Source Attribution and Critical Loads Assessment for Special Areas of Conservation and Special Protection Areas in the UK
February 2007

A methodology was developed for allocating site relevant critical loads of acidity and nutrient nitrogen to designated features to assess the impact of industrial emissions on European Sites (Special Areas of Conservation (SAC) and Special Protected Area (SPA)). Three systems of habitat classification, EUNIS, Annex I of the Habitats Directive, and BAP, were used in the linking of designated features to empirical critical loads of nitrogen and critical loads of acidity.

The process of allocating critical loads was divided into two phases relating to the use of two different methodologies. SAC Annex I habitats and Annex II plant species were treated together in one methodology, while SPA and SAC Annex II non-plant species were treated with a separate methodology.

Designated features for SACs and SPAs, were assessed for their sensitivity to acidification and eutrophication and suitable critical loads were then allocated for each. The assessment of sensitivity and linkages to critical load habitat classes were carried out by habitat and species specialists from English Nature, CEH, JNCC and Scottish Natural Heritage.

Critical loads for SPA features and SAC Annex II ‘non-plant’ features were assigned based on known linkages between the species and the habitat on which it depends either for breeding, feeding or roosting.

Six broad habitats for acidity were used in the assignment of critical loads of acidity (acid grassland, calcareous grassland, dwarf shrub heath, bog, montane, unmanaged coniferous and broadleaved woodland). Critical loads of acidity for these habitat classes were mapped for the whole of the UK at a resolution of 1 km.

Components of the critical load function, CLmaxS, CLminN, and CLmaxN,  were produced for all six habitats. By overlaying the 1 km critical loads data onto the polygons of the sites, maximum and minimum values for each polygon for each acidity habitat class were obtained.

Critical loads for nitrogen were based on the Berne empirical critical loads defined at the Berne workshop. Impacts of exceedance and reliability of a critical load were described.

Critical Loads for acidity for freshwater features were assessed using the results of previous work reported by Curtis et al., 2003 (Freshwater Screening and Assessment Based on Freshwater Critical Loads).

A database in Excel was produced to provide a lookup table for each for SAC, SPA to obtain lists of features and relevant critical load values. Written guidance was drafted to allow users to interpret their sites of interest. The Excel database was later transferred to the APIS web site.

A limitation of this assessment is the assumption that all European interest features are present in every polygon. Using the dominant soil type for producing acidity critical loads can be inaccurate for habitats not found on this dominant soil type. Furthermore, some features are found on more than one soil type, leading to the requirement to assess some features at a site specific level.

The present assessment does not address the integration between air pollution and other factors like land-use management and climate change in respect of exceedances of critical loads. However, combined with an assessment of air pollution conducted in parallel in this project (Dore et al 2005), the results provide the basis for a suitable screening approach  for assessing the impacts of air pollution on sensitive sites and features around the UK.

Copies of this report are available from the Foundation, in electronic format on CDRom at £20.00 + VAT or hard copy at £15.00, less 20% to FWR members.
N.B. The report is available for download from the SNIFFER Website