Abstract:
Freshwater mussels are burrowing, filter feeding organisms that were once widespread prior to river regulation but now face extinction or extirpation in many rivers of North America. The lifespan of some species can exceed a century and population densities have the potential to reach one hundred individuals per square meter in some rivers of Alabama. The functional traits of mussels, combined with their long lifespan and ability to occur in high-densities, suggest that they could impact reach-scale sediment processes, involving sediment transport and bank erosion, potentially leading to changes in channel morphology. Few studies, however, have examined ecosystem engineering by mussels and their potential effects on spatio-temporal changes in river morphology. We tested whether four, high-density mussel aggregations (> 14 ind/m2) influenced lateral river migration and bank erosion rates in a 48-km segment located in the Sipsey River of Alabama (USA). We digitized and compared riverbank positions of the study reach on georeferenced historical aerial images from 1965 and images from 2018. Above average rates of lateral migration (>0.2 cm per year) and bank erosion (>33 cm3 per year) occurred at all observed high-density mussel reaches. We observed the presence of mid-channel bars persisting for the duration of the 53-year study period immediately downstream of each high-density mussel location. Additionally, we tested whether mussel population densities can be used to predict locations of reach-scale riverbank erosion. We quantified bank erosion, mussel density, median particle size distribution, and determined bank erosion hazard index (BEHI) scores at 44 transects located within three reaches. We created a stepwise, linear regression model to determine the effect of mussel population density on bank erosion. Mussel density was a stronger predictor (r2= 0.25) of riverbank erosion than most BEHI metrics, including root depth (r2= 0.06) and bank height/bank full (r2= 0.01). The results of this study provide a critical step toward understanding reciprocal relationships between abiotic and biotic systems and new insights into the evolution of channel morphology not previously considered. Future river restoration projects should consider that many organisms, not just abiotic factors, can create biogeomorphic change of river geomorphology.
