244 Oxygen and Carbon Dioxide 



decomposition of organic matter, reduces the oxygen supply faster 

 than it can be replenished from the atmosphere. 



Aquatic Environment. The total amount of oxygen that water will 

 hold at saturation varies with temperature, salinity, and pressure. 

 Sample oxygen saturation values for fresh water and sea water at 

 temperatures within the range of ecological interest are given in 

 Table 17. When we realize that the 21 per cent of oxygen present 



in the atmosphere is equivalent to 210 cc per liter, the contrast be- 

 tween the amounts available in air and in water is brought into relief. 

 There may be 25 times as much oxygen in a liter of air as in a liter 

 of water. When the oxygen in the water is in equilibrium with 

 that in the air, however, the pressure of the oxygen is the same in 

 both media. 



From the foregoing it is clear that the world's main reservoir of 

 free oxygen is in the atmosphere. The chief source of oxygen for 

 the water environment is the oxygen which can be absorbed from 

 the air, but a second source of supply in aquatic habitats is the oxygen 

 released beneath the surface by the photosynthesis of submerged 

 and planktonic plants. Water may lose oxygen by diffusion from its 

 surface out into the atmosphere. Oxygen is also used up by the 

 respiration of aquatic organisms and by the decomposition of organic 

 matter in the water. Living organisms are thus seen to influence 

 both the supply and the depletion of oxygen in natural bodies of 

 water; oxygen is a modifiable factor in the aquatic environment but it 

 is essentially an unmodifiable factor in the air. The actual amount 

 of oxygen present at any one time and place in the water is the result 

 of a balance between the processes of supply and depletion. Here is 

 another situation in which an ecological factor is controlled by a 

 dynamic equilibrium involving both physical and biological processes. 



Three general situations may be discerned in regard to the supply 

 of oxygen in the water environment: the surface stratum, the sub- 

 surface zone, and the deep layers. The water at the surface tends 

 to be in equilibrium with the air above it so that the oxygen value 



