Department of Geography

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Browsing Department of Geography by Author "Atkinson, Carla"

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    Biogenic modification of sediments by unionid mussels and their implications for sediment transport in the Sipsey River of Alabama
    (University of Alabama Libraries, 2018) Koerner, Matthew Ryan; Davis, Lisa; University of Alabama Tuscaloosa
    Few studies have tested the influence of freshwater mussels on sediment properties and their potential influence on geomorphic processes outside of lab-based flume studies. Freshwater mussels have historically constituted a great proportion of the benthic biomass in many rivers in the southeastern United States and may maintain or transform the physical stability of river bed sediment indirectly by their biogenic structure or directly through processes including biodeposition and bioturbation. We tested the effects of freshwater mussels on sediment properties (D50 median grain size and sorting), sediment scour and reach scale bedload transport by deploying 36 mussel enclosures in the Sipsey River, Alabama, for a 9-week period. We employed a randomized design consisting of 4 replicates of the following treatments: 3 diversity treatments (Cyclonaias asperata; Fusconaia flava; Cyclonaias asperata and Fusconaia flava), 2 abundance treatments (24 indiv/m2, 48 indiv/m2), a sediment only control, and 2 sham mussel controls (24 indiv/m2, 48 indiv/m2). Sliding-bead monitors were installed in the center of each mussel enclosure to measure the amount of scour over the 9-week period, and biomodifications to surface layer D50 particle sizes and Folk and Ward sorting coefficients were measured using ImageJ photo analysis software. Hypothetical differences of bedload transport rates between reaches containing mussels (24 indiv/m2; 48 indiv/m2) and no mussels were computed using BAGS (Bedload Assessment in Gravel-bed Streams, Version 2008.11) bedload transport model. The results of this experiment suggest freshwater mussels contribute to bed scour, and increase surface layer D50 particle sizes and the degree of surface layer sorting. BAGS predicted that bedload transports rates in a reach with high abundances of freshwater mussels were lower than a reach containing no mussels (88.6% lower under baseflow conditions and 55.3% lower under bankfull flow conditions), based on biomodifications to surface layer D50 particle sizes and sorting by freshwater mussels. These results suggest that alterations to sediment properties by freshwater mussels are quantifiable in non-flume based settings, and that freshwater mussels may play a significant role as ecogeomorphic agents in the Sipsey River, AL, and other rivers that contain dense, long-lived communities of freshwater mussels.
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    Biogeomorphic Impacts of Freshwater Mussels on Reach-Scale Geomorphology in the Sipsey River of Alabama
    (University of Alabama Libraries, 2020) Shafer, Gregory Wayne; Davis, Lisa; University of Alabama Tuscaloosa
    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.