Response of Freshwater Mussel Assemblages to Dam Removal

Dam removals benefit stream ecosystems and biota via numerous mechanisms including increased ecological and biological connectivity, improvements to water and habitat quality as well as reducing navigational and recreational hazards. However, dam removals may also have significant impacts on stream ecosystems including increasing accessibility for invasive species as well as increased sediment inputs to downstream reaches and changes to stream geomorphology (i.e., bed scouring, channel alteration) that have implications for stream biota. Although pre-removal surveys are required by most agencies, project designs seldom include plans or funding for post-removal monitoring. As a result, few studies have attempted to quantify effects of short or long-term impacts of dam removals on stream biota. For example, of 185 dam removal projects conducted across 22 states, I was only able to find pre or post-removal surveys for 29 projects; only 13 of these were pre-removal projects whereas 22 projects examined mussel responses post-removal. In order to better understand the long-term consequences of dam removals on freshwater mussels, I obtained historical abundance and richness data from numerous data sources associated with 12 former impoundments in North Carolina and conducted primary mussel surveys where dam removals occurred. I used a combination of generalized linear models (GLMs), linear mixed-effects models (LMMs), and non-metric multidimensional scaling (NMDS) to assess the effects of low-head dam removals on freshwater mussel assemblages in North Carolina. These models incorporated both reach-scale and basin-scale variables—including time since removal, search effort, and spatial structure—to evaluate changes in mussel abundance, diversity, taxonomic composition, and shell length. While analyses revealed modest increases in mussel growth and weak trends in catch-per-unit-effort over time, diversity and assemblage composition remained highly variable and often distinct from reference sites. These findings challenge the assumption that dam removal universally promotes ecological recovery and instead underscore the need for species-sensitive, site-specific planning—particularly for imperiled mussels that may depend on legacy habitat conditions formerly maintained by small impoundments.<p></p>