USING HYDRAULIC PARAMETERS TO PREDICT MUSSEL DENSITY

D.J. Hornbach, Macalester College, Department of Biology
M.C. Hove, Macalester College, Department of Biology
K.R. MacGregor, Macalester College, Department of Geology

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. At each location we sampled 3 1-m2 quadrats quantifying mussels. We collected sediment samples from 1 quadrat to determine sediment size. 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, Reynolds number, shear velocity, boundary Reynolds number, shear stress and laminar flow) were calculated. The Froude number differentiates tranquil from turbulent flow in the water column. The Reynolds number differentiates laminar and turbulent flow in the water column (or at the sediment/water interface for the boundary Reynolds number). Shear stress is a measure of the tangential forces acting on the streambed by the water column and is likely important to substrate stability. The laminar flow layer describes the thickness of near-bed laminar flow, which may be important for filter-feeding organisms. 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. 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. 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.