THE UTILIZATION OF SOLAR ENERGY BY 



AQUATIC ORGANISMS 



By GEORGE L. CLARKE 



BIOLOGICAL LABORATORIES, HARVARD UNIVERSITY, CAMBRIDGE, MASS., AND WOODS HOLE OCEAXOGRAPHIC 



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 tlie utilization of it by 

 aquatic animals and plants. My more im- 

 portant desire in bringing together this 

 material, however, is to delineate the criti- 

 cal problems which have now arisen on this 

 subject and upon the solution of which 

 further significant progress depends. Re- 

 search 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 v/e 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" whicli 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 alti- 

 tudes greater than 30°) and the remainder 

 has been found to be caused by a greatly 

 increased rate of extinction in the upper- 

 most 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 



* ContriVtution Xo. 12(11. 



downward into the water, it is reduced in 

 intensity according to tlie following equa- 

 tion : 



7:=' 



-K-L 



wlun'e I,, 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 

 Avater through which it has passed is ex- 

 pressed 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 lov>er. For 

 example, after traversing 70 meters of dis- 

 tilled 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 tlian 

 3 meters of water. 



Now the energy of the sun as it reaches 

 the surface of a natural bodj' 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 



t Tiii.s quantity may be taken as the depth, but 

 if the mean path of the light departs from the ver- 

 tical, as is usually the case in natural waters, a 

 small correction is recjuired since the path length 

 is then greater than the vertical depth (of. Whitney 



