Poor air quality not only harms human health; it also impacts the structure and function of ecosystems, often far away from the emission sources. This report focuses on the deposition of airborne sulphur (S) and nitrogen (N) compounds and their negative effects on ecosystems. N- and S-containing air pollutants are released to the atmosphere by combustion processes such as the burning of coal or petrol, and by non-combustion processes such as agricultural fertiliser application (EEA, 2013a; EEA, 2014a).
The report evaluates how European ecosystems were affected by acidifying and eutrophying air pollutants in the past decades, and also how they are predicted to be affected in the future if the 2012 amended Gothenburg Protocol under the Convention on Long-range Transboundary Air Pollution (LRTAP) will be fully implemented by 2020. The presented results will be used to inform forthcoming updates of the EEA Core Set Indicator 'Exposure of ecosystems to acidification, eutrophication and ozone' (CSI 005) (EEA, 2012a) and the biodiversity indicator 'Critical load exceedance for nitrogen' (Streamlining European Biodiversity Indicators; SEBI 009) (EEA, 2010a).
Over the last century, rising anthropogenic air pollutant emissions accompanied increasing industrialisation in Europe. Polluted air masses containing sulphur (S) and nitrogen (N) compounds can travel long distances over national borders and damage other countries' resources: their freshwaters, forests, grasslands or cultural heritage (i.e. buildings, monuments, etc.).
Atmospheric deposition of such pollutants caused the past acidification of thousands of European lakes, rivers and streams. Fish species such as brown trout and Atlantic salmon, the latter a fish that spends most of its life in the sea before returning to fresh water to spawn, are particularly sensitive to acidification. 'Acid rain' led to the loss of fisheries in large regions of Sweden and Norway, with the issue being recognised as a major environmental problem by scientists and the public from the 1960s (e.g. Odén, 1968).
In the early 1970s symptoms of widespread forest decline appeared in central Europe, with the acidification of many forest soils first being regarded as a major cause in Germany (e.g. Ulrich et al., 1979). Scientific consensus subsequently emerged that acid deposition of S and N compounds was playing a significant role as predisposing or contributing factors leading to the observed impacts on trees.
To allow the quantification of environmental harm associated with air pollution, the so-called 'critical loads' concept was introduced as an effects-based tool for assessing the sensitivity of freshwater and terrestrial habitats to the harmful effects of S and N deposition (e.g. Nilsson and Grennfelt, 1988). A critical load is the upper limit of one or more pollutants, deposited to the Earth's surface, that an ecosystem such as a lake or a forest can tolerate without being damaged in its function (as for example the nutrient nitrogen cycle) or its structure (as for example with respect to plant species richness). A positive difference between the deposition loads of acidifying and/or eutrophying airborne pollutants and the critical loads is termed an 'exceedance' (see Boxes 1.1 and 1.3 in Chapter 1).
With respect to eutrophication, in different regions of Europe the atmospheric supply of nutrient N is in the range of around 2 to more than 40 kilograms per hectare per year (see Box 1.1 in Chapter 1). This has the potential to enrich the short- and long-term N content of soils, resulting in increased plant growth and hence species competition. The N supply via the air can therefore directly lead to eutrophication effects in terrestrial habitats, such as nutrient-poor grasslands. While excessive N stimulates the presence of nitrophilous (nitrogen‑loving) plant species, it reduces the occurrence of species adapted to low N availability. This in turn can lead to changes in species richness i.e. changes in biodiversity over time (e.g. Steven et al., 2004; Emmett et al., 2007).