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4 Ecosystem-based guidance

4.1 Introduction

The RMA requires consideration of the potential and actual effects of air pollutants on ecosystems as well as human health. This chapter presents new guidance for assessing the effects of air pollution on ecosystems. It must be read in conjunction with Effects of Air Contaminants on Ecosystems and Recommended Critical Levels and Critical Loads (Stevenson et al, 2000). The guidance is designed to encourage councils and others to investigate the potential impacts of air quality on ecosystems. However, the advice should be applied cautiously and flexibly as it is based on northern hemisphere research that may not be wholly applicable to New Zealand. It must also only be applied where areas of potentially sensitive ecosystems exist.

For the purposes of this discussion, an ecosystem is considered to include all the living organisms within natural and semi-natural systems and the environments in which they live and on which they depend. This specifically excludes the direct effects of pollutants when people inhale them, which are covered in the health-based guideline values.

On the basis of current scientific evidence it was only possible to assess a fraction of the organisms and impact pathways, and a limited number of common contaminants. Even for the various impacts that have been documented in the technical report (Stevenson et al, 2000), there is still a high degree of uncertainty, particularly when compared with human health-effects studies.

As a general principle, protecting the fundamental components of an ecosystem protects the ecosystem as a whole. The most fundamental components are climate and hydrology, and the physical and chemical characteristics of the soils and substrata. As the primary photosynthetic producers, plants are the next most fundamental component. To a large extent plants determine and exemplify the nature, diversity and health of ecosystems. Accordingly, if the characteristics of the fundamental components are protected, and substantially unaltered conditions for plant growth are assured, the health and quality of the ecosystem are also assured.

Animals are likely to be protected from the direct toxic effects of air pollutants by standards or guidelines established to protect human health. However, the possibility of some exceptionally sensitive species being adversely affected at such levels cannot be ruled out.

The discharge of toxic components such as heavy metals or persistent organic toxins into the environment can adversely affect animals in an ecosystem without adversely affecting plants. Toxic components are therefore a possible exception to the general principle that protecting the fundamental ecosystem components protects all plants and animals.

4.2 Recent research

There has been considerable research done in Europe and North America on developing guideline values to protect ecosystems. These are aimed at a variety of natural and semi-natural ecosystems and species, and are typically referred to as critical levels and critical loads.

Critical levels relate to the toxic effects of ambient air pollutants on plants. They are available for sulphur dioxide, oxides of nitrogen, ozone and ammonia. Critical loads relate to the toxic effects of deposition of air pollutants into ecosystems. These are available for acidity (the acidifying effect of sulphur and nitrogen air pollutants on ecosystems, particularly on soil/plant systems and natural waters), and nitrogen (the effects of nutrient enrichment).

The United Nations Economic Commission for Europe (UNECE) has developed the most comprehensive approach to ecosystem management as part of the Convention on Long Range Transboundary Air Pollution. The UNECE has used critical levels and critical loads for protecting natural and semi-natural ecosystems, which it defines as follows:

Critical levels:

The concentrations of pollutants in the atmosphere above which direct adverse effects on receptors such as plants, ecosystems or materials, may occur according to present knowledge.

Critical loads:

A quantitative estimate of an exposure, in the form of deposition, to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.

Generally, critical levels are based on relatively short-term (hours to a year) effects of a single pollutant species (for example, oxides of nitrogen, sulphur dioxide or ozone). Critical loads involve assessing the effect of several different pollutants over longer time periods (one year to decades). UNECE has defined critical loading for acidity deposition and nitrogen deposition. Both critical loads and critical levels vary from traditional air quality guideline values established for the protection of human health in several ways:

  • critical loads/levels are developed to protect both single organisms and ecosystems
  • critical loads/levels are established as close to the threshold point as possible with little or no margin of safety
  • critical loads/levels consider both direct and indirect impact mechanisms: they are effects-based approaches to environmental management
  • both critical loads and critical levels are defined in terms of their environment (for example, ecotype).

The critical load/critical level concept has subsequently been adopted by the World Health Organization (WHO, 1996). Due to their comprehensiveness and international acceptance, this approach seems to be a valid method for New Zealand to use as a management tool.

It is important, however, to recognise that the UNECE/WHO (1996) guidelines are based on central and northern European conditions. Therefore, in the absence of detailed specific New Zealand research, particularly for native species, it is difficult to gauge their applicability here. European ecosystems have developed with human influences at varying degrees of intensity over many centuries, and have almost certainly adapted to this situation to a certain degree. On the other hand, most New Zealand ecosystems have developed under conditions of low nitrogen supply, for example, and have only been subject to increased nitrogen supply over about the last century or less.

The guidelines should therefore be applied cautiously and users must review the more detailed discussions in Stevenson et al (2000). Where more specific information is available for New Zealand conditions, this should be used in preference to the guidelines given below.

4.3 Guidance for New Zealand

4.3.1 Critical levels

Critical levels for different airborne pollutants are presented in Table 4. They are based on the UNECE/WHO (1996) and Australia and New Zealand Environment and Conservation Council (ANZECC) guideline values. The fluoride guidelines presented with Table 6 are the same as those in the 1994 Guidelines.

Table 4: Critical levels for protecting ecosystems


Contaminant and land useCritical levelAveraging periodAdditional requirements

Sulphur dioxide:

  • agricultural crops

30 µg/m3

Annual and winter average

  • forest and natural vegetation

20 µg/m3

Annual and winter average

  • lichen

10 µg/m3



Sulphate particulate:

  • forests

1.0 µg/m3


Where ground-level cloud present > 10% of time

Nitrogen dioxide

30 µg/m3




8 µg/m3




  • forests

21,400 µg/m3-h


  • semi-natural vegetation

6420 µg/m3-h


  • crops (yield)

6420 µg/m3-h


  • crops (visible injury)

428 µg/m3-h


Mean daytime vpd below 1.5 kPa

1070 µg/m3-h


Mean daytime vpd above 1.5 kPa


  • special land use

1.8 µg/m3



1.5 µg/m3



0.8 µg/m3



0.4 µg/m3


0.25 µg/m3


  • general land use

3.7 µg/m3



2.9 µg/m3



1.7 µg/m3



0.84 µg/m3



0.5 µg/m3


  • conservation areas

0.1 µg/m3


Notes: Critical levels for nitrogen dioxide assume that either O3 or SO2 are also present at near guideline levels. Critical levels for ozone are expressed as a cumulative exposure over a concentration threshold referred to as AOT40 values (accumulative exposure over a threshold of 85.6 µg/m3, at 0°C), calculated as the sum of the difference between hourly ambient ozone concentrations and 85.6 µg/m3, when ozone concentrations exceed 85.6µg/m3). Ozone is only measured during daylight hours with a clear global radiation of 50 Wm-2 or greater; vpd = vapour pressure deficit.


In general, areas where sensitive ecosystems are located (rural and forest environments) are unlikely to experience pollution levels that breach these critical levels. There may be cause for concern if valued ecosystems are located near large individual sources or urban environments. It is difficult to make this same broad statement with regard to ammonia, since emission sources, emission rates and ambient concentrations in New Zealand are generally not well known. Intensive agriculture is a potentially significant ammonia source in rural areas, where sensitive ecosystems may also be located (although the little information available suggests that the critical level for ammonia is unlikely to be exceeded).

Of particular interest is the high sensitivity of forests and lichen to long exposures to sulphur dioxide. It is also important to note that the critical levels adopted by WHO (1996) are much lower than the 1994 guideline value for human health protection of 50 µg/m3 (annual average).

4.3.2 Critical loads

Critical loads for acidity are defined in terms of the acid-neutralising capacity of the soil, which depends on the nature of the parent material and the texture of the soil.

The probable critical loads for acidity for a range of New Zealand catchments are estimated in Technical Report 15 (Stevenson et al, 2000). They indicate that problems of soil/ecosystem acidification are unlikely to occur here in New Zealand, making it unnecessary to provide national guidelines for critical loads. However, where the area requiring protection is likely to be sensitive to acid deposition, as indicated by very low levels of alkalinity in water draining from the area (for example, less than 10 mg/l as calcium carbonate), or from other information, investigations of potential effects should be undertaken.

Where the potential effects of acid deposition and nitrogen enrichment need assessment, acid deposition and nitrogen enrichment can be estimated using approximate relationships between ambient air concentrations and their deposition rates for the pollutants of concern. In this way, ambient air monitoring data (modelled or monitored) can be used to estimate the worst-case deposition scenario. The results can then be compared with the critical load for the appropriate ecosystem. If the ambient air concentrations suggest that there may be problems from acid deposition or nitrogen enrichment (using the specified relationships), further investigations may be required, such as measuring actual deposition rates and the soil's acid-neutralising capacity.

Because of the low probability of significant soil acidification in New Zealand and the uncertainties about the nitrogen sensitivities of New Zealand ecosystems, the Ministry has not developed critical loads for use in New Zealand. However, councils are advised to identify valuable ecosystems and determine whether existing or predicted ambient air quality has the potential to affect them via nitrogen loads. Such assessments should ideally be undertaken as part of the assessment of environmental effects for consent applications for discharges to air.

Further local investigations need to be undertaken to refine the relationships between ambient air concentrations of nitrogen and sulphur compounds and deposition rates of acidity and nitrogen.

4.3.3 Applying ecosystem-based guideline values

As for the health-based guideline values, the critical levels provided above are mostly for investigating the potential region-wide effects of air pollution levels on ecosystems. With the exception of fluoride, they are not designed to assess individual discharges to air, and the same caveats as those given in section 3.6 apply to their use.

When they are applied, the following factors must be taken into account:

  • the potential for effects to occur on more sensitive sub-groups within an ecosystem
  • the inter-dependency between various species in an ecosystem
  • the value placed on the flora and fauna within an ecosystem (whether it is native or introduced and, if introduced, whether or not it is a pest species)
  • any key ecosystem role that sensitive at-risk species may have.

Further guidance on considering the impacts of individual discharges on ecosystems will be contained in guidance on assessing discharges to air.