Assessment of the Trophic Status by Monitoring of Reservoir’s Water Quality

Continuous long-term monitoring of aquatic systems is important for understanding their complete evolution in order to monitor changes in the trophic status and water quality. The continuous monitoring during a period of 20 years, by sampling once a month at two locations, the water quality of reservoir “Grlište”, which is used for the water supplying town Zaječar (Eastern Serbia), is observed and developmental stages in the life of the reservoir were determinated. It should be noted that the obtained results were used also in the purpose of finding a cause of cyanobacteria bloom, as a consequence of algal production. Limiting factors of algal production usually were nitrogen and phosphorus, however, in this study, obtained results of subtraction between trophic state index, calculated through total chlorophyll a (TSIChl-a) and trophic state index, calculated through total phosphorus (TSITP), indicated that limiting factor of algal production was light. On the basis of the concentrations of dissolved oxygen (DO), total phosphorus (TP) and chlorophyll a (Chl-a) in the surface and in the bottom, it was concluded that the reservoir passed through four development phases during the examined period. Results of long-term monitoring showed that in the first years after the formation of the reservoir, the highest trophicity was detected (hypereutrophic status), but later the reservoir mostly maintained eutrophic status