Kaffeklubben Sø, the world's most northerly lake, was entombed beneath a near-permanent layer of ice some 2400 years ago. Now it is beginning to thaw – and some of the organisms that disappeared from its waters are beginning to return. The finding is the latest evidence that warmer temperatures in polar regions can result in rapid ecological changes.
Located at 83° 37' north, on the coastal plain of northern Greenland, the 48-hectare Kaffeklubben Sø looks out over the Arctic Sea. "It's kind of the end of the earth," says Bianca Perren of the University of Franche-Comté in Besançon, France. One to two metres of ice cover the lake year-round, but a "moat" of water forms around the edge of the lake in summer when average temperatures rise to 1.6 °C.
The lake formed about 3500 years ago when local precipitation increased, says Perren. A few species of silica-shelled algae called diatoms lived in the young lake, but their populations declined as regional temperatures cooled, and they vanished entirely 2400 years ago. All that survived under the ice were hardy cyanobacteria, which require little light and can survive even under several metres of ice.
A couple of brief summer thaws allowed diatoms to return briefly, but the lake remained nearly barren until around 1960, when the first diatom species returned. The latest water samples, collected by Perren and her colleagues, contain some 20 species.