DEGRADATION OF ORGANIC-SUBSTANCES IN RESERVOIRS

Due to the retention of water in an impoundment, the degradation of ea sily decomposable organic substances expressed as BOD (''self- purific ation'') is higher in a reservoir compared to a similar stretch of flo wing river. For the same reason, the production of new organic matter by phytoplankton (PP) is also enhanced after impoundment. Data on rese rvoirs with theoretical retention time (RT) in the range of 1 - 535 da ys 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 i ncreasing RT, decreases with increasing mean depth and depends on the quality of inflows. During 10 years' period, the relation of BOD and P P to bacterial and algal respiration has been examined in a temperate, dimictic, canyon-shaped reservoir. Total budget showed that BOD decre ase fluctuated between 70 to 88% of TI, irrespective of the increasing PP during the whole period.