Malacocoenoses of large lowland dam reservoirs of The Vistula River basin and selected aspects of their function

Abstract

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.