The object of the study was Tyrsko Lake (area 18.6 ha, max. depth 30.4 m), located in the western part of Olsztyn (Olsztyn Lakeland). This lake is one of the clear water lakes in Olsztyn, but the progressive deterioration of water quality has been observed during recent years. The phosphorus concentration in the water-sediment interface, phosphorus fractions quantity and the amount of components which can bind this element was investigated in the upper (0–5 cm) layer of deposits. The aim of study was to analyze the potential influence of bottom sediment on the lake water quality. The obtained results revealed that the bottom sediment of Tyrsko Lake can be classified as mixed, silica-organic type, with a relatively high content of iron (over 4% Fe in d.w.). The total phosphorus content was ca. 3.5 mg P g,-1 d.w. on average. Phosphorus in the bottom sediment was bound mainly with organic matter (NaOH-nrP fraction), which had over 50% share in TP. Highly mobile fractions (NH4-Cl-P and BD-P) together only included ca. 5% to 7% TP. The obtained results show that the bottom sediment of Tyrsko Lake can bind phosphorus quite effectively. The calculated internal mineral phosphorus loading during summer stagnation period was 10.9 kg and it was lower that the assessed annual external phosphorus load (22.6 kg). The assessed annual phosphorus loading from both sources was still lower than the critical load according to Vollenweider criteria. However, due to the fact that the internal loading phenomenon is occurring in the lake, it should be taken into consideration that the lake water quality can deteriorate gradually during the longer time perspective. These findings should be accounted for in the future if the potential protection and restoration procedures will be developed.