240 



Light 



apparent that the zone in which productive growth of the crucially 

 important phytoplankton can take place is a relative skimming of 

 the surface. The photosynthesis in the upper few meters of the 

 ocean and of lakes accounts for the main portion of the initial pro- 

 duction of organic matter for the whole breadth, length, and depth 

 of the water. 



Surface 



100 m- 

 160 - 



250 - 



500 - 

 550 



800 - 



1,000 



1,700 



4,000 



10,860 



i 



Limit of growth for planktonic plants 

 Greatest depth for benthic plants 



Daylight 0.001% of surface value 



Approximate limit for vision of fish 

 Complete darkness for man 



Limit of response to day and night 

 by Crustacea 



Photographic plate blackened 

 after 80 minutes' exposure 



No perceptible light 

 from the surface 



Plate not affected after 120 mmutes' 

 exposure 



Average depth of ocean 



r I exposure 

 I I 



Deepest recorded sounding in ocean 



Fig. 6.29. Summary of limitation by light factor in aquatic environment. 



A summary of the conditions of light in the aquatic environment 

 and its critical limiting effects is given in Fig. 6.29. Here are indi- 

 cated the maximum depths for the growth of phytoplankton and 

 slightly deeper, of benthic plants. Since the mean illumination 

 necessary for the vision of aquatic animals is so very much smaller, 



