132 LIFE IN THE SEA [CH. 



is greatest in the oozes at the bottom. It is large 

 near the land. 



In considering the distribution of marine animals 

 and plants we noticed that the abundance of food 

 was a factor of importance and we see now that the 

 abundance of food, that is, the abundance of the 

 plants the producers of animal food depends on the 

 nitrogenous and other ultimate food-stuffs present in 

 the sea- water. We need only consider the distribution 

 of the nitrogen for this is the substance which rules 

 the production in the sea. It is true that carbonic 

 acid, mineral salts and oxygen are also required, and 

 that lime and silica are necessary for the shells or 

 skeletons of marine organisms. But when a number 

 of indispensable food-stuffs are required and when 

 one of these is present in small proportions, then the 

 production depends on the substance which is present 

 in minimal quantity, just as the strength of a chain 

 depends on that of its weakest link. The silica and 

 phosphates are present in the sea in very small traces, 

 but they are usually more abundant than is necessary 

 when the amount of nitrogen present is considered. 

 The other materials, and oxygen, are present in 

 relatively large amount. 



Let us return now to some of the physical 

 questions discussed in former chapters. We have 

 seen that light penetrates to a depth of about 

 400 fathoms at the most, and probably effective light 



