Evaluation of the influence of pollutants upon whole populations of 

 aquatic organisms makes it easier to extrapolate the experimental data to 

 natural waters. One may expect that similar disturbances will take place 

 in nature. 



It is necessary to take into account the fact that in nature, a pollu- 

 tant acts to influence a number of other factors. The first attempt at 

 classification of these factors was made by Wuhrman and Woker (1955). An 

 improvement of this classification is offered in Table 2. 



In addition, usually not one, but a number of pollutants are present in 

 natural waters. It is generally accepted in the USSR that it is possible 

 to estimate the sum of the effects of substances as an additive function. 

 The cases of synergism and antagonism are to be accounted for only in acute 

 lethal concentrations, since they are less important under the conditions 

 of chronic action. 



Pollutants exert simultaneous action on a number of species of or- 

 ganisms. The net result depends on the relative sensitivity of these 

 species to the pollutant. Therefore, creation of a scheme of classifica- 

 tion of organisms according to their sensitivity to toxicants is directly 

 related to the question under consideration. The relation of organims to 

 saprobic pollution is well considered in various saprobity systems. The 

 relation to toxicants requires special elaboration. Here a scheme of 

 division is proposed (Table 3) accounting for analogous elaborations in the 

 USSR (Stroganov, 1971) and in the USA (Muirhead-Thomson, 1971). 



A more detailed division of organisms according to their sensitivity is 

 desirable, but difficult, for two reasons: 1) the indicated classification 

 is a generalization, and the specific position of many organisms requires 

 further clarification, since within each of the groups there are further 

 gradations of sensitivity; and 2) the sensitivity of single species to 

 various types of toxic substances is different. It is possible that future 

 considerations will require the creation of not one, but a number of such 

 systems, while more useful for classification, such an addition will make 

 the system more cumbersome. 



Thus, in analyzing the actual differences between the conditions in 

 experiments and those of natural waters, the experimental data can be used 

 with greater assurance if the analytical situation employs natural condi- 

 tions. 



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