Deliverable 5.3

Abstract: Report on biophysical catchment-scale modelling in the River Dee catchment, Scotland

Here we report on the use of hydrochemical models to investigate the likely changes in hydrology and water quality under scenarios of climate and land use change in the River Dee demonstration catchment, in northeast Scotland. Specific novel objectives were to: (1) predict the effect of any changes in water quality on freshwater biota, by linking catchment-scale, process-based hydrochemical models to measures of ecological impact; and (2) to integrate modelling outcomes with cost-effectiveness analysis carried out in work package 6, to determine the most cost-effective way of reducing eutrophication, both now and under future scenarios of environmental change.

The Dee catchment is a large, relatively unspoilt area, famed for its salmon fishing, shooting and hill walking. It has been designated at European level for the species it supports, in particular Atlantic salmon (Salmo salar), freshwater pearl mussel (Margaritifera margaritifera) and otter (Lutra lutra). The catchment is subject to significant pressures, including morphological alterations and nutrient inputs from sewage and agriculture, and the area remains of top conservation priority.

Three ecological indicators were identified: freshwater pearl mussel (M. margaritifera), macroinvertebrates (ASPT score) and macrophytes (MTR score). To be able to link hydrochemical model output to ecological impact, relationships were developed between nutrients and these ecological indicators. These were based on literature values (freshwater pearl mussel) or on empirical relationships. A nitrogen leaching model was then used to explore possible changes in nitrate concentration throughout the Dee catchment, and associated changes in ecological status.

Two sub-catchments of the Dee, subject to different pressures, were selected for more in-depth modelling applications: (1) the Tarland Burn sub-catchment, in the middle reaches of the Dee catchment, where stream flow, nitrogen (N) and phosphorus (P) were simulated using the STREAM-N and INCA-P models; and (2) the Loch of Skene sub-catchment further downstream, where work focused on P and suspended sediment concentrations in the Corskie Burn, the main tributary to the loch. Another study in the upland Gairn sub-catchment looks at stream temperature and salmonids and will be published elsewhere, as will a study examining total phosphorus in the Loch of Skene.

Within the Tarland and Skene sub-catchments, a suite of model runs were carried out using the climate and land use scenarios in isolation, in combination, and with a series of cost-effective mitigation measures identified previously. Key findings are that:

  1. Climate change alone is not projected to be large enough in this region to bring about significant changes to water quality. Land use change driven by socio-economic factors and climate could have a more significant impact.
  2. Land use changes are unlikely to be evenly distributed, with some areas undergoing little change. Other areas may be suitable for a range of different uses and will be more vulnerable.
  3. In the Tarland Burn sub-catchment, climate and land use scenarios result in simulated changes in suspended sediment, total phosphorus and nitrate concentrations which are large enough to be significant despite uncertainty in model output; phosphate concentrations show a smaller response. In the Skene sub-catchment, projected changes in climate and land use are very small, leading to little change in simulated stream nutrient status.
  4. Under the worst case scenario, the measures to reduce nitrate concentration in the Tarland Burn result in similar nitrate concentrations to the baseline, as the reduction in fertilizer usage is balanced out by the increased area of arable land. However, under the best case scenario nitrate concentrations may decrease by up to 30%. In the Skene sub-catchment, the projected changes in climate and land use are small enough that the effectiveness of measures is unlikely to change.
  5. Predicted ecological response is highly uncertain, but preliminary findings are that: (i) neither the Tarland nor the Skene sub-catchments are likely to meet literature-derived safe water quality thresholds for the freshwater pearl mussel; (ii) macroinvertebrates in the Tarland Burn are likely to remain in the ‘Clean’ category, with a shift towards the ‘Clean’/’Doubtful’ boundary under some scenarios; (iii) macrophytes in the Skene sub-catchment may shift towards more oligotrophic communities were mitigation measures put in place.
  6. In both the Tarland and Corskie Burns, the primary measures required to improve ecological status under the Water Framework Directive relate to reducing suspended sediment inputs, improving morphology and, in the Skene sub-catchment, removing barriers to fish migration.

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