History of a presently slightly acidic lake in northeastern Alberta,Canadaas determined through analysis of the diatom record

The Holocene sedimentary diatom record from Otasan Lake, Alberta, has been analyzed to determine the development of this presently slightly acidic lak e. The changes in the lake have been linked to the development of the Sphag num-dominated catchment. Analysis of the stratigraphic data revealed four d istinct zones. The lake record began ca. 8200 yrs BP with a benthic and alk aline diatom assemblage dominated by Ellerbeckia arenaria (Moore) Crawford. At ca. 7300 yrs BP planktonic species began to increase and dominate indic ating increased water levels, decreased turbidity, and increased nutrient l evels. Throughout the Holocene the peatland in the catchment encroached tow ard the modern lake margin and by ca. 5000 yrs BP lake acidity had changed sufficiently such that acidic diatom species dominated. Tabellaria flocculo sa (Roth) Kutz.v. flocculosa Strain IIIp sensu Koppen dominated the record from ca. 5000 to ca. 3100 yrs BP. The lowest lake water pH was inferred for this zone. From ca. 3100 yrs BP to the present Fragilaria species, primari ly F. construens v. venter (Ehr.) Hustedt, dominated the diatom assemblage. Diatom productivity and inferred pH were interpreted as stable. From corre spondence analysis of the fossil samples, and from species assemblages, und erlying gradients of pH, nutrient level, and water depth were inferred. The change from alkaline to slightly acidic conditions took place between ca. 8200 and ca. 5000 yrs BP. From ca. 3000 yrs BP to the present, lake water p H has remained fairly constant. Nutrient levels and water depth were inferr ed to have altered together. After ca. 8200 yrs BP, nutrients and water lev el began to increase until ca. 6000 yrs BP. Then, there was a gradual decli ne in these variables over the most acidic zone until ca. 3000 yrs BP, afte r which they, too, have remained fairly constant. Dominant Boreal Upland Ve getation was established by ca. 7200 yrs BP, and it was inferred that domin ant climate patterns had been established at that time, but small changes i n climate have occurred and the landscape in northeastern Alberta has only been stable for the last 3000 years.