HEADWATER LAKE CHEMISTRY DURING THE SPRING FRESHET IN NORTH-CENTRAL ONTARIO J. R. M. KELSO* Great Lakes Fisheries Research Branch, 1219 Queen Street East, Sault Ste. Marie, Ontario P6A 5M7, Canada C. K. MINNS Great Lakes Fisheries Research Branch, 867 Lakeshore Road, P.O. Box 5050, Burlington, Ontario L7R 4A6, Canada J.H. LIPSIT Great Lakes Fisheries Research Branch, 1219 Queen Street East, Sault Ste. Marie, Ontario P6A 5M7, Canada and D. S. JEFFRIES National Water Research Institute, 867 Lakeshore Rd., P.O. Box 5050, Burlington, Ontario L7R 4A6, Canada (Received 26 July, 1985; in revised form 31 December, 1985) Abstract. From data on 30 headwater lakes in north-central Ontario we found that, during the spring snow melt of 1981, all lakes underwent serious declines in alkalinity. Generally, SO42- , alkalinity, Ca 2+ and Mg 2 + concentrations were reduced by runoff and rain then recovered to intermediate levels after the major inputs declined. As expected, a range in responses was evident with lower alkalinity systems showing the greatest changes. The observed changes, however, were consistent with acid loading having depleted alkalinity. In calculating an input-output budget for each lake, we found that changes in C1 , Na ÷ , and K ÷ were consistent with atmospheric inputs being the major source as the difference between the expected input and the actual contribution from rain and snow had a mean near zero. There appears to be a significant, ~ 45 %, watershed source of sulphate that we hypothesize is from dry deposition occurring prior to snowfall and is eluted with the melting process. With refinements to a mass balance approach explaining the watershed source of SO42- and A1, we feel it is possible to predict springtime lake changes given a few chemical and simple morphometric variables. 1. Introduction In Ontario, the snowpack melt in the spring can provide between 36 and 77~o of the annual acid export from a watershed to a lake (Jeffries et al., 1979). This short-term massive input, perhaps coupled with any concomitant change in trace metal levels, can result in mortality or impairment of exposed fisheries (Leivestad and Muniz, 1976; * Author for all correspondence Water, Air, and Soil Pollution 29 (1986) 245-259. © 1986 by D. Reidel Publishing Company.