diagram indicated in Table 1. With rare exception, the bacteria which take 

 part in the decomposition of an organic substance are more resistant to 

 toxicants than invertebrate animals and fish. Algae, as a rule, occupy an 

 intermediate position. Consequently, these empirical data make it possible 

 to construct a diagram to search for the weakest link in the functional 

 cycles of materials in the ecosystem, and provide for a means to forecast 

 changes in the structure of the ecosystem and the relationship of species 

 in the community. Various concentrations of phenylmercuric acetate cause 

 some groups of organisms to drop out. At 0.0005 mg/liter the vital pro- 

 cesses of all aquatic organisms will occur normally (see Table 1) and the 

 water can be considered acceptable biologically. If mercury should accumu- 

 late in commercially important organisms and damage their commercial qual- 

 ity, then the economic norm would be disturbed, but not the biological norm. 

 In this case, additional analysis should be conducted to limit the concen- 

 tration of the substance in food organisms. At 0.01 mg/liter the processes 

 of self-purification will occur normally, but some algae and crustaceans 

 such as Daphnia will die and the growth of some fish (trout fry) will be 

 poor. At 0.1 mg/liter the processes of self-purification will be disturbed 

 but the second phase of nitrification occurs. Some algae, macrophytes, 

 worms and crustaceans die, fish feeding is disturbed, and fish fry die. At 

 1 mg/liter all the organisms indicated in the table die, and only the sapro- 

 phytic microorganisms which carry out biological oxidation of the organic 

 material remain. 



The diagram of sensitivity of aquatic organisms of different systematic 

 groups to a toxicant provides a scientific basis for predicting potential 

 changes in communities exposed to toxicants at different concentrations. 

 By revealing the weak links in the community of aquatic organisms in the 

 food chain, such as through the effect of blue-green algae metabolites, it 

 is possible to foresee the nature of the structural rearrangements of the 

 community. 



The means we proposed for simulation of the potential changes in the 

 ecosystem of a reservoir with pollution cannot be without error. Like any 

 model, the proposed simulation has an approximate nature, and the natural 

 situation may be different. But we think that the proposed diagram of simu- 

 lation is more complete and reliable than those proposed by other re- 

 searchers to judge the potential changes based on the reaction of one or 

 two organisms, or on biophysical bases. The simulation has two weaknesses. 



1. In a natural water, several dozen, and sometimes even hundreds, of 

 species of aquatic organisms co-exist. We conduct tests only on representa- 

 tive species, i.e., predominant ones and species having commercial signifi- 

 cance. The rest of the species, a quantitatively larger group but less 

 dominant, does not determine the nature of the community. With a change in 

 conditions, such as the appearance of a toxicant, some dominant species can 

 disappear and less dominant species increase in numbers. 



2. We analyzed the toxicity of only one substance, but in natural 

 water under present conditions, several toxicants act simultaneously. 

 Mutual intensification or weakening of their effect is possible. We think 



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