2 - with filamentous algae ( mougeotia genuflexa ) and 



oligochaets ( Lumbriculus variegatus ). 



3 - with duckweed ( Lemna trisulca ) . 



4 - with elodea ( Elodea canadensis ) . 



5 - with elodea, duckweed and molluscs ( Limnea 



stagnalis and Planorbis sp.). 



6 - with the same content as in 5. 



It is quite natural that some microscopic organisms were introduced to- 

 gether with the macrocomponents (Elodea, duckweed, molluscs, oligochaets) 

 and with the water and sand. 



Phenol was added to the five aquaria first in doses of 1 mg/liter (1st 

 period - 59 days), then in doses of 5 mg/liter (2nd period - 112 days), and 

 then in doses of 10 mg/liter (3rd period - 194 days). Altogether, 2388 mg 

 of phenol per liter of medium were added to all the experimental aquaria 

 during the three periods (365 days). A year after the beginning of the ex- 

 periment, the addition of phenol was stopped (4th period - 140 days). 

 Aquarium #5 was the control; phenol was not added to it. 



The concentration of phenol was systematically measured by pyramidon 

 method (Kaplin and Fesenko, 1962). The contents of nitrogen, phosphorus, 

 oxygen, pH and B0D 5 in the medium were determined less regularly. The 

 bacterial population of the aquaria, especially the number of saprophytic 

 bacteria decomposing phenol, was constantly controlled. The numbers and 

 species composition of colorless flagellates, infusoria, algae and fungi 

 were also determined. 



As seen in Figure 3 which presents data on accumulation and decomposi- 

 tion of phenol, the destruction of this toxicant takes place faster in the 

 aquarium having the most diverse composition (5) than in other aquaria. 



The results of the second section of the research allows us to draw the 

 following conclusion. 



The rate of phenol destruction is, given other similar conditions, a 

 function of the diversity of the community taking part in the destruction 

 process. The living population of a water-body is able to cope with exter- 

 nal disturbances, acting as a self-regulating system only if it is diverse 

 enough. The basis of the self-regulation is biotic circulation, i.e., the 

 same processes which guarantee the yearly repeated cycles of biotic produc- 

 tion. The presence of oxygen and nutrients (nitrogen salts and phosphorus), 

 being a very important factor in the destruction of phenol, is by itself 

 not sufficient to guarantee its effectiveness. A very energetic decomposi- 

 tion of the toxicant may occur at relatively low values of these factors. 



190 



