Part III: Antarctica and Academe - Scott Polar Research Institute

T
he unique physics and chemistry of the Southern Ocean strongly influences the
biology and distribution of planktonic organisms. The most important factors,
described above, include the seasonal waxing and waning of the pack ice zone,
the extreme seasonality of the light regime, the extensive cloud cover, and the
temperatures of the various water masses and their role in the transport of nutrients
and oxygen. The food web of the Southern Ocean reflects the fact that incoming
energy from the sun, nutrients, bacteria, microplankton and phytoplankton are the
basis of all higher life in the sea. Although the shading effect of the ice cover prevents
significant primary production in the underlying water column, a layer of iceassociated
algae seeds the water when the sea ice melts, a process which may
contribute more than a tenth of the annual phytoplankton production within the
Southern Ocean. These layers of tiny brown pigmented organisms, by absorbing
solar radiant energy and so accelerating the melting of the pack ice, may also have a
disproportionately large effect on the global environment.
The phytoplankton is dominated by diatoms, dinoflagellates and silicoflagellates.
The diatoms are enclosed in siliceous containers, either centric (drum-shaped) or
pennate (boat-shaped) which are often finely sculpted and of great beauty under the
microscope. Nearly l00 species have been described from the Antarctic, many of
them cosmopolitan in distribution, but some limited to the Antarctic. Colonial forms
are common and a few species produce resting spores over-winter. A typical
example is Nitschia; under the microscope thin, flat cells are seen to joined end-to-end
or face-to-face in spiralling ribbon-like colonies. In contrast to the robust diatoms, the
dinoflagellates are much smaller structures, mostly lacking external coverings and
vulnerable to damage; a few are encased in cellulose plates. A species of
Protoperidinium is the most abundant but the species prevalent in other oceans,
Ceratium, is not found south of the Antarctic Polar Front. The level of endemism is
greater than in other oceans with 35 species, 80-85% of the total restricted to the
Antarctic. Another species, Phaeocystis, is a very abundant brown algae which can
form vast gelatinous blooms down to l00 or l50 m. The silicoflagellates have an
internal skeleton, often of great beauty under the microscope; a typical species is
Dictyocha speculum, an indicator of cold water.
Abundance is measured in terms of the amount of the photosynthetic pigment
chlorophyll-a. There are seasonal and regional variations and successional patterns.
In the open ocean in summer algal biomass, measured by the concentration of the
respiratory pigment, chlorophyll a, may be ten times the winter level. The highest
values are found near the surface in coastal waters, to 3.6 g/ m 2 - as high as
anywhere in the oceans - but the Antarctic as a whole is not an especially productive
area. Diatom blooms when they occur can be conspicuous, a thick "pea soup"
extending for miles even in the open ocean. In one such situation, at a depth of 5 m,
there were l.4 million cells per litre of the diatom Corethron criophilum.
Earlier studies were based on the so-called net plankton, mainly large robust forms
such as diatoms, large enough to be caught by plankton nets (more than 20 um).
262