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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.

Diatom ecology and species compositional patterns across current arctic treeline can provide important paleoecological information associated with climatic and environmental change. In this paper we examine the relationships between measured environmental variables and modern diatom assemblage composition from 77 lakes across the treeline ecozones of the Central Canadian Arctic. The weighted-average optima for selected environmental variables were calculated for 74 of the most common diatoms, and photographic plates of these taxa are included. Our results indicated that both forest-tundra and arctic tundra lakes differed significantly in diatom assemblage composition from boreal forest lakes. In general, planktonic diatom taxa (e.g., Cyclotella species) were more common in forested lakes, which may be due to ecological conditions related to climate. Small, benthic, alkaliphilic Fragilaria taxa reached their highest abundances in forested lakes, likely because of the more alkaline nature of these lakes. Arctic tundra lakes were characterized by higher abundances of circumneutral to acidophilic taxa. Heavily silicified Aulacoseira taxa (e.g., Aulacoseira lirata, Aulacoseira perglabra) were more common in deeper tundra lakes, likely because of the less alkaline nature of these lakes and greater wind-induced turbulence in this zone. These trends provide important information on the variability of aquatic ecosystems across this climatically sensitive vegetational gradient.Key words: arctic treeline, Canada, diatoms, paleolimnology, weighted-average optima, climate change.

Chrysophyte stomatocysts from the surface sediments of 49 lakes located on a north–south transect in the Yukon and Northwest Territories, Canada, were investigated for their potential use as indicators of environmental change in northern latitudes. Photographic plates and descriptions, following International Statospore Working Group guidelines, illustrate 19 new stomatocyst morphotypes. The main patterns of floristic variation in the data set were explored using canonical correspondence analysis, which indicated that gradients of chloride (r2 = 0.73), dissolved inorganic carbon (r2 = 0.63), and surface-water temperature (r2 = 0.55) were important in influencing species assemblages. Compared with the diatom-temperature inference model developed from the same set of lakes, the stomatocysts provided a slightly less robust model. These results suggest that stomatocysts are weakly, though significantly, related to some of the gradients in lake water chemistry in this data set and can provide a complement to other paleoecological markers. Key words: chrysophyte stomatocysts, inference models, northwestern Canada.