Illumination in the upper ocean is not continuous, but has a diel cycle (this is not
news). It increases rapidly as the sun rises, peaks at noon, and drops to very low levels
at night. Virtually all organisms, marine and terrestrial, have physiological cycles
matched to the illumination cycle, and most have internal time-keeping processes
allowing anticipation of events in the daily round. Algal growth and often
multiplication vary with a daily periodicity, since photosynthesis must vary with the
alternation of light and dark. Nelson and Brand (1979) have examined the phase
relationships between cell division and the light–dark cycle for a variety of planktonic
algae in culture. Timing of cell division tends to follow taxonomic lines (Fig. 3.12a &
b).
Fig. 3.12 (a) Division rate (h−1) throughout the day–night cycle in two diatom clones