DIVERSITY OF AQUATIC MALACOFAUNA WITHIN A FLOODPLAIN OF A LARGE LOWLAND RIVER (LOWER BUG RIVER, EASTERN POLAND)

This study analyses the composition and species richness of aquatic molluscan communities within a large sector (over 100 km) of the valley of the Bug, a large lowland river in eastern Poland. An attempt was also made to determine the number of samples sufficient to compile a representative mollusc list within the whole study area and in groups of water bodies representing different degree of permanence, as well as to test the completeness of sampling of the malacofauna. Within the study area, 54 mollusc species were found – 36 snails and 18 bivalves. Up to 27 species were recorded from individual sites with a mean of 8+5 species per site. Mean number of species was significantly higher in permanent water bodies than in the two other site groups. Dominance patterns in mollus-can communities and frequencies of individual species showed distinct differences within three groups of habitats differing in permanence. Species composition was related to permanence, size, depth, hydrological connectivity and successional stage of water bodies, as well as to the type of bottom sedi-ments and macrophyte abundance. The aquatic malacofauna found within the study area was rich, comprising almost 90 % of potential composition based on regional species lists. Rarefaction curves for the total malacofauna confirmed the low probability of finding additional species, whereas tem-porary and permanent water bodies showed incomplete species lists. About 20 samples should be suf-ficient to collect representative data in the study area and more than 60 samples would be necessary to collect a nearly complete dataset