Weerzien met het Naardermeer

The ‘Naardermeer’, a lake and nature conservacy in the centre of the Netherlands has been studied on molluscs by the author in 1944 and 1945 paying special attention to the Sphaeriidae, in particular Pisidium pseudosphaerium and the planorbid Gyraulus riparius. In september 2000 the area was revisited and re-sampled in order to fïnd out the effects of pollution on the malacofauna. The lake had less algae than before and molluscs were less abundant. New was the occurence of Potamopyrgus antipodarum. dium henslowanum [a more fluviatile species] seems to be more abundant than before. P. pseudosphaerium still occurs in the lake and finding P. moitessierianum was new for the ‘Naardermeer’. Analysing the results of the collecting done by a provincial organisation, who have done macrofauna research since 1984, is recommended.