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Minnesota Sea Grant: Lake Superior is cold, but not so icy

Lake Superior has a reputation that's getting harder to keep. People still talk about the lake's coldness, but, so far this year, there isn't much ice to speak of. Despite the thermometer readings, almost all of Lake Superior's surface remains li...

Lake Superior has a reputation that's getting harder to keep. People still talk about the lake's coldness, but, so far this year, there isn't much ice to speak of. Despite the thermometer readings, almost all of Lake Superior's surface remains liquid.

"Sure, these cold blasts have made some ice, but the lake is far from frozen," said Great Lakes Environmental Research Laboratory Scientist George Leshkevich.

Leshkevich manages the National Oceanic and Atmospheric Administration's Great Lakes CoastWatch program, which helps monitor ice cover on the Great Lakes. According to Leshkevich, ice has been scarcer, thinner, and of poorer quality on Lake Superior since 1997.

After poring over multiple historic data sets, Jay Austin, assistant physics professor at UMD's Large Lakes Observatory, concluded that the lake has been becoming gradually warmer, windier, and more iceless over the past 30 years.

Scientists like Leshkevich and Austin are working to understand what Lake Superior's recent warming temperatures and higher wind speeds might mean for the future of ice on this mammoth lake.

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In the interim, they know enough about ice to know what the lack of it means for the coming months. Here's what they and other experts are saying:

Evaporation

Ice puts a lid on evaporation, which crescendos in Lake Superior from fall to early winter. The "sea smoke" you see wisping up from the lake on cold days is the lake evaporating. On windy winter days, dry nor'westers whistle over the lake, sucking up water like a sponge. On average, a foot of water evaporates off of Lake Superior between November and February. Low ice cover and high winds mean even more water leaves the lake.

Lake behavior

Austin reports that ice cover is closely linked with the onset of Lake Superior's stratified season, the time from spring to fall when warmer water lies above much colder water. Less ice cover means stratification gets an earlier start; an earlier start means a longer warm season; a longer warm season means higher water temperatures. Conversely, more ice means a chillier summer swim. In fact, Austin suggests that winter ice cover is the primary indicator of how warm the lake will be the following summer.

Fisheries

Ice cover is generally good for both commercial and recreational fishing. Certainly, ice fishermen, whitefish eggs, and diatoms (microscopic algae) gain advantages when Superior freezes.

When ice is thick and far-reaching, people can catch lake trout a quarter mile or more off shore. More commonly, ice fishermen are confined to catching Kamloops and Coho within 20 yards of land.

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Ice cover is particularly important to the reproductive success of fall-spawning fish, such as whitefish, whose eggs lie dormant over winter. Open water exposes the eggs to more predators, and to wind and waves, which can damage them or toss them ashore.

Although ice limits photosynthesis by diminishing the sunshine of winter's short days, diatoms can still go about their business. Euan Reavie, a research associate with the UMD's Natural Resources Research Institute, studies phytoplankton. He said that diatoms have a competitive advantage over other microscopic organisms in cold, ice-covered environments because they can continue to reproduce.

"Certainly there will be changes in the Lake Superior phytoplankton community if ice cover continues to decline," Reavie said.

Other human concerns

Although ice keeps more water in the lake, which aids shipping, it bars shipping for about two months. Superior's ice also complicates hydropower production and damages shoreline structures.

Beyond its functional properties, Superior's ice can take your breath away (assuming the bitter winds haven't first). The formations can take a striking variety of configurations, from acres of glassy black ice to rafted sculptures over 10 feet high. One of the first ice types to form on Lake Superior is an ice foot, a conglomerate of ice balls and wave spray that freezes to shore.

By March, ice volcanoes sometimes form along the Minnesota and Wisconsin Point shorelines. These "volcanoes" are born as high surf slams against the face of an ice shelf, sending a mighty pulse of water underneath that bursts from cracks or weak points.

Lake Superior is never in a rush to freeze and rarely freezes all the way (the last time was in 1994; almost in 2003).

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For an accurate image of today's ice conditions on the lake, go to the Great Lakes Environmental Research Laboratory's CoastWatch Web site: http://coastwatch.glerl.noaa.gov .

Written by Sharon Moen, science writer for the Minnesota Sea Grant Program at UMD

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