For quantitative estimation of past water temperature of four Holsteinian (MIS 11) palaeolakes from eastern Poland, the oxygen isotope palaeothermometer was applied to shells of the aquatic gastropods Viviparus diluvianus and Valvata piscinalis. The δ18O composition of their shells demonstrated the average growth-season water temperatures during the mesocratic stage of the interglacial (Ortel Królewski Lake), during its climatic optimum – the Carpinus–Abies Zone (Ossówka-Hrud, Roskosz and Szymanowo Lakes), and in the post-optimum (Szymanowo Lake). The calculation was based on δ18OShell values and the δ18OWater assumed for the Holsteinian from the modern oxygen isotope composition of precipitation and the expected amount of evaporative enrichment. The mean oxygen isotope palaeotemperatures of Ortel Królewski lake waters were in the range of 18.1–21.9°C and were uniform for the Taxus and Pinus–Larix zones. Ossówka-Hrud and Roskosz Lakes had mean temperatures of 17.4–21.0°C during the climatic optimum, whereas the temperature of Szymanowo lake waters was estimated at 20.6–21.7°C at that time. These values are concordant with the pollen-inferred July air temperatures noted during the Holsteinian in eastern Poland. Relatively high values of ~25°C in the post-optimum noted at Szymanowo were connected with the presence of a shallow and warm isolated bay indicated bypollen and mollusc records.
