Source Attribution and Critical
Loads Assessment for Special Areas of Conservation and Special
Protection Areas in the UK
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
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
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
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.
The report is available for download from the SNIFFER Website