It is generally claimed that surface runoff in forested catchments is minimal and therefore nutrient fluxes via runoff are unimportant. However, significant catchment slope inclination may promote overland water flow and, therefore, surface runoff might be responsible for transferring important nutrient loads to the lake. It was found that surface runoff waters within the catchment of Piaseczno Małe Lake have low pH (4.7-5.6), brown colour (max. 240 mgPt·dm^-3), and high concentrations of dissolved organic carbon (max. ca 100 mgC·dm^-3) – a major component of humic acids. Moreover, considerably high concentrations of biogenic substances were noted. Compared to beech-growing areas, surface runoff from pine forests contained higher concentrations of organic carbon, but lower concentrations of biogenic substances – differences were statistically significant. Similar tendencies were observed in the laboratory experiment. Moreover, the release of chemical substances from forest litter was found to be rapid. The most significant increase in nitrogen and phosphorus ions concentration and water colour and a decrease of pH were observed after the first hour of the experiment. Fluctuations of the colour and pH of the lake water (with the most intense brown colour and the lowest pH of the lake water observed in the periods of the highest rainfall and surface runoffs) indicate a significant role of surface runoff in the determination of the dystrophic condition of Piaseczno Małe Lake.