Acidification of lakes and rivers is one of the most serious environmental problems in Norway. Acid rain is a threat to aquatic biodiversity: it has resulted in the loss or depletion of more than 15 000 fish stocks. Liming is an effective way of reducing the damage.
Acidification of Norway’s lakes and rivers is mainly caused by pollutants from other countries that are transported in the atmosphere and deposited as acid rain. This problem can only be reduced through international agreements to reduce releases of pollutants.
Critical loads for deposition of acid rain are still being exceeded in much of the southern half of Norway. Liming makes lakes and rivers less acidic, improving the water chemistry and providing better conditions for fish and other freshwater organisms. The Norwegian authorities provide substantial funding for liming of many rivers and lakes.
The Norwegian Environment Agency has drawn up an action plan for liming for the period 2011–15.
Acidification started in Norway with the Industrial Revolution in the 1800s, and continued to worsen until the 1970s. In 1925, it was discovered that fish mortality was caused by acid water, but the links with acid rain were not understood until the 1950s. More and more fish stocks were damaged as the problem spread, leading to losses of aquatic biodiversity generally.
When the situation was at its worst, 30 % of mainland Norway was affected, and one in five species disappeared from the most acidic river systems. More than 15 000 fish stocks were wiped out or depleted: Norway lost 25 of its salmon stocks, and at least 20 others were depleted. The damage was worst in Vest-Agder and Aust-Agder counties in the far south of the country.
Liming programmes started in the mid-1980s, and 21 salmon rivers and 2 500 lakes and streams are now limed regularly. The effects are documented by monitoring water chemistry, fish, benthic animals and aquatic vegetation.
Many years of liming combined with reductions in acid deposition have improved water quality to the extent that ecosystems are recovering. The lower acidity (higher pH) improves conditions directly, and also means that less aluminium – which is highly toxic to many aquatic species – is released from soils. Both salmon stocks and their prey species are recovering, although there is still a considerable potential for further improvement of species diversity.
Since 2000, however, the positive trend has levelled off, and no major improvement is expected after 2010. Acid rain (sulphur and nitrogen deposition) is still a serious threat to freshwater biodiversity in Norway. About 10 % of mainland Norway is now damaged by acidification, with the most serious effects in the southernmost and westernmost parts of the country.
Much of the southern half of Norway has thin soil cover on bedrock consisting of acidic rocks such as gneiss and granite. These areas are very sensitive to acid rain. The liming programme is most extensive in the counties that are worst affected – Aust-Agder, Vest-Agder, Rogaland and Hordaland – and will have to be continued until critical loads for acidification of surface water are no longer exceeded.
Critical loads are used to express how much acidification different ecosystems can absorb without damage. In Norway, freshwater ecosystems are particularly sensitive to acidification, and critical loads are therefore low.
Acid rain is mainly caused by combustion of fossil fuels. Power plants, industrial processes and transport are the major sources of the emissions that cause acid rain. About 90 % of the sulphur and nitrogen deposited in Norway originates in other countries, especially the UK, Germany and Poland. This means that the amount of acid rain falling on Norway is to a large extent determined by developments elsewhere in Europe.
Sulphur emissions in Europe have been greatly reduced in the past 20–30 years. In Eastern Europe, this is partly a result of economic problems that have led to the closure of factories and lower energy production. In Western Europe, cuts in emissions have generally been achieved by installing emission abatement technology at industrial plants and by switching to the use of low-sulphur fuels. Motor vehicles and other mobile sources are an important source of nitrogen emissions, and this has made it more difficult to reduce releases of nitrogen.
Since 1980, total deposition of sulphur in precipitation in Norway has been reduced by about 75 %, while the corresponding figure for nitrogen is about 35 %. The reductions have largely been brought about by countries taking action to meet their commitments under international agreements.
Several binding protocols have been adopted under the Convention on Long-range Transboundary Air Pollution, including the Gothenburg Protocol, which entered into force in 2005. There are 26 parties to the Protocol, which in its first phase set emission ceilings for 2010 for sulphur, nitrogen oxides (NOx) and other pollutants. In 2012, new national emission reductions were agreed to be achieved in 2020 and beyond.