T H E PRODUCTION O F MARINE PLANKTON
131
phyll and carbon, tends to be higher in the E:hade forms. The existence
of these shade and sun types of phytoplankton, particularly in the
deeper euphotic zones of highly stratified tropical waters, has been
noticed recently by many authors, for exam:ple, by Ryther and Menzel
(1959). These investigators show that in an area of the Sargasso Sea
during winter, when the euphotic zone may approach some 150 metres in
depth, the water is mixed and is virtually isothermal to a depth of
400 metres. During this time the phytoplankton at three depths,
corresponding to loo%, 10% and 1% respectively of the surface light,
was of a similar sun type, being light saturated at about 50 000 lux. In
summer, however, marked stratification occiirs so that a clear thermocline exists at a depth of about 25-50 metres. The surface plankton
during summer is still of the sun type, but below the thermocline, at a
depth of about 100-150 metres, which corretiponds to about 1% of the
surface radiation, the phytoplankton is now of the shade type and is
light saturated at less than 10 000 lux light intensity. At intermediate
depths, between the surface and 100 metres, the plankton is rather intermediate in character. Again, Saijo and Ichimura (1962) have shown
for the Japanese waters of the Kuroshio Current that photosynthesis
varies more or less directly with light energy at relatively low light
intensities. However, for plankton from the surface, 20 metres, and 50
metres depths respectively, there are clear differences in the light saturation values which reflect the sun (surface) type and shade (deeper) type
of phytoplankton.
Although many species of phytoplankton may be classed as sun or
shade types, the position is by no means simple in that one and the
same species can undoubtedly become adapted to different light intensities. Harvey (1955), for example, demonstrated that growth rate in
Biddulphia mobiliensis varied according to whether the cells had
previously been grown in dim or bright light. Thus Biddulphia grown
previously a t intensities of about 4 000 lux grew best in dim light, while
cells grown at relatively high light intensities showed their maximum
growth rate at about 18 000 lux. But light ie not the only factor which
affects the relationship between photosynthesis and light intensity:
there appear to be interactions between light, temperature and nutrient
level (cf. also Maddux and Jones, 1964). Nutrient level can affect the
ratio between chlorophyll and carbon in a pliytoplankton cell, lowered
nutrient level tending to reduce the relative amount of chlorophyll (cf.
Steele, 1962; Steele and Baird, 1963). Steele indeed considers that there
are peaks in the relative amount of chlorophyll in the phytoplankton of
temperate regions during spring and early autumn, when nutrient levels
tend to be high, and that nutrient limitation during summer may affect
the chlorophyll/carbon ratio more than having the usually recognized
131
phyll and carbon, tends to be higher in the E:hade forms. The existence
of these shade and sun types of phytoplankton, particularly in the
deeper euphotic zones of highly stratified tropical waters, has been
noticed recently by many authors, for exam:ple, by Ryther and Menzel
(1959). These investigators show that in an area of the Sargasso Sea
during winter, when the euphotic zone may approach some 150 metres in
depth, the water is mixed and is virtually isothermal to a depth of
400 metres. During this time the phytoplankton at three depths,
corresponding to loo%, 10% and 1% respectively of the surface light,
was of a similar sun type, being light saturated at about 50 000 lux. In
summer, however, marked stratification occiirs so that a clear thermocline exists at a depth of about 25-50 metres. The surface plankton
during summer is still of the sun type, but below the thermocline, at a
depth of about 100-150 metres, which corretiponds to about 1% of the
surface radiation, the phytoplankton is now of the shade type and is
light saturated at less than 10 000 lux light intensity. At intermediate
depths, between the surface and 100 metres, the plankton is rather intermediate in character. Again, Saijo and Ichimura (1962) have shown
for the Japanese waters of the Kuroshio Current that photosynthesis
varies more or less directly with light energy at relatively low light
intensities. However, for plankton from the surface, 20 metres, and 50
metres depths respectively, there are clear differences in the light saturation values which reflect the sun (surface) type and shade (deeper) type
of phytoplankton.
Although many species of phytoplankton may be classed as sun or
shade types, the position is by no means simple in that one and the
same species can undoubtedly become adapted to different light intensities. Harvey (1955), for example, demonstrated that growth rate in
Biddulphia mobiliensis varied according to whether the cells had
previously been grown in dim or bright light. Thus Biddulphia grown
previously a t intensities of about 4 000 lux grew best in dim light, while
cells grown at relatively high light intensities showed their maximum
growth rate at about 18 000 lux. But light ie not the only factor which
affects the relationship between photosynthesis and light intensity:
there appear to be interactions between light, temperature and nutrient
level (cf. also Maddux and Jones, 1964). Nutrient level can affect the
ratio between chlorophyll and carbon in a pliytoplankton cell, lowered
nutrient level tending to reduce the relative amount of chlorophyll (cf.
Steele, 1962; Steele and Baird, 1963). Steele indeed considers that there
are peaks in the relative amount of chlorophyll in the phytoplankton of
temperate regions during spring and early autumn, when nutrient levels
tend to be high, and that nutrient limitation during summer may affect
the chlorophyll/carbon ratio more than having the usually recognized