A fall in the level of acid raid is behind the increasingly brown colour of many of our lakes and streams, according to new research by a team of EU-funded scientists. The hypothesis suggests
that the colour change is the mark of a return to a more natural state.

‘A huge amount of carbon is stored in the form of organic deposits in soils, and particularly in the peatlands that surround many of our remote surface waters,’ explained Don Monteith of the
Environmental Change Research Centre at University College London (UCL). ‘In the past two decades an increasing amount of this carbon has been dissolving into our rivers and lakes, turning the
water brown.’

Writing in the journal Nature, the researchers note that many possible explanations for the colour change have been put forward over the years. ‘Some invoke anthropogenic forcing, through
mechanisms related to climate change, nitrogen deposition or changes in land use, and by implication suggest that current concentrations and fluxes are without precedent,’ they write.

To find out what was going on, the international team of researchers analysed water chemistry records from 1990 to 2004 from 522 remote lakes and streams in six north European and American
countries (the UK, Norway, Sweden, Finland, the US and Canada).

‘We’ve found that the dominant factor in the whole process is not global warming. The most important driver has actually been the major reduction in acid rain since the 1970s,’ said John
Stoddard of the US Environmental Protection Agency (EPA). ‘As acidity and pollutant concentrations in the soil fall, carbon becomes more soluble, which means more of it moves into our lakes and
rivers and more can be exported to the oceans. In some ways we’re seeing waters returning to their natural, pre-industrial state.’

However, Dr Stoddard warns that more research is needed into the implications of this change for our waters. ‘The environmental pathways of heavy metals like aluminium and mercury, for example,
are closely tied to dissolved organic carbon, and it’s too early to know how increasing organic matter will affect these toxic compounds.’

The results will also have impacts for water companies, as they will now be faced with the task of removing the colour from drinking water using equipment which is not used to coping with such
high concentrations of dissolved organic carbon.

EU support for the research came from the Euro-limpacs project (Integrated project to evaluate the impacts of global change on European freshwater ecosystems), which is funded under the Sixth
Framework Programme’s ‘Sustainable development, global change and ecosystems’ thematic area.