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Paleolimnological analysis of microfossils and physical sediment characteristics in 210Pb and Ambrosia dated sediment cores, along with diatom-inferred total phosphorus concentration [TP] reconstructions, were used to determine the trophic histories (ca 200 years) of four lakes within the Rideau Canal system, Ontario, Canada. Paleoecological information of the dominant diatom taxa that flourished during the pre-settlement period indicated that these lakes were naturally oligo-mesotrophic. At the estimated time of canal construction, all lakes demonstrated an increase in nutrients but their responses varied in magnitude. These differences were likely related to a number of variables, but the surface-area:watershed ratio appeared to be an important explanatory variable. Additionally, the similar trophic response of the control lake (not part of the canal), Otter Lake, illustrated the regional impact of past watershed disturbance (e.g. logging, settlement, mining, agriculture), not directly related to canal construction. In more recent years (~1970 to present), less productive planktonic species (e.g. Cyclotella comensis and Cyclotella aff. gordonensis) increased in all the study lakes. These recent water quality changes were attributed to improved nutrient retention of developing soils in secondary growth forests, the potential effects of climate warming, as well as mitigation efforts (e.g. decreased phosphorus concentrations in detergents, etc.). Eutrophication patterns determined for the deeper study lakes were similar to paleolimnological studies of other deep lakes in the canal system. However, the trophic response in the shallow lake, Lower Rideau Lake, is more pronounced at the time of canal construction than those of other shallow canal lake responses (e.g. nearby Lake Opinicon) and suggests that both alternative equilibrium states have occurred. This heightened response was attributed to increased nutrient export in Lower Rideau Lake’s limestone catchment and/or higher watershed disturbance. Finally, results from this study furthers our understanding of impacts in an integrated system of lakes and this information can be used to help set realistic mitigation targets for these and other lakes in the Rideau Canal system.
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