9
days despite high grazing pressure and sinking rates if the breakdown
of stability is gradual. Which mode of breakdown is typical for Antarctic waters is not known.
Pelagic blooms in polar oceans are usually dominated by fairly
large centric diatoms (or chains of them) or colonies of the haptophyte
Phaeocystis pouchetii. VULCAN and ACDA data have revealed that such
forms make up a higher percentage of the biomass the higher the total
phytoplankton biomass is. For non-bloom conditions, which are very
common in the Antarctic, nanoplankton «20nm) may constitute more than
50% of the phytoplankton biomass (Brockel 1981). By using epifluorescence
microscopy immediately after sampling (Hewes and Holm-Hansen 1983) during
the ACDA cruise it was observed that about 1/3rd of the nanoplankton biomass consisted of heterotrophic organisms (Hewes et al. 1984). Thus a
regenerative "heterotrophic" loop (Azam et al. 1983) may be of major importance in the Antarctic food web.
PHYSIOLOGICAL ECOLOGY OF POLAR PHYTOPLANKTON
This section deals with effects of the light regime, nutrients,
and temperature on the growth rate and the chemical composition of phytoplankton. The dominant factor is the light regime, which includes the
intensity, duration, and the submarine light gradient to which cells are
exposed by turbulent water movements and absence/presence of ice. Physiological adaptation to varying light intensity is of considerable importance in determining growth rates of phytoplankton (Neori et al.
1984). The importance of such adaptation has also been indicated by
Slags tad (1982), who found by running a numerical plankton model that
shade adaptation might allow blooms in open boreal waters to set in three
weeks earlier as compared to light adapted populations.
Light/shade adaptation is manifested in several ways. One is fluctuation in the cellular chlorophyll content as compared to carbon or
nitrogen (Falkowski and Owens 1980). During the ACDA cruise natural
water samples were incubated at ambient water temperature (0 to 4 0 C)
and attenuated sunlight. When the PFD (~E m- 2 s- 1 ) was decreased from
1050 to 6.5 (6 different light levels), the chl/N ratios increased 2.5-fold (from 0.031 to 0.072). The growth rates in these 6 cultures
varied from 0.23 to 0.41 doubl. per day. The chlorophyll decrease became particularly pronounced when the light flux was inhibitory. The
ATP/C ratios remained fairly constant (about 0.0043). Therefore the
chl/ATP ratios also reflect light/shade adaptation. The chlorophyll
levels of these cultures were quite low, however, when compared to
days despite high grazing pressure and sinking rates if the breakdown
of stability is gradual. Which mode of breakdown is typical for Antarctic waters is not known.
Pelagic blooms in polar oceans are usually dominated by fairly
large centric diatoms (or chains of them) or colonies of the haptophyte
Phaeocystis pouchetii. VULCAN and ACDA data have revealed that such
forms make up a higher percentage of the biomass the higher the total
phytoplankton biomass is. For non-bloom conditions, which are very
common in the Antarctic, nanoplankton «20nm) may constitute more than
50% of the phytoplankton biomass (Brockel 1981). By using epifluorescence
microscopy immediately after sampling (Hewes and Holm-Hansen 1983) during
the ACDA cruise it was observed that about 1/3rd of the nanoplankton biomass consisted of heterotrophic organisms (Hewes et al. 1984). Thus a
regenerative "heterotrophic" loop (Azam et al. 1983) may be of major importance in the Antarctic food web.
PHYSIOLOGICAL ECOLOGY OF POLAR PHYTOPLANKTON
This section deals with effects of the light regime, nutrients,
and temperature on the growth rate and the chemical composition of phytoplankton. The dominant factor is the light regime, which includes the
intensity, duration, and the submarine light gradient to which cells are
exposed by turbulent water movements and absence/presence of ice. Physiological adaptation to varying light intensity is of considerable importance in determining growth rates of phytoplankton (Neori et al.
1984). The importance of such adaptation has also been indicated by
Slags tad (1982), who found by running a numerical plankton model that
shade adaptation might allow blooms in open boreal waters to set in three
weeks earlier as compared to light adapted populations.
Light/shade adaptation is manifested in several ways. One is fluctuation in the cellular chlorophyll content as compared to carbon or
nitrogen (Falkowski and Owens 1980). During the ACDA cruise natural
water samples were incubated at ambient water temperature (0 to 4 0 C)
and attenuated sunlight. When the PFD (~E m- 2 s- 1 ) was decreased from
1050 to 6.5 (6 different light levels), the chl/N ratios increased 2.5-fold (from 0.031 to 0.072). The growth rates in these 6 cultures
varied from 0.23 to 0.41 doubl. per day. The chlorophyll decrease became particularly pronounced when the light flux was inhibitory. The
ATP/C ratios remained fairly constant (about 0.0043). Therefore the
chl/ATP ratios also reflect light/shade adaptation. The chlorophyll
levels of these cultures were quite low, however, when compared to