Due to the retention of water in an impoundment, the degradation of easily decomposable organic substances expressed as BOD (“self- purification”) is higher in a reservoir compared to a similar stretch of flowing river. For the same reason, the production of new organic matter by phytoplankton (PP) is also enhanced after impoundment. Data on reservoirs with theoretical retention time (RT) in the range of 1 - 535 days showed that PP reached 9 - 99% of the total input (TI) expressed as BOD (TI = PP + BOD load by inflows). The share of PP increases with increasing RT, decreases with increasing mean depth and depends on the quality of inflows. During 10 years' period, the relation of BOD and PP to bacterial and algal respiration has been examined in a temperate, dimictic, canyon-shaped reservoir. Total budget showed that BOD decrease fluctuated between 70 to 88% of TI, irrespective of the increasing PP during the whole period.