Part III: Antarctica and Academe - Scott Polar Research Institute

that there has probably been no significant change in the extent of the Antarctic sea
ice over the last l00 years - as far back as such records exist. Although there has been
no significant trend since satellite data have been studied, there are large variations
between regions. There is for example a shift in phase of 2-3 years from the Weddell
to the Ross Seas; a minimum seen in the former region in l977, which in the latter in
l980.
The sea ice is formed in three different ways. Frazil ice consists of small ice crystals
(about l mm) and often forms in stormy conditions, coagulating into small sugarylooking
floes, often with turned up edges due to their bumping together, called
pancake ice. These consolidate and thicken at the rate of several cm an hour.
Congelation ice is formed slowly by heat loss through an overlying layer of ice,
which may have originated as frazil ice. The crystals are larger - to l cm and as they
form express salts and exclude algae initially. The crystals have a columnar structure
which subsequently allows sea water (and therefore nutrients) to circulate upwards
and also downward flushing with freshwater from snow-melt; this produces
favourable conditions for algal growth. A third ice-formation process is when free
crystals form under inshore fast ice where conditions are sheltered and still.
Phytoplankton is trapped in this platelet ice among the crystals and incorporated into
the ice layer. Later in the summer, as the ice melt and decay continues, larger
channels open up so that it becomes riddled with holes - like gruyère cheese - further
enhancing nutrient exchange and the spread of the algae.
Conditions are better for algal blooms in ice formed by congelation or from platelets,
than in frazil ice. Later algae are also found in the surface layers as snow
accumulation depresses the older surface layers and sea water penetrates, forming a
loose ice-water matrix that is inoculated with algae. They are also found at various
levels within the ice, depending both on how the ice forms (and whether algae are
included or excluded during the process), as well as on the establishment of
circulation channels. Initially the sea ice contains about 30% of sea water, but by the
end of the winter this has fallen to l5%. Its thickness varies according to location and
season. It may be only 0.3 m thick in early winter at 60 - 65°S but as much as 2-3 m at
75°S in late spring or early summer. Little is known about maximum winter
thicknesses.
Seasonal changes in the chemistry of sea ice have been studied. The salinities
measured in melted ice cores taken from it ice vary from 0 to 30°/oo, while brine
collected from holes drilled in the sea ice may very from 64-l24°/oo. The nutrients
measured in these cores and brine fluctuate in an extreme way. In summer
conditions very high levels of ammonia (far above those found in seawater) and high
nitrate and phosphate levels are found. These high levels are evidence for the
presence of a unique interstitial microbial food chain of microorganisms adapted to
low temperatures and high salinity. In winter lower phosphate levels are found and
there is relative depletion of silica; some nutrients are exhausted, others regenerated.
It may be that the growth of diatoms may be nutrient limited and this may affect the
community structure.
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