trations effects such as death or abnormalities do not increase by more 

 than 25% as compared with the control. We regard the stimulation of 

 fertility, number of the species, growth rate, and other items as desirable 

 effects of the toxicant. Therefore, such stimulation does not limit the 

 permissible concentration. From this point of view, such concentrations 

 are considered harmless to the test species and the system as a whole. It 

 follows from Table 1 that different aquatic organisms possess different 

 sensitivities to the same toxicant and different manifestations of vital 

 activity are affected in different ways. For example, the processes of re- 

 production and fertility are disturbed more quickly than mortality. As a 

 result, aquatic organisms which differ in their resistance determine the 

 wide variation of potential changes that occur in the ecosystem. 



Variable damage to aquatic organisms from different systematic groups 

 eliminates some species and increases others in the system. An ecological 

 system is unified and all of its components are interdependent. The compli- 

 cated plant-animal complex not only depends on the abiotic environment, but 

 on the biotic factors (bacteria, protozoan, algae, and higher plant groups) 

 that compose the environment. The diversity of changes in the qualitative 

 and quantitative composition of the organisms is primary a result of the 

 variable sensitivity of the aquatic organisms composing the system. 



It is evident from Figure 1 that at a salicylanil ide concentration of 1 

 mg/lite>% biological oxidation of organic substances will occur completely 

 but nitrification will occur poorly and incompletely. Consequently, basic 

 self-purification processes will be disturbed and will not be complete. 

 This in turn will create unacceptable conditions for other aquatic or- 

 ganisms. If the salicylani lide concentration were 0.1 mg/liter, then min- 

 eralization will be complete and such algae as Scenedesmus , Anabaena and 

 Elodea die, and Ceratophyllum will exist normally. Mollusca , worms, crust- 

 aceans, and fish disappear from the community. 



In a natural situation, the changes which have been described will 

 occur in a more complex manner because the toxicant will decompose or de- 

 grade to another, less toxic state, and because non-resistant individuals 

 will be eliminated. As was evident from Figure 2 (curve 2), a portion of 

 the resistant individuals survive (2-3%), and re-establish the population 

 to normal. Such population responses to the effect of a toxicant have a 

 decisive effect on the structure of the community, and the change in its 

 structure with time. This reaction, as is evident, occurs because in- 

 dividuals have varying sensitivities. Apparent differences in degree of re- 

 action of different aquatic organisms are relative and time-dependent in 

 nature. 



By comparing the sensitivity of aquatic organisms of varying systematic 

 groups, one can see that bacteria and algae are less sensitive than fish 

 and Daphnia . However, the magnitude of the difference also depends on the 

 chemical nature of the toxicant. For example, the algae Scenedesmus and 

 Anabaena , are more sensitive to salicylanil ide than the bacteria Nitroso - 

 monas and Nitrobacter , while, conversely, the bacteria are more sensitive 

 to 8-oxyquinoline. We tested several dozen substances according to the 



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