A comparative study of freshwater mussel habitat along the Clinch River, Tennessee and Virginia, USA : hydraulic factors and their influences

Freshwater bivalves of the Superfamily Unionacea are the largest aquatic invertebrates in North America and make up a major part of the invertebrate biomass of lakes, rivers, and streams. Their high susceptibility to pollution and stream degradation makes them useful indicator taxa for assessing stream quality. However, there have been few quantitative studies of the habitat parameters that define freshwater mussel habitat limits, nor studies that have analyzed spatial patterns of mussel habitat throughout one river or basin. This study investigates the relationship between freshwater mussel success (measured in terms of species diversity) and local hydraulic forces at 12 sites along the Clinch River in Tennessee and Virginia. Site selection was facilitated through published population data from on-going diversity studies. I classified sites as high quality habitat if they had high species richness and endemic or endangered species present, and as low quality habitat if they had low species richness and no endemic or endangered species present.

This study compared hydraulic properties of high quality and low quality sites during the summer of 2000 to describe physical habitat characteristics, and to determine whether differences in these factors relate to quality of habitat. Spatial and temporal analyses of hydraulic properties at sites were conducted for both simple hydraulic parameters (depth, width, velocity, and slope) and complex hydraulic forces (shear stress, stream power, Froude number, and Reynolds number). The simple hydraulic parameters differed little between sites, though they aided in habitat description. Complex hydraulic forces were significantly different at high and low quality sites and showed some interesting temporal patterns.

During early summer, three out of the four complex hydraulic parameters measured showed significant differences between good sites and poor sites. The early summer measurements showed that shear stress and stream power at good sites were almost double the values at poor sites. In late summer, the means and ranges of hydraulic parameters at good and poor sites were similar, indicating a convergence of conditions at that time.

The converging of conditions over the summer may indicate that temporal changes and seasonal fluctuations are quite important for freshwater mussels. In examining temporal conditions at good sites, shear stress, stream power, and Froude number showed significant decreases from early to late summer measurements. At poor sites, all four hydraulic parameters showed significant changes between early and late summer as well; however, shear stress, stream power, and Froude number increased in magnitude from early to late summer (Reynolds number decreased).

These results indicate real differences in the hydraulic forces present at freshwater mussel habitats of contrasting quality along the Clinch River, and indicate that temporal or seasonal changes in physical habitat conditions may be important. They allow the recommendation of a seasonal range of shear stress values that may be optimal for both juvenile colonization of sites and adult persistence. The shear stress limits are provided as a guide for detection and conservation of quality habitat or to aid site selection in future reintroduction studies.