Absorption by pigments is not 100% perfect, and some wavelengths are more
effective than others at driving photosynthesis. This is shown in the “action spectrum”
for photosynthesis (Fig. 3.2b). The highest overall effectiveness is found at those
wavelengths centered at 465 nm, which are dominant in deeper ocean layers. This is
true for phytoplankton generally. Lewis et al. (1985) have shown this by producing
action spectra for natural phytoplankton assemblages at various ocean sites. All have
their peak at 425–450 nm, a broad shoulder to 550 nm, and a rise at 675 nm (Fig. 3.3).
Photosynthetic yield per quantum drops off rapidly below 425 nm and has low values
from 600 to 650 nm – wavelengths not important in pelagic habitats except at the very
surface.
Fig. 3.3 Photosynthetic action spectra for natural phytoplankton assemblages from