Long-term changes in the occurrence of unionid bivalves in a eutrophic lake

摘要: Earlier studies (1972–1987) on the occurrence of unionid bivalves in eutrophicated Lake Mikolajskie (Masurian Lakeland, NE. Poland) were continued 1992–2008. Of five recorded species (Unio tumidus, U. pictorum, Anodonta cygnea, A. anatina, and Pseudanodonta complanata), cygnea P. complanata disappeared by 1987, whereas pictorum was observed sporadically at that time, zone markedly reduced. Beginning 1992, a pronounced increase abundance tumidus observed, since 2002–2003, decline other two species. Since 2003 maximal original vertical distribution (down to ca. 5 m) re-established. The size age structure its population changed during study. No individuals shell length exceeding 55 mm aged more than 5–6 years 1987. Between 1972 1987 one-year-old prevailed, following slowly, 2008 twoto four-yearold most frequent. potential effects changes trophy factors, e.g. Dreissena polymorpha, Unionidae are discussed.