The study aimed at determining the changes in the trophic status of the processes associated with the risk of eutrophication in small retention reservoir in Korycin, characterized by agricultural performance of direct and indirect catchment. The study was conducted using the surface water samples that were collected systematically every month over four hydrological years (2008 to 2014) from three research points. Mean annual concentration of total phosphorus varied from 0.641 mg P/dm³ during research conducted in 2007/2008 to 0.874 mg P/dm³ in 2013/2014 showing an increasing trend from year to year. Taking into account the calculated average values, an upward trend can be seen along with particular years of the study from the annual average value of 1.44 mg N/dm³ determined in the first year to the value of 2.66 mg N/dm³ recorded in the last year of the study. It was observed during the study that in non-flowing parts of Korycin reservoir, plankton developed more abundantly than in the central fragments, where the flow of water is more intensive. A mild growth of phytoplankton in the waters of Korycin reservoir is provided by relatively low concentrations of chlorophyll “a” found during all research periods. Concentrations of chlorophyll “a” in waters of the reservoir were within the range of 4.08 to 5.21 g/dm³. At a Schindler coefficient > 2, this value, based on the Decree of the Minister of the Environment of 2011, should not exceed 7 and therefore waters of the Korycin reservoir during the research can be classified as the first class. Values of the general trophic level (Trophic State Index) during the first three years of the study ranged from 67.15 to 68.65, which enabled to count waters of the reservoir in Korycin to eutrophic ones. In the last year of the analyzes, this coefficient increased to a value of 72.43, classifying its waters as hypertrophic. The largest share in such a situation was expressed by TSI (TP), which ranged from 97.34 to 101.82, always classifying the waters to hypertrophic ones. In contrast, the indicator TSI (TN) ranged from 59.71 to 68.57 and it classified the reservoir waters to eutrophic, while TSI (Chl) to mesotrophic.