Oxygen isotope ratios of biogenic silica derived from planktonic diatoms living in the pelagial of a freshwater lake are used to determine the temperature effect on the isotope fractionation between water and biogenic silica under ecosystem conditions. Our data show a deterministic relation between seasonally changing water temperatures (4¿C--22¿C) and the oxygen isotope fractionation during valve formation. The temperature dependent fractionation appears to be independent of diatom cell sizes indicating a mere physical control of this process. The isotopic change induced per degree centigrade, the temperature coefficient, amounts to a value of -0.2?/¿C. This implies that previous studies have overestimated the temperature relationship of this proxy by using coefficients of up to -0.5?/¿C for climate reconstructions.