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.
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The work upon which this report is based was supported in part by funds provided by the Office of Water Research and Technology, United States Department of the Interior, Washington, D.D., as authorized by the Water Research and Development Act of 1978. Public Law 95-467.
Calkins, John; Colley, Ed; Wallingford, Jay; Hulsey, Carol; Lohr, Kathy; and Bolling, Marc, "Sunlight Induced Movement of Planktonic Organisms and Their Relationships to Water Movements" (1982). KWRRI Research Reports. 71.