452
Chapter 12: The Southern Ocean
subantarctic water was dominated by microbiota, as was the subtropical water to the
north of the STCZ in spring: here, up to 98% of phytoplankton was in the <20-m
fraction, with values of 0.9–1.9 for the prokaryote/eukaryote carbon ratio. In comparison,
the phytoplankton of the STCZ was at all seasons dominated by the < 20-m fraction
and the prokaryote/eukaryote carbon ratio was of order 0.25. These observations are
now supported at the basin-scale by the images shown by Alvain et al. (2005) of the
global distribution of four phytoplankton assemblages: the South Subtropical Convergence appears at all seasons, except briefly in late winter, as a linear zone dominated by
Synechococcus-like bacterial chlorophyll. It is only briefly in summer that zonal bands of
high diatom concentration partly obscure the dominance of small cells.
In each of the three water masses investigated, microzooplankton were consuming 78–
118% of daily primary production. Despite such observations, the STCZ does harbor large
populations of macrozooplankton, of which Neocalanus tonsus is a dominant component
that contributes significantly to the vertical flux of carbon. Although this copepod has a
latitudinal range of about 20
in the southern oceans, maximum biomass of >100 ind m
−3
in the mixed layer was located within the STCZ on each of 11 meridional transects in
all three oceans (Bradford-Grieve and Jillett, 1998): this distribution suggests that this is
one of the most abundant large copepods in all seas. Stage V copepodites of Neocalanus
tonsus descend to >500 m in austral autumn, there to moult into adults; reproduction is
then fueled by wax esters that have accumulated in the surface layer during spring and
summer. Eggs are laid at depth, and the cycle starts again with the rise of larvae and of
the surviving adult females to near-surface layers in austral spring. The annual descent
of this population in autumn represents a very large contribution to downward carbon
flux, lost to the interior of the ocean. This has been calculated as 25–89 g C m
−2 y
−1
by Bradford-Grieve (pers. commun., 1995), which is an order of magnitude larger than
computed for the seasonal vertical migration of large copepods in the North Atlantic
(Longhurst and Williams, 1992). But, because small cells form a larger fraction of the
subantarctic phytoplankton, the 102 Gt C y
−1 that Neocalanus contributes to the deep
carbon pool is a smaller proportion of total phytoplankton production than is represented
by the flux in the North Atlantic.
Since “Discovery” days, it has been known that zooplankton biomass is greater in
the STCZ than farther north and that it undergoes seasonal vertical migration. This
increase in the abundance of zooplankton coincides with the southern “Transition Zone”
of McGowan’s Pacific biogeography. The group of 12 species of zooplankton that we
encountered in the North Pacific Transition Zone Province (NPPF) occurs also here
in the transition between subtropical and subpolar gyres. Representative species are the
euphausiid Thysanoessa gregaria, copepods Eucalanus bungii, E. elongatus, and E. longiceps, the chaetognath Sagitta scrippsae, and the mollusk Limacina helicina. However, this
region affords a good example of the dubious support given to ecological geography by
biogeography: consider the euphausiid species pair Nematoscelis difficilis and N. megalops.
In his monumental work on Pacific euphausiids, Brinton (1962) considered the former
to be characteristic of the North Pacific transitional zone (the NPPF province), and
N. megalops to be characteristic of the southern transitional zone (the SSTC province).
He goes on to tell us that N. megalops occurs widely in the North Atlantic, a statement
supported by later writers, in the Mediterranean, and in the Benguela Current, whereas
N. difficilis is isolated in the North Pacific.
One might infer, from such a distribution, that one ocean is exceptional in having
ecological similarity between boreal and austral high-latitude provinces. On the other
hand, we may take comfort from Brinton’s global distribution of Thysanoessa gregaria,
which has three components. First, it occurs circumglobally in the SSTC, following the
pattern of the STCZ closely in each ocean, though with some drift into the eastern
boundary currents. Then, it occurs in the transitional zone of the North Pacific, where it
Chapter 12: The Southern Ocean
subantarctic water was dominated by microbiota, as was the subtropical water to the
north of the STCZ in spring: here, up to 98% of phytoplankton was in the <20-m
fraction, with values of 0.9–1.9 for the prokaryote/eukaryote carbon ratio. In comparison,
the phytoplankton of the STCZ was at all seasons dominated by the < 20-m fraction
and the prokaryote/eukaryote carbon ratio was of order 0.25. These observations are
now supported at the basin-scale by the images shown by Alvain et al. (2005) of the
global distribution of four phytoplankton assemblages: the South Subtropical Convergence appears at all seasons, except briefly in late winter, as a linear zone dominated by
Synechococcus-like bacterial chlorophyll. It is only briefly in summer that zonal bands of
high diatom concentration partly obscure the dominance of small cells.
In each of the three water masses investigated, microzooplankton were consuming 78–
118% of daily primary production. Despite such observations, the STCZ does harbor large
populations of macrozooplankton, of which Neocalanus tonsus is a dominant component
that contributes significantly to the vertical flux of carbon. Although this copepod has a
latitudinal range of about 20
in the southern oceans, maximum biomass of >100 ind m
−3
in the mixed layer was located within the STCZ on each of 11 meridional transects in
all three oceans (Bradford-Grieve and Jillett, 1998): this distribution suggests that this is
one of the most abundant large copepods in all seas. Stage V copepodites of Neocalanus
tonsus descend to >500 m in austral autumn, there to moult into adults; reproduction is
then fueled by wax esters that have accumulated in the surface layer during spring and
summer. Eggs are laid at depth, and the cycle starts again with the rise of larvae and of
the surviving adult females to near-surface layers in austral spring. The annual descent
of this population in autumn represents a very large contribution to downward carbon
flux, lost to the interior of the ocean. This has been calculated as 25–89 g C m
−2 y
−1
by Bradford-Grieve (pers. commun., 1995), which is an order of magnitude larger than
computed for the seasonal vertical migration of large copepods in the North Atlantic
(Longhurst and Williams, 1992). But, because small cells form a larger fraction of the
subantarctic phytoplankton, the 102 Gt C y
−1 that Neocalanus contributes to the deep
carbon pool is a smaller proportion of total phytoplankton production than is represented
by the flux in the North Atlantic.
Since “Discovery” days, it has been known that zooplankton biomass is greater in
the STCZ than farther north and that it undergoes seasonal vertical migration. This
increase in the abundance of zooplankton coincides with the southern “Transition Zone”
of McGowan’s Pacific biogeography. The group of 12 species of zooplankton that we
encountered in the North Pacific Transition Zone Province (NPPF) occurs also here
in the transition between subtropical and subpolar gyres. Representative species are the
euphausiid Thysanoessa gregaria, copepods Eucalanus bungii, E. elongatus, and E. longiceps, the chaetognath Sagitta scrippsae, and the mollusk Limacina helicina. However, this
region affords a good example of the dubious support given to ecological geography by
biogeography: consider the euphausiid species pair Nematoscelis difficilis and N. megalops.
In his monumental work on Pacific euphausiids, Brinton (1962) considered the former
to be characteristic of the North Pacific transitional zone (the NPPF province), and
N. megalops to be characteristic of the southern transitional zone (the SSTC province).
He goes on to tell us that N. megalops occurs widely in the North Atlantic, a statement
supported by later writers, in the Mediterranean, and in the Benguela Current, whereas
N. difficilis is isolated in the North Pacific.
One might infer, from such a distribution, that one ocean is exceptional in having
ecological similarity between boreal and austral high-latitude provinces. On the other
hand, we may take comfort from Brinton’s global distribution of Thysanoessa gregaria,
which has three components. First, it occurs circumglobally in the SSTC, following the
pattern of the STCZ closely in each ocean, though with some drift into the eastern
boundary currents. Then, it occurs in the transitional zone of the North Pacific, where it