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1 2009

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Long-term changes of mollusc assemblages in bottom sediments of small-isolated lakes of different trophic state

Kołodziejczyk A., Lewandowski K., Stańczykowska A.

Polish Journal of Ecology

2009

Vol. 57, nr 2

331-339

It is a common view that increase of the trophic state of lakes has a negative effect on littoral invertebrate macrofauna, molluscs among them. However, the available data are often contradictory, and the decline of particular species is often observed only at a very pronounced raise in trophic state. The aim of this work was to present the changes of the composition and abundance of bottom malacofauna taking place during last 30 years in five small (area from 0.12 to 1.74 km[^2]), mainly shallow (mean depth from 3.7 to 11.8 m) lakes of different trophic state (eutrophic and mesotrophic), connected by a small river (Masurian Lakeland, Northeastern Poland). The research were conducted in years 1997 and 2006 and were compared with published data, collected in 1976. The trophic state of the lakes studied was still constant during the period of investigations. In the studied lakes the decrease of number of bottom mollusc species was observed in subsequent years. The previously recorded there alien, invasive species, Potamopyrgus antipodarum (J.E. Gray, 1843) and Lithoglyphus naticoides (C. Pfeiffer, 1828) also disappeared. Dreissena polymorpha (Pallas, 1771) remained the dominant species in most of the lakes. These changes were recorded in four eutrophic lakes as well as inone mesotrophic lake. The decline of the species in individual lakes didn't follow any regularity. Some mollusc species disappeared and value of similarity index between malacocenoses in these lakes decreased. It seems, that the trophic state was not the cause of disappearance of some mollusc species from the studied lakes. However, the effect of frequent anoxia in littoral zone related to eutrophication was raised up as the possible cause. The decline is of long-term character, probably resulting from small size and relative isolation of the lakes, which impede their recolonisation.