T H E P R O D U C T I O N O F M A R I N E P L A N K T O N
133
phytoplankton. Similarly in the experiments of Antis et al. (1963), selfshading occurred with the very heavy crop of phytoplankton, although
with stirring, the algal cells could photosynthesize at remarkably low
light intensities. With the marked differential absorption of wavelengths of light in the sea, marine algae must not only be adapted to
photosynthesis at low light intensities but itlso at wavelengths which
may not be maximal for chlorophyll itself. It ;seems extremely likely that
the many carotenoids which are present in different forms and in different concentrations in the various algal groups in the phytoplankton,
are of significance in absorbing all light wavelengths. Although not
every carotenoid may be of equal importance in absorbing energy, it
appears to be the total quantity of solar radiation reaching the depths
which is of real importance. The marine algae therefore appear to have
become adapted to the generally low light intensities in the sea by increasing the chlorophyll content and by possessing active accessory
carotenoids.
2. Temperature
While light intensity must be of major significance in relation to
primary production in the sea, other factors may also play a part.
Temperature is probably of little direct importance since lowered
temperature will reduce the respiratory needs of the plant cells and
higher temperatures, by increasing respiratcry requirements, can have
a beneficial effect on photosynthesis only if very high light intensities
are available. It is obvious that photosynthe3is occurs a t high efficiency
in the Antarctic where the temperatures may be permanently below
O"C, and equally well in tropical regions where temperatures approach
30°C. On mudflats in tropical regions, much higher temperatures may
be experienced for at least part of the day. Experimentally it can be
shown that increased temperature may have a direct effect on photosynthetic rate provided light saturation is achieved. Thus in the experiments of Curl and McLeod (1961) already quoted, it was shown that the
temperature optimum for Skeletonema approached 20°C provided that
sufficient light was available. Similarly Wimpenny (1958) investigated
the carbon uptake of Rhizosolenia using a, standard illumination of
16 000 lux. Provided this high light intensitj. was available, Wimpenny
showed that if the photosynthetic rate at lO'C was regarded as loo%, a
reduction in temperature to 5°C lowered the photosynthetic activity to
about 45% and a rise to 15°C increased it; to approximately 140%.
Although temperature may not appear to have a direct effect on photosynthetic activity in the marine environment, it has extremely important indirect effects on production, particularly in relation to the establishment of stratification and the setting up of a thermocline during the
133
phytoplankton. Similarly in the experiments of Antis et al. (1963), selfshading occurred with the very heavy crop of phytoplankton, although
with stirring, the algal cells could photosynthesize at remarkably low
light intensities. With the marked differential absorption of wavelengths of light in the sea, marine algae must not only be adapted to
photosynthesis at low light intensities but itlso at wavelengths which
may not be maximal for chlorophyll itself. It ;seems extremely likely that
the many carotenoids which are present in different forms and in different concentrations in the various algal groups in the phytoplankton,
are of significance in absorbing all light wavelengths. Although not
every carotenoid may be of equal importance in absorbing energy, it
appears to be the total quantity of solar radiation reaching the depths
which is of real importance. The marine algae therefore appear to have
become adapted to the generally low light intensities in the sea by increasing the chlorophyll content and by possessing active accessory
carotenoids.
2. Temperature
While light intensity must be of major significance in relation to
primary production in the sea, other factors may also play a part.
Temperature is probably of little direct importance since lowered
temperature will reduce the respiratory needs of the plant cells and
higher temperatures, by increasing respiratcry requirements, can have
a beneficial effect on photosynthesis only if very high light intensities
are available. It is obvious that photosynthe3is occurs a t high efficiency
in the Antarctic where the temperatures may be permanently below
O"C, and equally well in tropical regions where temperatures approach
30°C. On mudflats in tropical regions, much higher temperatures may
be experienced for at least part of the day. Experimentally it can be
shown that increased temperature may have a direct effect on photosynthetic rate provided light saturation is achieved. Thus in the experiments of Curl and McLeod (1961) already quoted, it was shown that the
temperature optimum for Skeletonema approached 20°C provided that
sufficient light was available. Similarly Wimpenny (1958) investigated
the carbon uptake of Rhizosolenia using a, standard illumination of
16 000 lux. Provided this high light intensitj. was available, Wimpenny
showed that if the photosynthetic rate at lO'C was regarded as loo%, a
reduction in temperature to 5°C lowered the photosynthetic activity to
about 45% and a rise to 15°C increased it; to approximately 140%.
Although temperature may not appear to have a direct effect on photosynthetic activity in the marine environment, it has extremely important indirect effects on production, particularly in relation to the establishment of stratification and the setting up of a thermocline during the