Limnological conditions in a subarctic lake (northern Quebec, Canada) during the late Holocene: analyses based on fossil diatoms

A fossil diatom record covering the past 3000 cal. years BP was analyzed from a small lake in northwestern Quebec near the northern limit of present-day tree-line. Fragilaria virescens var. exigua Grunow in Van Heurck was the dominant species throughout the core with abundances ranging between 13-35% of the total valve count. There was a replacement of alkaliphilous taxa by acidophilous taxa beginning ca. 1300 cal. yr ago, probably reflecting long-term, natural acidification processes. A diatom-based transfer function was used to provide quantitative estimates of variations in lakewater dissolved organic carbon (DOC). These inferred values showed that DOC concentrations have remained stable over the past 3000 years (mean +/- S.D. = 5 +/- 0.43 mg C l(-1)), suggesting relatively constant allochthonous carbon inputs and underwater light conditions during the late Holocene. The reconstructed DOC data were compared to the palynological record from the same lake. Our study indicates that, in contrast to paleolimnological records from lakes in central and western Canada, climatic variations and associated vegetational shifts have been too subtle to cause pronounced variations in DOC in this northern Quebec site.