CHAPTER XIV 



THE ABYSSAL BENTHIC AND PELAGIC COM- 

 MUNITIES OF THE SEA 



The depth of penetration of light into sea water has been discussed 

 above (p. 160) . It is important to note that it is the threshold values 

 for optic sensation and for plant assimilation that are important for 

 animal distribution rather than the minute amounts of light which are 

 present at great depths. 1 The lighted stratum varies in depth diurnally 

 and annually, for a given locality, and notably with the latitude for 

 different localities. Any sharp delimitation is made still more impossible 

 by the varying threshold values for different animals and plants. The 

 lighted stratum is commonly divided into a euphotic stage (0 to 80 m.) 

 and a dysphotic stage (80-600 m.). It does not seem practical to 

 attempt a further subdivision of the lightless lower stratum, in which 

 even the great increase in pressure seems to have little effect on dis- 

 tribution. 



The penetration of light directly governs the presence of plants. 

 In both benthal and pelagial they are principally developed in the 

 upper stage of the lighted stratum, and extend to greater depths in the 

 tropical than in the polar oceans. 



Lohmann's figures 2 for nannoplankton, primarily composed of 

 plants, are as follows: 



Greater depths are reached only by the protected spores and dead 

 remains of plants. A great number of animals directly dependent on 

 these plants for their food are thus confined to the euphotic stage. 



The amount of water in the abyssal stratum exceeds that in the 

 lighted one. Of the total surface of the oceans the following percent- 

 ages correspond to various depths: 



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