THE POLAR DEEP SEAS
253
bottom melting. The density of these drop-stones
declines northwards, and their distribution influences
the population dynamics of encrusting taxa in the
Southern ocean. Unfortunately very little is known
of either the distribution or the fauna of these dropstones.
Hydrothermal vents
One habitat which was unknown before the mid1970s and which is currently the subject of intense
investigation is that of hydrothermal vent fields (Gage
and Tyler, 1991). The tectonic history of the Southern
Ocean suggests that hydrothermal vent fields are very
likely to exist, especially in the eastern Scotia Sea or
the South Shetland Islands. As yet, however, none has
been described.
Primary production
For many years the Southern Ocean was considered
to be highly productive. It is now known that this
conclusion was the result of a strong seasonal and
spatial bias in sampling (Smith and Sakshaug, 1990).
More extensive oceanographic measures and especially
remotely-sensed ocean colour data have shown that
the bulk of the Southern Ocean is oligotrophic.
Standing-crop chlorophyll biomass can reach high
levels (>40 mg m
−3 ) in nearshore waters, or in the
marginal ice zone, but otherwise levels are low.
A representative figure for annual production in pelagic
waters of the Southern Ocean is considered to be about
16 g C m
−2 yr
−1 (Holm-Hansen et al., 1977).
As with the Arctic Ocean, considerable primary
production is associated with sea ice. In the Southern
Ocean the marginal ice zone may fix carbon at rates five
times those of the open ocean (Smith et al., 1988).
Primary production in the Southern Ocean rarely
results in the depletion of macronutrients (N, P, Si) to
levels which would be regarded classically as limiting.
There is currently an intense, and as yet unresolved,
debate as to the relative influence of light limitation
through vertical mixing, grazing and limitation by
micronutrients (especially iron) in limiting Southern
Ocean production.
Overall, present knowledge of primary production
and vertical flux suggests that biogenic production in
surface waters is low, and that input to the deepsea system is also likely to be low, reflecting the
generally oligotrophic nature of the overlying ocean.
Low surface production is likely to lead to low
productivity in the underlying benthos (Rowe, 1971).
There are very few estimates of secondary production
in the deep sea, although model calculations suggest
that macrobenthic production in the Southern Ocean
decreases exponentially with water depth (Brey and
Gerdes, 1998).
The Southern Ocean deep-sea fauna
Although the deep sea forms the single largest benthic
habitat in the Southern Ocean, very little is known
of its biology. Sampling has largely been confined to
the continental shelves and slope, although some truly
abyssal material has been collected. Most significant
has been the extensive work by Russian biologists [still
largely inaccessible to western scientists, but recently
summarized by Vinogradova (1997)] and the series of
collections made in the Atlantic and Pacific sectors by
USNS Eltanin in the 1960s (Menzies et al., 1973).
The unusual depth of the continental shelves around
Antarctica means that many organisms elsewhere
associated with shelves are, in the Southern Ocean,
living at depths traditionally regarded as deep-sea
(for example, down to 900 m in some deep basins).
Indeed at such depths, organisms necessarily display
physiological adaptations to pressure which parallel
those found in true deep-sea organisms in other oceans.
From the perspective of faunal evolution, however, such
assemblages are best regarded as associated with the
continental shelf (albeit unusually deep), rather than
with the deep sea sensu stricto. Around Antarctica the
true deep-sea fauna should be regarded as that living at
2000 m or deeper, although recent sampling has shown
that some abyssal species extend up the continental
slope to the shelf-break at 1000 m (Brandt, personal
communication). Unfortunately the deep-sea fauna has
been sampled only sporadically, and knowledge of
the deep-sea assemblages remains a major gap in
understanding of the Southern Ocean.
Ekman (1935, 1953) was the first to propose
that the Antarctic deep-sea fauna forms a discrete
biogeographic entity, relatively isolated from deepsea faunas elsewhere. The northern boundary of
the Antarctic deep-sea was suggested to fall around
latitude 40ºS, emphasising the minimal impact of
the surface Antarctic Polar Front on deep-sea faunas.
Ekman based his conclusion on the distribution of
sponges, but the concept of a uniform Antarctic abyssal
253
bottom melting. The density of these drop-stones
declines northwards, and their distribution influences
the population dynamics of encrusting taxa in the
Southern ocean. Unfortunately very little is known
of either the distribution or the fauna of these dropstones.
Hydrothermal vents
One habitat which was unknown before the mid1970s and which is currently the subject of intense
investigation is that of hydrothermal vent fields (Gage
and Tyler, 1991). The tectonic history of the Southern
Ocean suggests that hydrothermal vent fields are very
likely to exist, especially in the eastern Scotia Sea or
the South Shetland Islands. As yet, however, none has
been described.
Primary production
For many years the Southern Ocean was considered
to be highly productive. It is now known that this
conclusion was the result of a strong seasonal and
spatial bias in sampling (Smith and Sakshaug, 1990).
More extensive oceanographic measures and especially
remotely-sensed ocean colour data have shown that
the bulk of the Southern Ocean is oligotrophic.
Standing-crop chlorophyll biomass can reach high
levels (>40 mg m
−3 ) in nearshore waters, or in the
marginal ice zone, but otherwise levels are low.
A representative figure for annual production in pelagic
waters of the Southern Ocean is considered to be about
16 g C m
−2 yr
−1 (Holm-Hansen et al., 1977).
As with the Arctic Ocean, considerable primary
production is associated with sea ice. In the Southern
Ocean the marginal ice zone may fix carbon at rates five
times those of the open ocean (Smith et al., 1988).
Primary production in the Southern Ocean rarely
results in the depletion of macronutrients (N, P, Si) to
levels which would be regarded classically as limiting.
There is currently an intense, and as yet unresolved,
debate as to the relative influence of light limitation
through vertical mixing, grazing and limitation by
micronutrients (especially iron) in limiting Southern
Ocean production.
Overall, present knowledge of primary production
and vertical flux suggests that biogenic production in
surface waters is low, and that input to the deepsea system is also likely to be low, reflecting the
generally oligotrophic nature of the overlying ocean.
Low surface production is likely to lead to low
productivity in the underlying benthos (Rowe, 1971).
There are very few estimates of secondary production
in the deep sea, although model calculations suggest
that macrobenthic production in the Southern Ocean
decreases exponentially with water depth (Brey and
Gerdes, 1998).
The Southern Ocean deep-sea fauna
Although the deep sea forms the single largest benthic
habitat in the Southern Ocean, very little is known
of its biology. Sampling has largely been confined to
the continental shelves and slope, although some truly
abyssal material has been collected. Most significant
has been the extensive work by Russian biologists [still
largely inaccessible to western scientists, but recently
summarized by Vinogradova (1997)] and the series of
collections made in the Atlantic and Pacific sectors by
USNS Eltanin in the 1960s (Menzies et al., 1973).
The unusual depth of the continental shelves around
Antarctica means that many organisms elsewhere
associated with shelves are, in the Southern Ocean,
living at depths traditionally regarded as deep-sea
(for example, down to 900 m in some deep basins).
Indeed at such depths, organisms necessarily display
physiological adaptations to pressure which parallel
those found in true deep-sea organisms in other oceans.
From the perspective of faunal evolution, however, such
assemblages are best regarded as associated with the
continental shelf (albeit unusually deep), rather than
with the deep sea sensu stricto. Around Antarctica the
true deep-sea fauna should be regarded as that living at
2000 m or deeper, although recent sampling has shown
that some abyssal species extend up the continental
slope to the shelf-break at 1000 m (Brandt, personal
communication). Unfortunately the deep-sea fauna has
been sampled only sporadically, and knowledge of
the deep-sea assemblages remains a major gap in
understanding of the Southern Ocean.
Ekman (1935, 1953) was the first to propose
that the Antarctic deep-sea fauna forms a discrete
biogeographic entity, relatively isolated from deepsea faunas elsewhere. The northern boundary of
the Antarctic deep-sea was suggested to fall around
latitude 40ºS, emphasising the minimal impact of
the surface Antarctic Polar Front on deep-sea faunas.
Ekman based his conclusion on the distribution of
sponges, but the concept of a uniform Antarctic abyssal