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Thanks to less ice last winter and an early spring, the top layer of the big lake will be "exceptionally warm by August," according to researchers at the Large Lakes Observatory at the University of Minnesota-Duluth.
Temperatures in the top 30 to 50 feet of water usually peak at 59 degrees in mid-August, but they hit that mark this week. The record of 68 degrees, reached in 1998, could well be matched or broken.
The heat is welcome news for swimmers and some species of fish, but streams feeding the largest Great Lake have seen some fish kills.
"It's going to mean a more pleasant year for tourism," said Jay Austin, associate professor of physics at UMD who is studying lake temperature trends. "It is going to mean a warmer year everywhere on the lake."
That extends to people who live or play along the North Shore, he said. Winds coming off the lake are not so cold this year because the lake is warmer, he said.
The reason for the warming is primarily because of a mild winter with less-than-normal ice on the lake, and a spring that arrived earlier and with higher temperatures.
That sped up the natural cycle, in which summer temperatures create an upper layer of warm water, floating above the bulk of the lake's much colder and denser water. Usually the warm layering begins in mid-July, but this year it started a month earlier. As a result, Lake Superior is about 20 degrees warmer than normal at this time of year.
Austin said the frequency of warmer-than-normal temperatures in recent years is probably an early sign of climate change. "This is a consequence of those larger climatic shifts," he said. "It's consistent with what has been going on everywhere else."
Austin and Steve Colman, director of the Large Lakes Observatory, collaborated on the latest report. In 2007 they found that summer water temperatures in the lake were warming twice as fast as air temperatures over the past 30 years, based on data from several buoys floating in the lake.
Fish could benefit
The effects of warmer lake water could be a mixed bag for different species, according to other researchers.
The lake's normal temperatures are too cold to support a rich array of plant and animal life. The warmth will probably boost the growth of algae, microscopic plants and animals, small fish and even larger predators, according to Jeff Gunderson, director of the University of Minnesota's Sea Grant program. "It's a food-chain response," he said.
The banquet might last longer if the warm water and higher productivity extends later into the fall than usual, he said.
Don Schreiner, Lake Superior fisheries supervisor for the Minnesota Department of Natural Resources, agrees that the higher productivity could fatten up species such as Pacific salmon and rainbow trout. But lake trout don't do well in warmer water, he said, so anglers may need to seek them in deeper areas and further offshore. "There's plenty of cold water in the lake" for the trout, he said. "I don't think they're running out of area at all."
Based upon what happened when temperatures peaked in 1998, said Schreiner, this year's warmth is not expected to have any drastic effects on adult fish populations.
However, the same may not be true for the offspring of salmon and steelhead, which migrate to spawn in Lake Superior's tributaries. After the eggs hatch, said Schreiner, the baby fish live and grow for six months to two years in the streams, which have also been warmed by this year's temperatures.
"The downside is that those fish that are still in the streams as nursery habitat are probably getting stressed more because of the high temperatures," he said. It's rare to see mortality in the young fish, said Schreiner, but this year the DNR has found about 100 dead steelhead in several streams near Duluth.
"This biology business, it's never all good or all bad," he said.
Austin said he'll continue to study the lake's temperatures, and will be interested to learn how long the warmth extends into the fall and whether it affects ice formation. Normally, Lake Superior cools rapidly in September and October, he said, when lower temperatures and strong winds cause the surface water to mix with colder, deeper layers.
Tom Meersman • 612-673-7388
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