Natural sources of Acid neutralizing capacity in low alkalinity lakes of the precambrian shield

A detailed alkalinity budget was constructed for Lake 239 in the Experimental Lakes Area of northwestern Ontario and for three small watersheds in its terrestrial basin. Alkalinity generation in the lake averaged 118 milliequivalents per square meter per year, 4.5 times as high as the areal rate in the terrestrial basin. Although acid deposition in the area is low, only one of the three terrestrial watersheds was a significant source of alkalinity. A second terrestrial watershed yielded very little alkalinity. The third watershed, which contains a wetland, was a sink for, rather than a source of, alkalinity. An analysis of ion budgets for the lake revealed that more than half of the in situ alkalinity production was by biological rather than geochemical processes. The major processes that generated alkalinity were: biological reduction of SO(4)(2-)(53%), exchange of H(+) for Ca(2+) in sediments (39%), and biological reduction of NO(3)(-) (26%). Comparison with experimentally acidified Lake 223 revealed that alkalinity production by sulfate reduction increased in response to increased inputs of sulfuric acid.