Effects of lake size, water clarity, and climatic variability on mixing depths in Canadian Shield lakes

Abstract

The depth of the summer mixed (Ed) of 21 Canadian Shield lakes in northwestern Ontario was examined in relation to lake size (surface area, Ao) and water clarity (extinction coefficient, Kd) for periods ranging from 2 to 23 yr (n = 1,408). The lakes range in Ao from 4 to 4.9 × 105 ha, and midsummer (mid‐June through mid‐August mean) Kd varied from <0.2 to >2.5 m‒1 with greatest variation in small lakes that were experimentally eutrophicated or acidified or whose terrestrial basins were burned. Over the full spectrum of lake sizes, Ao was the primary determinant of Ed; transparency significantly modified this relationship but only in small lakes (Ao <500 ha). In noneutrophic shield lakes, transparency is controlled by the concentration of dissolved organic C (DOC). Lower DOC concentrations (a likely consequence of 2 × CO2 climate change) would cause transparency to increase, resulting in 1–2‐m‐deeper epilimnia in small lakes; there would be no similar effect in large lakes.