472
Chapter 12: The Southern Ocean
the stratification afforded by the platelets, a miniature chlorophyll profile had maxima in
the range 7–36 mg chl m
−3 and with POC maxima of 250–1250 mg chl m
−3 . Chlorophyll
concentrations were highest below freshly frozen leads. The bloom was composed largely
of centric diatoms, and the only herbivore of consequence was a large heterotrophic
dinoflagellate. As nitrate became exhausted, the nutrient-depleted diatoms sank to form
a miniature DCM at the bottom of the platelet layer and hence at a nitracline. Such
bloom conditions, below continuous ice cover, extended over an area of 20 000 km
2
and contrasted with the oligotrophic conditions then obtaining throughout all the open
waters to the north. Other investigations of the below-the-ice autotrophs have revealed
abundant pico- and nanophytoplankton along with the overwintering very early larval
stages of copepods.
Blooms in the far south may be very extensive, but are not well seen by satellite
sensors because of cloud cover. The Ross Sea is perhaps the site of the greatest and most
consistent phytoplankton biomass in the Southern Ocean, marked by deep deposits of
richly siliceous sediments. Coastal blooms along Victoria Land are consistently diatomdominated, whereas offshore blooms are more likely to be dominated by Phaeocystis
antarctica (DiTullio and Smith, 1996); an event in the coastal polynya of the western Ross
Sea contained intense blooms of up to 10–40 mg chl m
−3 over an area of 106 000 km
2
and occurred before the polynya opened up. Later, about 10 mg chl m
−3 was seen over
an area of 126 000 km
2 (Arrigo and McClain, 1994). Diatom blooms of this kind may
be terminated rapidly by gales that destroy the stratification on which they form. Images
obtained in December and January, later in the season, show the very great extent of
the Phaeocystis pouchetii blooms that occur here and are advected into McMurdo Sound
after the spring bloom of centric diatoms is over. This appears to be a characteristic
sequence in many regions in this province. Phaeocystis also has a characteristic seasonal
morphological cycle: in spring a very high percentage of cells are flagellated, solitary
individuals, whereas late in the season almost all are in the colonial stage, when their
mortality rate is much lowered.
These under-ice cell concentrations are the food resource for a variety of organisms,
some having life cycles closely adapted to these singular conditions: the small copepod
Stephos longipes is one such. This copepod manages the alternation between ice-covered
and open-water conditions as follows. There are two concurrent generations, of which
the younger overwinters within and below the mass of unconsolidated platelets below
the fast ice as nauplii and early copepodites, while the older generation lies in the
water column just below the ice as C4s that become adults in very early spring as the
ice starts to break up. The summer population, in the upper 50 m of ice-free water,
comprises mostly C1–C3s. In autumn, the adults from this generation enter the freezing
ice to generate eggs and nauplii, whereas the slightly younger copepodites remain in the
water column to overwinter as C4s, closing the cycle (Schnack-Schiel et al., 1995). Using
such an apparently unlikely stratagem, the equally improbable ice-platelet bloom is well
utilized.
Around Elephant Island (61
S), < 20-m cells were found to consistently comprise
55–100% of total chlorophyll. In spring, prokaryote contribution is negligible, but in summer these cells represent about 5–10%, though at anomalous stations with very low chlorophyll values they may contribute <50% of total chlorophyll. In many areas, these small
cells are probably comprised largely by motile cells of Phaeocystis (<3 × 10
6 cells liter
−1 )
released from macroscopic colonies of this prymnesiophyte. Several other investigations
in the Weddell Sea have shown the existence of a rich protist fauna of heterotrophic
flagellates, choanoflagellates, dinoflagellates, and ciliates whose abundance is correlated
with autotrophic cells and is direct evidence of a well-developed microbial loop. We can
expect this association to have greater biomass and flux near the ice edge and progressively less biomass out into the open water. In the Ross Sea, although abundant protists
Chapter 12: The Southern Ocean
the stratification afforded by the platelets, a miniature chlorophyll profile had maxima in
the range 7–36 mg chl m
−3 and with POC maxima of 250–1250 mg chl m
−3 . Chlorophyll
concentrations were highest below freshly frozen leads. The bloom was composed largely
of centric diatoms, and the only herbivore of consequence was a large heterotrophic
dinoflagellate. As nitrate became exhausted, the nutrient-depleted diatoms sank to form
a miniature DCM at the bottom of the platelet layer and hence at a nitracline. Such
bloom conditions, below continuous ice cover, extended over an area of 20 000 km
2
and contrasted with the oligotrophic conditions then obtaining throughout all the open
waters to the north. Other investigations of the below-the-ice autotrophs have revealed
abundant pico- and nanophytoplankton along with the overwintering very early larval
stages of copepods.
Blooms in the far south may be very extensive, but are not well seen by satellite
sensors because of cloud cover. The Ross Sea is perhaps the site of the greatest and most
consistent phytoplankton biomass in the Southern Ocean, marked by deep deposits of
richly siliceous sediments. Coastal blooms along Victoria Land are consistently diatomdominated, whereas offshore blooms are more likely to be dominated by Phaeocystis
antarctica (DiTullio and Smith, 1996); an event in the coastal polynya of the western Ross
Sea contained intense blooms of up to 10–40 mg chl m
−3 over an area of 106 000 km
2
and occurred before the polynya opened up. Later, about 10 mg chl m
−3 was seen over
an area of 126 000 km
2 (Arrigo and McClain, 1994). Diatom blooms of this kind may
be terminated rapidly by gales that destroy the stratification on which they form. Images
obtained in December and January, later in the season, show the very great extent of
the Phaeocystis pouchetii blooms that occur here and are advected into McMurdo Sound
after the spring bloom of centric diatoms is over. This appears to be a characteristic
sequence in many regions in this province. Phaeocystis also has a characteristic seasonal
morphological cycle: in spring a very high percentage of cells are flagellated, solitary
individuals, whereas late in the season almost all are in the colonial stage, when their
mortality rate is much lowered.
These under-ice cell concentrations are the food resource for a variety of organisms,
some having life cycles closely adapted to these singular conditions: the small copepod
Stephos longipes is one such. This copepod manages the alternation between ice-covered
and open-water conditions as follows. There are two concurrent generations, of which
the younger overwinters within and below the mass of unconsolidated platelets below
the fast ice as nauplii and early copepodites, while the older generation lies in the
water column just below the ice as C4s that become adults in very early spring as the
ice starts to break up. The summer population, in the upper 50 m of ice-free water,
comprises mostly C1–C3s. In autumn, the adults from this generation enter the freezing
ice to generate eggs and nauplii, whereas the slightly younger copepodites remain in the
water column to overwinter as C4s, closing the cycle (Schnack-Schiel et al., 1995). Using
such an apparently unlikely stratagem, the equally improbable ice-platelet bloom is well
utilized.
Around Elephant Island (61
S), < 20-m cells were found to consistently comprise
55–100% of total chlorophyll. In spring, prokaryote contribution is negligible, but in summer these cells represent about 5–10%, though at anomalous stations with very low chlorophyll values they may contribute <50% of total chlorophyll. In many areas, these small
cells are probably comprised largely by motile cells of Phaeocystis (<3 × 10
6 cells liter
−1 )
released from macroscopic colonies of this prymnesiophyte. Several other investigations
in the Weddell Sea have shown the existence of a rich protist fauna of heterotrophic
flagellates, choanoflagellates, dinoflagellates, and ciliates whose abundance is correlated
with autotrophic cells and is direct evidence of a well-developed microbial loop. We can
expect this association to have greater biomass and flux near the ice edge and progressively less biomass out into the open water. In the Ross Sea, although abundant protists