Structure and Dynamics of Mollusk Communities of Small Oxbow Lakes and the Determining Factors (the Khoper River Valley, Penza Oblast)

The structure and dynamics of mollusk communities, as well as environmental factors influencing them, were analyzed in small oxbow lakes of the forest-steppe zone (a case of the Khoper river valley, Penza region, Russian Federation). 19 species of mollusks were found in the studied water bodies. The oxbow lakes were characterized by relatively low alpha-diversity (number of species 4 ± 2) and relatively high beta-diversity (Jacquard index 0.26 ± 0.18), which is typical for floodplain ecosystems. Lake communities were dominated by Planorbis planorbis (40%), Lymnaea stagnalis (15%), Anisus spirorbis (14%), Lymnaea saridalensis (9%), Anisus vortex (8%). The results of our analysis showed that the group of factors that determined the type of water body (stability, light intensity, water temperature, oxygen concentrations) had the greatest impact on the structure of malacocenoses. The seasonal dynamics of mollusk communities was determined by the number of adjoining water bodies, water temperature, and dissolved oxygen concentrations. The number of adjoining oxbows was the major factor influencing mollusk species diversity, as well as the total number and biomass of mollusks. Oxbow isolation was favorable for mollusk communities. The factors influencing the highest number of mollusks species (seven) were the light intensity and the presence of fish. Other significant factors were the stability of lakes (shown for five species of mollusks), the number of adjoining oxbows and water transparency (four species). Aquatic vegetation positively correlated with the abundance of only one species— Anisus vortex . Our data suggest that the presence of fish could influence the abundance of Lymnaea saridalensis . The mollusks, in their turn, apparently, had a positive effect on the abundance of leeches in water bodies. Mollusk species most sensitive to the external factors were Lymnaea stagnalis , Lymnaea saridalensis , Aplexa turrita , Anisus spirorbis and Anisus leucostoma , most tolerant were Bathyomphalus crassus , Anisus vortex , Planorbis planorbis and Planorbarius corneus.