Limnological succession in reservoirs: a paleolimnological comparison of two methods of reservoir formation

Analysis of diatoms, algal pigments, and chironomids in sediment cores from two otherwise similar prairie reservoirs demonstrated that differences in reservoir formation (river valley impoundment versus lake inundation) and h ydrological regime (variable versus stable water level) resulted in distinc t patterns of aquatic community change. Lake Diefenbaker, a 500-km(2) reser voir created by damming the South Saskatchewan River in 1968, experiences w ater level fluctuations of 6 m.year(-1). In contrast, impoundment of Buffal o Pound Lake in 1952 flooded a natural lake, raised mean water levels simil ar to 2.0 m, and reduced water level fluctuations from similar to 3 to <1 m .year(-1). Comparison of fossil records showed that reservoir formation did not inevitably lead to eutrophication. Lake Diefenbaker exhibited typical reservoir ontogeny with three trophic periods, including an initial similar to 4-year period of eutrophy, a decade of mesotrophy, and a gradual shift to modern productive conditions. Planktonic taxa dominated diatom communiti es at all times, whereas benthic chironomid and algal remains were rare. In contrast, pigment analyses suggested that phytoplankton standing crops dec lined after impoundment in Buffalo Pound Lake but that chironomid and macro phyte populations expanded. Such site specificity in trophic development ap pears to result from differences in the extent of inundation (500 versus si milar to 5 km(2)) and the magnitude of subsequent water level fluctuations (6.3 versus <1 m).