128
J . E . G . R A Y M O N T
latitude. Thus the experiment in Table I1 was carried out during the
spring; other experiments carried out later in the year, with increasing
length of day and greater light intensities, showed the compensation
depth much deeper in the water. Other similar experiments such as
those of Marshall and Orr (1 928, 1930) who worked on a single species of
diatom, Coscinosira polychorda, suggested that whereas in winter, the
compensation depth was only a metre or two beneath the surface,
during summer it might lie at about 20-30 metres.
One of the most complete studies on the effect of light on photosynthetic activity is that of Jenkin (1937), working with the diatom,
Coscinodiscus escentricus. For the English Channel, Jenkin found a
compensation depth a t about 45 metres, corresponding to a light intensity of approximately 0.13 g cal/cm2/h. Using the oxygen bottle
technique she showed that between this light value and some 1 * 8 g cal/
cm2/h ( 3 5 000 lux), oxygen production, as a measure of photosynthetic activity, increased practically linearly with light energy. At light
intensities above this value it continued to rise but at a somewhat
lower rate, indicating that some inhibition was occurring; maximal
photosynthesis occurred a t a light energy approximating to 7.2 g cal/.
cm2/h (of the order of 20 000 lux). Above this intensity, which may be
termed the light saturation value, inhibition began to be more obvious
and at high light intensities photosynthesis was markedly reduced.
These experiments have been confirmed by others. Thus Talling (1960)
using Chaetoceros afinis, showed photosynthetic activity to increase
linearly from a compensation intensity approaching that of Jenkin to
a value of some 5 000 lux; light saturation occurred at a mean value of
25 000 lux. Jenkin’s clear demonstration of inhibition at high intensities
shows that even in temperate latitudes about midday during the summer
some degree of inhibition of photosynthesis can occur, but this will be
confined to algae close to the surface (cf. Fig. 2).
The results of early experiments on primary production such as those
of Gaarder and Gran (1927), suggest that some species of diatoms in a
mixed phytoplankton population differ in their light requirements.
Thus the diatoms Lauderia and Thalassiosira gravida grew best in the
upper 2 metres, the latter species probably growing fastest right at the
surface. On the other hand, the related species, T . nordenskioldii,
appeared to grow best at a depth of 2-5 metres, and presumably had a
lower light optimum. Later work, particularly that of Steemann Nielsen
and his colleagues, has indicated clearly that different species of phytoplankton have different light optima. Although an average compensation intensity for phytoplankton species of some 0-13 g cal/cma/h, as
suggested by Jenkin, appears to be reasonable, the various species
exhibit considerable differences in their light optima and saturation
J . E . G . R A Y M O N T
latitude. Thus the experiment in Table I1 was carried out during the
spring; other experiments carried out later in the year, with increasing
length of day and greater light intensities, showed the compensation
depth much deeper in the water. Other similar experiments such as
those of Marshall and Orr (1 928, 1930) who worked on a single species of
diatom, Coscinosira polychorda, suggested that whereas in winter, the
compensation depth was only a metre or two beneath the surface,
during summer it might lie at about 20-30 metres.
One of the most complete studies on the effect of light on photosynthetic activity is that of Jenkin (1937), working with the diatom,
Coscinodiscus escentricus. For the English Channel, Jenkin found a
compensation depth a t about 45 metres, corresponding to a light intensity of approximately 0.13 g cal/cm2/h. Using the oxygen bottle
technique she showed that between this light value and some 1 * 8 g cal/
cm2/h ( 3 5 000 lux), oxygen production, as a measure of photosynthetic activity, increased practically linearly with light energy. At light
intensities above this value it continued to rise but at a somewhat
lower rate, indicating that some inhibition was occurring; maximal
photosynthesis occurred a t a light energy approximating to 7.2 g cal/.
cm2/h (of the order of 20 000 lux). Above this intensity, which may be
termed the light saturation value, inhibition began to be more obvious
and at high light intensities photosynthesis was markedly reduced.
These experiments have been confirmed by others. Thus Talling (1960)
using Chaetoceros afinis, showed photosynthetic activity to increase
linearly from a compensation intensity approaching that of Jenkin to
a value of some 5 000 lux; light saturation occurred at a mean value of
25 000 lux. Jenkin’s clear demonstration of inhibition at high intensities
shows that even in temperate latitudes about midday during the summer
some degree of inhibition of photosynthesis can occur, but this will be
confined to algae close to the surface (cf. Fig. 2).
The results of early experiments on primary production such as those
of Gaarder and Gran (1927), suggest that some species of diatoms in a
mixed phytoplankton population differ in their light requirements.
Thus the diatoms Lauderia and Thalassiosira gravida grew best in the
upper 2 metres, the latter species probably growing fastest right at the
surface. On the other hand, the related species, T . nordenskioldii,
appeared to grow best at a depth of 2-5 metres, and presumably had a
lower light optimum. Later work, particularly that of Steemann Nielsen
and his colleagues, has indicated clearly that different species of phytoplankton have different light optima. Although an average compensation intensity for phytoplankton species of some 0-13 g cal/cma/h, as
suggested by Jenkin, appears to be reasonable, the various species
exhibit considerable differences in their light optima and saturation