28
> 4,000 m deep, an immensely rich diatom assemblage of over 1.4 x 10 6
cells/l was reported (Fryxell and Hasle, 1971). These examples of Antarctic richness are not representative of the overall phytoplankton
activity; they remain the exception. Nevertheless, they point out that
the Southern Ocean is capable of higher levels of standing crop given
its rich supply of nutrient salts.
D. Water Column Stability
Several investigators have drawn attention to the importance
of the stability of the water column in controlling production (Braarud
and Klem, 1931; Gran, 1931; Sverdrup, 1953; Pingree, 1978). Of the
several processes which have been hypothesized to be important in initiating and sustaining near-ice bloom, the most important is the vertical stability induced by melt-water. According to this hypothesis,
first proposed by Marshall (1957) for Arctic waters, the low salinity
of melt-water contributes to the stability of the near-ice water column,
thus helping to retain the phytoplankton in near surface, photic water
and to promote a bloom. There is evidence that this mechanism is important in the initiation of ice-edge blooms in the Antarctic Ocean as
well (Jacobs and Amos, 1967; El-Sayed, 1971). Thus stability of the
upper layers plays an important role in the development of Antarctic
phytoplankton blooms.
Sakshaug and Holm-Hansen (personal communication)
reported that for all the "bloom stations" where chlorophyll a was
> 2 mg/m 3 , the pycnocline was between 20 and 40 m deep. They speculate
that 50 m is the maximum depth for the bloom to develop. They contend
that the presence of homogenous (i.e. isothermal) layers, reaching to
50-100 m depth for most of the year, hinders the development of a bloom
and contributes to the low primary production of the Antarctic waters.
On the other hand, the Polar Front has been cited by several investigators as a factor contributing to a low standing crop of phytoplankton.
This has been explained as a result of comparatively low
vertical stability of the water column, which prevents the phytoplankton from remaining in the optimal light zone long enough for extensive
production (Hart, 1942; EI-Sayed and Mandelli, 1965; Hasle, 1969). Paradoxically, during Cruises 17 and 19 of the ISLAS ORCADAS, higher-thanaverage values of chlorophyll a and primary production were found in
the vicinity of the Polar Front. Allanson et al., (1981) noted marked
increases in primary production and phytoplankton biomass at the Polar
Front in the southwestern part of the Indian Ocean; and Yamaguchi and
Shibata (1982) recorded high chlorophyll ~ at the Polar Front south of
> 4,000 m deep, an immensely rich diatom assemblage of over 1.4 x 10 6
cells/l was reported (Fryxell and Hasle, 1971). These examples of Antarctic richness are not representative of the overall phytoplankton
activity; they remain the exception. Nevertheless, they point out that
the Southern Ocean is capable of higher levels of standing crop given
its rich supply of nutrient salts.
D. Water Column Stability
Several investigators have drawn attention to the importance
of the stability of the water column in controlling production (Braarud
and Klem, 1931; Gran, 1931; Sverdrup, 1953; Pingree, 1978). Of the
several processes which have been hypothesized to be important in initiating and sustaining near-ice bloom, the most important is the vertical stability induced by melt-water. According to this hypothesis,
first proposed by Marshall (1957) for Arctic waters, the low salinity
of melt-water contributes to the stability of the near-ice water column,
thus helping to retain the phytoplankton in near surface, photic water
and to promote a bloom. There is evidence that this mechanism is important in the initiation of ice-edge blooms in the Antarctic Ocean as
well (Jacobs and Amos, 1967; El-Sayed, 1971). Thus stability of the
upper layers plays an important role in the development of Antarctic
phytoplankton blooms.
Sakshaug and Holm-Hansen (personal communication)
reported that for all the "bloom stations" where chlorophyll a was
> 2 mg/m 3 , the pycnocline was between 20 and 40 m deep. They speculate
that 50 m is the maximum depth for the bloom to develop. They contend
that the presence of homogenous (i.e. isothermal) layers, reaching to
50-100 m depth for most of the year, hinders the development of a bloom
and contributes to the low primary production of the Antarctic waters.
On the other hand, the Polar Front has been cited by several investigators as a factor contributing to a low standing crop of phytoplankton.
This has been explained as a result of comparatively low
vertical stability of the water column, which prevents the phytoplankton from remaining in the optimal light zone long enough for extensive
production (Hart, 1942; EI-Sayed and Mandelli, 1965; Hasle, 1969). Paradoxically, during Cruises 17 and 19 of the ISLAS ORCADAS, higher-thanaverage values of chlorophyll a and primary production were found in
the vicinity of the Polar Front. Allanson et al., (1981) noted marked
increases in primary production and phytoplankton biomass at the Polar
Front in the southwestern part of the Indian Ocean; and Yamaguchi and
Shibata (1982) recorded high chlorophyll ~ at the Polar Front south of