region is not particularly unusual, and that high rates of opal preservation in sediment
are the main factor.
(^) Northward from the winter ice to the PF and from there to the SAF, there is also
upwelling from global-scale thermohaline circulation that provides abundant major
nutrients. In the vicinity of the PF there are significant, but not huge, spring
(November–December) blooms (∼1.5 mg Chl m−3, Fig. 11.15) with increase above
the annual background (∼0.3 mg Chl m−3) lasting about a month. It is thought that, as
this bloom moves southward with seasonally increasing irradiance, depletion of trace-
metal nutrients, most importantly iron, terminates the increase of phytoplankton stock
well before major nutrients are consumed. Nitrate and phosphate remain in excess
throughout the year (e.g. >7 μM nitrate at 76°S in the Ross Sea, Gordon et al. 2000),
sustaining modest levels of phytoplankton stock (∼0.3 mg Chl m−3) until darkness
and ice return. Silicate, however, can be exhausted as far poleward as 64°S (Fig.
11.16), which is likely important in termination of diatom production all across the
northern extent of the ACC to the PF.
Fig. 11.15 A time-series of mean polar front-chlorophyll concentration in 1997–1998,
estimated from nine fluorescence sensors moored in the surface mixing layer from 60°
to 61°S near 170°W. A phytoplankton bloom was initiated in late November and
ended (Chl < 0.3 mg m−3) by 15 December.
(^) (After Abbott et al. 2000.)
Fig. 11.16 Silicic acid concentrations on spring (October) and summer (January)
transects along 170°W from well north of the polar front to the seasonal ice edge.
Depletion from 60° to 65°S results from the diatom bloom that follows the receding
ice edge.
(^) (After Franck et al. 2000.)