Composition of Natural Waters 93 



Since osmotic pressure is determined by the total concentration of 

 molecules and ions in solution, values for the aquatic medium will 

 vary according to the total salt content of the water body ( Table 5 ) . 

 Dissolved materials in soft fresh water vary in amount from prac- 

 tically zero to about 65 parts per million. Lakes and ponds with 

 hard water exhibit a variety of higher values for salt content, but 

 the value given in the table, 300 ppm, is representative. Salt lakes 

 may contain exceptionally large amounts of dissolved substances. 



In contrast to the extreme variability in the salt content of inland 

 waters, the open ocean tends to remain highly uniform in salinity 

 over huge areas (Fig. 4.1). A fish could swim up the middle of the 

 Atlantic Ocean, for example, from the Cape of Good Hope almost 



Fig. 4.1. The surface salinity of the Atlantic Ocean. 



