Studies carried out in the hydrological year 2006 aimed at investigating the impact that a storage reservoir, built on a watercourse flowing into a lake, makes on the phosphorus inflow from an agricultural catchment to the lake. Hydrological measurements were made once a week on the watercourse, and water samples for laboratory analyses were collected at two-month intervals at the sites of flow measurement. It was found that the mean concentrations of total phosphorus in the outflow from the catchment depended on the pattern and intensity of land use, and were higher in a drained agricultural catchment than in an afforested catchment. The total phosphorus concentrations were lower in the growing period than outside of this season. The phosphorus load carried away from the unit area of catchment depended on the land use pattern, being higher in the drained agricultural catchment 0.18 kg P· ha^–1 · year^–1) and lower in the afforested catchment (0.12 kg P· ha^–1 · year^–1). The load of phosphorus delivered from the catchment per unit area of the reservoir surface (0.5 g · m^–2) is almost 4 times heavier than the maximum allowable value, which results in accelerated eutrophication and accumulation processes. The reservoir forms a barrier that traps the phosphorus compounds running off from the catchment. The amount of P_tot accumulated during the year 2006 in the reservoir basin was 21.5 kg (i.e. 0.86 kg P_tot · ha^–1), with most of it being retained in the growing period.

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