ACCESS TO OXYGEN 13 



figured out for instance that by diffusion from the surface 

 the first oxygen molecules would reach a depth of 250 m in 

 42 years. If no oxygen was used up all waters would never- 

 theless have become fully saturated by diffusion. 



Convection takes place to a large extent by currents gen- 

 erated by the wind, but temperature variations are also an 

 important source of convection because of their effect upon 

 the specific gravity of water. Fresh water shows a maximum 

 specific gravity at 4°C. The presence of dissolved salts lowers 

 the temperature of maximum density. In brackish water of 

 18.6°/oo salinity the maximum is at 0°C, and in ocean water 

 at - 3.5°. 



In temperate regions the temperature of lakes will ap- 

 proach 4° during some period in the spring and the wind will 

 usually cause the whole body of water to become mixed, but 

 in the summer the upper layers of water are heated by day 

 while the surface itself is cooled at night by evaporation and 

 irradiation. This causes a vertical convection down to a 

 certain level, called the thermocline, depending upon the 

 nightly cooling and the exposure to strong winds — say be- 

 tween 10 and 20 m — above which the water is regularly 

 kept mixed and reaches during summer a fairly high tempera- 

 ture. Below the thermocline the water is stagnant and the 

 temperature remains low. 



The influence of organic processes on the oxygen content of natural 

 waters. While in air the processes of assimilation and respira- 

 tory metabolism have only a very slight and local effect upon 

 the oxygen content of the atmosphere, the effects in water 

 are often pronounced. In the upper layers both of fresh 

 water and of the sea it is the^ rule, at least in summer, for 

 assimilation to predominate and cause an increase in oxygen 

 content above that which would be in equilibrium with the 

 atmosphere. The thickness of the layer in which assimilation 

 can take place depends upon the transparency of the water, 

 and may vary from a couple of meters or even less in fresh- 

 water ponds to a couple of hundred meters in clear ocean 



