Freshwater Umbrella News

Browner waters are more natural — increased DOC in lakes and streams a response to recovery from acid rain

Thursday November 22nd, 2007

Rivers and lakes in northern Europe and North America that have turned increasingly brown, the result of increased amounts of dissolved organic carbon (DOC), and are returning to a more natural, pre-industrialised state, says a study published today in the science journal Nature.

An international collaboration between researchers from the UK, USA, Canada, Norway, Sweden and Finland, the study drew on water chemistry time series from over 500 sites across the northern hemisphere and clearly shows that the rise in DOC is a response to a reduction in the amount of acid rain. The UK data were drawn from the UK Acid Waters Monitoring Network funded by the UK Department for Environment, Food and Rural Affairs, the body which also funds the Freshwater Umbrella research programme.

The research was led by Don Monteith from the Environmental Change Research Centre at University College London (UCL) and John Stoddard of the US Environmental Protection Agency (EPA), and also involved Chris Evans from CEH, a member of the Defra Freshwater Umbrella.

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, said Don Monteith.

There have been numerous attempts to explain what's happening, with everything from global warming to changing land-use cited as the cause. Some studies have suggested that we're seeing an unprecedented phenomenon as soils destabilise with unpredictable consequences for the global carbon cycle, he added.

By analysing water chemistry records from over 500 sites across the northern hemisphere 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. 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, John Stoddard explained.

In some ways we're seeing waters returning to their natural, pre-industrial state. However, more research is needed into the implications for fresh 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, he added.

The scientists hope that the results published today will settle the debate about the cause of the DOC increase and help move the scientific agenda forward.

We believe that there should be a lot more work going into the consequences of the potential changes in the carbon cycle — we don't have any real idea as to the fate of this dissolved organic carbon, said Don Monteith.

Data for this study were drawn from nationally funded monitoring programs in the UK, USA, Canada, Norway, Sweden and Finland under the auspices of the UNECE ICP WATERS programme. Trends in dissolved organic carbon, air temperatures and a suite of other chemical variables were assessed using data from 1990-2004. The study is the largest of its kind and the data represents the main source of high quality, long-term information about the condition of our headwater systems. Ironically many of the study sites, including those in the UK, are under threat due to cuts in funding.

We have learnt an enormous amount about these upland ecosystems under Government funded acid rain research, and these unique long-term datasets now provide a perfect baseline to track the future influence of climate change and nitrogen deposition on water quality, biodiversity and soil processes, said Don Monteith.

Regrettably, as acid deposition begins to fall and these waters are beginning to show signs of improvement, funding is being cut back sharply, and we risk losing this invaluable national scientific resource.

Without these records, UK scientists and policy makers will struggle to determine how and why climate change may be affecting wide swathes of the UK landscape into the future, he added.

The research has been reported on the Telegraph and BBC News websites.

Further details on the research paper can be found on the Nature website.