Light as a Limiting Factor for Aquatic Animals and Plants

Abstract

Every school boy knows that light is required for the growth of green plants and that all animals, including ourselves, are dependent directly or indirectly upon the plants for their food supply. It is not so obvious, however, that exactly the same situation is encountered in the aquatic habitat. The ultimate source of energy for all the multifarious life in the sea and in every body of fresh water is sunlight. Furthermore most fish and many types of animals need enough illumination to see-at least part of the time-to catch their food and to avoid being caught themselves. But light does not penetrate into water indefinitely: it is absorbed by the water itself and further reduced by sediment and by stains. The aquatic biologist is thus concerned to know how much light exists at various depths in rivers, ponds, lakes and in the ocean itself, and what are the maximum depths at which fish can see and at which the all-important green plant can make a living. Since no natural body of water can be any more transparent than pure water itself, let us begin our study of the biological action of light in the aquatic habitat by imagining a lake full of distilled water. The penetration of sunlight into such a lake is represented diagramatically in Figure 1. Depth is indicated alonig the right hand side of the graph with the surface at the top. At the upper right hand corner of the figure the full (100%) intensity of the sunlight is indicated and diminishing percentages are found toward the left. As we descend into our hypothetical lake the illumination is progressively reduced in intensity, but the rate of the diminution is expressed by the slope of the line and this is very different in the various parts of the spectrum. Thus the rate of absorption of the red Color Wavelength Absorption/meter