Choice of reference levels for dissolved organic carbon - a critical step for acidification assessments

A quarter of a century ago, Krug and Frink published an article in Science, where they suggested that acid deposition would not acidify lakes and streams, but merely replace the natural organic acidity with strong acids of anthropogenic origin. The decadal trend of rising surface water DOC (dissolved organic carbon) that correlates with declining sulphate deposition is cause for reconsidering that controversial claim. If the observed increase in DOC is actually a recovery to pre-acidification levels, then more recent, higher DOC concentrations may better represent pre-industrial reference conditions than the lower DOC levels during the height of acidification. The choice of DOC reference level is a critical step when assessing acidification status. Here we show that if the pre-industrial reference levels for DOC are higher than they were previously thought to be, then the actual extent of surface water acidification in Sweden is substantially less than previously reported, if still far from unimpacted. We assessed the acidification status of 66 lakes with long-term observations which represent a population of c:a 12,700 acid sensitive lakes in nemoral/boreal Sweden. The lakes were classified by the deviation from the reference (i.e. pre-industrial) pH (ΔpH). Whereas 30 of the 66 study lakes were classified as severely acidified (ΔpH ≥ 0.8) in 1990 using the DOC levels observed then as the pre-industrial reference, only 18 of them would have been classified as severely acidified with the higher DOC observed in 2008 as reference level. Furthermore, by 2008, 17 of the lakes would be classed as significantly acidified (ΔpH ≥ 0.4) with the lower DOC reference. With the higher DOC reference, only 7 lakes would be classified as significantly acidified. With further progress in the recovery from acidification, in combination with climate changes, future DOC levels may rise even further. These findings emphasize the need to establish reference levels for DOC, which influences so many aspects of aquatic ecosystems, including acidity. A simple regression approach, where SO4- and Cl-concentrations, precipitation and temperature were used to predict DOC, indicates that pre-industrial levels were 2.6 mg/l (or 21 %) higher in median than what is observed today. Additional efforts with more sophisticated methods, such as hydrogeochemical modeling or paleolimnological reconstructions, are however needed to refine these estimates of DOC reference levels.