The world's largest lake is on course to be warmer than ever due to low ice cover last winter.
Lake Superior's surface waters may reach record temperatures this summer.
Low ice cover this winter gave the lake a head start in heating up.
The lake has been slowly warming over the last 30 years.
Lake Superior -- the world's largest lake by surface area -- may reach record-high temperatures this season, thanks to low ice cover last winter.
The prediction comes from Jay Austin of the University of Minnesota, Duluth, who has documented accelerated warming of Lake Superior over the last 30 years.
"Lakes have very distinctive seasons in them," he told Discovery News. "One of these seasons at low- to mid- latitudes is a season where you have a layer of warm water sitting on top of very cold water."
Austin's temperature tracking shows that the layers developed more than a month early this year. Once they develop, the more buoyant warm water can't mix with the colder water below, so all the summer sun is soaked up by the surface layer.
"That overturn occurs typically in mid-July," Austin said. "This year it occurred in early-to mid-June. The lake is going to have an extra month it's going to be heating up this year."
This date is a close match to the overturn date in 1998 -- a strong El Nino year and the year of the lake's warmest recorded surface water temperatures.
Depending on how the summer unfolds, this year may break that record. "It wouldn't surprise me at all," Austin said. "But we could have a series of systems that bring cool, dry air in and it wouldn't heat up that much."
The early overturn date resulted from reduced ice cover on the lake over the winter, he said. Ice reflects sunlight off the lake, while the darker water surface absorbs the sun's heat. With less ice, the exposed water can absorb more heat, which in turn melts the ice faster, creating a cycle of warming.
"We can't draw inference from one year," Austin said. "But it is part of a long-term trend of lower ice years and warmer temperatures over the last 30 years. When you have a very warm year on top of that trend, that's when you have record-breaking temperatures."
What do higher temperatures mean for the lakes inhabitants?
"That's sort of the $64,000 question for the lake," Austin said. "How does this trickle down for the ecology and the fisheries in the lake? I don't think any of us have a really good feel for how different things are going to be."
"Fish have a specific range of temperatures in which they like to spawn," Austin said. "It may be that for some fish this very warm year is going to be great for them, but for others, like trout which are a very cold-adapted fish, it's not going to be great."
Fish migrations may change, because fish follow temperature boundaries, agreed Randall Hicks, an aquatic microbiologist, also of the University of Minnesota, Duluth, but not a part of Austin's work.
"Temperature is only one of the factors," Hicks said, speaking of the lake's smallest inhabitants. "Microorganisms grow much faster in warming temperatures, but the lake has relatively few nutrients, so there is a limit to how much they can grow."
Changes in microorganism populations can cascade through the ecosystem, since they form the base of the food chain. Blooms of microorganisms can deplete the oxygen in water, killing or driving away other aquatic life. Lake Superior's characteristics make it unlikely to suffer such problems, even as it warms, Hicks said.
"We don't have the anthropogenic impacts on this lake like we do on the other Great Lakes. We don't have agriculture around this lake. We don't' have the large urban areas around the lake," he said "Lake Erie is the shallowest and there are very high nutrient conditions. There, when it warms they develop anoxic bottom waters. I don't think we're going to hit that situation here, even if temperature goes up."
The warmer lake may make at least one species especially happy: humans. "It's going to be, frankly, a more pleasant year to be recreating in the lake," Austin said.