Previous studies have shown the planktonic organisms of various natural water bodies to be quite sensitive to the levels of solar ultraviolet radiation incident at the water surface in summer. It has been assumed that the majority of the phytoplankton are simply carried about in the mass of the water which they inhabit with little control of their position or sunlight exposure. Photosynthetic microorganisms must strike a delicate balance with regard to solar radiation; they must obtain enough visible light for adequate photosynthesis while avoiding an excessive exposure to the injurious solar UV-B radiation.
It is proposed that the circulation in natural waters is such that organisms can ride the current to obtain radiation exposures approximating the average radiation level of the euphotic zone. If exposure tends to become excessive, the plankton drops out of the circulation into the relatively immobile thermocline region until conditions are :favorable for a return to the highly illuminated surface waters. Observations with two species of aquatic plants generally confirm the proposed model.
A radiation regime where the entire population receives essentially the same exposure to sunlight could be substantially more efficient in using the available light for photosynthesis than the static model where it is assumed that part of the population is overly exposed and another segment continuously dwells deep in the water in very dim light. The uniform dosage raises the potential problem that, should the average UV-B dose exceed the tolerance of the individual, virtually the entire population would be wiped out in a very short time. Perhaps such a species might gradually recover its prominence from the limited survivors or return the next season after being supplanted in its niche by other more resistant organisms
