CAN FRESHWATER MUSSEL DENSITY BE PREDICTED BY COMPLEX HYDRAULIC PARAMETERS?
Hornbach, Daniel J ¹; Hove, Mark C¹; MacGregor, Kelly R ²
1. Department of Biology, Macalester College, St. Paul, MN 55105, USA, E-mail:  hornbach@macalester.edu; mark_hove@umn.edu
2. Department of Geology, Macalester College, St. Paul, MN 55105, USA, E-mail: macgregor@macalester.edu

Freshwater mussels are among the most endangered animals in North America. Changes in water quality, land use, and modification of rivers have been implicated in their decline. The St. Croix River is home to a dense and diverse assemblage of mussels and serves as an important refuge for these organisms. Despite their importance in rivers little is known about factors that control their distribution and abundance. Simple habitat factors, such as water velocity, depth and substrate type, are not strong predictors of abundance or diversity. The use of more complex hydraulic parameters shows promise. We randomly selected 40 locations in a 5 km stretch of the St. Croix River between Interstate Park and Franconia, MN, USA. At each location we sampled 3 1-m2 quadrats quantifying mussels, and collected sediment samples from 1 quadrat. Depth and velocity were measured with an acoustic Doppler current profiler under different discharge levels allowing us to measure the range of hydraulic stresses experienced by the river bottom. Complex hydraulic parameters (Froude number, boundary Reynolds number, shear velocity, etc.) were calculated. Analyses of covariance using the hydraulic parameters as a covariate and location (Franconia or Interstate) showed that there was a significant interaction between location and the hydraulic parameters Froude number, shear velocity and shear stress. The Froude number, shear velocity and shear stress are all higher at Interstate which harbors a greater density of mussels than Franconia. Mussel density was significantly correlated with these parameters (except Reynolds number), with large numbers of mussels corresponding to high Froude number, shear stress, turbulent flow, and thin laminar flow layers. These hydraulic parameters suggest that Franconia is a more depositional riverbed environment, which may account for the overall lower mussel density there. These data suggest that reach-scale hydraulic measurements may be helpful in determining habitat suitability for mussels.