THE role of organic acids in surface-water acidification is a matter of recent controversy1-8. It has been suggested1,4-6 that lakes and streams in the northeastern United States and southern Scandinavia that have high mineral acidity resulting from acid deposition had, before acid deposition, high concentrations of dissolved organic carbon (DOC) and were acidified by natural organic acids. The suggestion is that deposition of strong mineral acids has been buffered by concurrent losses in organic acids and DOC4-6, resulting in little or no overall change in pH. Despite considerable debate6-8 and comparative analyses of lake chemistry2,7, this hypothesis has never been tested experimentally in the field. Here we present results from an experimental acidification of a brown-water stream that tests two of the major elements of the hypothesis. We find that DOC concentrations are not reduced by acidification, and that the organic acid-base system has only a very limited capacity to buffer inputs of strong mineral acids. In addition, mineral acids mobilize toxic forms of aluminium.
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