Over the last 20 years lakes and streams in remote parts of the UK, southern Scandinavia and eastern North America have been increasingly stained brown by dissolved organic matter. In this weeks Nature journal (22 November) an international team, led by researchers from UCL (University College London) and the US Environmental Protection Agency (EPA), demonstrates that the colour change is indicative of a return to a more natural, pre-industrial state following a decline in the level of acid rain.
Don Monteith, Senior Research Fellow at the UCL Environmental Change Research Centre, says: 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. In the past two decades an increasing amount of this carbon has been dissolving into our rivers and lakes, turning the water brown.
There have been numerous attempts to explain whats happening, with everything from global warming to changing land-use cited as the cause. Some studies have suggested that were seeing an unprecedented phenomenon as soils destabilise with unpredictable consequences for the global carbon cycle.
John Stoddard of the EPA says: By analysing water chemistry records from over 500 sites across the northern hemisphere weve 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. 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 were seeing waters returning to their natural, pre-industrial state. However, more research is needed into the implications for freshwaters. The environmental pathways of heavy metals like aluminium and mercury, for example, are closely tied to dissolved organic carbon, and its too early to
|Contact: Dave Weston|
University College London