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

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.

Hydraulic requirements of freshwater mussels (Unionidae) and a conceptual framework for how they respond to high flows

“…Thus, our results appear to provide validation of Sansom's et al. () hypothesis and a potential mechanistic understanding for how mussels are able to persist in habitats with varying levels of entrainment potential.…”

“…We show that species with certain morphologies (smooth and/or thin shells) plus demographic traits (i.e., short life spans, high fecundity, and high growth rates) were more associated with high RSS habitats than species that lacked these traits, which would suggest biological mechanisms for coping with high flows. Sansom et al (2018) examining persistence of mussel assemblages to bed load transport made a similar observation. The authors found that mussel beds within modeled rivers persisted over a 20-yr time span despite evidence that bed mobilization had occurred frequently (at least once every 2 yr or less) over that same time period.…”

“…Sansom et al. () examining persistence of mussel assemblages to bed load transport made a similar observation. The authors found that mussel beds within modeled rivers persisted over a 20‐yr time span despite evidence that bed mobilization had occurred frequently (at least once every 2 yr or less) over that same time period.…”

“…The authors argued based on this result that sediment stability by itself provides only a partial explanation for mussel occurrence and hypothesized that species traits may also contribute to persistence, even in stable habitats. Thus, our results appear to provide validation of Sansom's et al (2018) hypothesis and a potential mechanistic understanding for how mussels are able to persist in habitats with varying levels of entrainment potential.…”

Freshwater mussels increase survival of largemouth bass (Micropterus salmoides) in drying pools

“…Mussel larvae (glochidia) are obligate ectoparasites on fish (Barnhart, Haag, & Roston, ; Haag, ), and there is a strong positive relationship between mussel and fish diversity (Vaughn & Taylor, ). Mussels form dense, multispecies aggregations (mussel beds) in river reaches that are hydrologically stable (Sansom, Bennett, Atkinson, & Vaughn, ; Strayer, ), and these are often areas that become pool refugia as rivers dry. Thus, during droughts fishes are likely to be stranded in isolated pools with mussels.…”