Physical and Chemical Limnology of the Lakes of Manitoulin Island

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Abstract
Of 51 of the large lakes on Manitoulin Island investigated from 1965 to 1970, 10 were found to be dimictic and 41 polymictic, most of the latter being very shallow. Twenty of the polymictic lakes evidenced one or more trenches in the lake bottom. These trenches may be maintained by wind-induced water currents. In one such trenched lake, Smoky Hollow Lake, winter oxygen declined to less than 1 ppm before surface melt-water entered the lake under the ice sheet. Winter kills of fish such as yellow perch (Perca flavescens) and northern pike (Esox lucius) were observed in three lakes, but the presence of older age-classes of fish in most lakes suggested that extensive winter kills are not common. The survival of fishes in the many shallow lakes is made possible by the continued presence of at least a small amount of oxygen in winter and the timely arrival of surface melt-water under the ice sheet. Most of the lakes had a high pH (7.9–9.1) and were rich in magnesium (mean 17.9 ppm) in relation to calcium (34.2 ppm), consistent with the underlying dolomitic bedrock. Sodium (mean 1.5 ppm) and potassium (0.8 ppm) were present at unusually low concentrations.

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