Effects of water level fluctuation and short-term climate variation on thermal and stratification regimes of a British Columbia Reservoir and Lake

Stratification and thermal regimes of a reservoir with fluctuating water levels were compared to a natural lake of similar morphometry and trophic status over a two-year period (2000-2001) in coastal British Columbia, Canada. We compared the timing and duration of stratification, summer heat budgets and heat fluxes in two morphometrically contrasting basins of Sooke Lake Reservoir and Shawnigan Lake (one shallow and one deep basin per water body). In the second year of the study, a 100-year drought allowed us to compare responses of a reservoir and a take to contrasting years of climatic conditions. Loss of volume from the reservoir during summer and fall caused stratification and thermal regimes to differ from Shawnigan Lake, but the magnitude of these differences was mediated by basin morphometry. Duration of summer stratification, timing of heat content, and the relative importance of seasonal heat fluxes in the shallow basin of Sooke Lake Reservoir were most different from Shawnigan Lake. While there were no major differences between years for Shawnigan Lake, contrasting years in precipitation and hydrology caused Sooke Lake Reservoir stratification and thermal regimes to differ between years. The magnitude of differences between years was mediated by basin size, with the shallower reservoir basin having greater differences between years. Our results indicate that reservoir physical processes are sensitive to short-term changes in hydrology, and that the combined impacts of shortterm climate variation and anthropogenic manipulation of hydrology may be greater in shallow reservoir ecosystems.