The environmental control of near-surface thermoclines in boreal lakes

We report on the effect of lake size, water transparency, and wind on the f requency of transient nearsurface thermoclines in 39 boreal lakes from the Experimental Lakes Area (ELA) and Northwest Ontario Lake Size Series (NOLSS ). This study was based on more than 3000 archived temperature profiles ama ssed over a 25-year period for lakes ranging from 2 ha to 8 million ha in s urface area. The incidence of transient thermoclines decreased with increas ing lake size from 90% of all summer days in small lakes (less than 4 ha) t o 40% or less in the larger NOLSS lakes (up to 34,700 ha). No transient nea r-surface thermoclines were detected in Lake Superior. Forest fires and cli matic variability were also found to affect the frequency of nearsurface th ermoclines. Long-term trends indicate an increase in average annual wind ve locity in the area, possibly as the result of extensive forest fires and cl earcutting. The subsequent decrease in the frequency of shallow secondary t hermoclines in aquatic ecosystems has possible consequences for the lake bi ota, as the result of changes in radiation, turbulence, and the nutrient re gime.