Extinction and Modification of Light 195 



depth but also a shift in the position of the maximum Hght to the red 

 region of the spectrum (Fig. 6.7). 



Since natural waters differ very greatly in transparency and in se- 

 lective absorption, a study of the penetration of each part of the spec- 

 trum in each body of water would have to be made for a complete 

 description of the light conditions. An approximate comparison of 

 the illumination in various natural waters can be made on the basis 

 of the relative transparency to the central part of the spectrum. 

 Sample values are given in Table 14 for representative bodies of 



* Extinction coefficient, k. 



_ - p-kL 



where /o = initial intensity. 



/ = intensity at depth, L, in meters. 

 e = 2.7. 

 t Depth at which a 20-cm white disc disappears when lowered from the 

 surface. 



water. Since the great differences in the extinction of light per meter 

 have a cumulative effect, an even greater contrast is presented in the 

 depths at which a given fraction of the light is found. The depth at 

 which the light intensity is reduced to 1 per cent of its surface value 

 is of particular importance because it represents the approximate 

 lower limit for plants, as will be discussed later. In general the table 

 and the representative curves in Fig. 6.8 show the profound variation 

 in the intensity of light available for organisms at increasing depths 

 in different natural waters. 



