13
In true Antarctic offshore waters the minimum level is seldom below
SO ~M, indicating that Si limitation may be neglected. Near the Polar
Front Si levels are considerably lower (Jacques 1983), implying possible Si-deficiency for some diatoms. Also in sheltered, Antarctic
areas Si-deficiency may be possible when extremely dense blooms occur.
Recent studies (Olsen 1980, Glibert et al. 1982, Ronner et al.
1983) in the Antarctic have shown (1) that SO-80% of total nitrogen
assimilated by phytoplankton is in the form of ammonia, (2) that waters
with low standing stock «1.S ~g chI liter- 1 ) have a much greater relative uptake rate of ammonia as compared to nitrate than that found in
bloom conditions, and (3) that microplankton have a higher nitrate
uptake relative to total N uptake than do nanoplankton. Regenerative
communities (N uptake based on NH 3 ) are of such ubiquitous global distribution that they may be regarded as a "global plankton baseline"
with blooms to be regarded as anomalies superimposed and allowed by
extreme seasonality or hydrographical pecularities. The Antarctic regenerative and oligotrophic community (i.e. low standing stock) differs
from others, however, in probably being the only one existing in a vast
sea of nitrate, and it is difficult to explain this in terms other than
energy economy in a light limited regime due to deep-reaching turbulence (SO-100m). It also follows that algae belonging to that community should have an impressive ability of shade adaptation.
Antarctic phytoplankton blooms are then, in contrast, characterized
by species which base their nitrogen uptake on nitrate as a main source
and which have a rather high light requirement (turbulence not deeper
than 20-40m). One may also speculate to which extent these species
have a high energy requirement considering their high silica content.
In contrast, the small FragiLariopsis nana-like diatoms predominant
in oligotrophic communities do not appear very silicified in the
microscope. Future autecological studies of characteristic species
from each community may shed some light upon the mechanisms involved
and possibly resolve the apparent paradox of oligotrophic communities
in nutrient-rich waters.
Our photobiology studies on the ACDA and VULCAN cruises have indicated that the saturating light intensity for photosynthesis is about
10% of incident radiation, and that growth rates of 0.2 doublings per
day are still obtained at intensities as low as O.S% for Io. Growth
rates of Antarctic phytoplankton under optimum light conditions range
from 0.1 to 0.9 doublings per day (Jacques 1983, Bunt and Lee 1970,
Holm-Hansen et al. 1977, Jacques and Minas 1981, EI-Sayed and Taguchi
1981, VULCAN and ACDA data).
These growth rates are generally well
below the maximum rates predicted by Eppley's (1972) equation (0.7S to
In true Antarctic offshore waters the minimum level is seldom below
SO ~M, indicating that Si limitation may be neglected. Near the Polar
Front Si levels are considerably lower (Jacques 1983), implying possible Si-deficiency for some diatoms. Also in sheltered, Antarctic
areas Si-deficiency may be possible when extremely dense blooms occur.
Recent studies (Olsen 1980, Glibert et al. 1982, Ronner et al.
1983) in the Antarctic have shown (1) that SO-80% of total nitrogen
assimilated by phytoplankton is in the form of ammonia, (2) that waters
with low standing stock «1.S ~g chI liter- 1 ) have a much greater relative uptake rate of ammonia as compared to nitrate than that found in
bloom conditions, and (3) that microplankton have a higher nitrate
uptake relative to total N uptake than do nanoplankton. Regenerative
communities (N uptake based on NH 3 ) are of such ubiquitous global distribution that they may be regarded as a "global plankton baseline"
with blooms to be regarded as anomalies superimposed and allowed by
extreme seasonality or hydrographical pecularities. The Antarctic regenerative and oligotrophic community (i.e. low standing stock) differs
from others, however, in probably being the only one existing in a vast
sea of nitrate, and it is difficult to explain this in terms other than
energy economy in a light limited regime due to deep-reaching turbulence (SO-100m). It also follows that algae belonging to that community should have an impressive ability of shade adaptation.
Antarctic phytoplankton blooms are then, in contrast, characterized
by species which base their nitrogen uptake on nitrate as a main source
and which have a rather high light requirement (turbulence not deeper
than 20-40m). One may also speculate to which extent these species
have a high energy requirement considering their high silica content.
In contrast, the small FragiLariopsis nana-like diatoms predominant
in oligotrophic communities do not appear very silicified in the
microscope. Future autecological studies of characteristic species
from each community may shed some light upon the mechanisms involved
and possibly resolve the apparent paradox of oligotrophic communities
in nutrient-rich waters.
Our photobiology studies on the ACDA and VULCAN cruises have indicated that the saturating light intensity for photosynthesis is about
10% of incident radiation, and that growth rates of 0.2 doublings per
day are still obtained at intensities as low as O.S% for Io. Growth
rates of Antarctic phytoplankton under optimum light conditions range
from 0.1 to 0.9 doublings per day (Jacques 1983, Bunt and Lee 1970,
Holm-Hansen et al. 1977, Jacques and Minas 1981, EI-Sayed and Taguchi
1981, VULCAN and ACDA data).
These growth rates are generally well
below the maximum rates predicted by Eppley's (1972) equation (0.7S to