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 Europe1,2,3. Some invoke anthropogenic forcing through mechanisms related to climate change3,4,5, 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 deposition8,9,10. 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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We thank the LRTAP Working Group on Effects and the EU 6th Framework Programme Euro-limpacs for support in the production and analysis of international, quality-controlled, comparable data. We also acknowledge the work of the ICP Waters Programme Centre at the Norwegian Institute of Water Research (NIVA), where the data were collated, verified and archived. The authors are indebted to many colleagues and organisations who provided data for this assessment, including: T. A. Clair, S. Couture, C. Gagnon, D. K. McNicol, R. C. Weeber, A. Paterson (Canada); J. S. Kahl, J. Kellogg, K. Roy, M. R. Hale, D. R. DeWalle (USA); the Finnish Environment Institute (SYKE) and Regional Environment Centres; the Norwegian Institute of Water Research (NIVA); the Swedish Environmental Protection Agency (Naturvårdsverket); and the UK Acid Waters Monitoring Network (supported by the Department for Environment Food and Rural Affairs) and supporting laboratories at: Fisheries Research Services, Pitlochry; Centre for Ecology and Hydrology, Wallingford; and the Environment Agency Llanelli. The information in this document has been funded in part by the US Environmental Protection Agency. It has been subjected to review by the National Health and Environmental Effects Research Laboratory, and approved for publication. Approval does not signify that the contents reflect the views of the Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.
Author Contributions D.T.M. and J.L.S. formulated the working hypothesis and analysed and interpreted the trend data. C.D.E. and H.A.d.W. contributed to the development of the hypothesis, assisted in the interpretation of the data, provided additional text and edited the manuscript. M.F., T.H., A.W., B.L.S., D.S.J., B.K. and J. Vuorenmaa provided data and commented on the text. J.K. provided advice and ideas on processes and contributed data from the Czech Republic (not included in the final analysis due to number and length of time series) consistent with the hypothesis. J. Vesely contributed advice, ideas and data in the early stages of development of our work.
The file contains Supplementary Figures S1-S3 and Supplementary Tables S1-S5 with Legends.
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Natural organic matter fractions and their removal in full-scale drinking water treatment under cold climate conditions in Nordic capitals
Journal of Environmental Management (2019)