introduction 



DURING the course of geologic time, orga- 

 nisms which were able to adapt so they were 

 better fitted to live under existing environmental 

 conditions were the ones which survived and now 

 form the biota. Geologic change is a slow process 

 and biota developed which were adapted not only 

 to the physical and chemical but also to the bio- 

 logical factors of the environment. The environ- 

 mental factors to which organisms adapted through 

 the evolutionary process are now their environ- 

 mental requirements. Therefore, any relatively 

 rapid change in these conditions can be detri- 

 mental or even disastrous. Because the biota is the 

 result of long evolutionary processes during which 

 delicate balances were established, a change in 

 conditions or in a portion of the biota can have 

 far reaching effects. 



Man has now attained the ability to alter 

 drastically his environment and that of other 

 organisms. Many of his activities already have im- 

 paired seriously his own environment and that of 

 other living things. Water pollution engineering 

 works and other changes that modify the aquatic 

 environment rank high in causmg detrimental 

 effects. 



Water pollutants may be harmful through alter- 

 ations m natural environmental conditions (such 

 as temperature, dissolved oxygen, pH, carbonates, 

 etc.), through physiological and other changes due 

 to the addition of toxicants, or through both. Thus, 

 in determining the effects of pollutants we must 

 consider environmental, physiological, and ac- 

 cumulative effects. 



Substances in suspension and solution, whether 

 solid, liquid, or gas, largely determine the quality 

 of the water. Aquatic organisms are affected not 

 only directly by these materials, but also indirectly 

 through their effects on other forms of aquatic life 

 which comprise their food, competitors, and pred- 

 ators. Hence, the determination of water quality 

 requirements for aquatic life is a very involved 

 task. The problem is further complicated by the 

 fact that different species and different develop- 

 mental or life stages of the same or different 

 species may differ widely in their sensitivity or 

 tolerance to different materials, to ranges in en- 

 vironmental conditions, and to the cumulative 

 synergistic and antagonistic effects of toxicants. 

 In determining water quality requirements for 

 aquatic Ufe and wildlife, it is essential to recognize 

 that there are not only acute and chronic toxic 

 levels but also tolerable, favorable, and essential 

 levels of dissolved materials. Lethal, tolerable, and 

 favorable levels and conditions may be ascertained 

 by: (1) determining the environmental factors and 

 concentrations of materials which are favorable in 



29 



