1. To investigate, as far as possible, the influence of tested 

 substances on the whole life-cycle of aquatic organisms, or 

 on its most susceptible stages. 



Z. To conduct observations for the entire period of a complete 

 biological cycle (for crustaceans not less than 15 days), 

 or of its separate stages. 



3. To direct attention to the influence of toxic substances 

 not only on survival, but also on the main physiological 

 functions of an organism (heart beat, reproduction, breath- 

 ing). 



4. To use in the experiment various organisms, since different 

 organisms respond to the action of substances in different 

 ways. 



5. To fix permissible levels in waters according to the weakest 

 biological link. 



Later, all the indices of the action of substances on organisms were 

 subdivided into two categories, principal and supplemental. The princi- 

 pal category describes those characteristics of the existance of popula- 

 tions of organisms under natural conditions which are well understood, 

 i.e., rate of growth, reproduction, death. Supplementary indices are 

 used to clarify the character of action of a given substance. The supple- 

 mentary indices are very important in order to establish the causes of 

 death of organisms in natural waters. 



For deeper understanding of the character of action of a substance, 

 it is necessary to perform experiments on more than one species of aquatic 

 organism. When choosing test-organisms their role in the circulation of 

 substances and their relative sensitivity to pollutants was considered. 

 The organisms are divided into 3-4 groups according to their relative 

 sensitivity to toxic substances (Lesnikov, 1968, 1969, 1975; Stroganov, 

 1971). Organisms primarily from the first two groups (the most sensitive 

 organisms) are taken for the experiments. 



In the USSR, a set of tests is used to study the influence of hydro- 

 chemical indices of water (gaseous and ionic composition of water, pH, 

 content of organic substances, dynamics of nitrogen compounds, time of de- 

 composition of pollutant) on: 



1. Producers: Sce.ne.d(LAmuA quadiicauda . 



Z. Consumers: 



Zooplankton 



Zoobenthos 



Fish 



Vaplmia magna and V. longiA>pina. 



China noma* donAalAA, GammaAui puZzx. 



Salmo i/udcuA, Cofie.goyiu6 poind and 

 others, (parallel experiments of 

 influence on eggs, larvae, less 

 than 1 year age class and in some 

 instances, on yearlings). 



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