OTTAWA - Global warming is affecting North America's northernmost lake, where algae growth has increased dramatically in the last two centuries, scientists said on Wednesday.

The team -- led by researchers from Quebec's Laval University -- studied an 18-centimetre (7-inch) long sediment core from Ward Hunt Lake, which is on Ward Hunt Island in Canada's Arctic. The core went back 8,450 years.

Analysis of the deepest layers of sediment revealed a very small number of algae, which the scientists suggested meant the lake had been permanently frozen in the past.

But the top of the core, corresponding to the last 200 years, showed that a pigment found in every species in the lake had increased 500-fold.

"This is, of course, an extreme environment for living organisms, but our data indicate that current conditions make the lake a more favorable location for algae growth than it was in the past," Laval's Dermot Antoniades said in a statement.

"We cannot claim with certainty that these changes were brought on by human activity, but natural variations observed over the last millennia were never so abrupt and extensive."

The lake is permanently covered by a 4-meter (13-foot) layer of ice, except for a small peripheral zone that thaws out during a few weeks every summer.

The Laval team's article will be published in the Sept. 28 edition of the Geophysical Research Letters publications.