192 



ANALYSIS OF THE ENVIRONMENT 



slightly higher than in fresh water. Oxygen 

 IS about 17 per cent less soluble in sea 

 water than in fresh water. The decrease in 

 solubility with an increase of temperature 

 from zero to 25° C, a common enough 

 change in nature, is about 41 per cent for 

 fresh water. 



The partial pressure of the gas at the 

 water's surface, and the solubility of the 

 gas, together with the salinity and tem- 

 perature of the water, determine the 

 amount of gas dissolved at equilibrium. The 

 effect of hydrostatic pressure of the water is 

 negligible; water at any depth of the ocean 

 contains the amount of dissolved oxygen it 

 would have at surface-equilibrium, plus or 

 minus (always minus, in lightless depths) 

 the amount contributed or removed by or- 

 ganic matter, living or dead. Comparable 

 relations exist for dissolved nitrogen. 



NITROGEN 



The nitrogen dissolved in water comes 

 mainly from the atmosphere. Some is 

 brought in by ground water that is fully 

 saturated at low temperature and so be- 

 comes supersaturated when the temperature 

 rises. The liberation of nitrogen in water by 

 the action of denitrifying bacteria has been 

 reported. It now appears that, at least un- 

 der conditions found in the sea, which have 

 been much studied, there is little or no loss 

 of fixed nitrogen (Sverdrup, Johnson, and 

 Fleming, 1942; ZoBell, 1946). In a lake 

 with a thermocline (p. 94), the nitrogen 

 content of the epilimnion tends to be in 

 equilibrium with the air, as is the entire 

 lake during spring and autumn overturns. 

 The water of the hypolimnion becomes and 

 remains supersaturated with nitrogen as it 

 gets warmer in summer. Ventilation by 

 convection is lacking, and the diffusion rate 

 is low. Water may become warmed so 

 rapidly that nitrogen and oxygen escape as 

 bubbles. Fishes in such water are subject to 

 gas embolism from the gases that pass out 

 of solution in their blood and collect in 

 veins and sinuses as gas bubbles. 



OXYGEN 



Oxygen exists in chemical combination 

 with hydrogen to form water. Such oxygen 

 is effectively removed from the oxygen en- 

 vironment of animals and when we speak 

 in ecology of the oxygen content of water, 

 we refer only to the oxygen dissolved in 

 water. 



Certain generalizations regarding the 

 amount of dissolved oxygen in aquatic 

 habitats have already been stated. We know 

 that during the summer stagnation, the hy- 

 poUmnion of thermally stratified lakes may 

 have a low oxygen content (p. 94); that 

 there tends to be an inverse relationship be- 

 tween the oxygen and carbon dioxide con- 

 tent of water and frequently, therefore, be- 

 tween the oxygen content of water and 

 its pH (p. 173). Water obtains its dis- 

 solved oxygen both from the air and 

 from the oxygen released in photosyn- 

 thesis by green plants. Within the lighted 

 surface region, water is often super- 

 saturated with oxygen during daylight 

 There is an oxygen pulse that reaches 

 its peak in the afternoon of sunny days 

 and is at a minimum near dawn. In 

 lower levels of the lighted zone, the 

 organisms present consume more oxygen 

 than is produced by photosynthesis. The 

 depth at which intake and consumption of 

 oxygen are in balance is called the com- 

 pensation level. Normally this lies near the 

 surface at night and normally sinks up to 

 the time of maximum light penetration dur- 

 ing the day; seasonally, the compensation 

 level lies lower in summer and, geographi- 

 cally, descends in depth toward the equator. 



Below the compensation level, the fairly 

 large animal population that feeds on or- 

 ganisms drifting down from above, together 

 with the decay of these dead forms, serves 

 to reduce the oxygen content often far be- 

 low the saturation point characteristic of 

 surface waters. The deeper waters of the 

 ocean obtain their oxygen supply primarily 

 from the drift of sinking cold water from 

 polar seas and have a larger oxygen con- 

 tent than that at intermediate depths. The 

 oxygen profile with depth is shown for a 

 few stations in Figure 42. 



Oxygen is lost from water as a result of 

 the respiration of living organisms and 

 through the oxidation of organic matter 

 and of dead bodies; bacteria probably con- 

 sume more oxygen in sea water and bottom 

 deposits than all other organisms taken to- 

 gether (ZoBell, 1946). Oxygen is also ex- 

 tracted from water and carried to the upper 

 air by the bubbling of other gases like 

 methane, appropriately called marsh gas. 

 Water near the surface may become 

 warmed and itself give off its dissolved gas 

 in bubbles. Dissolved oxygen is also lost in 

 the oxidation of iron and perhaps of other 



