method for the whole area of the reservoir during the navigable period. It 

 is not high, (9600 ton C), which means 1.2% of the total organic matter 

 input. 



EXPENDITURE OF ORGANIC MATTER 



From the very beginning, a part of the organic matter is expended by 

 primary productivity, i.e., by phytoplankton. According to our data, this 

 is equal to 20% of the primary production for 24 hours incubation. When 

 analyzing the destruction of organic matter by the oxygen method in the 

 water mass, the given value is a general sum of the organic matter des- 

 troyed by all plankton organisms. Due to the low mean depth and the severe 

 wind regime, the water mass of the Rybinskiy reservoir is very well mixed 

 to the bottom, and the organic matter is mostly oxidized in aerobic condi- 

 tions by bacterial action. The average number of bacterioplankton by 

 direct counting (Fazumov, 1932) for 20 years of investigation is 1.5 mill/ 

 ml and ranged between 0.5-2.5 mill/ml in different years (Figure 3). 



By cultivating a sample of bacteria from the reservoir in sterile re- 

 servoir water, it is possible to prove that the amount of bacteria deter- 

 mined by direct count is true. In this medium, the bacteria prepared from 

 the water grow very quickly, and it is possible to determine the limits of 

 their development very easily (Romanenko, 1973b). 



The bacterial cell volumes in the water are small (0.3-0.5 y 3 ). The 

 horizontal and vertical distribution of bacterial biomass is uniform in the 

 reservoir, and the total number of bacteria changes little from one point 

 to another. Only near the shores and in the bottom layers is it a little 

 higher. Wet bacterial biomass is equal to 1-2 mg/liter of water. 



The generation time, i.e., time needed for the total number of bacteria 

 to double, fluctuated within wide limits and ranged from 5 to 10 hours for 

 individual periods. In the hottest period (July and August) with a doubling 

 in the number of bacteria, generation time is between 16-20 hours; but for 

 temperatures 5-13 C, it can be up to hundreds of hours. For 20 years of in- 

 vestigation, the mean value for the navigable period was 48 hours with temp- 

 erature fluctuations of 2 to 23 C. 



Data on heterotrophic assimilation of C0 2 was used for calculating bac- 

 terial biomass (Romanenko, 1964). Its mean value was 35 g C/m 2 or 145 x 10 3 

 ton C on the whole reservoir (Table 1). 



125 



