24
and to which Antarctic phytoplankton can hardly adjust their physiology.
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In their study of the e f fect of radiant energy on the photosynthetic activity of Antarctic phytoplankton, Holm-Hansen et al.
(1977) found a high degree of correlation between the intensity of
solar radiation and the photosynthetic rates in the euphotic zone.
For instance, they found that on days when light intensity was high,
photosynthetic rates were low in the surface waters, and these rates
increased with depth.
On the other hand, when incident light was low
during the in situ incubation period, photosynthetic rates remained
fairly constant in the upper waters of the euphotic zones or they were
highest in surface waters (Fig.3).
These results are most probably
due to photo-inhibition, whereby high intensity light causes a decrease
in the photosynthetic rates.
Holm-Hansen et al. (1977) found that on
bright days (100 to 160 cal /cm2 /half-light~a;l, maximum photosynthetic
assimilation occurred at depths corresponding to between 25 and 50%
of incident radiation. When, on the other hand, solar radiation was
low (20 to 30 cal/cm 2 /half-light day), there was no evidence of photoinhibition.
The threshold of photo-inhibition for Antarctic phyto-