464
Chapter 12: The Southern Ocean
This is, of course, the region within which Atlantic Deep Water upwells, within and
adjacent to the Antarctic Divergence. Lower Central Deep Water (LCDW) reaches the
surface at the AD and Upper Central Deep Water (UCDW) closer to the PF. The LCDW
water mass carries higher concentration of dissolved nutrients than UCDW.
Biological Response and Regional Ecology
Nelson and Smith (1991) reformulated the Sverdrup model with parameters suitable
for very high latitudes to estimate the maximum chlorophyll biomass permitted by the
relationship between critical and mixed-layer depths in the APZ. Their predictions closely
match observations, both for the marginal ice zone and for the open oceanic regions. At
60
S, where clear-sky irradiance in January would be ∼60 mol m
−2 d
−1 , this is reduced to
<50% on most days by cloud cover to around 25 mol m
−2 d
−1 . Critical depth (Z cr ) shoals
in response to a very small increase in chlorophyll, but this relationship becomes much
weaker at >2 g liter
−1 chlorophyll. In the absence of chlorophyll, critical depth would
exceed mixed depth, but Z cr = Z m at 05–15 g chlorophyll, depending on the value of
Z m . As Nelson and Smith comment, “This makes any bloom self-limiting” because above
a critical chlorophyll biomass “an unfavourable relationship between Z cr and Z m would
prevent further increase in phytoplankton biomass.” In open water, this may restrain
biomass to <10 g liter
−1 , although higher concentrations occur in the MIZ. Nelson and
Smith predict that mixed-layer depth and integrated chlorophyll biomass will be inversely
related, as indeed they appear to be now that we have adequate spatial information on
chlorophyll and wind speed from satellite observations.
The regime in the permanently open water and of that part of the migrating marginal
ice zone (MIZ) that lies north of the CWB is seasonally complex, as noted earlier. This
region lies poleward of the maximum silicate gradient (Si max ) in spring, but equatorward
in summer, because the strong uptake of silica by the diatom blooms that occur progressively further south during that period drives the silica gradient poleward. Progressive
SiO 3 depletion induces a shift from Fe limitation in spring to Fe repletion in austral
summer. This change is accompanied by the disappearance of diatoms from the phytoplankton assemblage, which comes to be dominated by small cells.
As suggested by such models of nutrient dynamics, the phytoplankton of the openwater areas in the ANTA province exhibits a complex response to changing seasons.
During winter, the low sun angle effectively stops phytoplankton growth, but upwelling
continues to bring UCDW water, relatively rich in both macro- and oligonutrients, toward
the surface. In the Pacific sector, the surface water mass at the end of winter comprises
almost entirely UCDW, with a small admixture of surface water from the austral polar
regime that flows equatorward from the APLR province (Hiscock et al., 2003). Thus,
when sun angle increases and intermittent stratification occurs in surface waters (recall
that the ice edge is usually not far distant), a diatom bloom develops in late November
or December, propagating poleward and progressively depleting SiO 3 as it goes. Highest
diatom biomass occurs at the silica gradient as the two features move poleward together,
but the bloom is liable to be interrupted by local mixing events. Of course, where the
zonal band of open water at the end of winter is relatively narrow, the open water and
MIZ blooms are initiated almost simultaneously. Where the zone is relatively wide, the
open-water bloom tends to be dominated by large diatoms (Corethron, Chaetoceros),
whereas along the ice edge Phaeocyctis is often dominant. Local chlorophyll enhancement
with values reaching about 4 mg chl m
−3 at a subsurface chlorophyll maximum (Smith
and Nelson, 1985). Sediment trap data indicate that the very brief spring bloom is
dominated by diatoms in December and January and is followed by a late-summer season
of regenerative algal growth of small cells from which sedimentation rates are two orders
of magnitude lower (Smetacek et al., 1990).
Chapter 12: The Southern Ocean
This is, of course, the region within which Atlantic Deep Water upwells, within and
adjacent to the Antarctic Divergence. Lower Central Deep Water (LCDW) reaches the
surface at the AD and Upper Central Deep Water (UCDW) closer to the PF. The LCDW
water mass carries higher concentration of dissolved nutrients than UCDW.
Biological Response and Regional Ecology
Nelson and Smith (1991) reformulated the Sverdrup model with parameters suitable
for very high latitudes to estimate the maximum chlorophyll biomass permitted by the
relationship between critical and mixed-layer depths in the APZ. Their predictions closely
match observations, both for the marginal ice zone and for the open oceanic regions. At
60
S, where clear-sky irradiance in January would be ∼60 mol m
−2 d
−1 , this is reduced to
<50% on most days by cloud cover to around 25 mol m
−2 d
−1 . Critical depth (Z cr ) shoals
in response to a very small increase in chlorophyll, but this relationship becomes much
weaker at >2 g liter
−1 chlorophyll. In the absence of chlorophyll, critical depth would
exceed mixed depth, but Z cr = Z m at 05–15 g chlorophyll, depending on the value of
Z m . As Nelson and Smith comment, “This makes any bloom self-limiting” because above
a critical chlorophyll biomass “an unfavourable relationship between Z cr and Z m would
prevent further increase in phytoplankton biomass.” In open water, this may restrain
biomass to <10 g liter
−1 , although higher concentrations occur in the MIZ. Nelson and
Smith predict that mixed-layer depth and integrated chlorophyll biomass will be inversely
related, as indeed they appear to be now that we have adequate spatial information on
chlorophyll and wind speed from satellite observations.
The regime in the permanently open water and of that part of the migrating marginal
ice zone (MIZ) that lies north of the CWB is seasonally complex, as noted earlier. This
region lies poleward of the maximum silicate gradient (Si max ) in spring, but equatorward
in summer, because the strong uptake of silica by the diatom blooms that occur progressively further south during that period drives the silica gradient poleward. Progressive
SiO 3 depletion induces a shift from Fe limitation in spring to Fe repletion in austral
summer. This change is accompanied by the disappearance of diatoms from the phytoplankton assemblage, which comes to be dominated by small cells.
As suggested by such models of nutrient dynamics, the phytoplankton of the openwater areas in the ANTA province exhibits a complex response to changing seasons.
During winter, the low sun angle effectively stops phytoplankton growth, but upwelling
continues to bring UCDW water, relatively rich in both macro- and oligonutrients, toward
the surface. In the Pacific sector, the surface water mass at the end of winter comprises
almost entirely UCDW, with a small admixture of surface water from the austral polar
regime that flows equatorward from the APLR province (Hiscock et al., 2003). Thus,
when sun angle increases and intermittent stratification occurs in surface waters (recall
that the ice edge is usually not far distant), a diatom bloom develops in late November
or December, propagating poleward and progressively depleting SiO 3 as it goes. Highest
diatom biomass occurs at the silica gradient as the two features move poleward together,
but the bloom is liable to be interrupted by local mixing events. Of course, where the
zonal band of open water at the end of winter is relatively narrow, the open water and
MIZ blooms are initiated almost simultaneously. Where the zone is relatively wide, the
open-water bloom tends to be dominated by large diatoms (Corethron, Chaetoceros),
whereas along the ice edge Phaeocyctis is often dominant. Local chlorophyll enhancement
with values reaching about 4 mg chl m
−3 at a subsurface chlorophyll maximum (Smith
and Nelson, 1985). Sediment trap data indicate that the very brief spring bloom is
dominated by diatoms in December and January and is followed by a late-summer season
of regenerative algal growth of small cells from which sedimentation rates are two orders
of magnitude lower (Smetacek et al., 1990).