Signals of climate trends and extreme events in the thermal stratificationpattern of multibasin Lake Opeongo, Ontario

Previous empirical observations on thermocline response to warmer condition s have been contradictory. Given that a deepening thermocline was observed for a small lake (0.5 km(2)) and a shallowing thermocline was observed for a larger system (73.6 km(2)), these contradictions may be attributable to d ifferences in lake size and the diminishing influence of water clarity on m ixing depth. The four basin systems of Lake Opeongo, Ontario, Canada, range in size from 4.4 to 22.1 km(2) and build an empirical suite of stratificat ion responses to climate variation for a wide range of lake sizes. From 195 8 to 1996, lake temperature profiles were taken in the four major basins of Lake Opeongo. Canonical correlation analyses linked earlier iceout dates, warm July-August air temperatures, and relatively high amounts of bright su nshine in July and August with warmer midsummer epilimnia and shallower mid summer thermoclines for all four basins. The occurrence of stratification p atterns associated with warmer climatic conditions increased (r = 0.44, p < 0.01) in parallel with the recent (post-1965) global warming trend. Such p atterns also tended to occur in El Nino years. Stratification conditions wi th cooler midsummer epilimnia and deeper thermoclines tended to occur in La Nina years and provided a pronounced signal in 1992, the year that experie nced a global cooling effect due to the eruption of Mount Pinatubo.