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

The depth of the summer mixed ( E d ) of 21 Canadian Shield lakes in northwestern Ontario was examined in relation to lake size (surface area, A o ) and water clarity (extinction coefficient, K d ) for periods ranging from 2 to 23 yr ( n = 1,408). The lakes range in A o from 4 to 4.9 × 10 5 ha, and midsummer (mid‐June through mid‐August mean) K d varied from 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, A o was the primary determinant of E d ; transparency significantly modified this relationship but only in small lakes ( A o <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 × CO 2 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.