Long-term decline of native freshwater mussel assemblages in a federally protected river

Freshwater systems are increasingly threatened by human impacts. To effectively monitor and manage these systems, reference or baseline conditions need to be documented to establish restoration or management goals. Freshwater mussel (Unionoida) communities are in decline in many river systems worldwide. Yet there are few long-term quantitative studies in relatively pristine habitats that could serve as reference locations. Here, we present the results of a 20-year quantitative monitoring study of mussel assemblages in a federally protected river in the United States. We found significant declines in mussel abundance (41%) and species richness (16%) over the 20-year period, while mussel species evenness and Shannon's diversity index values increased over time (19.3% and 12.1%, respectively). This, combined with an analysis of community composition change trajectories, suggests the declines were disproportionately higher for common species than for rare species. Variation in life-history traits of macroinvertebrates is often used to assess the degree of degradation of aquatic systems. Using Haag's (2012) classification of mussel life-history traits, we found the proportion of opportunistic species (fast growth, high reproductive output, short lifespan) decreased by 65.3%, while the proportion of equilibrium species (slow growth, long lifespan, lower investment in reproduction) increased by 54.6% and the proportion of periodic species (moderate-to-high growth rates, low-to-intermediate fecundity, lifespan and age at maturity) increased by 82.4%. There were modest changes in human population and land use in the catchment during our study period, and a change in the operation of a hydroelectric dam reduced discharge variability. Sediment size changed at several study sites, increasing at some sites while decreasing at others. This study occurred in one of the best-protected and most pristine rivers in the upper Midwest of the United States, harbouring one of the region's most intact mussel fauna that could be used as a reference to set goals for the restoration of mussel fauna in other river systems. Despite the high quality and protection of this river system, we found slow declines in the mussel community but found no evidence that changes in land use, flow regime, water quality, the spread of invasive species or increased sedimentation were responsible for these declines. The slow decline of mussels and lack of specific causes in such a river underscore the imperilment of this fauna, and the need for more aggressive conservation, management and research approaches.