Composition of Natural Waters 91 



organism and the outside world is necessary. The crucial need for a 

 proper water balance is demonstrated by the consequences of even 

 small water losses in some instances. During starvation, for example, 

 man may lose as much as 40 per cent of his body weight including 

 half of the proteins and nearly all the glycogen and fat without serious 

 danger, but if 10 per cent of the water content of the human body 

 is lost, serious disorders result. If as much as 20 per cent of the 

 water is lost, death follows. 



The concentration of water divides the environment into aquatic 

 and terrestrial habitats. At first sight, one might say that more than 

 enough water exists in the aquatic environment and that water is a 

 problem only on land. Closer scrutiny shows, however, that water 

 is tending to enter or to leave the organism too fast in almost every 

 situation. In certain terrestrial habitats the water supply may be 

 excessive for many organisms, as in some tropical rain forests where 

 the air is often 100 per cent saturated and the ground is completely 

 permeated. A visitor to such a habitat sees moisture condensing on 

 every surface and hears the steady dripping of water from the 

 vegetation. 



In the ocean in spite of the thousands of cubic miles of sea water 

 a scarcity of water would nevertheless exist for many plants and 

 animals! For these organisms the concentration of water is too low 

 relative to the abundance of salt for proper osmotic equilibrium. 

 Fresh-water environments, viewed similarly in relation to osmotic 

 balance, tend to contain too much water. This contrast between 

 marine and inland aquatic situations is, in a sense, analogous to the 

 difference between dry and humid climates on land. The mainte- 

 nance of the proper water balance is thus a problem that must be 

 considered in all habitats for all types of organisms. 



WATER PROBLEM IN THE AQUATIC ENVIRONMENT 



Since the water in natural environments contains a varying amount 

 of dissolved materials and usually a different amount from the fluids 

 of the organism, the resulting differences in osmotic pressure raise a 

 problem in regard to water exchange. In order to determine the 

 tendency of water to enter or to leave the organism we must know 

 something of the composition of the surrounding medium. 



Composition of Natural Waters 



Representative values for the amounts of the more numerous ions 

 found in sea water and in the water of ponds and lakes are shown in 



