Winter and Spring pH Depressions in Northern Wisconsin Lakes Caused by Increases in pCO2

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Abstract
Seasonal variation in pH in six clearwater northwern Wisconsin lakes (mean pH 5.9-7.8; mean total alkalinity 9-830 .mu.eq .cntdot. L-1) encompasses an annual range of 1.0-1.5 units in each lake, with lowest values characteristically occurring during the late winter and early spring. In detailed investigations of one lake, we evaluated strong acid inputs, shifts in pCO2, and changes in organic acids as possible mechanisms for pH depression. Our results indicate the seasonal pH depressions are caused largely by pCO2 increases under the ice. Despite an acidic snowpack the pH depression could not be attributed to input of acid meltwater because total alkalinity did not change in the lake. Our results emphasize the importance of using measurements other than pH in investigating the causes of acid dynamics of natural waters.