Aquatic Environment 245 



is at or near the saturation concentration for the existing conditions 

 of temperature and saHnity. The thickness of the surface stratum 

 in which saturation equihbrium is found varies greatly according to 

 the amount of turbulent mixing of the water in contact with the air. 



In the subsurface zone great variation in oxygen concentration may 

 occur because the water does not have easy exchange with the at- 

 mosphere and factors causing both decrease and increase in oxygen 

 are present. Respiration and decomposition, tending to deplete 

 the oxygen supply, occur at all levels, and photosynthesis, tending to 

 increase the abundance of oxygen, takes place down to the com- 

 pensation depth, that is, to the lower limit of the euphotic zone, as 

 described in the previous chapter. When large populations are 

 respiring or large quantities of dead material are decomposing, the 

 oxygen concentration is reduced to a low level. In stagnant ponds 

 and swamps choked with organic matter, and in rivers or other 

 bodies of water receiving excessive amounts of sewage or other 

 pollutants, the available free oxygen often becomes completely ex- 

 hausted. Under other circumstances supersaturation may occur in 

 subsurface water layers. If water which was saturated during the 

 winter becomes warmed as the season advances, more oxygen will be 

 present than can be held in true solution at the higher temperatures. 

 Similarly, when photosynthesis exceeds respiration at intermediate 

 levels in the euphotic zone, oxygen may be released into the water 

 faster than it can be carried away. As a result of these physical and 

 biological influences, supersaturated values as high as 180 per cent 

 in the sea and 300 per cent in inland lakes have been reported. 



Often cyclic fluctuations in oxygen content are observed in natural 

 bodies of water. Seasonal changes in temperature and circulation 

 coupled with differences in rates of photosynthesis, respiration, and 

 decomposition result in changes both in the absolute amount of 

 oxygen present in the water and in the degree of saturation. Under 

 certain circumstances a pronounced diurnal fluctuation in oxygen 

 concentration is found; this is known as an oxygen pulse. During the 

 daylight hours photosynthesis tends to cause an increase in the 

 amount of oxygen present, but, after the sun has set, the respiration 

 of the aquatic organisms and the decomposition processes going on 

 draw on the free oxygen in the water. The amplitude of the oxygen 

 pulse may reach considerable proportions in quiet waters in which 

 green plants are growing in abundance. Measurements in an Ohio 

 lake, for example, revealed a diurnal fluctuation in oxygen concentra- 

 tion extending from 6.7 ppm at 8:00 a.m. to 13.0 ppm at 5:00 p.m. 

 (Fig. 7.1). 



