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.
Browning
of surface waters because of increasing terrestrial dissolved
organic carbon (OC) concentrations is a concern for drinking water
providers and can impact land carbon storage. We show that positive
trends in OC in 474 streams, lakes, and rivers in boreal and subarctic
ecosystems in Norway, Sweden, and Finland between 1990 and 2013 are
surprisingly constant across climatic gradients and catchment sizes
(median, +1.4% year–1; interquartile range, +0.8–2.0%
year–1), implying that water bodies across the entire
landscape are browning. The largest trends (median, +1.7% year–1) were found in regions impacted by strong reductions
in sulfur deposition, while subarctic regions showed the least browning
(median, +0.8% year–1). In dry regions, precipitation
was a strong and positive driver of OC concentrations, declining in
strength moving toward high rainfall sites. We estimate that a 10%
increase in precipitation will increase mobilization of OC from soils
to freshwaters by at least 30%, demonstrating the importance of climate
wetting for the carbon cycle. We conclude that upon future increases
in precipitation, current browning trends will continue across the
entire aquatic continuum, requiring expensive adaptations in drinking
water plants, increasing land to sea export of carbon, and impacting
aquatic productivity and greenhouse gas emissions.
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