Does Local Bed Shear Stress Predict the Occurrence of Freshwater Mussels?

The relationship between near-bed hydrodynamics, pore-water chemistry and the presence of unionids was examined in a well-characterized multispecies mussel bed (26 species; ~24 mussels m⁻²; 70% gravel, 20% cobble and 10% sand) within a lowland river (Sydenham River) in southern Ontario, Canada. Local bed shear stress (τ_b) was determined from velocity profiles at ~1-m intervals along four laterals and three longitudinal transects in the reach providing 118, 14.3-cm diameter quadrats in which porewater was sampled and sediments were excavated for unionids. Ninety-four unionids (10–135 mm long) from 14 species were found in 56 quadrat locations that had significantly higher mass of very fine gravel (2000-μm sieve) and lower dissolved oxygen in a comparison of individual environmental factors. Larger adult mussels (i.e., > 30 mm long; n = 68) were found in quadrats with τ_b = 0.022–1.34 Pa and an excavated very fine gravel mass = 616–2821 g, but smaller, juvenile mussels (< 30 mm; n = 26) were found in a more restricted range of τ_b and sediment content (0.075–0.77 Pa and 1151–2561 g, respectively). Using forward and backward stepwise habitat variable selection in logistic regression, models incorporating τ_b, τ_b² and/or very fine gravel mass provided the highest probability of locating juvenile mussels. The final model, which is based on body size, is informative at the community level rather than on a species-specific basis. These results provide insights into the relationship between near-bed hydrodynamics and these important benthic invertebrates, the location of small juvenile and larger adult unionids and their habitats in riverbeds. Conservation efforts should focus on preserving and enhancing these habitats and those of the most vulnerable early life stages of freshwater mussels.