460
Chapter 12: The Southern Ocean
chain-forming and often of complex cell form. As has been proposed, the formation of
chains and the development of long spines and bristles on the cell wall may reduce their
rate of consumption by herbivores (Smetacek, 2001). It is also possible, of course, that in
these seas where deep mixing rather than stratification is the rule, cell spines may function
also to slow sinking rates as well as to inhibit ingestion. Iron-enrichment experiments in
the PFZ have, nevertheless, demonstrated the usual fate of these large cells in episodic
blooms. In EIFEX 2004 at 45
S in the Atlantic sector the summer mixed layer was very
deep and “the rate at which the bloom sank through the water column exceeded our
expectations”—to quote the Weekly Report from Polarstern for 22 March 2004. Within
one week of the peak of the induced bloom, a mass of apparently healthy diatoms was
found to be sinking at >500 m d
−1 and was subsequently followed clear to the bottom
at 3500 m. No sustained natural bloom can occur here, simply because euphotic zone
regeneration cannot occur: one or another element would be rapidly stripped down to
limiting level by removal in sinking phytoplankton. It is not surprising, therefore, that
a zone of silica-rich sediment lies on the deep-sea floor below the PFZ. Sedimentation
rates appear to be of order 80 g C m
−2 y
−1 , or about an order of magnitude higher
than in the oligotrophic zone of the Subantarctic Water Ring (Wefer and Fischer, 1991;
Tréguer and Jacques, 1992). Sporadic blooms in oligotrophic, perhaps Fe-limited places,
a small number of unusually large diatom species are characteristic: Fragilariopsis kerguelenensis, Thalassiothrix antarctica, T. lentiginosa, Corethron pennatulum. Such species
have a relatively high silica demand, because of their large size and extravagant cell
architecture.
The high-resolution (15-km scale) distribution of diatom species in the PF at 10–11
W
at austral midsummer of 1995–96 was resolved by the German JGOFS team, who observed
three species complexes of diatoms biomass being dominated by the species Fragilariopsis
kerguelenensis, Corethron inerme and C. criophilum (Bathman et al., 1997; Peeken, 1997).
Even in the absence of significant stratification in October–November, chlorophyll concentrations increased from 0.7 to 40 mg chl m
−3 to biomass stabilize at 20 mg chl m
−3 .
Maximum concentration of a Thalassiothrix species complex occurred north of the frontal
jet, Chaetoceros biomass was highest within the jet itself, while Pseudo-nitzschia was more
uniformly distributed. The dominant diatoms in each species complex were all very large
and either needle-shaped or spinous (Smetacek et al., 2002). Only euphausiids would
have been capable of clearing blooms of these species and they were present only in small
patches. Diatom biomass were therefore able to reach biomass of 2 mg chl m
−3 against a
pico- and nanophytoplankton background of 05 mg chl m
−3 , much more susceptible to
herbivore consumption.
Historically, and until the quite recent past, attention has been focused on what were
thought to be the dominant Antarctic herbivores, comprising copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachi, and Rhincalaus gigas), the chaetognaths Sagitta
gazellae and Eukrohnia hamata, and swarming tunicates (Salpa thompsoni). Though
implicated in regulating diatom blooms (and providing food for whales and seabirds),
the crustacea of this assemblage are also facultative predators and (in the case of some
species) able to feed on detritus and algae from the lower surface of ice floes. The 1992
JGOFS investigations suggested that despite their abundance, the large herbivorous copepods (mostly Calanoides acutus and Calanus propinquus) consumed only 1% of the daily
primary production in the diatom blooms at the Polar Front in spring and summer
(Dubischar and Bathman, 1997). It was concluded that the massive swarms of salps
(Salpa thompsoni) were the principal constraint on phytoplankton biomass: but these
organisms, previously thought of as microherbivores, were found to consume important
quantities also of small zooplankters.
This model was derived from results obtained with the large-mesh (200 m) plankton
nets habitually employed in the Southern Ocean, but the use of smaller-meshed nets by
Chapter 12: The Southern Ocean
chain-forming and often of complex cell form. As has been proposed, the formation of
chains and the development of long spines and bristles on the cell wall may reduce their
rate of consumption by herbivores (Smetacek, 2001). It is also possible, of course, that in
these seas where deep mixing rather than stratification is the rule, cell spines may function
also to slow sinking rates as well as to inhibit ingestion. Iron-enrichment experiments in
the PFZ have, nevertheless, demonstrated the usual fate of these large cells in episodic
blooms. In EIFEX 2004 at 45
S in the Atlantic sector the summer mixed layer was very
deep and “the rate at which the bloom sank through the water column exceeded our
expectations”—to quote the Weekly Report from Polarstern for 22 March 2004. Within
one week of the peak of the induced bloom, a mass of apparently healthy diatoms was
found to be sinking at >500 m d
−1 and was subsequently followed clear to the bottom
at 3500 m. No sustained natural bloom can occur here, simply because euphotic zone
regeneration cannot occur: one or another element would be rapidly stripped down to
limiting level by removal in sinking phytoplankton. It is not surprising, therefore, that
a zone of silica-rich sediment lies on the deep-sea floor below the PFZ. Sedimentation
rates appear to be of order 80 g C m
−2 y
−1 , or about an order of magnitude higher
than in the oligotrophic zone of the Subantarctic Water Ring (Wefer and Fischer, 1991;
Tréguer and Jacques, 1992). Sporadic blooms in oligotrophic, perhaps Fe-limited places,
a small number of unusually large diatom species are characteristic: Fragilariopsis kerguelenensis, Thalassiothrix antarctica, T. lentiginosa, Corethron pennatulum. Such species
have a relatively high silica demand, because of their large size and extravagant cell
architecture.
The high-resolution (15-km scale) distribution of diatom species in the PF at 10–11
W
at austral midsummer of 1995–96 was resolved by the German JGOFS team, who observed
three species complexes of diatoms biomass being dominated by the species Fragilariopsis
kerguelenensis, Corethron inerme and C. criophilum (Bathman et al., 1997; Peeken, 1997).
Even in the absence of significant stratification in October–November, chlorophyll concentrations increased from 0.7 to 40 mg chl m
−3 to biomass stabilize at 20 mg chl m
−3 .
Maximum concentration of a Thalassiothrix species complex occurred north of the frontal
jet, Chaetoceros biomass was highest within the jet itself, while Pseudo-nitzschia was more
uniformly distributed. The dominant diatoms in each species complex were all very large
and either needle-shaped or spinous (Smetacek et al., 2002). Only euphausiids would
have been capable of clearing blooms of these species and they were present only in small
patches. Diatom biomass were therefore able to reach biomass of 2 mg chl m
−3 against a
pico- and nanophytoplankton background of 05 mg chl m
−3 , much more susceptible to
herbivore consumption.
Historically, and until the quite recent past, attention has been focused on what were
thought to be the dominant Antarctic herbivores, comprising copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachi, and Rhincalaus gigas), the chaetognaths Sagitta
gazellae and Eukrohnia hamata, and swarming tunicates (Salpa thompsoni). Though
implicated in regulating diatom blooms (and providing food for whales and seabirds),
the crustacea of this assemblage are also facultative predators and (in the case of some
species) able to feed on detritus and algae from the lower surface of ice floes. The 1992
JGOFS investigations suggested that despite their abundance, the large herbivorous copepods (mostly Calanoides acutus and Calanus propinquus) consumed only 1% of the daily
primary production in the diatom blooms at the Polar Front in spring and summer
(Dubischar and Bathman, 1997). It was concluded that the massive swarms of salps
(Salpa thompsoni) were the principal constraint on phytoplankton biomass: but these
organisms, previously thought of as microherbivores, were found to consume important
quantities also of small zooplankters.
This model was derived from results obtained with the large-mesh (200 m) plankton
nets habitually employed in the Southern Ocean, but the use of smaller-meshed nets by