27
Even at the peak of phytoplankton growth, the concentration of nutrient
salts remains well above the limiting values.
For instance, during
the heavy bloom of phytoplankton in the southwestern Weddell Sea,
mentioned earlier, the concentrations of nutrient salts were still
high (El-Sayed, 1968). During the Phaeocystis pouchetti bloom encountered by El-Sayed et al. (1983) in the Ross Sea, nitrate concentration
for instance was still high, with euphotic zone concentrations averaging 17 ~g at./l. Thus it is unlikely that these nutrient salts are
sufficiently low at anyone time to become limiting factors to the
growth of the phytoplankton.
However, according to Walsh (1971), and
Allanson et al. (1981), the pattern of silicate distribution indicates
that Si0 3 may be the most limiting of the major nutrients for the growth
of Antarctic diatoms, thus corroborating a similar conclusion arrived
at earlier by Hart (1942).
With regard to the trace elements, Jacques (1983) carried out enrichment experiments using Zn, Mo, Co, Mn, and Fe, and showed that they
are not limiting factors.
It is possible, however, that organic
factors, e.g. vitamin B12 and thiamin (see Carlucci and Cuhel, 1977)
may alter the species composition of the phytoplankton without changing
the overall rate of primary production. It is also possible that the
availability of tracelmicronutrients may be altered by pack-ice and
iceberg melt-water, thus affecting the productivity or species composition of the waters in the vicinity of pack-ice and icebergs.
El-Sayed et al. (1964), El-Sayed and Jitts (1973) and El-Sayed
and Weber (1982) reported elevated values in the standing crop and
primary production in the Antarctic coastal waters and in the vicinity
of Antarctic and sUbantarctic islands. These authors attributed this
increase to the so-called "island-mass effect" (Doty and Oguri, 1956)
and the attendant increase in nutrient concentrations.
However, increased concentrations of inorganic salts cannot be invoked here, as
is usually the case in tropical or temperate waters, since these nutrients as mentioned above, are in abundant supply. Nor are the elevated values of the phytoplankton standing crop and primary production
in the Southern Ocean limited to the Antarctic coastal waters and the
vicinity of islands. During ISLAS ORCADASCruises 17 and 19 in the
southwestern Atlantic and the Scotia Sea, several deep stations were
noted for their rich phytoplankton populations (El-Sayed and Weber,
1982). Further, many other deep oceanic stations in the Atlantic,
Pacific and Indian sectors of the Antarctic also exhibited higher-thanaverage values of chlorophyll ~ and primary production. For example,
at a station in the Weddell Sea located 600-800 km from the coast and
Even at the peak of phytoplankton growth, the concentration of nutrient
salts remains well above the limiting values.
For instance, during
the heavy bloom of phytoplankton in the southwestern Weddell Sea,
mentioned earlier, the concentrations of nutrient salts were still
high (El-Sayed, 1968). During the Phaeocystis pouchetti bloom encountered by El-Sayed et al. (1983) in the Ross Sea, nitrate concentration
for instance was still high, with euphotic zone concentrations averaging 17 ~g at./l. Thus it is unlikely that these nutrient salts are
sufficiently low at anyone time to become limiting factors to the
growth of the phytoplankton.
However, according to Walsh (1971), and
Allanson et al. (1981), the pattern of silicate distribution indicates
that Si0 3 may be the most limiting of the major nutrients for the growth
of Antarctic diatoms, thus corroborating a similar conclusion arrived
at earlier by Hart (1942).
With regard to the trace elements, Jacques (1983) carried out enrichment experiments using Zn, Mo, Co, Mn, and Fe, and showed that they
are not limiting factors.
It is possible, however, that organic
factors, e.g. vitamin B12 and thiamin (see Carlucci and Cuhel, 1977)
may alter the species composition of the phytoplankton without changing
the overall rate of primary production. It is also possible that the
availability of tracelmicronutrients may be altered by pack-ice and
iceberg melt-water, thus affecting the productivity or species composition of the waters in the vicinity of pack-ice and icebergs.
El-Sayed et al. (1964), El-Sayed and Jitts (1973) and El-Sayed
and Weber (1982) reported elevated values in the standing crop and
primary production in the Antarctic coastal waters and in the vicinity
of Antarctic and sUbantarctic islands. These authors attributed this
increase to the so-called "island-mass effect" (Doty and Oguri, 1956)
and the attendant increase in nutrient concentrations.
However, increased concentrations of inorganic salts cannot be invoked here, as
is usually the case in tropical or temperate waters, since these nutrients as mentioned above, are in abundant supply. Nor are the elevated values of the phytoplankton standing crop and primary production
in the Southern Ocean limited to the Antarctic coastal waters and the
vicinity of islands. During ISLAS ORCADASCruises 17 and 19 in the
southwestern Atlantic and the Scotia Sea, several deep stations were
noted for their rich phytoplankton populations (El-Sayed and Weber,
1982). Further, many other deep oceanic stations in the Atlantic,
Pacific and Indian sectors of the Antarctic also exhibited higher-thanaverage values of chlorophyll ~ and primary production. For example,
at a station in the Weddell Sea located 600-800 km from the coast and