De zoetwater-mollusca (Gastropoda, Bivalvia) van het Zuid- Limburgse heuvellandschap. – Het Zuidoostelijke gebied –

During the last week of October 1995 a survey was held to record the freshwater mollusca of Zuid Limburg. In this report the spatial coverage of the survey extends to the area south of the town of Valkenburg and east of the river Maas (the Maas not included). Because of the very sparse data reported from this area over the last 50 years (especially stagnant water species and Sphaeriidae) the survey was aggravated to gather data on distribution and ecology and, in this manner bridge the gap in observations. To this end we sampled running waters (brooks, brooklets and sources), stagnant waters (ponds, pools and puddles) and water-throughs in the meadows. Futhermore we were looking for trends. During the sixties and seventies however, most of the brooks were heavily polluted. Hence we characterized changes in distribution with respect to 1945. Due to a lack of quantitative data in literature only qualitative changes in distribution are considered. Because of the extension of the area only a limited number of random choosen sites were visited. A total of 21 species (13 Gastropoda; 8 Bivalvia) were found on 36 locations (table 1). Compared with 1945 the Gastropoda show an increase of 62 % whereas the Bivalvia show a reduction of 20% (Table 6). Pisidium casertanum (Poli), local density up to 200 spec. per 0.2 m² and Ancylus fluviatilus (Müll.), local density up to 60 spec. per 1 m² are considered to be the most abundant species of the brooks. Pisidium personatum Malm is only present in the neighbourhood of sources, local density up to 212 spec. per 0.2 m². Potamopyrgus antipodarum (Gray) , local density 17 spec. per 0.2 m² and Physella acuta (Drap.), 2 spec., are considered to be new in the brooks. Unionidae and Pisidium amnicum (Müll.) are absent in the brooks (table 2; Table 3 for brooklets). The inspection of 2 sources (Noor and St. Anna bron, Valkenburg) yielded nothing. Modern nature management like re-arranging the original pools and importing new aquatic plants in newly digged pools has probably led to an increase of Gastropoda in the environment of Bemelen and Cadier & Keer. It is conceivable that new species for the area like Lymnea stagnalis (L.), Stagnicola palustris (Müll.), Planorbis planorbis (L.) and Gyraulus crista (L.) are imported together with the aquatic plants (Elodea etc.). The small amount of species and the high number of individuals found in some of the pools (table 5) may be due to instabilities in biotopes caused by management measures. It is not understand why no other Bivalvia are found in the pools besides Musculium lacustre (Müll.) while Pisidiums are collected in the castle ponds (table 4). Anodonta anatina (L.) was found in a commercial fishing pond probably due to the import of fish parasitized by glochidium. Adres schrijver: Poortenaarlaan 94 3431 RJ Nieuwegein