The link between functional groups is expressed diagrammatical ly in 

 Figure 1 using only the most general aspects that occur in the water body. 



The following considerations are relevant to the interrelations in ques- 

 tion: 



1. The primary organic material (algae, macrophytes) produced is al- 

 most entirely transformed into a "new organic material", which is partially 

 utilized by man in the form of commercial animals (fish, crayfish, Mollusca , 

 etc.). 



2. Organisms which have died, as well as food residues, are destroyed 

 by bacteria, fungi, and protozoans into simple organic and mineral 

 materials, which enter again into the cycle of materials in the reservoir 

 as nutrients for algae, macrophytes, etc. 



3. A portion of the organic material of animal and plant origin does 

 not mineralize and is deposited in the bottom sediment. Man, using a given 

 water body, is interested in high quality water and commercial organisms of 

 good quality. If a given system of interrelationships is in equilibrium, 

 then nearly all of the organic material is transformed and little forms 

 bottom residues. Such a situation exists in oligotrophic reservoirs. The 

 introduction of toxicants into such a reservoir sharply changes the rela- 

 tionship between functional groups because the species have both low and 

 high sensitivity. Some increase in number, others disappear or decrease, 

 and still others remain in their previous state (Figure 2). 



In each functional group, there are several dozens of species in un- 

 equal numbers. Usually, 2-5 species are dominant in numbers and biomass 

 and the rest are supplementary species thst play a small or negligible role 

 in the interrelationships between aquatic organisms. With a change in the 

 environment as a result of pollution, the relationship between species 

 changes depending on their sensitivity. The resistant species increase and 

 achieve dominance (the development of blue-green algae, inferior fish, 

 etc.). The new relationship of species affects water quality and commer- 

 cial species. As a rule, this change is less desirable for man's uses. 



One can, with sufficient reliability, identify the physiological basis 

 of the change in relationship of species by means of laboratory bioassays 

 of the sensitivity of the principal aquatic organisms which cycle materials 

 in the water body. Based on these data, the weak link in the chain of 

 transformations and interrelationships during exposure to toxic substances 

 can be identified. 



Given in Table 1 are data on the sensitivity of various aquatic organ- 

 isms resulting from bioassays of toxicants. Shown in the table are the per- 

 missible, i.e., almost harmless, concentrations of the toxicants in water 

 based on the aquatic organism's sensitivity. The numerical data given in 

 the table reflect both the no-effect and permissible concentrations of the 

 toxicants in the water body at which vital life processes are possible. In 

 accordance with the accepted method (N.S. Stroganov, 1971), at these concen- 



153 



