Oxidation-Reduction Potential 251 



of the open Atlantie Ocean is the oxygen completely used up, and 

 minimum values are generally above 3 cc per liter. In the Pacific 

 Ocean, however, oxygen is reduced to zero in certain oxygen minimum 

 layers, as for example, at depths between 300 and 1300 m off the 

 coast of lower CaHfornia. 



Other instances in the marine environment of the complete lack 

 of oxygen, or of very low concentrations, are found only in special 

 situations in which circulation is largely or entirely cut off and de- 

 composing organic matter has accumulated. The classic example 

 of this condition is in the free water of the Black Sea, in which no 

 measurable oxygen is found from a depth of about 150 m to the 

 bottom at 2200 m. Vertical stirring is prevented by a layer of fresh 

 water at the surface supplied by the discharge of the Danube and 

 other large rivers entering the Sea, and deep exchange with outside 

 water is stopped by the shallow entrance at the Bosporus. Organic 

 particles carried in with the river water use up the oxygen as they 

 sink to deeper layers, and anaerobic decomposition produces great 

 quantities of hydrogen sulphide. Similiar conditions on a smaller 

 scale are found within deep fjords with shallow sills and within other 

 estuaries in which the circulation does not supply the oxygen as fast 

 as it is used up (Fig. 7.6). The bottom material of the marine en- 

 vironment is also likely to develop a shortage of oxygen unless it is 

 sufficiently coarse to allow good movement of water through it. 

 Fine muds containing decomposing matter contain little or no 

 oxygen at depths of more than a few centimeters from their surfaces. 



Oxidation-Reduction Potential. Another aspect of oxygen avail- 

 ability in the aquatic environment, but less well known, is the oxida- 

 tion-reduction potential, or redox potential. The redox potential 

 (Eh), measured in volts, is an expression of the electropositivity or 

 negativity of a substance in solution as referred to a hydrogen stand- 

 ard. A positive redox potential indicates a condition tending to cause 

 oxidation, and a negative potential indicates the reverse condition, 

 tending to bring about reduction. As used to describe an aquatic 

 habitat, redox is the overall summation of the redoxes of all the solutes 

 present in the water. Redox potentials have two general properties 

 that must be considered: intensity or Eli as defined above; and ca- 

 pacity or poising, which is analogous to the buffering of a pH system. 

 A high capacity reflects the tendency of the system to retain its in- 

 tensity despite minor changes in its constituents. 



In well-aerated natural waters oxygen concentration usually gov- 

 erns the redox potential and produces positive values. However, fer- 

 rous complexes or oxygen shortage may allow various inorganic and 



