132
J . E . G . RAYIIZONT
effect on photosynthetic rate. Steemann Nielsen and Park (1964) have
demonstrated that the adaptation of phytoplankton to changed light
conditions can be quite rapid. They took plankton from an area in
Friday Harbour (Washington State) where there is strong vertical
turbulence and where with the lack of stratification the whole phytoplankton is adapted to relatively high light intensities. This plankton
was isolated in bottles and transferred to a depth where the light was
only 5% of the surface value. The phytoplankton cells showed a change
to the dark adapted type within three days, the main change being a
marked increase in chlorophyll content. But although surface (“sun”)
plankton generally has rather lower chlorophyll content, inactivation of
plankton exposed to high light intensities does not necessarily involve
an immediate destruction of the chlorophyll. Steemann Nielsen (19624
demonstrated that Chlorella grown in low light intensities and then
transferred to high intensities showed a depression in the rate of photochemical and of enzymatic processes; after a short period of being
returned to darkness, however, these processes were completely
reactivated.
The complex relationship existing between light optima, temperature
and nutrient levels, and possibly other factors, is illustrated by the
experiments of Curl and McLeod (1961) with the diatom Skeletonema
costatum. At temperatures between 5 and 18°C the photosynthetic rate
increased with temperature, and the light saturation value was fairly
stable a t intensities of 12 000 to 16 000 lux. At temperatures from
20-30°C photosynthesis was diminished but the saturation intensity
was also reduced to a value of only about 5 000 lux. Provided nutrients
(nitrate and phosphate) were present in maximal amounts, the temperature optimum approached 20°C, but when nutrients were in
limited supply, not only did the photosynthetic rate fall off but the
temperature optimum was lowered (cf. also Smayda, 1963). Lanskaya
(1963) has shown that even when optimum conditions for such factors as
light and nutrients exist, phytoplankton species (diatoms, dinoflagellates, and flagellates) show marked variations in rates of division.
Although phytoplankton species adapted to higher as well as to
lower light intensities exist in the oceans, in general the very rapid light
absorption means that much phytoplankton must carry out photosynthesis a t relatively low light intensities. Some diatoms for instance
have been found in the Arctic which will actively photosynthesize
under ice (cf. Smayda, 1963). Moreover, when phytoplankton blooms
occur, the cells will cause shading to those deeper in the water layers.
Thus in experiments using artificial addition of fertilizers (Raymont and
Miller, 1962; Ansell et al., 1964) the compensation depth can be very
close to the surface because of self-shading by the very heavy crop of
J . E . G . RAYIIZONT
effect on photosynthetic rate. Steemann Nielsen and Park (1964) have
demonstrated that the adaptation of phytoplankton to changed light
conditions can be quite rapid. They took plankton from an area in
Friday Harbour (Washington State) where there is strong vertical
turbulence and where with the lack of stratification the whole phytoplankton is adapted to relatively high light intensities. This plankton
was isolated in bottles and transferred to a depth where the light was
only 5% of the surface value. The phytoplankton cells showed a change
to the dark adapted type within three days, the main change being a
marked increase in chlorophyll content. But although surface (“sun”)
plankton generally has rather lower chlorophyll content, inactivation of
plankton exposed to high light intensities does not necessarily involve
an immediate destruction of the chlorophyll. Steemann Nielsen (19624
demonstrated that Chlorella grown in low light intensities and then
transferred to high intensities showed a depression in the rate of photochemical and of enzymatic processes; after a short period of being
returned to darkness, however, these processes were completely
reactivated.
The complex relationship existing between light optima, temperature
and nutrient levels, and possibly other factors, is illustrated by the
experiments of Curl and McLeod (1961) with the diatom Skeletonema
costatum. At temperatures between 5 and 18°C the photosynthetic rate
increased with temperature, and the light saturation value was fairly
stable a t intensities of 12 000 to 16 000 lux. At temperatures from
20-30°C photosynthesis was diminished but the saturation intensity
was also reduced to a value of only about 5 000 lux. Provided nutrients
(nitrate and phosphate) were present in maximal amounts, the temperature optimum approached 20°C, but when nutrients were in
limited supply, not only did the photosynthetic rate fall off but the
temperature optimum was lowered (cf. also Smayda, 1963). Lanskaya
(1963) has shown that even when optimum conditions for such factors as
light and nutrients exist, phytoplankton species (diatoms, dinoflagellates, and flagellates) show marked variations in rates of division.
Although phytoplankton species adapted to higher as well as to
lower light intensities exist in the oceans, in general the very rapid light
absorption means that much phytoplankton must carry out photosynthesis a t relatively low light intensities. Some diatoms for instance
have been found in the Arctic which will actively photosynthesize
under ice (cf. Smayda, 1963). Moreover, when phytoplankton blooms
occur, the cells will cause shading to those deeper in the water layers.
Thus in experiments using artificial addition of fertilizers (Raymont and
Miller, 1962; Ansell et al., 1964) the compensation depth can be very
close to the surface because of self-shading by the very heavy crop of