loo A TEXTBOOK OF OCEANOGRAPHY 



of ail marine plants are therefore présent in solution in sea- 

 water, and those plants flourish according to the quantity of 

 that indispensable food substance which is présent in minimum 

 amount. The question now arises, Which of the indispensable 

 éléments is présent in minimum quantity in sea-water? The 

 most important éléments for prolific growth are phosphorus 

 and nitrogen. Silica is apparently présent at times in such 

 small quantities as to corne under considération with respect 

 to the growth and increase of certain planktonic plants which 

 require large quantities of silica for their shells — e.g., diatoms. 

 After giving due considération to the various éléments, there 

 seems, according to Brandt, to be no doubt that nitrogen is 

 présent in sea-water in such minute quantities as to be the 

 controlling factor in the development of marine plants. In 

 carp cultivation, which is especially developed on the Continent 

 of Europe, those ponds which are provided with abundant 

 nitrogen (in the form of manure) are found to be much more 

 productive of carp flesh per unit of surface than similar ponds 

 not so provided with dung. In the case of fresh-water lakes, 

 a détermination of nitrates by means of the diphenylamine- 

 sulphuric acid reaction shows that those lakes which are rich 

 in nitrogen are precisely those which hâve the most abundant 

 plankton. Since ail nitrogen compounds are easily soluble, 

 we should expect the sea to be very rich in nitrogen on account 

 of the waste of nitrogenous material from the land, but as a 

 matter of fact sea-water is remarkably poor in nitrogen 

 compounds. 



This deficiency of nitrogen in the sea can be attributed to 

 the présence of denitrifying bacteria. In nature the abundance 

 and présence of nitrogen compounds is correlated with the 

 activities of nitrogen bacteria. 



Thèse latter are of two kinds — Nitrifying bacteria, which 

 oxidise ammonia, producing nitrous and nitric acids ; and 

 denitrifying bacteria, which reduce nitrogen compounds, 

 liberating free nitrogen. If it had not been for the activity 

 of thèse denitrifying bacteria the waters of the océan would 



