The implications of fine sediment inputs for freshwater mussel populations have long been the subject of speculation by aquatic biologists. However, because long term monitoring, quantification of in-stream habitat parameters and controlled experiments have only recently become incorporated into studies designed to assess the status and health of freshwater mussel populations, these relationships have remained somewhat equivocal. Over the course of the past 5 years (2017-2021) we have been conducting monitoring of Appalachian elktoe (Alasmidonta raveneliana) and habitat conditions in the South Toe River in Yancey County, North Carolina in an effort to understand how this population is responding to the expansion of an existing highway corridor in the middle of the watershed. A wide range of methods were used to examine how this project may be affecting mussel populations and their habitats. Substrate conditions were examined using both pebble counts to quantify streambed conditions and freeze cores to quantify interstitial substrates. Mussel population trends were monitored using qualitative (timed-search) surveys at historically occupied and regularly sampled sites. A mark recapture study was conducted to examine the role of habitat, season and surveyor experience in estimating mussel detectability as well as to estimate the precision of prior population size estimates. Occupancy surveys were used to determine the extent and size of Appalachian elktoe populations at other sites in the South Toe River. Finally, a combination of field and hatchery-based experiments were used to assess the effect of fine sediments on Appalachian elktoe growth and survival.
Freeze core and pebble count data revealed largely congruent patterns. The proportion of surface and interstitial fine substrates was higher at sites downstream of the highway project compared to upstream reference sites but fines were highest near the confluence of a sediment-impacted tributary rather than immediately downstream of the highway overpass. Mussel populations remained relatively stable at upstream reference sites but declined substantially at the 4 sites downstream of the highway corridor. Mark-recapture and occupancy studies revealed that substrate conditions and surveyor experience were both important factors influencing mussel detectability; mussels were more likely to be detected in habitats with coarse substrates but more experienced personnel were also better able to detect mussels irrespective of habitat conditions. Finally, field and hatchery trials revealed that although survival rates were lowest at downstream sites, fine sediments do appear to have a direct influence on Appalachian elktoe mortality. Instead, cumulative effects of habitat changes (e.g., alteration of habitat availability and stream geomorphology) resulting from disturbance and sedimentation from the highway corridor may be influencing mussel populations in the lower South Toe River. Surveys in the upper river indicate that Appalachian elktoe populations appear to be stable and are larger than previously believed so it will be interesting to see if populations at downstream sites are capable of recovering once habitat conditions stabilize. Efforts to restore degraded stream channel and riparian zone habitats as well as population augmentations at sites may enhance recovery rates in the lower South Toe River.
