246 Oxygen and Carbon Dioxide 



How does oxygen get to the bottom of the ocean? We have seen 

 that the sources of oxygen are at or near the surface, but we know 

 that many kinds of aerobic animals live at the bottom of some lakes 

 and in the ocean abyss at depths of several miles beneath the surface. 

 Although oxygen moves slowly through the water by direct diffusion, 

 it can reach the deeper levels at an adequate rate only by the circula- 

 tion of the water itself. 



6 pm 



12 mid 



6 am 



12 m 



5 pm 



Modified from Tressler, Tiffany, and Spencer, 1940 



Fig. 7.1. Diurnal cycle of temperature, oxygen, hydrogen ion (as pH), and car- 

 bon dioxide in the surface waters of Buckeye Lake, Ohio, during August 12—13, 

 1930. Negative values for CO2 represent amount of CO2 required to make v^^ater 

 neutral to phenolphthalein. 



Oxygen-rich surface water is carried downward by wind stirring, 

 eddy conduction, and mass sinking. Wind stirring is very effective 

 in aerating the upper layers of lakes and of the ocean at times when 

 the water circulates freely. Whenever a water body is covered by 

 ice, no turbulence caused by wind or waves is possible. At other 

 seasons stirring is greatly curtailed whenever the water is stratified in 

 respect to density— as a result of salinity differences occurring at any 

 time of year, or as a result of a thermal gradient arising primarily 

 in summer. During periods of pronounced stratification the circula- 

 tion produced by wind and waves is limited to the layers above the 

 density discontinuity (Fig. 7.2). When stratification has been de- 



