172 



ANALYSIS OF THE ENVIRONMENT 



solution of salts, particularly for marine 

 invertebrates. In the concentration in which 

 they occur in sea water, salts are about 90 

 per cent ionized. There are four principal 

 electropositive ions (cations), Ca", Mg*\ 

 K* and Na*, and four main electronega- 

 tive ones (anions) COa"", SOr", CI' and 

 Br'. Forty chemical elements, gases ex- 

 cluded, are dissolved in sea water in small- 

 er amounts; many of those in trace con- 

 centrations probably are completely ionized 

 (Sverdrup, Johnson, and Fleming, 1942). 

 Water itself is slightly dissociated into H* 

 and OH, ions (10' normal). 



The ions present exert a joint force in 

 aflFecting the osmosis of water through a 

 semipermeable membrane; in addition, they 

 often act also to produce characteristic ef- 

 fects on living organisms. Even ions 

 closely related in their physicochemical 



HYDROGEN ION CONCENTRATION 

 (pH) 



Hydrogen ion concentration as measured 

 on the pH scale runs from normal acid to 

 normal base. Some of the pertinent relations 

 between different pH values are illustrated 

 in Table 12, which makes the reciprocal 

 relations between positive H ions and nega- 

 tive OH ions apparent at a glance. 



Many pH measurements have been taken 

 in environments fairly free from biotic con- 

 ditioning as well as in those influenced by 

 plants and animals. We shall limit attention 

 to the former. Differences in the use of 

 physicochemical constants, in methods of 

 measurement, and frequent disregard of 

 errors resulting from different salinities, 

 especially with colorimetric methods, pre- 

 vent a close comparison of results reported 



Table 12. The pH Scale and Some of Its Equivalents 



Neutrality. 



properties may affect certain living proc- 

 esses differently, and quite unlike ions may 

 yield closely similar results. The relations 

 between cations and organisms are much 

 better known than are those of anions, and 

 both antagonistic and synergistic effects 

 exist (Heilbrunn, 1943). 



The end result is that the physiological 

 balance of ions in sea water, together 

 with the isotonicity of this medium for most 

 marine invertebrates, helps strongly to make 

 sea water an excellent aquatic environment 

 for many diverse plants and animals. It is 

 true that these organisms have evolved in 

 adjustment to this environment. 



by the many different people who have 

 made pH determinations. 



The pH of sea water in free contact with 

 the atmosphere varies between about 8.1 

 and 8.3. There is, in general, a decrease 

 with depth to about the region of minimum 

 dissolved oxygen (p. 192), and then an in- 

 crease in deeper water. The pH is not 

 greatly changed even in intermediate 

 depths; it reaches about 7.5 in the north 

 Pacific at a level at which the oxygen 

 content of the water is reduced to some 

 tenths of a cubic centimeter per liter 

 (Sverdrup, Johnson, and Fleming, 1942). 

 In inshore waters, particularly those of the 



