Part III: Antarctica and Academe - Scott Polar Research Institute

vertical water-filled channels and grows in spring. The algae are adapted to the cold
and saline conditions; during winter they may experience temperatures as low as -
l0°C and salinities as high as l50°/oo (for comparison sea water salinities are about
34°/oo; and even in winter, whenever the sun is above the horizon, they can make use
of its energy. However peaks in standing crop (chlorophyll-a) have been described in
some sea ice regions, occurring in April to June and October to November. In one of
these studies 98% of the chlorophyll-a was concentrated in the bottom of the ice,
mainly formed by congelation during May. There was an increase in the upper
consolidated snow layer from October to January. All these layers are clearly
exposed when the ice is turned over by the passage of a ship and this also shows one
how patchy the algal blooms are in their horizontal distribution. When melting of the
ice begins in spring a stable meltwater surface layer forms which is fresher and
lighter than ordinary sea water; the algae trapped in this layer are exposed to
sunlight, become more active and their growth rates increase, so that dense green
blooms form along the ice edge. Later when the ice disappears and wind-induced
turbulence stirs up the surface water the water column is seeded with phytoplankton
organisms. In this way levels of chlorophyll-a may be enhanced up to 250 km away
from the edge of the ice and this can be an indication of recent ice cover.
Zooplankton is the animal plankton which feeds upon the minute plants in the
phytoplankton; it includes protozoans, small crustaceans (like copepods, amphipods
and euphausiids, including the Antarctic krill), chaetognaths (arrow worms), pelagic
gastropods (allied to snails, slugs), salps, siphonophores, and fish larvae (the
ichthyoplankton). Strictly-speaking plankton drifts passively with the water
movements, but some species included in the zooplankton are quite actively moving
- and should more properly be referred to as nekton (free-swimming organisms). The
Antarctic krill, Euphausia superba, as we shall see is the most important member of
this latter group. The literature is dominated by E. superba because of its enormous
ecological interest and its value as a resource. There are ten other species of
crustaceans, collectively known as krill (the term derives from the Norwegian word
for whale food). The most important are Euphausia frigida, E. crystallorophias, E.
valentini and Thysanoessa macrura . The species of Euphausia demonstrate the
latitudinal zonation characteristic of many groups of zooplankton: E.valentini is
confined to water above 3°C north of the Antarctic Polar Front; E. triacantha and E.
frigida are warm water species found south of the Polar Front, the former straddling
it, the latter in colder regions; E. superba which is found in large numbers anywhere
south of the Polar Front particularly in the open ocean; and E. crystallorophias which
occurs over the continental shelves. Other groups, for example copepods, rival krill
in importance in terms of their biomass and show similar zonation. There is a
gradual transition from north to south, but many species occupy clearly defined
temperature zones which are circumpolar; analysis of net- hauls has confirmed this.
Leaving aside Euphausia superba for the moment (as part of the nekton), about 70% of
the meso-plankton (middle-sized organisms) are copepods and the three most
common genera are Calanoides acutus, Calanus propinqus andRhincalanus gigas; these
are herbivores. The arrow worms, chaetognaths, are second only to them in
importance and are carnivorous predators. Of the l8 species described from Antarctic
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