankton dynamics.
Despite the importance of smaller forms, it is the larger copepods that have attracted
most attention in the Southern Ocean until now. The seasonal and ontogenetic vertical migrations, which allow them to feed near the surface and to overwinter deep,
have been studied in detail since the early days of Antarctic oceanography. Two related
species of calanoids have fundamentally different strategies: I deal with them together for
comparison, though it must be noted that these species have somewhat different zonal
distributions. Calanus propinquus occurs principally in the open water of the PFZ and
immediately poleward, whereas Calanoides acutus is the more austral species, at home in
the coastal water flow (Mackintosh, 1937).
In line with this observation, C. propinquus departs from the high-latitude norm and
stores its lipid reserves as triglycerides rather than as wax esters as does C. acutus. Both
species migrate deep in autumn, but differently. Calanoides acutus descends to great
depths (500–1000 m) to overwinter in diapause so as to conserve lipid reserves that
will be required for reproduction at the surface in spring. Calanus propinquus goes less
deep (100–500 m) and does not enter physiological diapause; some feeding, often by
predation on microzooplankton, may continue and reproduction occurs over a longer
period than for C. acutus. Both species are concentrated near the surface in summer
(usually at 0–50 m), and C. acutus both ascends and descends earlier than C. propinquus.
The ascent of C. acutus females coincides with the onset of sea-ice melting, or with the
very earliest spring bloom (October) in permanently open water: eggs are matured and
shed quite rapidly. In the austral autumn (March), C. acutus descends (as copepodite 4s or
5s), although there is still sufficient phytoplankton in the surface waters for C. propinquus
to continue feeding (Smith and Schnack-Schiel, 1990; Schnack-Schiel et al., 1991).
In contrast, Rhincalanus gigas is equally at home in both SANT and ANTA provinces
and may produce two generations per year, in spring and autumn. It lies somewhat deeper
in summer (50–100 m) than does either C. acutus and C. propinquus, and in winter it
descends to 500–1000 m. Several species perform regular diel migrations in this water
body between the near-surface chlorophyll layer at night and the warmer subsurface layer
by day at >200 m. These are familiar diel migrant genera: Pleuromamma robusta together
with the small krill species Euphausia triacantha and E. frigida that preferentially inhabit
the PFZ.
Here, sei whales feed on swarming zooplankton, largely copepods, that are present
in sufficient concentrations to support their unique “swallowing and skimming” feeding
technique that merges that of right whales and the blue whale. Sei whales are more
omnivorous than right whales, taking not only herbivorous crustacea but also amphipods,
461
Dubischar et al. (2002) showed that the large calanoids are—as it were—just the tip of the
copepod iceberg. In the PFZ of the Atlantic sector, dense populations of small copepods
occur within the frontal system, although diversity is not high: a few dominant species
take abundances five to ten times higher than the “common” species. Concentrations of
up to 50000 ind m
−3 , mostly copepodites (and mostly those of Oithona similis, O. frigida,
and Ctenocalanus citer) were recorded in obliquely hauled nets. Even higher concentrations must occur at some depths, since profiles of copepod abundance are rarely
uniform. The abundance of small copepods in the PFZ is therefore comparable with
the greatest abundances reported for productive coastal ecosystems. Retention in the
warm, downwelling side of meanders along the frontal jet assists in maintaining such
concentrations, but the fivefold increase in numbers of small calanoids and cyclopoids
in austral spring suggest that these may play an important role in the controlling diatom
accumulation. Although the daily carbon requirement of the <2-mm copepods exceeds
the 750–950 mg C m
−2 d
−1 typical of such blooms, at least Oithona is known to feed
largely on fecal pellets of larger zooplankters. Whatever they consume in these seas, we
cannot neglect the role that small copepods must play in regulating Southern Ocean
plankton dynamics.
Despite the importance of smaller forms, it is the larger copepods that have attracted
most attention in the Southern Ocean until now. The seasonal and ontogenetic vertical migrations, which allow them to feed near the surface and to overwinter deep,
have been studied in detail since the early days of Antarctic oceanography. Two related
species of calanoids have fundamentally different strategies: I deal with them together for
comparison, though it must be noted that these species have somewhat different zonal
distributions. Calanus propinquus occurs principally in the open water of the PFZ and
immediately poleward, whereas Calanoides acutus is the more austral species, at home in
the coastal water flow (Mackintosh, 1937).
In line with this observation, C. propinquus departs from the high-latitude norm and
stores its lipid reserves as triglycerides rather than as wax esters as does C. acutus. Both
species migrate deep in autumn, but differently. Calanoides acutus descends to great
depths (500–1000 m) to overwinter in diapause so as to conserve lipid reserves that
will be required for reproduction at the surface in spring. Calanus propinquus goes less
deep (100–500 m) and does not enter physiological diapause; some feeding, often by
predation on microzooplankton, may continue and reproduction occurs over a longer
period than for C. acutus. Both species are concentrated near the surface in summer
(usually at 0–50 m), and C. acutus both ascends and descends earlier than C. propinquus.
The ascent of C. acutus females coincides with the onset of sea-ice melting, or with the
very earliest spring bloom (October) in permanently open water: eggs are matured and
shed quite rapidly. In the austral autumn (March), C. acutus descends (as copepodite 4s or
5s), although there is still sufficient phytoplankton in the surface waters for C. propinquus
to continue feeding (Smith and Schnack-Schiel, 1990; Schnack-Schiel et al., 1991).
In contrast, Rhincalanus gigas is equally at home in both SANT and ANTA provinces
and may produce two generations per year, in spring and autumn. It lies somewhat deeper
in summer (50–100 m) than does either C. acutus and C. propinquus, and in winter it
descends to 500–1000 m. Several species perform regular diel migrations in this water
body between the near-surface chlorophyll layer at night and the warmer subsurface layer
by day at >200 m. These are familiar diel migrant genera: Pleuromamma robusta together
with the small krill species Euphausia triacantha and E. frigida that preferentially inhabit
the PFZ.
Here, sei whales feed on swarming zooplankton, largely copepods, that are present
in sufficient concentrations to support their unique “swallowing and skimming” feeding
technique that merges that of right whales and the blue whale. Sei whales are more
omnivorous than right whales, taking not only herbivorous crustacea but also amphipods,