AbstractThe freshwater pearl mussel (Margaritifera margaritifera) is an aquatic invertebrate with a potential lifespan of over 100 years. It thrives in coarse sand or fine gravel substrates in clean, fast-flowing and well-oxygenated waters. This species has undergone a dramatic decline throughout its geographical range and is now listed as Critically Endangered in the IUCN Red Data Book. Irish populations, while representing 46% of the EU’s total mussel numbers, have low recruitment levels and are in unfavourable conservation status.
One of the principal pressures impacting on freshwater pearl mussel (FPM) populations are believed to be diffuse sediment losses associated with agriculture and forestry. The contribution of anthropogenic activity to river sediment load is of increasing global concern (Duerdoth et al. (2015). Excessive sedimentation of watercourses can reduce habitat quality, in part due to the limitation of exchange between the hyporheic zone (the porous substrate adjacent to flowing water) and the water column. This effect is of crucial importance to the FPM which inhabits the hyporheic zone at critical stages in its life cycle. Other impacts associated with excessive transport of sediment, include changes in turbidity, disruption of primary productivity, and alteration of substrate and channel morphology.
The primary aim of this study was to assess critical suspended sediment concentrations in representative habitat in two internationally important and protected FPM catchments: the Caragh and the Kerry Blackwater catchments in SW Ireland. Each catchment contains populations of over 2,750,000 mussels. However, despite the large populations, recruitment levels are insufficient to ensure the survival of the species.
Firstly, this study aimed to evaluate sediment transfer in three chosen catchments using turbidity as a proxy for suspended sediment concentrations. This approach provided metrics for sediment concentration and areal flux allowing for comparison between catchments (with a gradient of mussel recruitment) to known sediment yields elsewhere. The results indicated that annual suspended sediment yields in the area are low in the context of contemporary European yields. The results are discussed in terms of suggested suspended sediment concentrations thresholds for the conservation of the freshwater pearl mussel. Suspended sediment thresholds are exceeded most often in the Bridia catchment, and least frequently in the Kealduff catchment, which has a mussel population with some levels of juvenile recruitment.
In the second part of this study, each catchment was surveyed to identify critical sources areas and areas of sediment storage. Source samples were categorized both by land-use (e.g. grassland, extensive land and forestry) and by surface and subsurface areas (e.g. channel banks). To determine non-point sources of sediment, sediment analysis was undertaken to ascertain the most appropriate properties (e.g. trace and heavy metals, organic content) for sediment fingerprinting. Unmixing models were employed to analyse the properties of source and suspended sediments. The combination of fingerprinting with multivariate ‘unmixing models’ allowed for discrimination between sources, as well as the evaluation of relative contribution of sediment from each source. Forestry was found to be a dominant source in the catchment containing a significant conifer plantation, while key sources in other catchments ranged from extensive areas to improved grassland and road verges. Results are discussed in the context of potential management implications for conservation of the freshwater pearl mussel.
In the third and final part of this study, the River Habitat Survey method was applied to representative stretches in three priority freshwater pearl mussel catchments in south–west Ireland to investigate habitat modification scores, along with the potential for modifications to disrupt sediment pathways. River channel modifications have previously been considered to have contributed to the species’ decline. River Habitat Survey results suggest that there are significant levels of modifications in these priority habitats, which may have contributed to loss of function; a catchment with a small remnant adult population achieved highest overall habitat modification scores, while a catchment with recent recruitment recorded the lowest levels of modification. Additionally, survey results are compared statistically for a range of habitats as indicated by levels of the freshwater pearl mussel. Findings are combined with the outcomes of sediment source and yield studies, with the aim of contributing to sustainable land management strategy for priority aquatic species.
Overall, the study aims to deliver new information regarding insight into selective erosion and preferential deposition in river channels and a provenance methodology across land-use gradients in an Irish context. Sediment flux analysis provides information on concentration, flux and magnitude-frequency analysis of high-resolution turbidity and suspended sediment data. Additionally, the study aims to contribute to the development of ecosystem-based adaptation measures that both aid the restoration of endangered species and provide broader sustainable land management benefits.
|Date of Award
|Teagasc - Irish Agriculture and Food Development Authority
|John Rowan (Supervisor), Daire Ó hUallachán (Supervisor) & John Finn (Supervisor)