Antarctic Westerly Winds Biome
459
that here, too, the model of Johnson et al. (1997) for the continuous regeneration of
dissolved Fe from sinking particles must apply. So also does the model for the recycling
of oligonutrients that goes back to Ramadhas and Venugopalam (1972). These authors
demonstrated that “body-equivalent excretion time” of Fe in zooplankton is a negative
function of body size, so we may anticipate very rapid turnover of iron through the
microbial food web. Iron excreted by the macrozooplankton is largely in particulate form
and this is available for recycling, according to the model of Johnson et al.
It was, of course, the oligotrophic regime of the SAZ in the northern part of the
Subantarctic Water Ring that first attracted attention to the “HNLC” properties of the
Southern Ocean. Essays such as Karl Banse’s “Low seasonality of low chlorophyll concentrations in the Subantarctic Water Ring” leave no doubt that this is where the emphasis
lay. The reader is left with the impression that this is the dominant condition of the open
Southern Ocean, but the serial SeaWiFS images now give us a more realistic vision of
that region. You will observe that the oligotrophic region of the SAZ is no wider than
the two frontal zones of higher chlorophyll that bound it north and south. You will also
note that some seasonality, much between-year difference, and many mesoscale patches
of enhanced chlorophyll are typical of the SAZ, which is not a monotonous blue desert,
as is abundantly confirmed by a viewing of any annual series of 8-day images of surface
chlorophyll in movie format.
In the oligotrophic regime (seasonal in the PFZ, quasipermanent in the SAZ), the
phytoplankton is dominated by very small cells despite the relative absence in these cold
seas of the prokaryotic autotrophs. A relatively invariant background flora of pico- and
nanoplankton contributes up to 90% in the SAZ, where overall chlorophyll concentrations are low (02–04 g liter
−1 ), but < 50% in the PFZ where chlorophyll biomass is
higher (>18 g liter
−1 ) during the bloom period. As Detmer and Bathman (1997) put
it, the pico and nano fractions “represent abundant and stable constituents of the phytoplankton assemblage of the Southern Ocean.” However, their diversity appears to be
somewhat greater in the PFZ (prymnesiophytes, green algae, dinoflagellates, cryptophytes,
pelagophytes, and micromonads) than in the SAZ, where only the first two of these taxa
are significant.
The principal consumers of these small autotrophs are protists and very small zooplankton, often the larval stages of larger species. Imposed upon the flora of the microbial
loop, when nutrient, irradiance, and stratification conditions are suitable, is a flora of
larger diatoms and the colonial cyanobacterium Phaeocystis pouchetti. The autotrophs of
the microbial loop are here dominated not by cyanobacteria, but rather by small flagellates and diatoms that maintain biomass levels of the order of 02 mg chl m
−3 , based on
production rates of ∼ 200 mg C m
−2 d
−1 . This biomass is maintained against the constant grazing pressure of abundant heterotrophic nanoflagellates, themselves consumed
by larger ciliates and flagellates (Smetacek et al., 2004).
In assessing the relative abundance of autotrophic flagellates in the Southern Ocean,
there is a potential for misleading counts, because extracellular chloroplasts from damaged cells continue to fluoresce and may be mistaken for monads for some days at these
very cold ambient temperatures. Estimates of the contribution of nanoflagellates and very
small diatoms (3–20 m) to total water column chlorophyll and primary production are
on the order of 50–75% and 50–65%, respectively. Since small cells are able to utilize available irradiance more efficiently than large cells, it should be no surprise that, generally,
nanoplankton (fueled by the microbial loop) contribute most in a well-mixed water column. Micro- or net plankton contribute more as soon as stability is introduced at periods
longer than the potential time scale of bloom development (Weber and El-Sayed, 1987;
Jochem et al., 1995) or if trace nutrient deficiency should cease to limit nitrogen uptake.
It is the diatom flora that has, of course, attracted much attention in the Southern Ocean and is dominated by cells around 10–50 m in dimension, frequently
459
that here, too, the model of Johnson et al. (1997) for the continuous regeneration of
dissolved Fe from sinking particles must apply. So also does the model for the recycling
of oligonutrients that goes back to Ramadhas and Venugopalam (1972). These authors
demonstrated that “body-equivalent excretion time” of Fe in zooplankton is a negative
function of body size, so we may anticipate very rapid turnover of iron through the
microbial food web. Iron excreted by the macrozooplankton is largely in particulate form
and this is available for recycling, according to the model of Johnson et al.
It was, of course, the oligotrophic regime of the SAZ in the northern part of the
Subantarctic Water Ring that first attracted attention to the “HNLC” properties of the
Southern Ocean. Essays such as Karl Banse’s “Low seasonality of low chlorophyll concentrations in the Subantarctic Water Ring” leave no doubt that this is where the emphasis
lay. The reader is left with the impression that this is the dominant condition of the open
Southern Ocean, but the serial SeaWiFS images now give us a more realistic vision of
that region. You will observe that the oligotrophic region of the SAZ is no wider than
the two frontal zones of higher chlorophyll that bound it north and south. You will also
note that some seasonality, much between-year difference, and many mesoscale patches
of enhanced chlorophyll are typical of the SAZ, which is not a monotonous blue desert,
as is abundantly confirmed by a viewing of any annual series of 8-day images of surface
chlorophyll in movie format.
In the oligotrophic regime (seasonal in the PFZ, quasipermanent in the SAZ), the
phytoplankton is dominated by very small cells despite the relative absence in these cold
seas of the prokaryotic autotrophs. A relatively invariant background flora of pico- and
nanoplankton contributes up to 90% in the SAZ, where overall chlorophyll concentrations are low (02–04 g liter
−1 ), but < 50% in the PFZ where chlorophyll biomass is
higher (>18 g liter
−1 ) during the bloom period. As Detmer and Bathman (1997) put
it, the pico and nano fractions “represent abundant and stable constituents of the phytoplankton assemblage of the Southern Ocean.” However, their diversity appears to be
somewhat greater in the PFZ (prymnesiophytes, green algae, dinoflagellates, cryptophytes,
pelagophytes, and micromonads) than in the SAZ, where only the first two of these taxa
are significant.
The principal consumers of these small autotrophs are protists and very small zooplankton, often the larval stages of larger species. Imposed upon the flora of the microbial
loop, when nutrient, irradiance, and stratification conditions are suitable, is a flora of
larger diatoms and the colonial cyanobacterium Phaeocystis pouchetti. The autotrophs of
the microbial loop are here dominated not by cyanobacteria, but rather by small flagellates and diatoms that maintain biomass levels of the order of 02 mg chl m
−3 , based on
production rates of ∼ 200 mg C m
−2 d
−1 . This biomass is maintained against the constant grazing pressure of abundant heterotrophic nanoflagellates, themselves consumed
by larger ciliates and flagellates (Smetacek et al., 2004).
In assessing the relative abundance of autotrophic flagellates in the Southern Ocean,
there is a potential for misleading counts, because extracellular chloroplasts from damaged cells continue to fluoresce and may be mistaken for monads for some days at these
very cold ambient temperatures. Estimates of the contribution of nanoflagellates and very
small diatoms (3–20 m) to total water column chlorophyll and primary production are
on the order of 50–75% and 50–65%, respectively. Since small cells are able to utilize available irradiance more efficiently than large cells, it should be no surprise that, generally,
nanoplankton (fueled by the microbial loop) contribute most in a well-mixed water column. Micro- or net plankton contribute more as soon as stability is introduced at periods
longer than the potential time scale of bloom development (Weber and El-Sayed, 1987;
Jochem et al., 1995) or if trace nutrient deficiency should cease to limit nitrogen uptake.
It is the diatom flora that has, of course, attracted much attention in the Southern Ocean and is dominated by cells around 10–50 m in dimension, frequently