Abstract
Lac Saint-Augustin provides a unique record of anthropogenic change during the settlement and evolution of Québec City, one of North America’s oldest cities. Lac Saint-Augustin is an urban lake that has been subject to three centuries of direct human impacts, with the most pronounced changes in trophic status and chemistry occurring within the last few decades. This accelerated eutrophication has greatly limited the present recreational opportunities for lake visitors and residents. A paleolimnological study of Lac Saint-Augustin was undertaken to identify the causes of this degradation and to place these changes within the context of historical development in the lake’s catchment. Biological and chemical indicators identified three major periods in the recent history of Lac Saint-Augustin. The first period (1670–1750) corresponds to the arrival of the first settlers at the lake and the construction of a diversion canal; at that time, the lake was characterized by oligo- to meso-trophic conditions (as indicated by diatom species of the genus Aulacoseira ) and low concentrations of chlorophyll a, total carotenoids, total phosphorus, and metals. The second period (1750–1904) corresponds to increased human colonization and was marked by slight increases in sedimentary metal and total phosphorus concentrations. The third period (1904–2002) coincides with an intensification of agriculture, the expansion of Québec City, and the urbanization of the catchment. At this time, there was a complete change in the diatom flora (predominance of Stephanodiscus hantzschii ) as well as major increases in fossil pigment concentrations, nutrients, and sedimentary chemical levels. Diatom species typical of saline waters also appeared coincident with the use of road salts on a highway that passes through the lake’s catchment. With the increasing demands for greater use of inland waters, multiproxy, long-term data of the kind provided here are essential to make informed management decisions.
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