| Abstract | Freshwater mussels have been found capable of improving water quality in river environments by filtering particles from the water column and depositing them on the river bed as pseudofaecal pellets. Growing recognition of the ecosystem services that freshwater mussels provide in river habitats has led to their inclusion in several river restoration projects. The focus of many of these projects in Europe has been on the freshwater pearl mussel, Margaritifera margaritifera with attempts to repopulate rivers with this critically endangered species. However, the very specific habitat requirements of M. margaritifera mean that many European rivers are not suitable environments for this species. In comparison to M. margaritifera, very little conservation effort has been directed at some of our more common, yet declining freshwater mussel species, and little is known about how these species influence river sediments and habitat conditions. Some of the more common European freshwater mussel species, such as Anodonta anatina and A. cygnea are capable of living in a much broader range of habitat conditions compared with M. margaritifera, meaning they could potentially be of benefit in river remediation projects not suitable for M. margaritifera.
To improve understanding of how Anodonta species may influence river environments through bioturbation, filtration, and biodeposition, their influence on river sediment characteristics, sediment dynamics, and habitat conditions was investigated in two lowland English rivers and a laboratory-based flume environment. Anodonta anatina and A. cygnea from Markeaton Brook, Derbyshire and the River Sence, Leicestershire were translocated from mussel-dense reaches to locations within each river where mussels were absent. In both rivers, quadrats where mussels had been removed were compared with control quadrats where mussels were present, whilst quadrats at the sites where mussels had been introduced were compared with control quadrats where mussels were absent.
At sites in both rivers where mussels had been removed from the river bed, significant decreases in hyporheic oxygen saturation were found in all quadrats, compared with the control quadrats that contained mussels. Half of all quadrats where mussels had been introduced to river sites showed significantly increased hyporheic oxygen saturation, with the remaining quadrats showing non-significant increases compared with the control quadrats without mussels. Grain-size distribution patterns of sediment cores taken from the stream bed indicated that the introduction of mussels to the River Sence increased the textural heterogeneity of the river bed sediment, whereas removal of mussels reduced textural heterogeneity of the river bed sediment. No significant differences in river bed textural heterogeneity were found in Markeaton Brook. Mean percentages of organic matter, inorganic carbon, and fine-grained sediment were not significantly affected by the presence of mussels in the majority of quadrats. Significant reductions in water turbidity were found in Markeaton Brook, where mussels were introduced, but no significant differences in turbidity were found in the River Sence. Significantly higher BMWP scores were found where mussels were present in the River Sence but differences in Markeaton Brook were non-significant. The variation in results between the river sites suggest that the extent to which freshwater mussels influence river bed conditions and water quality may be mussel density-dependent and site-specific.
A recirculating flume-based study using fifty A. anatina investigated the impact of this species on substrate characteristics, hydrological conditions, and particle flux of a polymodal substrate. River seston was added to the flume at weekly intervals to provide food for the mussels, and water and substrate conditions were monitored for the eight-week duration of the study. A control experiment was also set up with mussels absent from the flume. It was found that the presence of A. anatina led to reduced near-bed, 0.6 depth and 0.4 depth velocities, and reduced suspended and dissolved solids in the water column. Anodonta anatina reduced the entrainment of fine and organic material but increased the entrainment of sand and gravel compared with the control study. Although water velocities were reduced with mussels in the flume, calculations based on the grain-sizes entrained into the flume’s sediment trap indicated that critical boundary shear stresses were significantly increased with mussels in the flume. Additionally, sediment grain-size distribution patterns and topographical measurements of the substrate surface indicated that the mussels increased the heterogeneity of the substrate.
The results of the research described in this thesis indicate that bioturbation, filtration and biodeposition by Anodonta species may positively influence hyporheic oxygen saturation levels, water quality and habitat heterogeneity in river environments. Increased mixing and mobilisation of river bed sediment, and the transferral of material from the water column to the substrate by mussels implies they constitute a critical element in the sediment and nutrient dynamics of fluvial systems. |
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