aquatic invertebrates and fish are still more sensitive than the nitrifiers. 

 Therefore, we can evaluate water on the basis of the line of sufficient 

 self-purification. More complex analysis, than is currently used, is re- 

 quired. We must also include toxicologic testing. 



Certain toxic substances do not break down (e.g., metals) or break down 

 poorly (some pesticides, detergents, etc.). In these cases, toxicologic 

 testing will reveal their presence above impermissible concentrations. 

 Chemical analysis is important and necessary for an overall description of 

 the quality of water, but the indexes of self-purification and toxicity 

 reflect another aspect, yery important for the course of normal hydro- 

 biologic processes. 



3. The principle of assurance of conditions of life for commercial 

 species. This principle falls entirely in the area of human evaluation. In 

 addition to pure water of good quality, man also needs biologic resources 

 found in reservoirs, particularly commercial species as a source of food and 

 industrial raw materials. Valuable commercial organisms react sensitively 

 to chemical pollution. They decrease their population or disappear as a re- 

 sult of death and migration to other water areas. Assurance of the condi- 

 tions of life means the presence of water of a quality such that commercial 

 species can continue to exist throughout their entire life cycle and do not 

 lose their valuable qualities (growth rate, fertility, maintenance of high 

 population, nonaccumulation of substances harmful to man, e.g., metals, 

 pesticides, hydrocarbons, detergents, etc.). Chemical pollution may have 

 both a direct effect on commercial organisms and an indirect effect through 

 their food and the water in which they live. 



It might be thought that, if the second principle is fulfilled, the 

 third is not needed. However, the problem is more complex. Valuable com- 

 mercial organisms and their sources of food are more sensitive than microor- 

 ganisms participating in the decomposition of organic matter in water. 

 Therefore, even if the second principle is fulfilled, though it is quite 

 important, it is not sufficient to assure the third. 



The qualitative and quantitative characteristics of this third principle 

 are: the specific composition of commercial species, their population and 

 biomass, ichthyofauna and the dimensions of the catch. Usually, the catch 

 of aquatic organisms is the first sign of deterioration in water quality for 

 commercial species, at a level at which the processes of self-purification 

 reflect no danger. 



At the present time, the importance of this principle is great, since 

 the catch of aquatic organisms will become increasingly concentrated in in- 

 land bodies of water and the littoral waters of the oceans and seas in the 

 near future. Hydrobiologic analysis encompasses essentially the entire 

 ecologic system and catch and, therefore, most completely characterizes a 

 given ecosystem with respect to its suitability for effective and complete 

 utilization in the national economy. 



4. The principle of suitability of water for drinking. The estimation 

 of the suitability of water is usually performed by sanitary organizations. 



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