Indicator assessment
Critical loads are thresholds above which
significant harmful effects occur on sensitive UK habitats. The
percentage of sensitive habitat area on which acid deposition
exceeded critical loads fell from 73% in 1996 to 60% in 2000. It
has subsequently shown little or no overall change and was at 58%
in 2005. The percentage of sensitive habitat area where eutrophying
pollutants (i.e. nutrient nitrogen) exceeded critical loads fell
from 66% in 1996 but has remained at between 59 and 61% since 2000.
Both measures are assessed as improving in the long-term, although
they show little or no overall change since 2000.
Description of trends
Approximately 78,000 km2 of
terrestrial habitats is sensitive to acid deposition. About
74,000 km2 is sensitive to eutrophication; many of these
areas are sensitive to both. The percentage area of sensitive
habitats for which acid deposition exceeded critical loads fell
from 73% in 1996 to 58% in 2005. During the same period, the
percentage area of sensitive habitats where eutrophying pollutants
exceeded critical loads fell from 65% in 1996 to 61% in 2005.
.
Relevance
The air pollutants sulphur dioxide, nitrogen
oxides and ammonia can contribute to acidification, and nitrogen
oxides and ammonia can contribute to terrestrial eutrophication.
Both problems can adversely affect semi-natural ecosystems.
Exceeding the critical load for acid deposition may have direct
effects, for example by altering the soil pH and therefore the
range of associated species or indirect effects, for
example by increasing dissolved aluminium concentrations in soil
and water. Excess nitrogen as a nutrient can lead to leakage of
nitrogen from soil systems, causing eutrophication of inland waters
or affect species composition, for example, by triggering
accelerated growth of some species at the expense of others.
Background and data sources
Critical loads are thresholds above which
significant harmful effects on sensitive UK habitats are known to
occur, given current levels of scientific understanding. Critical
loads are established separately for acidification and nutrient
nitrogen (eutrophication effects). The pollutants causing
acidification and eutrophication mainly arise as a result of
emissions from livestock waste, and burning fossil fuels.
There are three main steps in the assessment
of the area of sensitive habitat exceeding critical loads: the
calculation of critical loads for each of the sensitive habitats;
the mapping of the habitats and the identification of the area of
habitat where deposition exceeds the critical load.
Critical loads for acidity and nutrient
nitrogen are calculated for 11 broad habitats considered sensitive
to deposition. These include woodland, semi-natural grassland,
heathland, freshwater and coastal habitats. A variety of methods
are used to calculate critical loads, based either on empirical
(observational or experimental data) or mass balance (input/output)
data. Further information on how critical loads are
calculated are given on the Centre for Ecology and Hydrology
website (see below).
To identify the area exceeding critical loads,
deposition maps based on a 5km grid covering the UK are produced
based on the sum of wet, dry and cloud deposition. These deposition
data are overlain on maps of critical loads for each broad habitat
to calculate critical load exceedances.
In general the area of sensitive habitat where
critical loads are exceeded for both acidity and eutrophication is
lower in Scotland than elsewhere in the UK (Table 10). Detailed
critical load exceedance maps are available on the Centre for
Ecology and Hydrology website.
Since 2002 (2001-3), the inclusion of nitric
acid deposition in the assessment has increased the area of
estimated critical load exceedance compared to earlier periods. The
deposition values from 2003 (2002-2004) additionally include
aerosol deposition of NH4, NO3 and
SO4. In all years, the 3-year average deposition is used
to smooth substantial year-to-year variability.
Table 10. Percentage of area of sensitive UKhabitats exceeding
critical loads for acidification and eutrophication for 2005 (2004
to 6)
|
|
Acidification (%)
|
Eutrophication (%)
|
|
UK
|
58
|
61
|
|
England
|
70
|
89
|
|
Wales
|
82
|
87
|
|
Scotland
|
49
|
42
|
|
Northern Ireland
|
67
|
80
|
Further development planned
The methodology for calculating critical loads is reviewed and
updated periodically as new research data become available. In
particular there are plans to develop dynamic modelling approaches
to the calculation of critical load values for a range of broad and
priority habitats.
There are also plans to apply this methodology to the
protected site network in the UK – this work is under discussion
with the UK conservation agencies.