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. 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. 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. 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 balances. The increase in DOC concentrations in these regions appears unrelated to other climatic factors.

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Skjelkvåle, B. L.; Stoddard, J. L.; Jeffries, D. S.; Tørseth, K.; Høgåsen, T.; Bowman, J.; Mannio, J.; Monteith, D.T.; Mosello, R.; Rogora, M.; Rzychon, D,; Vesely, J.; Wieting, J.; Wilander, A.; and Worsztynowicz, A., "Regional scale evidence for improvements in surface water chemistry 1990-2001" (2005 Data demonstrates a continued regional-scale chemical response of acid sensitive lakes and streams to emissions controls programs which is conducive to biological recovery. AbstractThe main aim of the international UNECE monitoring program ICP Waters under the Convention of Long-range Transboundary Air Pollution (CLRTAP) is to assess, on a regional basis, the degree and geographical extent of the impact of atmospheric pollution, in particular acidification, on surface waters. Regional trends are calculated for 12 geographical regions in Europe and North America, comprising 189 surface waters sites. From 1990e2001 sulphate concentrations decreased in all but one of the investigated regions. Nitrate increased in only one region, and decreased in three North American regions. Improvements in alkalinity and pH are widely observed. Results from the ICP Waters programme clearly show widespread improvement in surface water acid-base chemistry, in response to emissions controls programs and decreasing acidic deposition. Limited site-specific biological data suggest that continued improvement in the chemical status of acid-sensitive lakes and streams will lead to biological recovery in the future.

For more than 50 years, scientific insights from surface water monitoring have supported Swedish evidence-based environmental management. Efforts to understand and control eutrophication in the 1960s led to construction of wastewater treatment plants with phosphorus retention, while acid rain research in the 1970s contributed to international legislation curbing emissions. By the 1990s, long-time series were being used to infer climate effects on surface water chemistry and biology. Monitoring data play a key role in implementing the EU Water Framework Directive and other legislation and have been used to show beneficial effects of agricultural management on Baltic Sea eutrophication. The Swedish experience demonstrates that well-designed and financially supported surface water monitoring can be used to understand and manage a range of stressors and societal concerns. Using scientifically sound adaptive monitoring principles to balance continuity and change has ensured long-time series and the capability to address new questions over time.

Sulphate deposition has decreased by about 60% in the Nordic countries since the early 1980s. Nitrogen deposition has been roughly constant during the past 20 years, with only a minor decrease in the late 1990s. The resulting changes in the chemistry of small lakes have been followed by national monitoring programmes initiated in the 1980s in Finland (163 lakes), Norway (100 lakes) and Sweden (81 lakes). These lakes are partly a subset from the survey of 5690 lakes in the Northern European lake survey of 1995. Trend analyses on data for the period 1990-1999 show that the non-marine sulphate concentrations in lakes have decreased significantly in 69% of the monitored lakes. Changes were largest in lakes with the highest mean concentrations. Nitrate concentrations, on the other hand, were generally low and showed no systematic changes. Concentrations of non-marine base cations decreased in 26% of the lakes, most probably an ionic-strength effect due to the lower concentrations of mobile strong-acid anions. Acid neutralising capacity increased in 32% of the lakes. Trends in recovery were in part masked by large year-to-year variations in sea-salt inputs and by increases in total organic carbon concentrations. These changes were most probably the result of climatic variations. Nordic lakes, therefore, show clear signs of recovery from acidification. Recovery began in the 1980s and accelerated in the 1990s. Reductions in sulphur deposition are the major "driving force" in the process of recovery from acidification. Further recovery can be expected in the next 10 years if the Gothenburg protocol on emissions of acidifying pollutants is implemented.