OR/14/047 Critical loads

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Farr, G, and Hall J. 2014. Atmospheric deposition and groundwater dependent wetlands: implications for effective catchment management and future Water Framework Directive groundwater classification in England and Wales. British Geological Survey Internal Report, OR/14/047.

Critical loads are a quantitative estimate of exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge (Nilsson & Grennfelt, 1988). In the UK critical loads are applied to broad habitats sensitive to acidification and/or eutrophication; this report considers only empirical critical loads for eutrophication (nutrient nitrogen) from atmospheric nitrogen.

Critical loads for nitrogen are published as a range (Bobbink & Hettelingh, 2011)[1] to encompass the variability in response of habitats to nitrogen. In the UK, a single value within these ranges has been chosen for the calculation of critical load exceedances; this ‘mapping value’ is based on UK evidence of nitrogen impacts (Hall et al. 2011[2]; 2014 in press). Nitrogen Critical loads are reviewed and updated under the Convention on Long Range Transboundary Air Pollution (CLRTAP), the last review being in 2010 (Bobbink & Hettelingh, 2011)[1]. Critical loads have been derived for seven ecosystem types: mire, bog and fen; grasslands; heathland, scrub and tundra; woodland and forest; inland surface waters; coastal; marine. In the UK critical loads have been mapped for N habitat types: acid grassland, calcareous grassland, dwarf shrub heath, montane, bog, managed and unmanaged woodlands, dune grassland and saltmarsh (Table 3). The critical loads are applied to each 1 x 1 km square of national-scale habitat distribution maps (Hall et al. 2011[2]; 2014 in press) and then compared with national 5 x 5 km resolution atmospheric N deposition maps. Where the deposition is greater than the critical load (ie, the critical load is 'exceeded') the habitat is considered to be at risk from adverse impacts from excess nitrogen deposition (Figure 4a).

The latest analysis (based on CBED annual mean deposition for 2010–12) shows that:

  • N deposition exceeds the critical loads across 65% of the total area of UK habitats sensitive to eutrophication

The above national-scale analysis is based on the areas of all nitrogen sensitive broad habitats mapped in the UK. The current study in relation to GWDTEs considers all designated habitat features (ie, not just ‘wetland’ habitats) found within SSSIs in England and Wales that are sensitive to nitrogen, and for which nitrogen critical loads are available. Critical loads are not available for some habitat feature types due to a lack of sufficient published data and evidence of nitrogen impacts. In addition, some SSSIs may contain habitat features that are not sensitive to nitrogen deposition. Consequently, in this study, critical loads were applied to 2355 of the 3320 sites, with no critical loads available for 965 sites.

The critical load values applied to the sensitive habitat features of SSSIs may differ from those applied to the broad habitats mapped nationally (Table 3). The Statutory Nature Conservation Bodies have set ‘recommended’ values from within the published ranges, for use in Article 17 reporting for the Habitats Directive, and these are the critical load values that have been used in this study. In many cases these ‘recommended’ values are the same as the ‘mapping values’, but for some habitats they may be the minima of the published range; in particular for habitats where there is less UK evidence available, or for habitat types not mapped nationally. Critical loads have been applied (where available) to each nitrogen-sensitive feature habitat within each site; these critical loads may vary from one habitat to another (Table 3). This also means that critical load exceedance may vary from one habitat to another. For national scale work the exceedance metric ‘Average Accumulated Exceedance’ is frequently mapped (see Figure 4b); in this study a precautionary approach has been taken, by using the maximum exceedance (rather than AAE) per site (SSSI). It should also be noted that historically due to a lack of digital data on the spatial location of feature habitats within each site, it is assumed in this data analysis that all feature habitats can occur anywhere and everywhere within each site.

As with many other target values not all sites may follow the rules and in some cases impacts (on designated sites) can be seen below the critical loading value for nitrogen (Stevens et al. 2011)[3].

Table 3    Critical loads of nutrient nitrogen showing published ranges (Bobbink & Hettlingh, 2011[1])
and values applied in the UK (Hall et al. 2011[2])
Habitat type EUNIS code[4][1] Critical load range
(kg N ha-1 year-1)
UK Mapping Value[5][2].
(kg N ha-1 year-1)
Recommended Value[6]
(kg N ha-1 year-1)
Marine habitats
Mid-upper saltmarshes A2.53 20–30[7] 25 N/A
Pioneer & low saltmarshes A2.54/55 20–30[7] 25 N/A
Coastal habitats
Shifting coastal dunes B1.3 10–20[8] not mapped 10
Coastal stable dune grasslands B1.4[9] 8–15 [10] 9 acid dunes

12 non-acid dunes

8
Coastal dune heaths B1.5 10–20[8] not mapped 10
Moist to wet dune slacks B1.8[11] 10–20[8] not mapped 10
Inland surface water habitats
Softwater lakes (permanent oligotrophic) C1.1[12] 3–10[13] not mapped 3
Permanent dystrophic lakes, ponds, pools C1.4[14] 3–10[13] not mapped 3
Mire, bog & fen habitats
Raised & blanket bogs D1[15] 5–10[16] 8,9,10
(rainfall dependent)
5
Valley mires, poor fens & transition mires D2[17] 10–15[18] not mapped 10
Rich fens D4.1[19] 15–30[20] not mapped 15
Montane rich fens D4.2[21] 15.25[22] not mapped 15
Grassland & tall forb habitats
Semi-dry calcareous grassland E1.26 15–25[23] 15 15
Dry acid & neutral closed grassland E1.7[24] 10–15[18] 10 10
Inland dune pioneer grassland E1.94[25] 8–15[10] not mapped 8
Inland dune siliceous grassland E1.95[26] 8–15[10] not mapped 8
Low & medium altitude hay meadows E2.2 20–30[7] not mapped 20
Mountain hay meadows E2.3 10–20[8] not mapped 10
Molinia caerulea meadows E3.51 15–25[23] not mapped 15
Juncus meadows & Nardus stricta swards E3.52 10–20[8] 15 10
Moss & lichen dominated mountain summits E4.2 5–10[16] 7 7
Alpine & subalpine acid grassland E4.3 5–10[16] not mapped 5
Alpine & subalpine calcareous grassland E4.4 5–10[16] not mapped 5
Heathland, scrub & tundra habitats
Arctic, alpine & subalpine scrub F2 5–15[27] not mapped 5
Calluna dominated upland wet heaths F4.11[28] 10–20[8] 10 10
Erica tetralix dominated lowland wet heaths F4.11[28] 10–20[8] 10 10
Dry heaths F4.2[29] 10–20[8] 10 10
Forest habitats
Broadleaved woodland G1 10–20[8] 12 10
Beech woodland G1.6 10–20[8] 15 15
Acidophilous oak dominated woodland G1.8 10–15[18] 10 10
Coniferous woodland G3 5–15[27] 12 10
Scots Pine woodland G3.4 5–15[27] 12 12
File:OR14047fig4.jpg
Figure 4    Examples of national-scale critical load exceedance maps generated under Defra contract AQ0826: (a) Average Accumulated Exceedance (AAE) of nitrogen critical loads for sensitive UK habitats by CBED total nitrogen deposition for 2010–12; (b) Maximum AAE of nitrogen critical loads per SSSI by CBED total nitrogen deposition (area-weighted value per SSSI) for 2010–12. AAE is an exceedance metric that averages exceedance over the entire sensitive habitat area; it is calculated as: AAE=(∑exceedance*exceeded area) ÷ total sensitive habitat area.
Contains OS data © Crown Copyright and database right [2015].

References and footnotes[edit]

  1. 1.0 1.1 1.2 1.3 BOBBINK, R, HETTELINGH, J P. (Eds.). 2011. Review and revision of empirical critical loads and dose-response relationships. Proceedings of an expert workshop, Noordwijkerhout, 23e25 June 2010, ISBN 978-90-6960-251-6. http://www.rivm.nl/bibliotheek/rapporten/680359002.pdf
  2. 2.0 2.1 2.2 2.3 HALL, J, EMMETT, B, GARBUTT1, A, JONES, L, ROWE, E, SHEPPAR, L, VANGUELOVA, E, PITMAN, R, BRITTON, A, HESTER, A, ASHMORE, M, POWER, S, and CAPORN, S. 2011. UK Status Report July 2011: Update to empirical critical loads of nitrogen. Report to DEFRA under contract AQ801 Critical Loads and Dynamic Modelling. 5th July 2011. http://cldm.defra.gov.uk/PDFs/UK_status_report_2011_finalversion_July2011_v2.pdf
  3. STEVENS, C J, SMART, S M, HENRYS, P, MASKELL, L C, WALKER, K J, PRESTON, C D, CROWE, A, ROWE, E, GOWING, D J, and EMMETT, B A. 2011. Collation of evidence of nitrogen impacts on vegetation in relation to UK biodiversity objectives. JNCC Report, No. 447. http://jncc.defra.gov.uk/pdf/jncc447_web.pdf
  4. Habitat class of the European Nature Information System (EUNIS); these are the habitat classes for which the nutrient nitrogen critical load ranges have been set within Europe (Bobbink & Hettelingh, 2011).
  5. The single value from within the range used for national-scale critical loads mapping for UK broad habitats, based on UK evidence of nitrogen impacts (Hall, 2011).
  6. 7.0 7.1 7.2 Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.
  7. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem. Cite error: Invalid <ref> tag; name "10–20" defined multiple times with different content Cite error: Invalid <ref> tag; name "10–20" defined multiple times with different content Cite error: Invalid <ref> tag; name "10–20" defined multiple times with different content Cite error: Invalid <ref> tag; name "10–20" defined multiple times with different content Cite error: Invalid <ref> tag; name "10–20" defined multiple times with different content Cite error: Invalid <ref> tag; name "10–20" defined multiple times with different content
  8. For acidic dunes, the 8–10 kg N ha-1 year-1 range should be applied; for calcareous dunes the range 10–15 kg N ha-1 year-1 should be applied.
  9. 10.0 10.1 10.2 Quite reliable: when the results of some studies were comparable. Cite error: Invalid <ref> tag; name "8–15" defined multiple times with different content Cite error: Invalid <ref> tag; name "8–15" defined multiple times with different content
  10. Apply the lower end of the range to habitats with low base availability, and the higher end to those with high base availability.
  11. This critical load should only be applied to oligotrophic waters with low alkalinity with no significant agricultural or other human inputs.
  12. 13.0 13.1 Reliable: when a number of published papers of various studies showed comparable results. Cite error: Invalid <ref> tag; name "3–10" defined multiple times with different content
  13. This critical load should only be applied to waters with low alkalinity with no significant agricultural or other direct human inputs.
  14. Apply the high end of the range to areas with high levels of precipitation and the low end of the range to those with low precipitation. Apply the low end of the range to systems with a low water table and the high end of the range to those with a high water table.
  15. 16.0 16.1 16.2 16.3 Reliable: when a number of published papers of various studies showed comparable results. Cite error: Invalid <ref> tag; name "5–10" defined multiple times with different content Cite error: Invalid <ref> tag; name "5–10" defined multiple times with different content Cite error: Invalid <ref> tag; name "5–10" defined multiple times with different content
  16. For EUNIS category D2.1 (valley mires) use the lower end of the range.
  17. 18.0 18.1 18.2 Quite reliable: when the results of some studies were comparable. Cite error: Invalid <ref> tag; name "10–15" defined multiple times with different content Cite error: Invalid <ref> tag; name "10–15" defined multiple times with different content
  18. For high latitude systems apply the lower end of the range.
  19. Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.
  20. For high latitude systems apply the lower end of the range.
  21. Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.
  22. 23.0 23.1 Reliable: when a number of published papers of various studies showed comparable results. Cite error: Invalid <ref> tag; name "15–25" defined multiple times with different content
  23. Apply the lower end of the range to habitats with low base availability, and the higher end to those with high base availability.
  24. Apply the lower end of the range to habitats with low base availability, and the higher end to those with high base availability.
  25. Apply the lower end of the range to habitats with low base availability, and the higher end to those with high base availability.
  26. 27.0 27.1 27.2 Quite reliable: when the results of some studies were comparable. Cite error: Invalid <ref> tag; name "5–15" defined multiple times with different content Cite error: Invalid <ref> tag; name "5–15" defined multiple times with different content
  27. 28.0 28.1 Apply the high end of the range to areas with high levels of precipitation and the low end of the range to those with low precipitation. Apply the low end of the range to systems with a low water table and the high end of the range to those with a high water table.
    Apply the high end of the range to areas where sod cutting has been practiced; apply the lower end of the range to areas with low-intensity management.
  28. Apply the high end of the range to areas with high levels of precipitation and the low end of the range to those with low precipitation. Apply the low end of the range to systems with a low water table and the high end of the range to those with a high water table.
    Apply the high end of the range to areas where sod cutting has been practiced; apply the lower end of the range to areas with low-intensity management.