350 



[(JHAP. 17 



some 90% of the light is absorbed by the water. High transparency, then, is 

 symptomatic of, but not in itself a cause of, low productivity. It merely reflects 

 a paucity of light-absorbing organisms, but does not explain why they are 

 scarce. 



As a corollary to the above statement we may say that organic production 

 beneath a unit of surface area is inversely proportional to the depth of the 

 euphotic zone. This was well demonstrated by Steemann Nielsen and Jensen 

 (1957), whose illustration is reproduced here (Fig. 2), and by Berge (1958), but 



50 100 



Depth of photosynthetic layer (m) 



Fig. 2. The relation between productivity and the depth of the euphotic zone. (After 

 Steemann Nielsen and Jensen, 1957.) 



it is true only to the extent that increased turbidity is due to increased con- 

 centrations of photosynthetic organisms. Such is not necessarily true, as 

 discussed above. 



Riley (1956) has determined empirically the relationship between trans- 

 parency and phytoplankton density (expressed as chlorophyll) in regions where 

 he assumed light absorption by material other than water and plants to be 

 negligible. His expression is 



k = 0.04 + 0.0088(7 + 0.054C 2 ^, 



where C is chlorophyll in micrograms per liter and the extinction coefficient of 



