ACCESS TO OXYGEN 15 



less contain any free oxygen. Further down anaerobic bac- 

 teria reduce oxides and often change the colour of the deposit 

 from a light grey to black. The production of hydrosulphide 

 and methane in these layers is quite common, and in many 

 ponds bubbles of methane and carbon dioxide rise regularly 

 from the bottom and may reduce the oxygen content and 

 increase the C0 2 content of the water. 



When oxygen is used up in the bottom at a fairly rapid rate 

 the water just above the bottom loses oxygen and, as pointed 

 out by Alsterberg (1922), a "microstratification" develops, in 

 which there is a steep gradient in oxygen content over a dis- 

 tance of a few cm. Certain animals like the Tubificidce studied 

 by Alsterberg and the larvae of some of the Chironomidce (K. 

 Berg, 1938) react to this condition by building mud tubes up 

 to 1 cm in height. These are ventilated by undulating move- 

 ments, and where the animals are sufficiently numerous 

 (3,000 per sq. m.) they may succeed in drawing down suffi- 

 cient oxygen (at a very low concentration) to keep the walls 

 of the tubes and the surface of the mud in the oxidized state. 



In the following chapters many instances will be given of 

 the adaptation of the respiratory systems of animals to the 

 accessibility of oxygen in their natural habitat. 



Carbon dioxide in natural waters. While the quantity and 

 pressure of CO2 in the air is everywhere quite low, we find in 

 many waters a fairly large quantity at a low tension and in a 

 few cases even a tension which is too high to be negligible 

 from the point of view of respiratory adaptation. 



The C0 2 of natural waters is only to a slight extent derived 

 from the atmosphere, but mainly from carbonates in solution. 

 It is not necessary for our purposes here to go into the com- 

 plicated reactions between carbonates and free C0 2 , but it is 

 necessary to emphasize that at neutral reaction an equi- 

 librium will be established by which only a fraction of the 

 total will be present as free C0 2 which can pass the surface 

 and come into equilibrium with an atmosphere. When CO2 

 is added (e.g., by a respiring animal) most of this enters into 



