Dissolved materials 



fresh water 

 organisms 



Water devoid of dissolved materials is intoler- 

 able in nature because pure water will not support 

 aquatic life. Natural waters contain endless varie- 

 ties of dissolved materials in concentrations that 

 differ widely from one locality to another as well 

 as from time to time. Many of these dissolved ma- 

 terials are essential for growth, reproduction, and 

 the general well-being of aquatic organisms. The 

 chlorides, carbonates, and silicates of sodium, po- 

 tassium, calcium, and magnesium are generally 

 the most common salts present. Traces of most 

 other essential substances are also found. 



Aquatic organisms live in different concentra- 

 tions of dissolved substances but productivity de- 

 clines as the concentrations move away from the 

 optimum. Seldom, if ever, are the dissolved sub- 

 stances at the optimum concentrations as we know 

 them. The range of tolerance may be relatively 

 wide, but when the concentrations reach too low or 

 too high a level, organisms degenerate and die. 

 Different organisms vary in their optimum require- 

 ments as well as in their ability to live and thrive 

 under variations from the optimum. Some orga- 

 nisms are equally at home in sea water and in 

 fresh water. Other organisms will tolerate only one 

 or the other. 



Any of the substances necessary to aquatic or- 

 ganisms has a range of concentration that is both 

 essential and tolerable. The tolerance levels for 

 any one substance vary depending on the concen- 

 trations of other substances present. The presence 

 of certain substances synergizes the effects of some 

 materials but antagonizes the effects of others. 

 Under optimal concentrations, the synergistic and 

 antagonistic effects are in balance and relatively 

 high concentrations can be tolerated without ad- 

 verse effects. 



Although several measures of dissolved mate- 

 rials are available, no measure in itself is adequate 

 as an index of optimum concentration nor is any 

 single measure adequate to express the range of 

 tolerance. The biological effects depend on the 

 concentrations of the individual solutes, some of 

 which are tolerated in terms of grams per liter but 

 others only in nanograms per liter. Some exert con- 

 siderable osmotic pressure, but for others the 

 osmotic effect is negligible. Some substances con- 

 tribute greatly to conductivity, while others have 

 little or no effect. 



In general, the concentrations of dissolved ma- 

 terials in natural fresh waters are below the opti- 

 mum for maximum productivity. In many in- 

 stances, therefore, the addition of any of a large 



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