The aim of the present study was to describe the diversity and abundance of the mollusc communities in long and diverse series of riparian water bodies (59 in total) and to compare them with previous (two years before) studies based on much shorter series of sites (21 in total). An attempt was made to check whether the previous analysis of mollusc groupings is suitable to the classification of the riparian sites of a large area. The studies were carried out in the years 2004–2006 in left-bank part of the valley between 53 and 152 km of the course of the lowland medium sized natural river (Bug River, Eastern Poland) in 59 water bodies differing as regards size (from about 0.01 to a few ha), location in the valley, hydrological regime and successional stage. The samples were collected from the bottom (with dredge) and from macrophytes. The high species richness and diversity were found within the study area despite that the natural floodplain fragments were quite small (about 10% of the whole investigated fragment of the river valley). 52 mollusc species were noted: 36 snails and 16 bivalves (over 50% more than in the preliminary series). Relatively rich malacofauna existed in larger and younger water bodies, as well as in some temporary ones, but located in the active floodplain. Species diversity of individual habitats was comparable to that reported from the earlier study (Jurkiewicz-Karnkowska 2006), i.e. H’ values amounted to 2–3 in most cases. However, there were more cases where H’ values exceeded 3, as well as H’ values were higher at larger scale, i.e. site groups and the total study area (3.5–4.5). The most common species (F≥ 50%) within the investigated area were Lymnaea stagnalis, Bithynia tentaculata, Planorbarius corneus, Anisus vortex, Radix balthica and Viviparus contectus. The most frequent species usually constituted significant components of mollusc communities of individual water bodies. The only species with percentages exceeding 10% of total mollusc abundance at the scale of the study area were P. corneus and L. stagnalis. The abundance of molluscs was of the order of several to a few hundred individuals per sample (0.5–1 m2 of bottom). Basing on the occurrence of molluscs in the investigated habitats I–V groups of water bodies were distinguished applying multivariate analysis. They differed in abundance, dominance pattern and diversity of malacocoenoses, as well as frequencies of individual species. Especially high distinctness of malacocoenoses of small temporary water bodies was found.
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