The impact of freshwater mussels (order Unionoida) on river bed characteristics and sediment flux: A flume-based study.

Abstract

Unionoid mussels are considered keystone species due to their ability to modify and link pelagic, benthic and hyporheic environments in freshwater systems, [1,2,3] yet empirical data to determine their influence on river bed dynamics and sediment flux is lacking.

A recirculating flume-based study using fifty individuals of the unionoid species Anodonta anatina investigated the impact of this species on bedform development and particle flux of a polymodal substrate representative of the grain size distribution of the mussel's river habitat. River seston was added to the flume at weekly intervals, and water and substrate conditions were monitored for the eight-week duration of the study. The control experiment had mussels absent from the flume. It was found that the presence of A. anatina increased the organic content of the substrate through deposition of pseudofaeces, and led to significant reductions in near-bed velocity, boundary shear-stress and the amount of suspended and dissolved solids in the water column. However, despite these impacts a greater quantity of sediment and a larger range of grainsizes entered the flume's sediment trap compared to the control experiment when mussels were absent. The impact of mussel bioturbation appears to outweigh any sediment stabilisation effects arising from the increased organic content of the substrate and the reduced near bed velocities. Additionally, sediment grainsize and longitudinal wetted profile measurements indicate that the mussels increased bed roughness and heterogeneity of the substrate. Given that freshwater mussels can exist at very high densities within rivers, [3] increased mixing and mobilisation of bedload, improved habitat heterogeneity and the transferral of material from the water to the substrate by mussels implies they constitute a critical element in the sediment and nutrient dynamics of fluvial systems.

References:

1. Vaughn, C.C., Nichols, S.J. & Spooner, D.E., 2008. Community and foodweb ecology of freshwater mussels. Journal of the North American Benthological Society, 27(2), pp.409-423.

2. Gutierrez, J.L. et al., 2003. Mollusks as ecosystem engineers: the role of shell production in aquatic habitats. Oikos, 101(1), pp.79-90.

3. Aldridge, D.C. et al, 2007. Freshwater mussel abundance predicts biodiversity in UK lowland rivers. Aquatic Conservation-Marine and Freshwater Ecosystems, 17(January), pp.554-564.