Natural and anthropogenic causes of lake acidification in Nova Scotia

Controversy has arisen over the recent acidification of lakes 1–4 , ascribed by many to anthropogenic acid deposition from the atmosphere 5,6 , and by some to natural processes of soil acidification enhanced by the regrowth of forests after cutting and burning 7–10 . Here we show, by analysing the chemistry of Nova Scotian lakes and ponds on base-poor terrains, that both anthropogenic and natural acidification can be important. We calculated correlations and regressions between hydrogen ion (H + ) concentrations and each of four predictors: dissolved organic carbon (DOC) (a surrogate for complex coloured organic acids, often of high molecular weight 11,12 ), non-marine sulphate (denoted by the prefixed asterisk as * SO 4 2− ; a surrogate for acid deposition 13 ), non-marine calcium ( * Ca 2+ ; the major basic cation from soil weathering and ion exchange), and the sum of the other non-marine base cations sodium, magnesium and potassium ( * (Na + + Mg 2+ + K + )). The results indicate that acidity in these waters is affected both by organic acids from peatland catchments and by acid deposition from long-range and local sources (see ref. 14).