HOLOCENE EVOLUTION OF THE PEAT-BOG IN SŁUPIAŃSKA BAY (WIGRY LAKE, NE POLAND) IN THE LIGHT OF MALACOLOGICAL ANALYSIS

Three types of molluscan assemblages have been identified in a profile of calcareous gyttja and peat penetrated by drilling within the raised peat-bog of the Słupiańska Bay. Numerous shells of molluscs have been identified within the 5.35 m thick intervals of fine detritus of the calcareous gyttja. The shells belonged primarily to the bivalves and water snails. Terrigenous species appeared only rarely and have been more frequent in the upper part of the penetrated interval. The three distinguished molluscan assemblages have been described and interpreted in terms of representing different environmental conditions. Assemblage with Gyraulus laevis that occurs within the lower part of the interval consists of the cold-loving forms representing early Holocene. Assemblage with Gyraulus albus that occurs in the middle part of the interval consists of warm-loving species linked to the Atlantic Phase. Assemblage with terrigenous species, mainly snails, occurs in the upper part of the interval. The distinguished types of molluscan assemblages are composed of species characteristic of littoral habitat of ephemeral lakes and some of terrestrial habitats. Lake-level fluctuations are expressed by changing of molluscan fauna. Variable proportions of Bithynia tentaculata shells and opercula indicates low-level water stages of the Wigry Lake that took place during the Boreal Phase, the middle part of the Atlantic Phase and during the termination of the Sub Boreal and Sub Atlantic phases. The completion of gyttja accumulation and the beginning of peat deposition are associated with the latter period.