Long‐term persistence of freshwater mussel beds in labile river channels

Abstract

Abstract Freshwater mussels are among the world’s most imperilled species, and much effort has been expended to understand their precipitous decline. The current paradigm is that relative river bed stability over decades is critical to maintaining mussel beds at a given river reach. Such information, however, is in stark contrast to the fundamental understanding of self‐formed rivers. Here, we examine the relationship between long‐term occurrence of aggregated freshwater mussel communities (mussel beds) and the dynamic rivers in which they live. We surveyed mussel assemblages in two streams with historical records to document their long‐term persistence, and we simulated bed mobility with a two‐dimensional flow and sediment transport model to estimate bedload transport during channel‐forming discharges. We found evidence of long‐term mussel bed persistence, in time and in kind, within these two streams and that significant bed mobility within mussel beds occurs every 1–2 years, as to be expected for self‐formed rivers with labile beds. Flow refugia, or bed immobility, thus cannot completely explain the persistence of these mussel beds. Our results suggest that mussels have adapted to proliferate in river channels that often experience significant bed mobility, but further studies are needed to understand the biophysical mechanisms enabling the long‐term persistence of mussel communities.