4
R. K. DELL
by the Antarctic Convergence result from a pattern of subsurface water
movement. Mainly because of the westerly wind, the surface waters of
the Southern Ocean move in an easterly direction to form the Antarctic
Circumpolar Current. Cold sea water originating along the edge of the
Continent is cold and dense and sinks to move towards the north as
Antarctic Bottom Water. Some of the Antarctic Surface Water has a
northerly component which sinks to an intermediate level at the Antarctic Convergence to form Subantarctic Intermediate Water. Highly
saline deep waters from the north, rich in nutrients, move southwards
at intermediate levels to replace the northward moving waters mentioned above, to rise close to the surface near the Continent. This injection of nutrient salts in the Antarctic, together with the effect of sunlight for 24 h of the day in southern summer results in an extremely
high period of summer productivity. In marked contrast to other areas
the supply of nutrient salts from this source is so great that productivity
is probably never limited from a lack of nutrients in the Antarctic.
There is of course a marked contrast in productivity between summer
and winter. The results as recorded by a human diver working throughout the year have been vividly expressed by Peckham (1964). During
the winter the absence of phytoplankton meant that the sea water was
exceptionally clear. A small lighted torch bulb was still visible to a
depth of almost 100 m. The first sunlight in September allowed exceptionally clear observation but by mid-summer the heavy phytoplankton
growth often cut visibility to less than 10 ft. The unique contribution
of Antarctic waters in this respect can better be appreciated by the
objective estimation (El-Sayed, 1968) that the waters south of the
Antarctic Convergence (about 5% of the oceans of the world) support
a total production equivalent to about 20% of that produced by all the
oceans of the world.
This abundance of phytoplankton provides an almost unlimited
source of food, directly or indirectly, to the benthos. The fact that it is
seasonal imposes physiological rhythms which have as yet been investigated in only a relatively few organisms.
The continental shelf around Antarctica is narrow everywhere
except in the two embayments of the Ross and Weddell Seas (Brodie,
1965). As Brodie has noted the sudden change in slope which marks the
outer limit of the shelf and the beginning of the continental slope is
much deeper than the 150 m or so that is commonly found elsewhere.
Around the Antarctic Continent the change in slope may occur at depths
between 400 and 600 m, and in the Ross Sea it occurs at nearly 800 m.
In most areas the continental slope descends without any very complicated bottom features to depths of about 3 000 m. Three areas of
R. K. DELL
by the Antarctic Convergence result from a pattern of subsurface water
movement. Mainly because of the westerly wind, the surface waters of
the Southern Ocean move in an easterly direction to form the Antarctic
Circumpolar Current. Cold sea water originating along the edge of the
Continent is cold and dense and sinks to move towards the north as
Antarctic Bottom Water. Some of the Antarctic Surface Water has a
northerly component which sinks to an intermediate level at the Antarctic Convergence to form Subantarctic Intermediate Water. Highly
saline deep waters from the north, rich in nutrients, move southwards
at intermediate levels to replace the northward moving waters mentioned above, to rise close to the surface near the Continent. This injection of nutrient salts in the Antarctic, together with the effect of sunlight for 24 h of the day in southern summer results in an extremely
high period of summer productivity. In marked contrast to other areas
the supply of nutrient salts from this source is so great that productivity
is probably never limited from a lack of nutrients in the Antarctic.
There is of course a marked contrast in productivity between summer
and winter. The results as recorded by a human diver working throughout the year have been vividly expressed by Peckham (1964). During
the winter the absence of phytoplankton meant that the sea water was
exceptionally clear. A small lighted torch bulb was still visible to a
depth of almost 100 m. The first sunlight in September allowed exceptionally clear observation but by mid-summer the heavy phytoplankton
growth often cut visibility to less than 10 ft. The unique contribution
of Antarctic waters in this respect can better be appreciated by the
objective estimation (El-Sayed, 1968) that the waters south of the
Antarctic Convergence (about 5% of the oceans of the world) support
a total production equivalent to about 20% of that produced by all the
oceans of the world.
This abundance of phytoplankton provides an almost unlimited
source of food, directly or indirectly, to the benthos. The fact that it is
seasonal imposes physiological rhythms which have as yet been investigated in only a relatively few organisms.
The continental shelf around Antarctica is narrow everywhere
except in the two embayments of the Ross and Weddell Seas (Brodie,
1965). As Brodie has noted the sudden change in slope which marks the
outer limit of the shelf and the beginning of the continental slope is
much deeper than the 150 m or so that is commonly found elsewhere.
Around the Antarctic Continent the change in slope may occur at depths
between 400 and 600 m, and in the Ross Sea it occurs at nearly 800 m.
In most areas the continental slope descends without any very complicated bottom features to depths of about 3 000 m. Three areas of