Man-made lakes cause direct and indirect changes in environmental conditions downstream of the water reservoir. Management must consider these water quality and environmental effects as well as uses of the river below the water reservoir.

Eutrophication is one of the main problems of water deterioration in accumulation lakes. Phosphorus is considered as the key factor for eutrophication of lacustrine water. Its concentration in lakes originates from external input and the emission from the sediment. It was necessary to determine the substrates of phosphorus by sequential extraction in order to determine the dynamics and bioavailability of phosphorus. On the basis of results obtained the proposals for the preparation of land were made in order to minimise the input of phosphorus.

Water samples for chemical and microbiological analyses as well as material of the water community to be considered in the assessment of qualitative and quantitative composition of phyto- and zooplankton, and bottom fauna were take on 30 May 2001 at two profiles. Judging by the most of the analyzed chemical parameters, with the exception of BOD5 and oxygen saturation values indicating class 2, water of the Bovan lake accumulation belongs to the 1st class of water quality. According to the presence of heterotrophic bacteria, water was characterized as class 1-2 (after Kohl). Also, 6 groups of phyto- and 4 groups of zooplankton were recorded. Relative abundance and qualitative composition values of plankton show that saprobic index ranged from 1.54 to 1.98 pointing out oligo-beta, namely beta-mesosaprobic character of the accumulation. Saprobic index when oligochaetes were considered amounted up to 3.6 (alfa-mesosaprobic water).

According to the Programme of systematic examination of surface water quality of the Republic Hydrometeorological Service of Serbia the examinations of Medjuvrsje water reservoir have been carried out during October 2002 and August 2003. The examinations have been carried out at the dam, in central part and at the entrance of the water reservoir, on three depths. This paper shows the results on phytoplankton composition, chemicophysical properties and the trophic status of water reservoir. The qualitative analysis of the phytoplankton shows the presence of 96 taxa from six divisions of algae. Following the standard of Swedish Agency for Environmental Protection, the high concentrations of primary nutrients belong to the fifth class, by the categorization. According to OECD classification trophic status of water reservoir may be characterized as eutrophic and hypereutrophic.