254 



DEPTHS OF THE OCEAN 



At 30° C. a litre of water which is saturated with oxygen 

 contains little more than half as much as at 0° C. There is 

 therefore normally more oxygen in the cold water-masses of the 

 Arctic and Antarctic regions than in the warm water-masses of 

 the tropics. The salinity is not such an important factor in the 

 solubility of oxygen as the temperature. 



Marine animals need oxygen for respiration, and therefore 

 consume some of that contained in the water. By the act of 

 respiration carbonic acid is produced and dissolved in the water. 

 The same thing goes on through the respiration of plants. 

 These are some of the principal oxygen-consuming processes. 

 But plants assimilate besides breathing ; that is to say, they 

 make use of the carbonic acid by dissociating it into oxygen 

 and carbon ; they employ the carbon for building up cells, while 

 the oxygen is again dissolved in the water. This is the chief 

 oxygen-producing process, but it is carried on only through the 

 influence of light-rays. It is doubtful what rays are the most 

 important for marine plant life, and in what quantity they are 

 necessary. Experiments have shown that many higher aquatic 

 plants assimilate much better in yellow light than in blue or 

 violet light ; this is the case with most adherent green algse, 

 and hence they are found in the upper water-layers near the 

 surface, where there is enough yellow light. The red alga^, on 

 the other hand, assimilate better in blue light than in yellow, 

 and therefore live in deeper water than the former. We know 

 nothing of the assimilation by the plankton-algae of the various 

 light-rays ; we only know that they need light, and that they 

 are found in the upper water-layers, but not in deep water. 

 The production of oxygen in the sea is thus limited to the 

 upper layers, while the consumption of oxygen takes place 

 wherever there are living organisms (excepting certain bacteria). 

 Now, supposing the processes of assimilation and of respiration 



