136 LIFE IN THE SEA [OH. 



of water from the Arctic seas, and this stronger drift 

 would bring with it a greater supply of food-stuff. 



But why do we not get a corresponding abundance 

 of plant life in equatorial seas where the upwelling 

 of bottom water is maximal, and where nitrogen and 

 carbon food-stuff must be transported from the bottom 

 to the surface ? As we have seen, the opposite is the 

 case, and the tropical seas are relatively poor in 

 plant life. This poverty might be explained in more 

 than one way, by the temperature hypothesis for 

 instance, which we have already noticed in Chapter III. 

 But if this were true we should still have an 

 abundance of nitrogen compounds produced by the 

 wasteful metabolism of the organisms there, and it is 

 the case that nitrogen compounds are scarcer in 

 equatorial seas than elsewhere in the ocean. A 

 further possible explanation is that these substances 

 are destroyed by bacteria. 



Nitrogen compounds must be removed from the 

 sea in some way for they are continually being added 

 to it in the water flowing down from the land in the 

 rivers, and it has been estimated that about 39 

 millions of tons of this element in the form of 

 dissolved inorganic and organic compounds are added 

 to the seas of the world every year. The amount 

 that flows into the North Sea itself is said to be at 

 least about 390 millions of kilograms annually, while 

 the amount of nitrogen in the form of fish and other 



