THE UTILIZATION OF SOLAR ENERGY BY AQUATIC ORGANISMS

By GEORGE L. CLARKE

BIOLOGICAL LABORATORIES, HARVARD UNIVERSITY, CAMBRIDGE, MASS., AND WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MASS.*

My first object in this paper is to present a brief summary of our present knowledge of the availability of radiant energy in natural waters and the utilization of it by aquatic animals and plants. My more important desire in bringing together this material, however, is to delineate the critical problems which have now arisen on this subject and upon the solution of which further significant progress depends. Research in this field falls roughly into two parts, namely, (1) the determination of the amount and nature of the light actually present at various depths in all types of water bodies, and (2) the measurement of the extent to which submerged organisms are able to utilize the light present. From the biological point of view we need to know not only the range of light intensity at any point but also its spectral composition, its angular distribution, and its distribution in time.

The solar energy which falls upon a body of water is subject first of all to a "surface loss" which in the case of the ocean may amount to as much as 60 per cent in rough weather. Only about 3 to 9 per cent of this is ordinarily due to reflection (for solar altitudes ereater than 30°) and the remainder has been found to be caused by a greatly increased rate of extinction in the uppermost meter of water ( Powell and Clarke 1936). It was originally suggested that this effect was due to bubbles existing, near the surface but this explanation has been questioned by Poole ( 1938). Whether a similar increase in extinction coefficient of the subsurface stratum occurs in lakes at times when waves exist is a question which invites investigation.

As the light passes from the surface downward into the water, it is reduced in intensity according to the following equation:

where Io is the initial intensity, I is the final intensity, k is the extinction coefficient, L is the thickness of the layer in meters, and e is 2.7. When this relationship between the reduction in the light and the thickness of water through which it has passed is expressed graphically on a semilogarithmic plot, a straight line is obtained (Fig. 1). The slope of the line is determined by the value of the extinction coefficient, k, which is thus an index of transparency. The extinction coefficient varies widely in the different parts of the spectrum--even for pure water--and its actual value depends upon the precise wavelength considered. In Fig. 1 the rate of absorption of red light by distilled water is seen to be very high, that for yellow light lower, and that for blue light very much lower. For example, after traversing 70 meters of distilled water blue light has suffered only a slight reduction to 70 per cent of its initial value, whereas yellow light has been reduced to 6 per cent. In the case of red light a reduction to 6 per cent has already taken place after passing through less than 3 meters of water.

Now the energy of the sun as it reaches the surf ace of a natural body of water is not equal in all parts of the spectrum but is distributed as shown by the uppermost curve in Fig. 2. We therefore start with

____________________
*
Contribution No. 201.
This quantity may be taken as the depth, but if the mean path of the light departs from the vertical, as is usually the case in natural waters, a small correction is required since the path length is then greater than the vertical depth (cf. Whitney 1939).

-27-

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