Geoscience Reference
In-Depth Information
living in the permanent darkness of the abyss. In the Southern Ocean, little is known
about the animals that swim in the deep ocean but there has been a surprising
amount of study of the plants and animals that inhabit the surface layers and we will
concentrate here on what is known about the ecosystems that occupy the sunlit layer
of the waters that surround Antarctica.
Life in the ocean is dependent on light from above. Sunlight gets absorbed very
rapidly by water, and by the particulate and dissolved material, so there is a gradient
of light from the surface into the deep. In the Southern Ocean, there is another
factor that comes into play
-
sea ice. During autumn, the surface of the ocean freezes
and a 1
3m thick layer of sea ice extends over the most productive parts covering
an area of up to 22million km
2
-
roughly the same size as the Antarctic
continent itself. Sea ice has a number of effects on the Southern Ocean. As seawater
freezes as a result of being in contact with the very cold air, the ice formed is
fresher than the ocean and the salt that is extruded in the freezing process makes the
underlying water very cold, saline and, hence, very dense. This water sinks down
to the ocean
-
floor and seeps through the ocean basins of the world, coming to the
surface in the northern hemisphere many centuries later. The formation of
this
'
Antarctic Bottom Water
'
is an important driving element of what has become
known as the
of globally interlinked ocean current systems.
Because of the sinking of this dense water there has to be a complementary rising
of water to compensate, and this water which upwells around the Antarctic
continent is rich in nutrients and is thought to be one of the reasons why some areas
of the Southern Ocean are so highly productive. Deeper waters are richer in
nutrients because there is a tendency for all the plant and animal production in the
surface layer to sink and the nutrients that are incorporated into biological particles
-
'
global conveyor belt
'
to fall out of the sunlit zone and into the abyss.
This strips the nutrients out of the surface layer and they have to be replenished
where upwelling occurs. Upwelling is vital because in summer the surface of the
ocean warms in the sunlight and can become thermally isolated from the deeper
layer, thus preventing vertical movement of water and nutrients. Although ice has a
positive effect on plant growth by initiating the upwelling circulation, it also has a
negative effect by cutting off light from the surface layer as it grows in autumn and
winter. The amount of light that is absorbed by the ice depends on its thickness and
the amount of snow that falls on top of it, but ice also forms a substrate on which
algae can grow. The algal communities that develop over winter can colour the ice
brown and can absorb signi
cells, waste material, dead bodies
-
cant amounts of light, as well as supporting a
considerable faunal community including, for instance, being one of the main
nursery grounds for krill, one of the
species of open water pelagic
ecosystems in the Southern Ocean. This sea ice ecosystem is one of the features that
makes the Southern Ocean unique.
'
keystone
'
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