Lake trophic status and hydrological connectivity modify mechanisms underlying mollusc assemblage structuring

Species distribution and assemblage structuring are influenced by a combination of species dispersal mode and the dispersal routes used. Habitat connectivity is particularly important for passively dispersing taxa such as freshwater molluscs. In addition, current anthropogenic eutrophication affects the structure of assemblages by reducing native fauna and promoting the spread of generalist species. Here, we examined mollusc assemblages in two systems of small lakes differing in hydrological connectivity. The assemblages of 22 isolated lakes in Albania were mainly controlled by the distance between the lakes with a lower contribution of environmental conditions and lake area. In contrast, assemblages of 52 interconnected lakes in Czechia were driven primarily by environmental conditions. However, as lake trophic status increased, the assemblages in Czechia became more homogeneous in species composition as high trophic status filtered out all species except generalists. These assemblages from 33 eutrophic lakes were strongly determined by environmental variables, whereas the remaining assemblages from 19 low trophic lakes were structured by a combination of spatial and environmental variables. We conclude that hydrological connectivity between lakes is crucial for the distribution of molluscs, but eutrophication may influence the importance of individual mechanisms structuring mollusc assemblages.