CHEMISTRY OF THE SEA 169 



The expended carbon dioxide is replaced in the sea partly from the 

 air, partly by that produced by the marine animals, and large amounts 

 are constantly brought in by the rivers. Carbon dioxide is used by 

 plants only in the euphotic and dysphotic strata of water. The animals 

 inhabiting the aphotic depths give off carbon dioxide, and the decom- 

 position of the sinking bodies of plants and animals adds to this, so 

 that carbon dioxide is stored in the depths. This store becomes avail- 

 able to marine plants only where vertical currents bring the bottom 

 water to the surface. The observations of Atkins 9 show that the carbon 

 dioxide in the upper layers may be rapidly used up. He found that 

 the reaction of the surface water on the English coast varied with the 

 season from pH 8.14 in December to pH 8.27 in May at the time of 

 the highest amount of sunshine. This seemingly small variation really 

 amounts to a fall of 25% in the hydrogen-ion concentration and is 

 due to the accumulation of the carbon dioxide produced by the excess 

 of animals in winter and its use by the plants which undergo a flourish- 

 ing growth in summer. Marine plants will accordingly flourish most 

 where there is a constant inflow of water with available carbon 

 dioxide. 



Nitrogen compounds. — The situation is similar with the nitrogen 

 compounds 10 which are unconditionally necessary to plant life, together 

 with carbon dioxide and phosphoric acid. Nitrogen in combination as 

 nitrates, nitrites, or ammonia, and thus available to plants, is present 

 in the North Sea to the amount of about 300 mg. per cu. m., and rather 

 less in the waters of the open ocean. The amount in milligrams varies 

 with the temperature, about as follows: 



In view of the great requirements of plants for nitrogen, it seems 

 likely that the smallness of the amounts present in sea water limit the 

 amount of plant life and thus also the animal life of a given part of 

 the sea. 



The sources of nitrogenous compounds in the sea are various. They 

 originate in the seas themselves by the metabolism of animals and by 

 the decay of dead organisms. The supply from without is brought in 

 by rivers in the form of nitrites, nitrates, and ammonia, and perhaps 

 also in organic compounds, and by rains, which contain a little nitric 

 acid, especially in the zones of frequent electric storms. Nitrate-fixing 

 bacteria do not normally occur in the sea water but may be present 

 on the ocean bottom. The supplies are diminished by loss of ammonia, 



