Background
The authors write that "several hypotheses have been proposed to explain recent, wide-spread increases in concentrations of dissolved organic carbon (DOC) in the surface waters of glaciated landscapes across eastern North America and northern and central Europe," noting that "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," which hypotheses imply that "DOC levels will continue to rise, with unpredictable consequences for the global carbon cycle."

What was done
Working with water chemistry data from 522 individual stream and lake sites that were located within the six countries they studied and were "largely free of local disturbance," Monteith et al. derived trends and mean concentrations of various pertinent parameters for the period 1990-2004.

What was learned
The thirteen researchers from the United Kingdom, United States, Norway, Finland, Sweden, Canada and the Czech Republic report 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," thereby demonstrating that "DOC concentrations have increased in proportion to the rates at which atmospherically deposited anthropogenic sulphur and sea salt have declined," the latter as a result of changes in "meteorological factors that affect Atlantic storminess," but with the former being responsible for more than 85% of the total effect in most of the studied regions.

What it means
Monteith et al. say their findings suggest that "threats of wide-spread destabilization of terrestrial carbon reserves by gradual rises in air temperature or CO2 concentration (Freeman et al., 2001, 2004; Worrall et al., 2003) may have been overstated," and that the DOC fluxes from the regions they studied may simply "be returning to levels more typical of pre-industrial times."

References
Freeman, C., Evans, C.D., Monteith, D.T., Reynolds, B. and Fenner, N. 2001. Export of organic carbon from peat soils. Nature 412: 785.

Freeman, C., Fenner, N., Ostle, N.J., Kang, H., Dowrick, D.J., Reynolds, B., Lock, M.A., Sleep, D., Hughes, S. and Hudson, J. 2004. Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels. Nature 430: 195-198.

Worrall, F., Burt, T. and Shedden, R. 2003. Long term records of riverine dissolved organic matter. Biogeochemistry 64: 165-178.