ACID-INDUCED CHANGES IN DOC QUALITY IN AN EXPERIMENTAL WHOLE-LAKE MANIPULATION

Fluorescence analyses of archived water samples were used to typify di ssolved organic carbon (DOC) quality in experimentally acidified lakes and reference lakes at the Experimental Lakes Area, in northwestern O ntario. Carbon-specific DOC fluorescence (CSF) during peak acidificati on (pH 4.5) was 40-50% of that for a high-DOG reference lake and simil ar to a low-DOC reference lake. Reference lakes showed similar but sma ller decreases in CSF during several years of prolonged drought in the late 1980s. During the 1990s, recovery from acidification resulted in increased CSF, whereas reference lakes remained unchanged during the same time period. In addition to causing decreased [DOC], acidificatio n causes changes in fluorescence-peak geometry that indicate a switch in DOC quality from allochthonous to ''autochthonous-like'' during aci dification. The acid-induced change in DOC quality was likely due to i ncreased chemical oxidation or precipitation of the UV-absorbent aroma tic portions of allochthonous DOC molecules, leaving more UV-transpare nt aliphatic chains. The change in the nature of DOC following acidifi cation and drought may have an important role in physical, biological, and chemical processes within these lakes. With recovery from acidifi cation, DOC quality has also recovered.