As per European Commission (2001) and Department of the Environment, Heritage and Local Government (2009) guidance, Appropriate Assessments (AA) are to “assess whether there will be adverse effects on the integrity of the site as defined by the conservation objectives and status of the site”. Important indicators of freshwater pearl mussel integrity include:
- will the plan or project cause delays in progress towards achieving the conservation objectives of the site?
- will the plan or project interrupt progress towards achieving the conservation objectives?
As AAs are to be undertaken in view of the site’s conservation objectives, regard must be given to published SSCOs (see above). AA should include scientific analysis of how the plan or project could impact upon the relevant attributes and targets, in view of their current condition. The current condition, which informs the ‘integrity’ of a qualifying freshwater pearl mussel population can be considered to consist of the condition of the species’ habitat and the condition of the population itself. The Department is currently preparing guidance on AA and the freshwater pearl mussel.
AA must consider the current unfavourable condition of populations and their habitat, particularly whether the plan or project could:
- Prolong the poor condition of the freshwater pearl mussel habitat
- Result in further deterioration in freshwater pearl mussel habitat condition
- Increase the area of freshwater pearl mussel habitat negatively effected
And in so doing:
- Prevent juvenile recruitment, owing to unsuitable juvenile habitat condition
- Cause stress to adult mussels resulting in reproductive failures
- Cause mortalities of adult mussels, impacting population size
- Result in an extended ‘gap’ in the population’s age profile, impacting population size and future reproductive potential
- Increase the patchiness of mussel distribution, impacting future reproductive potential.
AAs involving the species are particularly challenging owing to its sensitivity and the potential for many developments and activities to have indirect effects on populations and their habitats. Detailed scientific analyses may be required, in particular, of potential hydrological and morphological impacts, as well as pollutant sources (fine sediment, dissolved and particulate nutrients and organic matter).
Alteration of the natural hydrological regime can have significant negative effects on freshwater pearl mussels. Sediment and nutrient mobilisation, transport and deposition are integrally linked to hydrology. Soil compaction, creation of preferential flow paths and drainage can increase run-off rates. This has the effect that:
- flood peaks increase in energy/erosive power and occur more rapidly, resulting in
- Increased river bank erosion and slumping, leading to river widening and increased sediment load
- Scour of freshwater pearl mussel habitat
- lower base flows, resulting in
- exposure (emersion) of mussels and their habitat
- increased sedimentation of mussel habitat
- increased macrophyte and macroalgal colonisation of mussel habitat.
Sub-surface/shallow groundwater flows to rivers help maintain oxygen levels in the substratum and influences sediment mobilisation and deposition, thus contributing to favourable juvenile freshwater pearl mussel habitat condition. As well as providing hydrological function, the importance of fringing wetlands as a food source to the freshwater pearl mussel is increasingly being recognised. Water flowing through and over such wetlands accumulates detritus that has been shown to play an essential role in sustainable juvenile growth and survival. Restoration of a near-natural hydrological regime is necessary to the achievement of the conservation objective for most freshwater pearl mussel populations.
The interactions of hydrological and morphological impacts are complex. In recent years examples have been recorded of river bank erosion and slumping, owing to a combination of grazing and trampling of the riparian vegetation and altered flow regime. The artificial widening of the channel has resulted in lower water depths and emersion (exposure) and death of mussels at low flows.