a body of water, and the more diverse this life quantitatively and qualita- 

 tively, the slower is the transition from normalcy to pathology. This sug- 

 gests that eutrophic systems should be less liable to the effects of toxic 

 substances than oligotrophic and distrophic ones. In this connection the 

 unstudied problems of toxicity criteria (normalcy and pathology) at the 

 sapra-organism level of life organization arise. The difficulty of their 

 formulation lies in the fact that the scientific fundamentals of functional 

 community studies are not established, and the present knowledge of commu- 

 nity structure is mainly the knowledge of morphology, composition, quantity, 

 biomass, occurrence, and various indices or relationship between the major 

 components in the structure. It concerns planktonic as well as bottom com- 

 munities, and also the other less studied group of aquatic animals. 



Nevertheless, even the morphological approach and related experimental 

 investigations permits discovery of some of the specific features of commu- 

 nity reactions to toxic effects. To understand these reactions, it is 

 necessary to use the concepts of dominant, subdominant, and "shelf" forms. 

 The results of ecological investigations show that in ecosystems not in- 

 fluenced extensively by man, the structure of communities and the character 

 of seasonal changes are rather stable, and may be of the same type over a 

 period of many years. In waters polluted by toxic substances, or in eco- 

 systems under conditions of experimental influence, characteristic features 

 become visible, including a shift of the dominant forms. Occasionally, 

 shifts are very abrupt and conditioned by the fact that the dominate forms 

 are inhibited or eliminated completely, whereas forms of minor importance 

 reach the maximum of abundance and biomass (Braginsky 1975; Braginsky, et 

 al. 1979). The shift of other community components may be observed, anB~ 

 tFese changes occur spasmodically as well as slowly in accordance with the 

 degree of toxic effect, toxicant concentration, selectivity of action, com- 

 munity specific composition, and many other factors. Moreover, there is a 

 change in total numbers, and in biomass of organisms, as well as an exchange 

 of roles in the structural components of biocenosis, i.e., a change in 

 hierarchical relationships. Under the influence of \/ery strong toxicants, 

 the community may be completely destroyed, and then the system becomes non- 

 structural. Apparently, the latter may be considered as an indicator of ob- 

 vious pathology, whereas the shift of dominant forms is not a pathological 

 process, but represents a form of community stabilization under new condi- 

 tions. The second case is the typical manifestation of degradation, the 

 mechanism of which has been studied in detail (Stroganov 1974). 



Experimental investigations and mathematical modeling had demonstrated 

 that aquatic communities, generally speaking, may exist in three stable 

 states: 1) initial, 2) functionally and structurally reversibly altered, 

 and 3) irreversibly altered. The second level of change is characterized as 

 ecological fluctuation, the third as a shift of dominant forms. These do 

 not represent pathology, but simply the normal range of community vari- 

 ability related to adaptational changes. Apparently, "pathology" begins 

 when the system passed the third level of stability and approaches the non- 

 structural level. In mathematical models this process is shown by a para- 

 bola and indicates the approach of ecological catastrophe. 



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