Something seems amiss with mighty Superior, the deepest and coldest of the Great Lakes, which together hold nearly 20 per cent of the world's fresh surface water.

Superior's surface area is roughly the same as South Carolina's, the biggest of any freshwater lake on Earth. It's deep enough to hold all the other Great Lakes plus three additional Lake Eries. Yet over the past year, its level has ebbed to the lowest point in eight decades and will set a record this fall if, as expected, it dips 7.6 more centimetres.

Its average temperature has surged about 4.5 degrees Farenheit since 1979, significantly above the 2.7-degree rise in the region's air temperature during the same period. That's no small deal for a freshwater sea that was created from glacial melt as the Ice Age ended and remains chilly in all seasons.
As the research boat bobs up and down on grey, choppy Lake Superior, Michigan Tech University chemist Noel Urban and two students drop a metal cylinder over the side to retrieve a water sample from the bottom.

They are measuring carbon dioxide content - an unspectacular statistic by itself, yet an important piece of a highly complex puzzle.

"It helps us develop a model that can say what's going to happen as the lake warms up," Urban says.

Plenty of people are wondering the same thing.

A weather buoy on the western side recently recorded an ''amazing" 24 degrees Celsius, "as warm a surface temperature as we've ever seen in this lake," says Jay Austin, assistant professor at the University of Minnesota at Duluth's Large Lakes Observatory.

Water levels also have receded on the other Great Lakes since the late 1990s. But the suddenness and severity of Superior's changes worry many in the region; it has plunged more than 30 centimetres in the past year. Shorelines are dozens of metres wider than usual, giving sunbathers wider beaches but also exposing mucky bottomlands and rotting vegetation.

"C'mon, girls, get out of the mud," Dan Arsenault, 32, calls to his two young daughters at a park near the mouth of the St. Marys River on the southeastern end of Lake Superior. Bree, five, and three-year-old Andie are stomping in puddles where water was waist-deep a couple of years ago. The floatation rope that previously designated the swimming area now rests on moist ground.

"This is the lowest I've ever seen it," says Arsenault, a lifelong resident of Sault Ste. Marie in Michigan's Upper Peninsula.

Superior still has lots of water. Its average depth is 147 metres and it reaches 406 metres at the deepest point. Erie, the shallowest Great Lake, is 64 metres at its deepest and averages only 19 metres. Lake Michigan averages 85 metres and is 281 metres at its deepest.

Yet along Superior's shores, boats can't reach many mooring sites and marina operators are begging the U.S. Army Corps of Engineers to dredge shallow harbors. Ferry service between Grand Portage, Minn., and Isle Royale National Park was scaled back because one of the company's boats couldn't dock.

Sally Zabelka has turned away boaters from Chippewa Landing marina in the eastern Upper Peninsula, where not long ago 8.2-metre vessels easily made their way up the channel from the lake's Brimley Bay. "In essence, our dock is useless this year," she says.

Another worry: As the bay heats up, the perch, walleye and smallmouth bass that have lured anglers to her campground and tackle shop are migrating to cooler waters in the open lake.

Low water has cost the shipping industry millions of dollars. Vessels are carrying lighter loads of iron ore and coal to avoid running aground in shallow channels.

Superior's retreat creates a double whammy in Grand Marais, where the only deepwater harbour of refuge along a 145-kilometres, shipwreck-strewn section of the lake already was filling with sand because of a decaying breakwall.

Burt Township, the local government, is extending the harbour's boat launching ramp an additional 12 metres, Supervisor Jack Hubbard says. Sand and shallow water are choking off aquatic vegetation that once provided habitat for hefty pike and trout.

Puffing on a pipe in a Grand Marais pub, retiree Ted Sietsema voices the suspicion held by many in the villages along Superior's southern shoreline: Someone is taking the water. The government is diverting it to places with more people and political influence - along Lakes Huron and Michigan and even the Sun Belt, via the Mississippi River.

"Don't give me that global warming stuff," Sietsema says. ''That water is going west. That big aquifer out there is empty but they can still water the desert. It's got to be coming from somewhere."

A familiar theory - but all wet, says Scott Thieme, hydraulics and hydrology chief with the Corps of Engineers district office in Detroit. Water does exit Lake Superior through locks, power plants and gates on the St. Marys River, but in amounts strictly regulated under a 1909 pact with Canada.

The actual forces at work, while mysterious, are not the stuff of spy novels, Thieme says.

Precipitation has tapered off across the upper Great Lakes since the 1970s and is nearly 15 centimetres below normal in the Superior watershed the past year. Water evaporation rates are up sharply because mild winters have shrunk the winter ice cap - just as climate change computer models predict for the next half-century.

Yet those models also envision more precipitation as global warming sets in, says Brent Lofgren, a physical scientist with the Great Lakes Environmental Research Laboratory in Ann Arbor. Instead there's drought, suggesting other causes.

Cynthia Sellinger, the lab's deputy director, suspects residual effects of El Nino, the warming of equatorial Pacific waters that produced warmer winters in the late 1990s, just as the lakes began receding.

Both long-term climate change and short-term meteorological factors may be driving water levels down, says Urban, the Michigan Tech researcher.

But he and Austin are more concerned about effects than causes. There's a big knowledge gap about how food webs and other aquatic systems will respond to warmer temperatures, they say.

"It's just not clear what the ultimate result will be as we turn the knob up," says Austin, the Minnesota-Duluth professor. "It could be great for fisheries or fisheries could crash."

That's a question Urban and his colleagues want to help answer with their carbon dioxide measurements on Lake Superior. Plugging those and other statistics into comprehensive ecosystem models will give scientists a basis for making predictions.

"We're always reacting to what's already happened instead of looking forward," Urban says. "As long as we have a poor understanding of the basic functions of the lake, we won't be able to say whether this warming is of major concern or not."