With less than a week left in April, ice still covers about two-thirds of Lake Superior —  more than any year in recent memory, according to an American researcher.

Great Lakes Environmental Research Laboratory scientist George Leshkevich said 2009 was what he calls a "fairly severe" ice year, but Superior was only about six per cent frozen on this date.

"This year … is quite different. [It’s] quite unusual, compared to recent years,” he said.

George Leshkevich

George Leshkevich says the fact that Lake Superior remains mostly ice-covered in late April is "quite different" from recent years. (Supplied)

"If you plot a trend line through the maximum ice cover for each year … it's downward. That trend line is going to be adjusted this year."

Cold air masses that hovered over the Lake Superior basin in recent months are to blame, Leshkevich added. As of Wednesday, Lake Superior was 67.8 per cent covered in ice.

Last year, Lake Superior was 2.7 per cent ice-covered on April 22, he said.

Cooler summer ahead?

The consequences of lingering ice cover on Lake Superior are "economic, societal and ecological,” Leshkevic added, citing the late start to shipping season.

"The shipping industry has had a rough time this spring.”

However, there are numerous ecological benefits to extensive ice cover, including more protection for spawning beds for several fish species, including whitefish, Leshkevic noted.

"So that could help the recruitment of that species in the coming season.”

But Jay Austin, an associate professor at the Large Lakes Observatory in Duluth, Minn., said extensive ice cover usually precedes a cool summer.

"The summer will start a lot later in these high-ice years," he said, "And you end up with lower summer temperatures."

Leshkevich said some ice could stay on Lake Superior as late as the middle of May.

lake superior satellite image april 24, 2014

This image from NASA's Aqua satellite shows Lake Superior on Sunday, April 20. An American researcher says the lake was 67.8 per cent ice-covered as of Thursday, April 24. (Supplied)