Interannual variation in the thermal structure of clear and colored lakes

We used end-of-summer temperature profiles to examine the thermal structure of 86 small (500 ha) lakes in Killarney Park, Ontario, Canada, during one cool (1997) and two extremely warm years (1998 and 1999). The main effect of the warm years, which had unusually high air temperatures during the spring, relative to the cool year was to create warmer surface waters, shallower mixing depths, and stronger metalimnetic thermal gradients in nearly all lakes. Changes in deep water temperatures differed between clear (DOC , 2m g L 21 ) and colored (DOC . 4 21 ) lakes. During warm years, the volume of cold water (,108C) was reduced in clear lakes. In colored lakes, deep water temperatures were more stable, and cold water volume actually increased during one warm year. We suggest that clear lakes will be more sensitive than colored lakes to the warming effects of climate change. Because clear lakes exhibit large thermal changes in response to small differences in DOC, they will also be more sensitive to changes in DOC levels associated with altered hydrological inputs, climate change, or acidification.