Studies on spatial diversity in species composition, dominance patterns and abundance of malacocenoses in the Zegrzyñski Reservoir were carried out in the late 1990s, and changes since the 80s were assessed. The malacocenoses of the reservoir in the 90s were characterized by a considerable structural diversity and varied abundance. The species diversity was higher in the littoral while the abundance was generally higher in habitats located farther from the shore. After a period of rapid development of the 80s, in the 90s the malacofauna stabilised at a lower level of abundance and species richness, while the dominance patterns became more pronounced. The phenomenon could be related to progressive eutrophication as well as ageing of the reservoir. Compared to other dam reservoirs the mollusc fauna of the Zegrzyñski Reservoir is still quite rich in terms of both abundance and number of species.

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 molluscan 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 sediments 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 temporary and permanent water bodies showed incomplete species lists. About 20 samples should be sufficient to collect representative data in the study area and more than 60 samples would be necessary to collect a nearly complete dataset.

1. The distribution and species richness of aquatic molluscs were analysed within a 186-km-long section of a large (755-km-long) lowland river floodplain using GIS maps. Special attention was paid to threatened species.2. An attempt was made to identify local 'hotspots' of molluscan diversity and to determine the most important areas for conservation based on species richness, number of threatened species and indices of synthetic conservation (Q) and centres of population density (ICD).3. Over four years of investigation, carried out in 176 water bodies, a rich malacofauna was identified: 58 species including 20 of elevated conservation concern.4. GIS maps revealed numerous 'hotspots' of mollusc species richness and higher incidence of threatened species, as well as areas characterized by raised Q and ICD indices.5. Water body size and hydrological connectivity positively influenced species richness and Q values. The highest values of these variables were found in large water bodies having limited surficial connectivity with the river channel.6. Species richness, numbers of threatened species, and Q and ICD values were significantly higher in younger permanent water bodies compared with semi-permanent or temporary ones representing advanced successional stages. 7. Spatial visualization data collected in the study area indicate that the most valuable sites for the conservation of aquatic molluscs include both sides of the river in the lower part of the middle stretch of the lower Bug River valley.

Spatio-temporal variation in qualitative and quantitative occurrence of malacofauna was analysed in three large lowland dam reservoirs. Differences between the malacofauna of flooded land areas and former river beds persist even in middle-aged reservoirs. Quick turnover of water has a positive effect on mollusc abundance and species richness of the whole reservoir, and on the frequency of occurrence in the former river bed. Considerable dynamics of the malacocoenoses and a possibility to revert to earlier development stages, as a result of considerable disturbance, were observed at an advanced stage of biocoenosis development. Besides water dynamics and composition and distribution of bottom deposits, a significant effect on the malacocoenoses is exerted by dominant species, especially D. polymorpha. Abundance and dominance structure of malacocoenoses determine their role in the ecosystem, including accumulation of phosphorus and heavy metals, and their cycling as a result of filtration activity, faeces production, excretion and trophic transfer. Generally, the quantities of elements, especially heavy metals, accumulated in molluscs, are much smaller than the quantities which flow through the malacocoenoses. A large part of the pool of these elements contained in the shells is excluded from circulation for many years. Food chains seem to have relatively little effect on the transfer of heavy metals in the reservoir.

Warniak is a shallow eutrophic pond-type lake, part of the Mazurian Great Lakes. Since the late 1960s it has been subject to long-term experiments: introducing fish stocks of varied size and composition, resulting in massive physical, chemical and biotic changes in the lake's ecosystem. The aim of this study was an assessment of changes in the lake's mollusc assemblages 17 years after the first investigations (1998). The comparison of malacofauna in 1998 and 2015 showed changes in its composition, a significant decline in species richness and abundance, reduction in vertical distribution, increase in spatial variation of composition, richness and abundance. The proportion of rare species (i.e. found in one or two samples) increased from 17.4% to 64.7%. The dominance pattern became simplified and the frequencies of most species decreased. In 2015 Anodonta cygnea (L.) was still the most widespread unionid and one of the three most frequent species. The changes in the malacofauna occurred despite the lack of significant changes in the lake trophy. The deterioration of conditions for molluscs may result mainly from the impoverishment of macrophytes and the decrease in their abundance.