Diatom shifts as evidence for recent Subarctic warming in a remote tundra lake, NWT, Canada

Changes in diatom assemblage composition were examined from the sediments of Slipper Lake, an isolated tundra lake located ∼50 km north of current treeline in Canada's Northwest Territories. Diatom shifts over the last ca. 5600 years were related to the response of this ecosystem to climatic and environmental change during the late Holocene, with particular emphasis on the last few hundred years. To date, a detailed diatom analysis for this recent time frame has not been undertaken for the central Canadian Subarctic. The first ca. 5400 years of the sediment record was marked by modest shifts between benthic, alkaliphilous diatom taxa (e.g., Fragilaria, Achnanthes, and Navicula species) and heavily silicified, tychoplanktonic Aulacoseira species. In the 19th century, abrupt changes were delineated by a marked shift to a diatom assemblage characteristic of more planktonic habitats consisting of the Cyclotella stelligera complex (C. stelligera, C. pseudostelligera), which was absent in earlier sediments. Several possible mechanisms for these recent changes are examined, including atmospheric deposition of acidifying compounds and anthropogenically derived nutrient enrichment, however we conclude that climatically induced limnological changes associated with shorter duration of ice cover and a longer growing season can best explain the direction and magnitude of changes in our diatom record.