Aim of the study:
The aim of the study was to evaluate changes in the concentration of selected ions in the waters of the Słupsko dam reservoir and to determine its role as a "primary sedimentation tank" for the Pławniowice reservoir.

Material and methods:
Determination of the factors causing these changes was made on the basis of measuring a number of physical and chemical parameters between March 2019 and February 2020. The research was carried out in terms of checking the potential of unfavorable phenomena for surface water and checking whether the Słupsko reservoir fulfills the assumed role, i.e. the function of the "initial settler" for the Pławniowice Reservoir.

Results and conclusions:
The Słupsko reservoir contributes only to a small extent to the equalization of the analyzed ions' concentrations . It is difficult to explain the lack of a clear relationship between sodium ion concentrations and conductivity. High concentrations of orthophosphate ions in the summer may suggest that the reservoir's canopy is enriched with this element internally in the summer. High concentrations of orthophosphates at the inflow to the reservoir may indicate the inflow of insufficiently treated municipal sewage or runoff from agricultural areas. The Słupsko reservoir plays only a marginal role as a "preliminary sedimentation tank" for the Pławniowice reservoir. The short time of water retention does not allow the removal of nutrients from it. Currently, the Słupsko reservoir is a potential source of nutrients for the Pławniowice reservoir. Therefore, it seems necessary to constantly monitor the quality of water in the reservoir.