Life in a cold environment - Antarctica: Global Science from a Frozen Continent - page 164

Box 6.1 Sea ice: an unexpectedly productive habitat

Research in recent years has indicated that sea ice plays a crucial role in the seasonal cycle
of production of the Antarctic marine ecosystem. As ice forms in autumn, some of the algae
and other microbes in the water become incorporated in the crystallising ice and thrive in
the complex habitat of the brine channels which permeate it. The ice floating on top of the

ocean keeps these plant communities as close to the sunlight as possible and during the
dark months this is the best place to be to encourage growth. In spring with longer hours of
sunlight, the sea ice communities begin to develop and they can then become the gardens on
which the ocean is herbivores can graze. During winter there is little food available in the

water column for the herbivores to feed upon so they employ many strategies to cope with
this long, dark period of food shortage. Some species hibernate, some can retreat to the
ocean

'

floor where the remains of the summer production may lie, some starve and some
rely on supplies laid down over summer. Adult krill ( Figure 6.1c ) can probably adopt a range
of these strategies but larval krill, which are hatched in late summer, need food to allow
them to grow and develop over winter. For them, the sea ice ecosystem is probably the critical
food source and it has been suggested that there is a strong relationship between the amount
of sea ice in winter and the number of larvae that survive into spring.
When the sea ice melts in spring it releases into the water all the organisms that have
been growing over the winter, and
these are thought to be the seeds
that initiate the bloom of primary
productivity. The melting sea ice also
affects the physical structure of the
ocean because the melt water is much
fresher than seawater and it tends to
form a less dense layer, separated from
and

(a)

floating on the underlying ocean.
The algae thus tend to be circulated
within this shallow surface layer and
are kept in the sunlit zone, which further
enhances their growth. As the sea ice
retreats from north to south it leaves
in its wake a bloom of phytoplankton
and it is this intense burst of
productivity that provides the base of
the food chain that eventually leads
to the great whales, and even to
humans.