10
Skeletonema which may reach chl/N values as high as 0.25 when shade
adapted (Sakshaug and Andresen, in prep.). They are also low compared
to a set of cultures from the VULCAN cruise (Holm-Hansen and Foster
1981) which had chl/N ratios close to 0.1. This may reveal differences
in ecological strategy for different groups of species. The ACDA cultures consisted of pennate diatoms from stable, ice-filled waters, presumably they might have been ice algae from melted ice, whereas the
VULCAN 7 cultures were "monads and flagellates" and centric diatoms
(Thalassiosira spp. and Chaetoceros tortissimus) from rather turbulent
offshore waters.
Fluctuations in cellular chlorophyll have also been observed for
natural populations. Table 4 presents data for various high latitude
areas. As expected the cellular chlorophyll level is lower in surface
waters than below the pyknocline (quite often by a factor of 2 or more) ,
but we also se that cellular chlorophyll levels in the homogeneous layer
tend to increase with the depth of this layer (VULCAN data). There is
also a pronounced seasonal trend, quite spectacular for surface Skeletonema, but apparent also from the other data. With both depth and
season playing a role the annual range of variation for the cellular
chlorophyll level can become quite high. Hegseth and Sakshaug (1983)
found that the chl/N ratio ranged through the year from 0.03 to 0.21
for Skeletonema and from 0.06 to 0.31 for Thalassiosira gravida when
grown in in situ dialysis cultures in the Trondheimsfjord at 0.5 and
4m depth.
Light/shade adaptation may also be revealed by the shape of the
P vs. I curve derived from short-term experiments.
This has been demonstrated for Arctic as well as Antarctic phytoplankton (Burkholder
and Mandelli 1965, Platt et al. 1982, El-Sayed 1984). Typically phytoplanton will exhibit different curves for different depths when waters
are stratified and no difference when waters are homogeneous. Data of
Platt et al. (1982) for Baffin Bay are most illustrative (Fig. 5):
carbon uptake normalized to chlorophyll (assimilation number) implies
a high maximum number for light adapted populations and also a somewhat
higher initial slope.
In terms of carbon turnover (right diagram) the
picture ,becomes altered since the shade-adapted populations had 1.7
times more chlorophyll than the light-adapted ones, and here the superior efficiency of shade adapted populations at low light becomes evident
On the other hand, both curves also reveal the high susceptibility to
photoinhibition for shade-adapted populations.
Since phytoplankton is constantly exposed to light regime fluctuations, diurnal or by vertical movement, the time course of adaptation
Skeletonema which may reach chl/N values as high as 0.25 when shade
adapted (Sakshaug and Andresen, in prep.). They are also low compared
to a set of cultures from the VULCAN cruise (Holm-Hansen and Foster
1981) which had chl/N ratios close to 0.1. This may reveal differences
in ecological strategy for different groups of species. The ACDA cultures consisted of pennate diatoms from stable, ice-filled waters, presumably they might have been ice algae from melted ice, whereas the
VULCAN 7 cultures were "monads and flagellates" and centric diatoms
(Thalassiosira spp. and Chaetoceros tortissimus) from rather turbulent
offshore waters.
Fluctuations in cellular chlorophyll have also been observed for
natural populations. Table 4 presents data for various high latitude
areas. As expected the cellular chlorophyll level is lower in surface
waters than below the pyknocline (quite often by a factor of 2 or more) ,
but we also se that cellular chlorophyll levels in the homogeneous layer
tend to increase with the depth of this layer (VULCAN data). There is
also a pronounced seasonal trend, quite spectacular for surface Skeletonema, but apparent also from the other data. With both depth and
season playing a role the annual range of variation for the cellular
chlorophyll level can become quite high. Hegseth and Sakshaug (1983)
found that the chl/N ratio ranged through the year from 0.03 to 0.21
for Skeletonema and from 0.06 to 0.31 for Thalassiosira gravida when
grown in in situ dialysis cultures in the Trondheimsfjord at 0.5 and
4m depth.
Light/shade adaptation may also be revealed by the shape of the
P vs. I curve derived from short-term experiments.
This has been demonstrated for Arctic as well as Antarctic phytoplankton (Burkholder
and Mandelli 1965, Platt et al. 1982, El-Sayed 1984). Typically phytoplanton will exhibit different curves for different depths when waters
are stratified and no difference when waters are homogeneous. Data of
Platt et al. (1982) for Baffin Bay are most illustrative (Fig. 5):
carbon uptake normalized to chlorophyll (assimilation number) implies
a high maximum number for light adapted populations and also a somewhat
higher initial slope.
In terms of carbon turnover (right diagram) the
picture ,becomes altered since the shade-adapted populations had 1.7
times more chlorophyll than the light-adapted ones, and here the superior efficiency of shade adapted populations at low light becomes evident
On the other hand, both curves also reveal the high susceptibility to
photoinhibition for shade-adapted populations.
Since phytoplankton is constantly exposed to light regime fluctuations, diurnal or by vertical movement, the time course of adaptation