De verspreiding der Sphaeriidae in Nederland

The distribution maps of Sphaeriidae are based on about 4000 entries in the E.I.S.-Netherlands data bank. The samples have been collected by about 120 field workers from 1910 on. Most of the material is in the Rijksmuseum van Natuurlijke Historie, Leiden, and in the Instituut voor Taxonomische Zoölogie/ Zodlogisch Museum, Amsterdam. The identification of all samples has been verified by the author. Hydrographically, the Netherlands are characterized by continuous human control of the water level. For this reason the sphaeriid distribution is more or less artificially conditioned. If there were no dikes, nearly half of the country would be flooded by sea water (fig. 1) and thus become uninhabitable to the halophobous Sphaeriidae. It is in these lowlands, some of which are situated as much as six meters below sea level, that most sphaeriid species find relatively favourable environmental conditions. With a view to land reclamation, the former saline inland sea, the Zuiderzee, has been separated from the North Sea by means of a dam in 1932, the so-called Afsluitdijk. The Zuiderzee then became IJsselmeer. The former peninsula of Noord-Holland gradually turned into a freshwater area as well. The marine fauna of the Zuiderzee disappeared soon, making room for a freshwater fauna. Within a couple of years the IJsselmeer was colonized by several species of Pisidium and Sphaerium (fig. 23, column 29). Another important hydrographic change has been the starting of the so-called Delta-works in 1954, which are still going on. These works aim at giving a better protection of the lowlands by a considerable reduction of the total length of the sea dikes. The southwestern islands which were formerly surrounded by seawater, are now connected by dikes. Tidal movement in the lower course of the rivers Rhine and Maas has been stopped. All these changes imply a potential extension of the range of the sphaeriids. Human influence on the waters of the higher eastern part of the Netherlands is, though not so spectacular, nevertheless important. For the above-mentioned reasons no distinction has been made on the maps between records from before and after 1950.