Oxygen isotope ratios (δ¹⁸ O) of deep-lake ostracod valves can provide excellent estimates of the oxygen isotopic composition of past atmospheric precipitation (δ¹⁸ O _P ). One of the major steps of such a δ¹⁸ O _P reconstruction is the calculation of the δ¹⁸ O of the lake water (δ¹⁸ O _L ) from the valve δ¹⁸ O, which includes an appropriate estimate of the past water temperature during valve formation. In order to simulate changes in lake vertical temperature profile and specifically the bottom water temperature during ostracod valve formation, we have redeveloped a one-dimensional eddy diffusion thermal lake model including a lake ice model and refined it by including a seasonal light extinction model. This model was tested for two different sites, Lac d'Annecy and Ammersee. Sensitivity studies show that the ostracod calcification temperature is approaching constant 3.7°C for colder, but may significantly vary for warmer than present conditions. In Lac d'Annecy, such temperature effects can lead to a significant underestimation of positive δ¹⁸ O _P excursions. In Ammersee the underestimation is smaller because the hypolimnic temperatures are less sensitive to surface temperature changes. Our model can be used to better constrain such temperature effects and such further increase the quality of the δ¹⁸ O _L and δ¹⁸ O _P reconstruction.