14 RESPIRATORY MECHANISMS 



water. The very transparency of the water means a low 

 content of organisms and a correspondingly low intensity of 

 the processes of assimilation. Where the light is insufficient 

 for assimilation, oxidative processes by bacteria and animals 

 predominate and the oxygen content is reduced. 



Speaking generally it can be said that in the sea the supply 

 of oxygen from the atmosphere and by assimilation and its 

 distribution by convection are sufficient to ensure practically 

 everywhere an oxygen concentration which is ample for 

 animal life. Exceptions are the depths of certain basins like 

 the Black Sea where circulation is practically absent. In 

 the Pacific there exists over a large area outside the west 

 coast of North America an intermediate body of water at 

 depths from 300 to 1000 m in which the oxygen concentration 

 is below 1 ml/1 and in some places, as first discovered by the 

 Dana expedition in 1922 in the Gulf of Panama, even prac- 

 tically (Thompson, Thomas, and Barnes, 1934). A study 

 of the fauna inhabiting these regions might show interesting 

 adaptations. 



In rivers a fairly high concentration of oxygen is also the 

 rule, although slow-flowing rivers in the tropics carrying an 

 abundance of organic material may have very low concen- 

 trations. In lakes the oxygen in the stagnant water below 

 the thermocline is often used up during summer, and in 

 smaller ponds conditions are very variable, and low concen- 

 trations of oxygen quite common. This is especially true of 

 tropical marshes where the high temperature favours a rapid 

 decay of organic material (Carter, 1935) which not only uses 

 up oxygen, but also causes C0 2 to accumulate at the excep- 

 tionally high tensions of 16-32 mm. 



Into the bottom deposits both in the sea and in fresh waters 

 oxygen can only penetrate by diffusion from the water just 

 above, as convection is practically absent. These deposits 

 always contain organic material, and often the quantity of 

 such material is quite high. Oxygen is used up, mainly by 

 bacterial processes, and the result is that generally only the 

 surface down to a depth of a few mm and sometimes even 



