Sediments, being the most important source of phosphorus (P) in the shallow (max. depth 7.2 m), polymictic Swarzędzkie Lake during the summer, were thoroughly studied in laboratory experiments with intact cores sampled at 7 stations in four seasons of the year. Under anaerobic conditions P release rates peaked in the samples from the deepest part of the lake at 26.86 mgP m^-2d^-1. However, the highest rates were determined at one of the littoral sites (near the mouth of a polluted stream) at 59.5 mgP m^-2d^-1. Littoral P release coincided with occasionally low oxygen concentrations (0.2 mgO₂ l^-1) above the bottom. On the scale of the whole lake, P release from the littoral zone, where both temperature and oxygen concentration were highly variable, had the strongest influence on the trophic state. It accounted for 63% of the annual internal load but only 55% of total lake area.