Malacofauna of the Holsteinian lake deposits at Hrud II (Eastern Poland) and its palaeoecological significance

Mollusc-bearing deposits at Hrud II (Eastern Poland) accumulated in the western part of a palaeolake of Holsteinian age (MIS 11). The faunal assemblage is typical of freshwater environments and presumably represents a part of the interglacial climatic optimum (Carpinus–Abies Zone). On the basis of its varied composition, the evolution of the water body is described. Reinterpretation of the data from the eastern part of the lake (Hrud I) and combining of the results from both sites enable a wider palaeoecological reconstruction. Fluctuations in lake level, water dynamics and changes in the aquatic vegetation are inferred from quantitative relations between selected molluscs, especially Lithoglyphus jahni, Valvata piscinalis and Bithynia tentaculata, which prevail in the deposits investigated. A predominance of Valvata piscinalis indicates a rise in water level, whereas the communities with abundant Bithynia opercula are typical of a more shallow lake with rich reed fields. Amelioration of the trophic conditions between the pre-optimal and optimal part of the Holsteinian Interglacial can also be recognised.