Antarctic Westerly Winds Biome
453
closely matches the distribution of the NPPF and northern California Current Province.
Finally, in the North Atlantic, it inhabits the terminal part of the Gulf Stream Province
and the whole of North Atlantic Drift Province across the ocean. Whether these distinct
populations belong to the same species, or to several cognate species, is a conundrum for
the taxonomists that will not delay us here.
The enhanced biomass of lower trophic levels in the STCZ supports concentrations of
large pelagic fish, including the mackerel Trachurus picturatus murphyi, which maintains
a large population from Chile out to at least 160
W (Parrish, 1989; Evseenko, 1987).
The STCZ is also the home range of the warm-blooded southern bluefin tuna (Thunnus
maccoyi), which exploits the relatively high biomass of small fish and squid that must
occur here. This species leaves the STCZ only to enter warmer water, principally to
the northwest of Australia, to spawn in the austral winter, when fish appear in Tasman
Sea Province and New Zealand Coastal Province as far north as the tropical convergence that passes across the northern Tasman Sea. The region of the STCZ adjacent to
the Agulhas Retroflection appears to carry an especially large population of southern
bluefin.
In the pristine ocean, before whaling, this was the preferred habitat of sei whales
(Balaenoptra borealis), whose slender baleen plates and specialized tongue grooves are
adapted to a wider range of planktonic organisms than those of the euphausiid specialists,
blue and fin whales. Although the feeding mechanism of sei whales is thus specialized
rather for obtaining copepods than swarming euphausiids, they utilize both skimming
and engulfing behavior according to the prey species actually present: these include
herbivorous copepods (e.g., Drepanopus), but also amphipods, especially Parathemisto
gaudichauda, one of the three main swarming nekton species of the Southern Ocean, and
in the SW Atlantic, Munida gregaria that is another swarming species—obviously! In the
Southern Ocean, copepods are also swarm-forming: calanoids here form swarms of order
100 m in horizontal extent, at concentrations sufficient to support “skimming” feeding of
sei and right whales. Nemoto and Harrison (1981) suggest that the absence of Calanoides
acutus and Euphausia triacantha in whale stomachs is due to their nonswarming habit.
Sei whales have now, after decimation of blue whales by whalers, to some extent shifted
their range southward into the range of Euphausia superba.
This is also the feeding range of the bulk of the population of southern sperm
whales (Physeter macrocephalus), although the larger males penetrate farther south into
cold water. Sperm whales feed almost exclusively on squid, obtained by deep diving
to at least 1200 m, where it is presumed that prey must be found entirely by echolocation. Very complex social structure is revealed by the composition of individual
schools: specialized groups of individuals form nursery, juvenile, bachelor, harem, and
bull schools that have some permanence and may each include >100 individuals (Brown
and Lockyer, 1984).
Synopsis
Case 3—Winter-spring production with nutrient limitation—The archived climatology
illustrated in synopsis form in Fig. 12.3 shows that the mixed layer undergoes a major
winter excursion to 175 m while photic depth remains at 40–50 m, so that the thermocline
is illuminated only briefly in austral summer. Productivity rate increases progressively
during the period of increasing illumination from June to the November maximum;
thereafter, productivity rapidly declines to winter minimum. There is a rather weak
seasonal change in chlorophyll biomass (025–035 mg m
−3 ), with some accumulation
occurring during the period of the productivity rate increase. It is not clear why the
seasonal changes in production and accumulation should not be matched more closely;
in some years, consumption appears to require a period of about 60 days to come into
453
closely matches the distribution of the NPPF and northern California Current Province.
Finally, in the North Atlantic, it inhabits the terminal part of the Gulf Stream Province
and the whole of North Atlantic Drift Province across the ocean. Whether these distinct
populations belong to the same species, or to several cognate species, is a conundrum for
the taxonomists that will not delay us here.
The enhanced biomass of lower trophic levels in the STCZ supports concentrations of
large pelagic fish, including the mackerel Trachurus picturatus murphyi, which maintains
a large population from Chile out to at least 160
W (Parrish, 1989; Evseenko, 1987).
The STCZ is also the home range of the warm-blooded southern bluefin tuna (Thunnus
maccoyi), which exploits the relatively high biomass of small fish and squid that must
occur here. This species leaves the STCZ only to enter warmer water, principally to
the northwest of Australia, to spawn in the austral winter, when fish appear in Tasman
Sea Province and New Zealand Coastal Province as far north as the tropical convergence that passes across the northern Tasman Sea. The region of the STCZ adjacent to
the Agulhas Retroflection appears to carry an especially large population of southern
bluefin.
In the pristine ocean, before whaling, this was the preferred habitat of sei whales
(Balaenoptra borealis), whose slender baleen plates and specialized tongue grooves are
adapted to a wider range of planktonic organisms than those of the euphausiid specialists,
blue and fin whales. Although the feeding mechanism of sei whales is thus specialized
rather for obtaining copepods than swarming euphausiids, they utilize both skimming
and engulfing behavior according to the prey species actually present: these include
herbivorous copepods (e.g., Drepanopus), but also amphipods, especially Parathemisto
gaudichauda, one of the three main swarming nekton species of the Southern Ocean, and
in the SW Atlantic, Munida gregaria that is another swarming species—obviously! In the
Southern Ocean, copepods are also swarm-forming: calanoids here form swarms of order
100 m in horizontal extent, at concentrations sufficient to support “skimming” feeding of
sei and right whales. Nemoto and Harrison (1981) suggest that the absence of Calanoides
acutus and Euphausia triacantha in whale stomachs is due to their nonswarming habit.
Sei whales have now, after decimation of blue whales by whalers, to some extent shifted
their range southward into the range of Euphausia superba.
This is also the feeding range of the bulk of the population of southern sperm
whales (Physeter macrocephalus), although the larger males penetrate farther south into
cold water. Sperm whales feed almost exclusively on squid, obtained by deep diving
to at least 1200 m, where it is presumed that prey must be found entirely by echolocation. Very complex social structure is revealed by the composition of individual
schools: specialized groups of individuals form nursery, juvenile, bachelor, harem, and
bull schools that have some permanence and may each include >100 individuals (Brown
and Lockyer, 1984).
Synopsis
Case 3—Winter-spring production with nutrient limitation—The archived climatology
illustrated in synopsis form in Fig. 12.3 shows that the mixed layer undergoes a major
winter excursion to 175 m while photic depth remains at 40–50 m, so that the thermocline
is illuminated only briefly in austral summer. Productivity rate increases progressively
during the period of increasing illumination from June to the November maximum;
thereafter, productivity rapidly declines to winter minimum. There is a rather weak
seasonal change in chlorophyll biomass (025–035 mg m
−3 ), with some accumulation
occurring during the period of the productivity rate increase. It is not clear why the
seasonal changes in production and accumulation should not be matched more closely;
in some years, consumption appears to require a period of about 60 days to come into