incident on the surf ace of lakes or oceanic areas is absorbed by the water itself or by detritus and that only a very small part can be utilized by plants or animals. We conclude that aquatic organisms are existing under very unfavorable circumstances in regard to the utilization of solar energy. It is for this reason that the intensity, amount, and composition of the light are so frequently found to be limiting or highly significant factors in the aquatic environment. Attack on the unsolved problems outlined above is therefore urgent, and the extension of our present observations into other bodies of water and particularly over longer periods of time is of the greatest importance.
The amount and nature of daylight in natural waters depends upon the surface loss, the selective absorption of the water itself, and the selective action of particulate and dissolved material. Further changes in the illumination result from differences in transparency with depth and with the season, differences in the length of day, and differences in the angular distribution of the light.
The biological significance of ultraviolet in natural waters is in doubt, but the visible component of light is important in the regulation of the activity of many animals, in the vision of fish, and especially in the photosynthesis of the plants. Since the utilization of light is very low even under the most favorable circumstances, it is understandable that light is so frequently a limiting, factor in the aquatic environment.
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