1. Environmental Change Research Centre, UCL, London, WC1E 6BT, UK
2. US EPA, Corvallis, Oregon 97333, USA
3. Centre for Ecology and Hydrology, Bangor, LL57 2UW, UK
4. Norwegian Institute for Water Research, N-0349 Oslo, Norway
5. Finnish Environment Institute, PO Box 140, FI-00251 Helsinki, Finland
6. Department of Environment Assessment SLU, SE-75007 Uppsala, Sweden
7. Environment Canada, Burlington, Ontario, L7R4A6, Canada
8. Ontario Ministry of the Environment, Laurentian University, Sudbury, Ontario, P3E 2C6, Canada
9. Biology Centre, Institute of Hydrobiology, 37005 eské Budjovice, Czech Republic
10. Czech Geological Survey, 152 00 Prague, Czech Republic
11. These authors contributed equally to this work.
12. Deceased.

Correspondence to: Donald T. Monteith^1,11 Correspondence and requests for materials should be addressed to D.T.M. (Email: dmonteit@geog.ucl.ac.uk).

Several hypotheses have been proposed to explain recent, widespread increases in concentrations of dissolved organic carbon (DOC) in the surface waters of glaciated landscapes across eastern North America and northern and central Europe^{1, 2, 3}. Some invoke anthropogenic forcing through mechanisms related to climate change^{3, 4, 5}, nitrogen deposition⁶ or changes in land use⁷, and by implication suggest that current concentrations and fluxes are without precedent. All of these hypotheses imply that DOC levels will continue to rise, with unpredictable consequences for the global carbon cycle. Alternatively, it has been proposed that DOC concentrations are returning toward pre-industrial levels as a result of a gradual decline in the sulphate content of atmospheric deposition^{8, 9, 10}. Here we show, through the assessment of time series data from 522 remote lakes and streams in North America and northern Europe, that rising trends in DOC between 1990 and 2004 can be concisely explained by a simple model based solely on changes in deposition chemistry and catchment acid-sensitivity. We demonstrate that DOC concentrations have increased in proportion to the rates at which atmospherically deposited anthropogenic sulphur and sea salt have declined. We conclude that acid deposition to these ecosystems has been partially buffered by changes in organic acidity and that the rise in DOC is integral to recovery from acidification. Over recent decades, deposition-driven increases in organic matter solubility may have increased the export of DOC to the oceans, a potentially important component of regional carbon balances¹¹. The increase in DOC concentrations in these regions appears unrelated to other climatic factors.

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