new questions. For reasons that are largely political and coupled to interest in the
common international “ownership” of the Antarctic continent, of whale stocks and the
krill resource, it has been studied more thoroughly than the subantarctic that we
consider next.
Subantarctic
(^) The subantarctic is a roughly circular band of ocean between the SAF front and the
STC, varying (and oscillating) in width from ∼14° of latitude (38° to 52°S) at 135°W
(eastern Pacific) to very narrow in the Cape Horn Current where all the circumpolar
currents squeeze through Drake Passage. It stretches across the “roaring forties” with
persistently high winds (recorded on some of the islands in excess of 50 km hr−1 for
150 days per year). That ensures persistently deep mixing. Its pelagic ecosystem
functions somewhat similarly to the subarctic Pacific. Surface temperatures vary with
latitude from 4 to 10°C in winter, rising to ∼14°C above a weak seasonal thermocline
in summer, but deeper than that in the subarctic Pacific. Surface salinity is reduced
mostly by abundant rainfall. There is a halocline, but again deeper, ∼200 m, than in
the subarctic Pacific. In terms of production ecology, the subantarctic is a cold, high-
nitrogen low-chlorophyll (HNLC) region (Moore & Abbott 2000). Major nutrients,
with the exception of silicate, are never exhausted, chlorophyll varies around 0.3–0.4
mg m−3 (Plate 11.2), and isolation from land results in iron limitation of growth rates
of larger phytoplankton. While there are chlorophyll “hot spots” during spring–
summer blooms around islands in antarctic waters south of the SAF (South Georgia,
Kergulen and Crozet islands), the main hot spots in the subantarctic are downstream
of New Zealand and South America. Even near the Crozet Plateau, well into usual
subantarctic latitudes at 46°S, the subantarctic front steers north well to the west,
putting the archipelago into fully antarctic waters. However, iron from the New
Zealand Plateau and from South America generates eastward-billowing flags of
elevated chlorophyll in satellite images of the subantarctic during spring and summer.
Deeper mixing and winter darkness limit production and stocks in fall and winter.
(^) Unlike other HNLC areas, silicate has very low availability in the subantarctic, <15
μM at the seasonal maximum and <1–3 μM after mid-summer. The general
explanation (Zentara & Kamykowski 1981) is that the siliceous shells of diatoms sink
to much greater depth – even to the seafloor – before significant dissolution, than do
organic nitrogen and phosphate (including the organic portions of diatoms). Also,
near-surface N and P regeneration allows those elements multiple cycles of
incorporation, each cycle exporting a larger fraction of Si than of N or P. Thus, silicon
becomes much less accessible to upward seasonal mixing: the silicocline is much
deeper than the nitricline. South of the SAF, the strong vertical velocity over most of
the water column compensates for this process, so that levels after the winter darkness