12
on phytoplankton growth in the Arctic than in the Antarctic. The question of N vs. P as a limiting factor is actually dependent on the species in question (Sakshaug et al. 1983). One may assume that the supply of Nand P is fairly balanced relative to the needs of an "average"
community since the A values in Table 2 are not too far from the Redfield ratio of 16. One may note that the values in Table 2 actually
tend to be somewhat higher than 16, i.e. pointing towards P as a potential minimum factor. Data in Table 2 show that silica consentrations
in the Arctic are very low as compared to the Antarctic. Paasche (1980)
found a Si/C ratio (atoms) of 0.047 to 0.17 for 4 common species along
the North Atlantic coasts, and the variation was highly species and
temperature dependent. A SkeZetonema bloom in the Oslofjord exhibited
a range of 0.077 to 0.18 (Paasche and 0stergren 1980).
In terms of
N/Si ratios the total range becomes 3.7to 1 for non-starved cells (at
N/C = 0.15, atoms) compared to 2.8 - 2 for the A values in Table 2.
This makes it likely that some diatoms in arctic waters may be growthlimited by insufficient silica.
In the. Antarctic Ocean nutrient depletion does not occur in offshore waters, and shipboard cultures may grow exponentially without
enrichment until surpassing 40 ~g chI liter- 1 • There have been no
chemical indications suggesting nutrient limitation in Antarctic waters.
Natural populations from the Scotia Sea (VULCAN 6 and 7) and around
the Antarctic continent (ACDA cruise) yielded a range for N/C of 0.11
- 0.17 with 0.15 as an average (225 samples with PON >10 ~g 1- 1 ), corrected for detritus according to Sakshaug (1978) and Olsen et al. (1983).
N-deficiency is not evident until N/C <0.1 (Sakshaug et al. 1983). On
the VULCAN 7 cruise the protein/carbohydrate ratio was measured and
was always well above 5, which indicates limitation by light rather
than by Nand P. Nutrient deficiency yields values below unity (Myklestad 1977).
The N/Si ratio in Antarctic samples (ACDA and VULCAN cruises) have
ranged between 0.5 to 4.8.
It is difficult to attach much significance
to these ratios as many phytoplankton groups (flagellates, e.g.) do
not require silica for growth.
In those samples which were dominated
by diato~s, the N/Si ratio ranged from 0.48 to 1.1, values which are
higher than the corresponding nutrient ratio. Data on Si-kinetics
also support the view that diatom growth rates are not limited by
silica availability. Jacques (1983) has reported half saturation constants of 12 ~M for Nitzschia tupgiduZa and 12-22 ~M for the extremely
silicified Nitzschia kepgueZensis.
These are very high values although
nevertheless indicating growth close to the maximum rate for Si >50 ~M.
on phytoplankton growth in the Arctic than in the Antarctic. The question of N vs. P as a limiting factor is actually dependent on the species in question (Sakshaug et al. 1983). One may assume that the supply of Nand P is fairly balanced relative to the needs of an "average"
community since the A values in Table 2 are not too far from the Redfield ratio of 16. One may note that the values in Table 2 actually
tend to be somewhat higher than 16, i.e. pointing towards P as a potential minimum factor. Data in Table 2 show that silica consentrations
in the Arctic are very low as compared to the Antarctic. Paasche (1980)
found a Si/C ratio (atoms) of 0.047 to 0.17 for 4 common species along
the North Atlantic coasts, and the variation was highly species and
temperature dependent. A SkeZetonema bloom in the Oslofjord exhibited
a range of 0.077 to 0.18 (Paasche and 0stergren 1980).
In terms of
N/Si ratios the total range becomes 3.7to 1 for non-starved cells (at
N/C = 0.15, atoms) compared to 2.8 - 2 for the A values in Table 2.
This makes it likely that some diatoms in arctic waters may be growthlimited by insufficient silica.
In the. Antarctic Ocean nutrient depletion does not occur in offshore waters, and shipboard cultures may grow exponentially without
enrichment until surpassing 40 ~g chI liter- 1 • There have been no
chemical indications suggesting nutrient limitation in Antarctic waters.
Natural populations from the Scotia Sea (VULCAN 6 and 7) and around
the Antarctic continent (ACDA cruise) yielded a range for N/C of 0.11
- 0.17 with 0.15 as an average (225 samples with PON >10 ~g 1- 1 ), corrected for detritus according to Sakshaug (1978) and Olsen et al. (1983).
N-deficiency is not evident until N/C <0.1 (Sakshaug et al. 1983). On
the VULCAN 7 cruise the protein/carbohydrate ratio was measured and
was always well above 5, which indicates limitation by light rather
than by Nand P. Nutrient deficiency yields values below unity (Myklestad 1977).
The N/Si ratio in Antarctic samples (ACDA and VULCAN cruises) have
ranged between 0.5 to 4.8.
It is difficult to attach much significance
to these ratios as many phytoplankton groups (flagellates, e.g.) do
not require silica for growth.
In those samples which were dominated
by diato~s, the N/Si ratio ranged from 0.48 to 1.1, values which are
higher than the corresponding nutrient ratio. Data on Si-kinetics
also support the view that diatom growth rates are not limited by
silica availability. Jacques (1983) has reported half saturation constants of 12 ~M for Nitzschia tupgiduZa and 12-22 ~M for the extremely
silicified Nitzschia kepgueZensis.
These are very high values although
nevertheless indicating growth close to the maximum rate for Si >50 ~M.