66 



The Sea-)vater ami its Physical and Chemical Properties 



Table 25. Saturation values for oxygen and for nitrogen in sea-water in mill Hit res per 

 litre for a dry standard atmosphere 



Temp. 



Oxygen 

 salinity (%o) 



Nitrogen 

 salinity (%„) 



less reactive than oxygen and is biologically inert. Water samples from different depths 

 show mostly only minor deviations in nitrogen content from the saturation values. 

 This could be used to draw conclusions about the origin of deep layers and about the 

 vertical and horizontal displacements that they have undergone since their last 

 contact with the atmosphere, but since very few nitrogen determinations have been 

 made in the open ocean the method has so far been of little use. In any case, care 

 would be required in the interpretation of such results since super-saturation or in- 

 complete saturation may be due to other causes: to subsequent heating and cooling, 

 to the mixture of saturated water masses at diflTerent temperatures which always 

 leads to small super-saturation, to variations in atmospheric pressure and to the 

 production of nitrogen by bacteria that decomposes nitrite or nitrate. Since the equal- 

 ization of existing differences in saturation always proceeds slowly these deviations 

 will be conserved for a long time and can simulate water movements that would 

 otherwise be quite impossible. 



The oxygen is also for the most part in equilibrium between the air and water at the 

 surface of the sea, but the deviations are more frequent and more marked than for 

 nitrogen. Besides the causes of more physical factors mentioned above (temperature 

 and pressure alterations, mixing, etc.), there are also biological factors which cause 

 variations. The respirations of plants and animals produces carbon dioxide and uses 

 up oxygen. Animals, however, obtain their essential carbon compounds from in- 

 gested organic material, while plants, on the other hand, obtain it by the assimilation 

 of carbon dioxide. This is converted with the help of sunlight into organic substances 

 and oxygen, which is set free, raising the oxygen content of the water. 



The oxygen in sea-water is consumed not only by the respiration of living organisms 

 but also by the bacterial oxidation of dead organic matter and of organic compounds 

 in solution. This oxygen consumption is proportional to the rate of oxidation, which is 

 in the first place dependent on the temperature and also on the amount and nature of 

 the organic material present. 



In the assimilation layer (the upper layer of the sea), usually down to the thermo- 

 cline (rapid density change in the vertical) conditions are rather complicated due to 

 the mutual interaction of the different factors. Oxygen is being steadily absorbed from 



