Communities of aquatic molluscs in floodplain water bodies of lowland river [Bug River, East Poland]

The present study aimed at describing composition, species richness, dominance patterns and diversity of mollusc communities as well as frequency of individual species within selected water bodies of the valley of large Polish lowland river (lower Bug River). Suitability of molluscs in the ordination of the investigated habitats was evaluated. The studies were carried out in the years 2003–2004 in left-bank part of the valley between 2.5–6.5 and 90–107 km of the river course in 21 water bodies heterogenous as regards size, location and successional stage. The dredge was used. 34 mollusc species were found: 23 snails and 11 bivalves. At individual sites from 2 to 15 species were noted (4 – 18 including molluscs found only as empty shells). Relatively rich malacofauna existed in larger water bodies in comparison with small stagnant and isolated ones located outside the embankment. Malacocoenoses of the investigated habitats exhibited considerable species diversity (H’ = 2–3 in most habitats). Lymnaea stagnalis, Bithynia tentaculata, Planorbarius corneus and Viviparus contectus were the most frequent species (F≥ 50%). These species as well as moderately frequent ones constituted significant components of mollusc communities of individual water bodies. Frequencies and dominance patterns of molluscs inhabiting younger and older water bodies distinctly differed. The abundance of molluscs was mostly of the order of several to a few dozen individuals per sample (0.5–1 m² of bottom). Principal component analysis and classification (PCCA) basing on the occurrence of molluscs expressed as the classes of abundance enabled to distinguish 4 groups of water bodies, among which 2 were more distinct. The comparison of malacofauna of these two groups exhibited significant difference in frequencies. Species (S) and percentage (Psc) similarities among malacocoenoses of the investigated water bodies were generally weak or moderate, which presumably results from isolation among majority of them. More cases of high values of similarity indices were found within the group of small and shallow water bodies in comparison with larger and deeper ones. This may result mainly from higher habitat homogeneity at later stages of hydrarch succession. Generally higher species than percentage similarity was observed indicating the differences in dominance relations among sites with similar composition of malacofauna.