Abstract

Dredging of the river to remove macrophytic vegetation and bottom sediment is a common anthropogenic disturbance in the river ecosystem that directly and indirectly influences benthic invertebrates, including molluscs. We assessed the effect of dredging on malacofauna during the year following such an intervention on the river Krąpiel (NW Poland) and describe the process of gradual recolonization of the dredged parts by gastropods and bivalves as well as its possible sources. Molluscs were adversely impacted immediately after the dredging: relative abundance of rheophilic and species typical of stagnant water or slow-flowing rivers changed and the overall species richness decreased. The fauna recovered to its pre-management state within a year. The BACI analysis showed no long-term effect of the intervention on the total abundance and diversity of the molluscs. As many as 17 mollusc species, among them Unio crassus, were present in the river before and after the dredging. An additional 12 taxa were noted for the first time following dredging indicating that the removal of deoxygenated sediments from the channel provided an opportunity for the establishment of more diverse mollusc assemblages. Habitat preferences, mobility, and life cycle characteristics of species determine how they survive disturbances and how fast they are able to recolonize the managed sites.