Mosses provide early warning of air pollution

21 September 2010 by Adele Walker

Soft to sit on and often picturesque, mosses can also provide important early warnings of increased atmospheric pollution.

Mossy woodland

A mossy woodland floor

The European Moss Survey has been monitoring heavy metal deposition from the atmosphere since 1990. Now researchers have carried out the first detailed analysis of factors affecting nitrogen concentration in mosses.

Nitrogen pollution is a big threat to biodiversity, ecosystem services and human health. It can leach from soils into lakes and rivers and cause eutrophication (overfertilisation) - a state where too much algae starves the water of oxygen, with dire results for fish and other water life.

"Eutrophication of ecosystems is a real problem at the moment and will continue to be in the future," says Dr Harry Harmens at the Centre for Ecology & Hydrology (CEH), a lead author of the report.

Ecosystems dominated by mosses and lichens are amongst the most sensitive to nitrogen pollution. Because mosses are so good at absorbing nitrogen they prevent it from leaching into ground water, but if they get overloaded they quickly deteriorate. Some moss species are especially sensitive to increased levels of nitrogen.

Harmens and his European colleagues have found that mosses are reliable indicators of air pollution risks to ecosystems, because they get most of their nutrients direct from the air and rain, rather than the soil.

They have other benefits too. Monitoring pollutants in rain or airborne particles requires frequent sampling and is expensive, so it's only done at a limited number of sites across Europe. On the other hand, collecting mosses is relatively cheap, there are thousands of sampling sites across Europe, and scientists can get useful information by sampling them just every five years.

River running though a fell

Moss covers the banks of a stream

So mosses are ideal for monitoring trends in the deposition of pollutants over time and across difference places.

Since 2000 the European moss survey has been conducted by a special international programme (ICP Vegetation), which is coordinated from CEH in Bangor.

ICP Vegetation is part of a United Nation Convention on Long-range Transboundary Air Pollution. This Convention provides the framework for controlling and reducing damage to the environment and human health caused by air pollution. ICP Vegetation looks at the effects of air pollutants on crops and other vegetation, and supplies some of the science that underpins emission reduction policies across Europe.

The main purpose of the survey is to map the distribution of pollutants in mosses across Europe, identify the most polluted areas and monitor trends over time.

"We measure element concentrations in the last three years of growth and use these to estimate the annual deposition of air pollutants," says Harmens, who is chair of ICP Vegetation.

The researchers ran a series of statistical analyses to account for factors such as the moss species used or distance from emission sources, which might cause variations in the relationship between the amount of a pollutant in the atmosphere and the amount the moss has absorbed. They cross-checked their moss data against modelled air pollution data. This data is provided by European nations to another branch of the Convention - the European Monitoring & Evaluation Programme (EMEP).

The results have been encouraging and confirm that mosses are reliable indicators of levels of airborne pollution.

Moss data provides a better geographic coverage than measured deposition data and can reveal more about actual atmospheric pollution at a local level. This is important for monitoring the condition of, for example, designated conservation areas such as the network of Natura 2000 sites, which was established by the European Union to protect Europe's most valuable and threatened species and habitats.

"So this makes moss an excellent early-warning system of ecosystems at risk of high deposition levels," Harmens concludes.

The study on nitrogen is published in Atmospheric Environment. ICP Vegetation reports on mosses and heavy metals have recently been published in Environmental Pollution and the Journal of Atmospheric Chemistry.

'First Europe-wide correlation analysis identifying factors best explaining the total nitrogen concentration in mosses' - Schröder, Winfried et al. Atmospheric Environment, 2010. doi:10.1016.j.atmosenv.2010.06.024