Water bodies of a city park as habitats of rare, protected and alien species of molluscs

Representatives of fifteen species of gastropods and ten taxa of bivalves were found in five small water bodies in Skaryszewski Park in Warsaw. They included Sphaerium rivicola (Lamarck) and Anodonta cygnea (Linnaeus) (empty shells only) which are very rare in Poland, as well as three alien species – Dreissena polymorpha (Pallas), Physa acuta Draparnaud and Menetus dilatatus (Gould). The first of the alien species may have been brought by humans and by waterfowl which is numerous in the Park, the second probably by birds alone. Bithynia troschelii (Paasch), recently elevated to species rank, was also recorded. Despite the similar water chemistry, the species composition and dominance structure of molluscs were greatly diversified both among the studied neighbouring ponds and within the largest of them – Lake Kamionkowskie. Two smallest ponds, emptied for winter, were inhabited only by snails. The diverse occurrence of molluscs might be an effect of different stability of particular water bodies and within Lake Kamionkowskie – of local accumulation of large amounts of organic matter (fallen leaves) on the bottom.