it is correct to sum the toxicity of toxicants. The occurrence of anta- 

 gonism and synergism is not encountered frequently, and can be disregarded. 

 It is difficult to predict the hydrochemical situation into which the toxi- 

 cant enter. Its effect on the aquatic organisms will depend on tempera- 

 ture, dissolved 2 , pH, total hardness and other indicators. Prediction is 

 facilitated by the inclusion of changes in the toxicity of toxicants 

 (metals, pesticides, etc.) caused by a series of environmental factors. 

 Thus, for example, a reduction in temperature usually lowers the toxicity 

 of the substance; a decrease in the environment's pH increases the toxi- 

 city of metals; and an increase in the water's hardness leads to a reduction 

 in toxicity. One can also list other factors which have an effect on the 

 final toxicity. We should not be discouraged by the difficulties in apply- 

 ing laboratory tests to forecast changes in the ecosytem when exposed to 

 toxicants. For specific water bodies perhaps it will be necessary to 

 change the experimental organisms. For examination of aquatic ecosystems, 

 considerable difficulty arises in finding major components which determine 

 its behavior. Simplifying the approach to solving the problem, I selected 

 basic functional blocks in the cycle of materials and the energy flow in the 

 system. They were known long ago, and have not been disputed. However, we 

 know little about the links within the blocks, and they often appear as 

 "black boxes", about which we judge based on what comes in and goes out. 

 For better predictions it would be desirable to know more. 



The matter of the stability of the system and the mechanisms of its reg- 

 ulation arises in connection with predicting potential changes in the eco- 

 system. It seemed correct to me to single out the two major aspects in 

 this matter--the abiotic environment and the organisms. The stability of 

 the ecosystem is determine primarily by the stability of the environment. 

 The introduction of toxicants (pollutants) changes the aquatic organism's 

 living environment and, as a result, a change in the aquatic community 

 occurs. Specific diversity is primarily determined, apart from the histori- 

 cally established conditions, by hydrochemical and hydrological conditions. 

 However, the organisms of the functional groups can maintain the stability 

 of the system within known limits. This is demonstrated especially well 

 when some species increased their numbers or when a new species appears. 

 But the abiotic environment always plays the largest role in the stability 

 of the ecosystem. Changes in this environment often have a decisive effect 

 on the nature of the links in the system. The different relationships of 

 aquatic organisms to the environment, which contains toxicants, is the 

 basic relationship. 



TABLE 1. COMPARISON OF HARMLESS CONCENTRATIONS OF SUBSTANCES 



(MG/LITER) 



Organisms Indicators Toxicants 



12 3 4 



BOD 1 50 20 50 5 



Bacteria NO formation 0.5 10 0.01 0.1 



NO formation 0.5 15 0.01 0.01 



158 



