252 Oxygen and Carbon Dioxide 



organic ions and molecules to set more negative values. Such low 

 redox systems represent reducing agents that would tend to combine 

 with any introduced oxygen. An aerobic organism, needing oxygen 

 for its respiration, would find such competition exceedingly difficult, 

 if not insuperable, if the redox system were well poised. Facultative 

 and obligative anaerobes, however, have no such difficulty and are 

 able to tolerate low redox values. 



Fig. 7.6. (A) Basin with local formation of heavier water and deep outflow 

 across the sill, as in the Mediterranean Sea, the Red Sea, and the Inner Gulf of 

 California. ( B ) Basin with surface outflow of lighter water and occasional inflow 

 of denser water across the sill, as in the Black Sea, Baltic Sea, and many fjords. 

 (From The Oceans by Svedrup et al., 1942, Copyright Prentice-Hall, N. Y. ) 



Zonation of organisms according to redox potentials has been shown, 

 especially in sediments, but whether the oxygen concentration or the 

 redox potential is the determining factor is hard to ascertain since the 

 latter generally depends chiefly upon the former. In marine muds 

 the redox potential decreases with depth in the sediment with the 

 most rapid change occurring in the few uppermost centimeters. The 

 abundance of aerobic organisms correspondingly drops off sharply, 

 and at increasing depths the anaerobes become relatively more 



