Abstract
Mollusc assemblages of the Orava reservoir, the biggest dam reservoir in Slovakia, were studied in 2020. The reservoir, built in 1941–1953, is situated in north-western Slovakia. Sixteen aquatic mollusc species (13 gastropods, 3 bivalves) were recorded. Valvata piscinalis (O. F. Müller), Radix auricularia (Linnaeus) and Gyraulus parvus (Say) were the most frequent; they were recorded in more than ten sites each. Rich mollusc assemblages, composed mainly of gastropods, were found in sites with rich littoral vegetation of macrophytes. The bivalve diversity and abundance were very small, especially compared to the other studied dam reservoirs. The current results were compared with the previous surveys. The occurrence of Radix lagotis (Schrank) is its first reliable record in Slovakia. Viviparus acerosus (Bourguignat), Aplexa hypnorum (Linnaeus), Anodonta cygnea (Linnaeus), non-native Potamopyrgus antipodarum (Gray) and Physa acuta (Draparnaud) were recorded in this area for the first time and their sites are outside their known range in Slovakia.
Highlights
Dams have been altering river ecosystems since humans started constructing them
The Orava dam reservoir provides appropriate habitats for abundant populations of species, which prefer standing waters; it differs from natural habitats of the area, such as nutrient-poor small rivers, brooks, springs, or small wetlands
It is situated at higher altitude, the reservoir is inhabited by richer mollusc assemblages and by many species, which are more common in lowlands
Summary
Dams have been altering river ecosystems since humans started constructing them. The earliest dams were built 5,000 years ago (Petts & Gurnell 2005). In 1990 Šteffek (1991) analysed the sediments near the dam after emptying of the reservoir; his publication included the results of several earlier visits in 1982, 1986 and 1989. The lack of data on the current composition of the mollusc assemblages, the fact that the previous surveys were mostly based on the contents of sediments and the possibility to compare changes in the assemblages during more than 30 years were the main reasons for a more detailed survey of 2020. The results of this survey are presented here
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