368 DEPTHS OF THE OCEAN chap. 



tion at a given moment need not necessarily be proportionate 

 to the conditions of existence prevailing. There may be after- 

 effects of a previous set of conditions. Indeed it is possible to 

 point to places totally destitute of vegetation, owing to former 

 unfavourable circumstances having destroyed all germs, while 

 new germs have not yet found their way there. Still this is the 

 only reservation we need to make, when asserting the universality 

 of this natural law. 



The necessary nutritive substances which are most likely 

 to occur "in minimum" in the sea are nitrogen, phosphoric 

 acid, and, in the case of diatoms, silicic acid ; all others occur 

 even to superfluity. Brandt in his works on metabolism 

 in the sea discusses at some length the importance of nitrogen, 

 phosphoric acid, and silicic acid, and his assistants at Kiel 

 have carried out a number of tests to ascertain the extent 

 to which these substances are present in sea- water. Not 

 only the nitrogenous compounds (organic compounds, ammonia, 

 and nitrates), but also phosphoric acid and silicic acid, occur in 

 extremely minute quantities, so that it is particularly difficult 

 to get accurate values representing them. We have therefore, 

 unfortunately, no proper conception as yet of the way in which 

 these substances vary in different parts of the sea. According 

 Raben. to Rabcn's latest investigations the total quantity of combined 



nitrogen (ammonia, nitrates, and nitrites) in true North Sea 

 water varies between o. no mg. and 0.314 mg. per litre, of 

 which 0.047 to 0.124 mg. is saline ammonia, the whole being 

 reckoned as free nitrogen. Even if we assume that the quantity 

 of nitrogen in the Atlantic is considerably less, these values are 

 high compared with the quantity of nitrogen to be found 

 combined in the cells of the plankton -algs. It seems, 

 therefore, hardly possible that the nitrogenous compounds are 

 entirely consumed by thealgse. It is, however, quite conceivable 

 that the variations in the total quantity of nitrogen, or in the 

 quality of such compounds as are easiest to absorb, may hasten 

 or retard the augmentation of the algse. The same is the case 

 with silicic acid, which Raben found to vary between 0.30 mg. 

 and 1.03 mg. per litre in thirty samples from the North Sea. 

 The quantity of phosphoric acid, according to Raben's investi- 

 gations, is as a rule below i mg. per litre, though it slightly 

 exceeds the quantity of nitrogen. 



Brandt starts by discussing the occurrence of nitrogenous 

 compounds in the sea. He calculates that large quantities of 

 combined nitrogen are carried out from the land by the 



