mechanism, a factor which often decreases the actual toxicity of a sub- 

 stance. By this is meant a change in the toxicity of the substance owing 

 to decrease of its actual concentration in solution, or its physical and 

 chemical transformation. The decrease in the toxicity of many heavy metal 

 ions in hard water and in sea water due to formation of precipitates is 

 illustrative of this mechanism. A change in the toxicity of various 

 metals as a result of a complete or partial hydrolysis, formation of poorly 

 soluble carbonates, and precipitation from solutions having the pH value 

 far from neutral is also well known. 



Experimental data on the dependence of the degree of toxicity of 

 various substances and resistance of aquatic organisms (mostly fish) upon 

 ecological factors are long known, although these data are still not used 

 in establishing the MPC. However, a skillful use of existing information 

 when determining the MPC of a given substance would allow not only a 

 reduction of the duration of experiments by conducting them under extreme 

 conditions (high temperatures, low oxygen content, etc.), but would also 

 invest the proposed MPC with an "ecological factor of safety", i.e., with 

 due consideration of the range of fluctuations of physical and chemical 

 factors of the environment. 



Such an "ecological MPC" would guarantee the relative well-being of 

 the ecosystem as a whole, as well as its separate components. It would 

 protect the components from poisoning even under conditions of deteriora- 

 tion of the main abiotic factors which usually leads to a decrease in the 

 toxic-resistance of aquatic organisms. 



Another aspect of the so-called ecological setting of standards for 

 harmful substances in water became an object of special discussions only 

 recently. By this is meant the so-called ecological MPC designed to 

 secure cleanness or "health" of a water body as a whole, i.e., preserva- 

 tion of natural ecosystem of the water-body and not only important com- 

 mercial organisms. However, the attempts made to clarify this concept 

 lead to the conclusion that one must speak not of "ecological MPC", but of 

 ecological foundations of establishing the MPC as noted by M.M. Kamshilov, 

 "Determination of concentrations of foreign substances not disturbing 

 natural biological circulation in aquatic ecosystem". The fisheries MPC 

 are, in essence the "ecological MPC", since they must secure protection 

 from toxicants of not only fish, but also the ecosystem as a whole. 



It is quite a different matter, when we speak of search and standardi- 

 zation of the indices of "ecological well being" which are very important 

 for the estimation of the efficiency of biological standard setting rela- 

 tive to toxicants discharged into waters. The investigation of the 

 polluted water bodies of the effect of domestic and industrial wastes on 

 the ecosystems of these water-bodies has been performed by sanitary hydro- 

 biology. Aquatic toxicology, including ichthyo-toxicology was born as an 

 outgrowth of sanitary hydrobiology. 



The development of these disciplines became possible as a result of 

 the realization of the fact that no investigation and no description of 

 the changes occurring in the communities residing in polluted waters, 



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