Association between the occurrence of the Thick-shelled River Mussel (Unio crassus) and macroinvertebrate, microbial, and diatom communities

Unionoid mussels play an important ecological role in freshwater ecosystems. They are considered keystone species and ecosystem engineers because they modify their surrounding environment by burrowing in the substrate (bioturbation), filtering suspended solids in the water column, and excreting organic matter via biodeposition available to primary producers. We used the endangered Thick-shelled River Mussel (Unio crassus, Philipsson 1788) to test the effect of this species on physical substrate variables and the community structure of microorganisms, diatoms, and macroinvertebrates in 5 streams in Bavaria, Germany. Our core hypothesis was that the presence of mussels increases the diversity and abundance of benthic micro- and macroorganisms. We compared physicochemical characteristics of habitats and taxonomic composition of benthic communities in plots with and without mussels. Redox potentials and substrate compaction were similar between mussel plots and plots without mussels. Furthermore, species richness, abundance, and taxonomic composition of benthic communities did not differ between plots with and without mussels, with the exception that diatom species richness was lower in plots with mussels. In contrast to previous studies, we found no correlation between mussel density and macroinvertebrate density. In conclusion, our results suggest that abiotic habitat variables have a stronger influence on benthic communities than the presence of U. crassus in the studied streams.