Freshwater mussel burrow position and its relation to streambed roughness

Freshwater mussels live partially or fully buried in river sediments, and burrowing by mussels causes bioturbation, increases benthic complexity, provides niche partitioning among other mussel species, and may reduce the chance of mussel dislodgment during high flows. However, there remains a need to better understand what influences the burrow position of mussels and how burrow position modifies physical and hydraulic habitat. In this study, we examined how mussel burrow position varied across species in 2 gravel-bedded rivers in the northeastern USA and how burrow position modified benthic habitat. We quantified the amount of benthic habitat mussels contributed to the riverbed and determined the contribution of burrow position to bed roughness and near-bed flow patterns. We found that burrow position varied by species, which was likely influenced by shell morphology, sculpture type, and river characteristics. We observed that the amount of shell exposed into the water column was similar to the coarse-sized sediment where mussels were found. We also found that mussel shells were exposed at a height greater than the median grain size and could, therefore, influence near-bed flows. Moreover, because larger grain sizes control sediment stability, the similar exposed length of burrowed mussels as coarse sediment could contribute to enhanced bed stability and reduce the risk of dislodgement. Better understanding of how mussel burrow position influences these important biophysical processes may help identify important Allee effects or minimum viable population indices, which may aid conservation and restoration efforts.